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Access Control Lists (ACLs) are defined in a separate section of the run time configuration file, headed by "begin acl". Each ACL definition starts with a name, terminated by a colon. Here is a complete ACL section that contains just one very small ACL:
begin acl small_acl: accept hosts = one.host.only |
You can have as many lists as you like in the ACL section, and the order in which they appear does not matter. The lists are self-terminating.
The majority of ACLs are used to control Exim's behaviour when it receives
certain SMTP commands. This applies both to incoming TCP/IP connections, and
when a local process submits a message using SMTP by specifying the -bs
option. The most common use is for controlling which recipients are accepted
in incoming messages. In addition, you can define an ACL that is used to check
local non-SMTP messages. The default configuration file contains an example of
a realistic ACL for checking RCPT commands. This is discussed in chapter
The default configuration file.
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The -bh
command line option provides a way of testing your ACL
configuration locally by running a fake SMTP session with which you interact.
The host relay-test.mail-abuse.org provides a service for checking your
relaying configuration (see section Checking a relay configuration for more details).
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In order to cause an ACL to be used, you have to name it in one of the relevant options in the main part of the configuration. These options are:
| ACL for non-SMTP messages |
| ACL for non-SMTP MIME parts |
| ACL at start of non-SMTP message |
| ACL for AUTH |
| ACL for start of SMTP connection |
| ACL after DATA is complete |
| ACL for ETRN |
| ACL for EXPN |
| ACL for HELO or EHLO |
| ACL for MAIL |
| ACL for the AUTH parameter of MAIL |
| ACL for content-scanning MIME parts |
| ACL for non-QUIT terminations |
| ACL at start of DATA command |
| ACL for QUIT |
| ACL for RCPT |
| ACL for STARTTLS |
| ACL for VRFY |
For example, if you set
acl_smtp_rcpt = small_acl |
the little ACL defined above is used whenever Exim receives a RCPT command in an SMTP dialogue. The majority of policy tests on incoming messages can be done when RCPT commands arrive. A rejection of RCPT should cause the sending MTA to give up on the recipient address contained in the RCPT command, whereas rejection at other times may cause the client MTA to keep on trying to deliver the message. It is therefore recommended that you do as much testing as possible at RCPT time.
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The non-SMTP ACLs apply to all non-interactive incoming messages, that is, they
apply to batched SMTP as well as to non-SMTP messages. (Batched SMTP is not
really SMTP.) Many of the ACL conditions (for example, host tests, and tests on
the state of the SMTP connection such as encryption and authentication) are not
relevant and are forbidden in these ACLs. However, the sender and recipients
are known, so the senders
and sender_domains
conditions and the
$sender_address
and $recipients
variables can be used. Variables such as
$authenticated_sender
are also available. You can specify added header lines
in any of these ACLs.
The acl_not_smtp_start
ACL is run right at the start of receiving a
non-SMTP message, before any of the message has been read. (This is the
analogue of the acl_smtp_predata
ACL for SMTP input.) In the case of
batched SMTP input, it runs after the DATA command has been reached. The
result of this ACL is ignored; it cannot be used to reject a message. If you
really need to, you could set a value in an ACL variable here and reject based
on that in the acl_not_smtp
ACL. However, this ACL can be used to set
controls, and in particular, it can be used to set
control = suppress_local_fixups |
This cannot be used in the other non-SMTP ACLs because by the time they are run, it is too late.
The acl_not_smtp_mime
ACL is available only when Exim is compiled with the
content-scanning extension. For details, see chapter Content scanning at ACL time.
The acl_not_smtp
ACL is run just before the local_scan()
function. Any
kind of rejection is treated as permanent, because there is no way of sending a
temporary error for these kinds of message.
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The ACL test specified by acl_smtp_connect
happens at the start of an SMTP
session, after the test specified by host_reject_connection
(which is now
an anomaly) and any TCP Wrappers testing (if configured). If the connection is
accepted by an accept
verb that has a message
modifier, the contents of
the message override the banner message that is otherwise specified by the
smtp_banner
option.
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The ACL test specified by acl_smtp_helo
happens when the client issues an
EHLO or HELO command, after the tests specified by helo_accept_junk_hosts
,
helo_allow_chars
, helo_verify_hosts
, and helo_try_verify_hosts
.
Note that a client may issue more than one EHLO or HELO command in an SMTP
session, and indeed is required to issue a new EHLO or HELO after successfully
setting up encryption following a STARTTLS command.
If the command is accepted by an accept
verb that has a message
modifier, the message may not contain more than one line (it will be truncated
at the first newline and a panic logged if it does). Such a message cannot
affect the EHLO options that are listed on the second and subsequent lines of
an EHLO response.
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Two ACLs are associated with the DATA command, because it is two-stage
command, with two responses being sent to the client.
When the DATA command is received, the ACL defined by acl_smtp_predata
is obeyed. This gives you control after all the RCPT commands, but before
the message itself is received. It offers the opportunity to give a negative
response to the DATA command before the data is transmitted. Header lines
added by MAIL or RCPT ACLs are not visible at this time, but any that
are defined here are visible when the acl_smtp_data
ACL is run.
You cannot test the contents of the message, for example, to verify addresses
in the headers, at RCPT time or when the DATA command is received. Such
tests have to appear in the ACL that is run after the message itself has been
received, before the final response to the DATA command is sent. This is
the ACL specified by acl_smtp_data
, which is the second ACL that is
associated with the DATA command.
For both of these ACLs, it is not possible to reject individual recipients. An error response rejects the entire message. Unfortunately, it is known that some MTAs do not treat hard (5xx) responses to the DATA command (either before or after the data) correctly - they keep the message on their queues and try again later, but that is their problem, though it does waste some of your resources.
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The acl_smtp_mime
option is available only when Exim is compiled with the
content-scanning extension. For details, see chapter Content scanning at ACL time.
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The ACL for the SMTP QUIT command is anomalous, in that the outcome of the ACL
does not affect the response code to QUIT, which is always 221. Thus, the ACL
does not in fact control any access. For this reason, the only verbs that are
permitted are accept
and warn
.
This ACL can be used for tasks such as custom logging at the end of an SMTP
session. For example, you can use ACL variables in other ACLs to count
messages, recipients, etc., and log the totals at QUIT time using one or
more logwrite
modifiers on a warn
verb.
Warning: Only the $acl_c
x variables can be used for this, because
the $acl_m
x variables are reset at the end of each incoming message.
You do not need to have a final accept
, but if you do, you can use a
message
modifier to specify custom text that is sent as part of the 221
response to QUIT.
This ACL is run only for a "normal" QUIT. For certain kinds of disastrous failure (for example, failure to open a log file, or when Exim is bombing out because it has detected an unrecoverable error), all SMTP commands from the client are given temporary error responses until QUIT is received or the connection is closed. In these special cases, the QUIT ACL does not run.
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The not-QUIT ACL, specified by smtp_notquit_acl
, is run in most cases when
an SMTP session ends without sending QUIT. However, when Exim itself is is bad
trouble, such as being unable to write to its log files, this ACL is not run,
because it might try to do things (such as write to log files) that make the
situation even worse.
Like the QUIT ACL, this ACL is provided to make it possible to do customized
logging or to gather statistics, and its outcome is ignored. The delay
modifier is forbidden in this ACL, and the only permitted verbs are accept
and warn
.
When the not-QUIT ACL is running, the variable $smtp_notquit_reason
is set
to a string that indicates the reason for the termination of the SMTP
connection. The possible values are:
‘acl-drop’ | Another ACL issued a |
‘bad-commands’ | Too many unknown or non-mail commands |
‘command-timeout’ | Timeout while reading SMTP commands |
‘connection-lost’ | The SMTP connection has been lost |
‘data-timeout’ | Timeout while reading message data |
‘local-scan-error’ | The |
‘local-scan-timeout’ | The |
‘signal-exit’ | SIGTERM or SIGINT |
‘synchronization-error’ | SMTP synchronization error |
‘tls-failed’ | TLS failed to start |
In most cases when an SMTP connection is closed without having received QUIT,
Exim sends an SMTP response message before actually closing the connection.
With the exception of the ‘acl-drop’ case, the default message can be
overridden by the message
modifier in the not-QUIT ACL. In the case of a
drop
verb in another ACL, it is the message from the other ACL that is
used.
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The value of an acl_smtp_
xxx option is expanded before use, so
you can use different ACLs in different circumstances. For example,
acl_smtp_rcpt = ${if ={25}{$interface_port} \ {acl_check_rcpt} {acl_check_rcpt_submit} } |
In the default configuration file there are some example settings for providing an RFC 4409 message submission service on port 587 and a non-standard "smtps" service on port 465. You can use a string expansion like this to choose an ACL for MUAs on these ports which is more appropriate for this purpose than the default ACL on port 25.
The expanded string does not have to be the name of an ACL in the configuration file; there are other possibilities. Having expanded the string, Exim searches for an ACL as follows:
acl_smtp_data = /etc/acls/\ ${lookup{$sender_host_address}lsearch\ {/etc/acllist}{$value}{default}} |
This looks up an ACL file to use on the basis of the host's IP address, falling back to a default if the lookup fails. If an ACL is successfully read from a file, it is retained in memory for the duration of the Exim process, so that it can be re-used without having to re-read the file.
acl_smtp_vrfy = accept |
in order to allow free use of the VRFY command. Such a string may contain newlines; it is processed in the same way as an ACL that is read from a file.
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Except for the QUIT ACL, which does not affect the SMTP return code (see section The QUIT ACL above), the result of running an ACL is either "accept" or "deny", or, if some test cannot be completed (for example, if a database is down), "defer". These results cause 2xx, 5xx, and 4xx return codes, respectively, to be used in the SMTP dialogue. A fourth return, "error", occurs when there is an error such as invalid syntax in the ACL. This also causes a 4xx return code.
For the non-SMTP ACL, "defer" and "error" are treated in the same way as "deny", because there is no mechanism for passing temporary errors to the submitters of non-SMTP messages.
ACLs that are relevant to message reception may also return "discard". This has the effect of "accept", but causes either the entire message or an individual recipient address to be discarded. In other words, it is a blackholing facility. Use it with care.
If the ACL for MAIL returns "discard", all recipients are discarded, and no
ACL is run for subsequent RCPT commands. The effect of "discard" in a
RCPT ACL is to discard just the one recipient address. If there are no
recipients left when the message's data is received, the DATA ACL is not
run. A "discard" return from the DATA or the non-SMTP ACL discards all the
remaining recipients. The "discard" return is not permitted for the
acl_smtp_predata
ACL.
The local_scan()
function is always run, even if there are no remaining
recipients; it may create new recipients.
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The default actions when any of the acl_
xxx options are unset are not
all the same. Note: These defaults apply only when the relevant ACL is
not defined at all. For any defined ACL, the default action when control
reaches the end of the ACL statements is "deny".
For acl_smtp_quit
and acl_not_smtp_start
there is no default because
these two are ACLs that are used only for their side effects. They cannot be
used to accept or reject anything.
For acl_not_smtp
, acl_smtp_auth
, acl_smtp_connect
,
acl_smtp_data
, acl_smtp_helo
, acl_smtp_mail
, acl_smtp_mailauth
,
acl_smtp_mime
, acl_smtp_predata
, and acl_smtp_starttls
, the action
when the ACL is not defined is "accept".
For the others (acl_smtp_etrn
, acl_smtp_expn
, acl_smtp_rcpt
, and
acl_smtp_vrfy
), the action when the ACL is not defined is "deny".
This means that acl_smtp_rcpt
must be defined in order to receive any
messages over an SMTP connection. For an example, see the ACL in the default
configuration file.
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When a MAIL or RCPT ACL, or either of the DATA ACLs, is running, the variables
that contain information about the host and the message's sender (for example,
$sender_host_address
and $sender_address
) are set, and can be used in ACL
statements. In the case of RCPT (but not MAIL or DATA), $domain
and
$local_part
are set from the argument address. The entire SMTP command
is available in $smtp_command
.
When an ACL for the AUTH parameter of MAIL is running, the variables that
contain information about the host are set, but $sender_address
is not yet
set. Section The AUTH parameter on MAIL commands contains a discussion of this parameter and
how it is used.
The $message_size
variable is set to the value of the SIZE parameter on
the MAIL command at MAIL, RCPT and pre-data time, or to -1 if
that parameter is not given. The value is updated to the true message size by
the time the final DATA ACL is run (after the message data has been
received).
The $rcpt_count
variable increases by one for each RCPT command received.
The $recipients_count
variable increases by one each time a RCPT command is
accepted, so while an ACL for RCPT is being processed, it contains the number
of previously accepted recipients. At DATA time (for both the DATA ACLs),
$rcpt_count
contains the total number of RCPT commands, and
$recipients_count
contains the total number of accepted recipients.
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When an ACL is being run for AUTH, EHLO, ETRN, EXPN, HELO, STARTTLS, or VRFY,
the remainder of the SMTP command line is placed in $smtp_command_argument
,
and the entire SMTP command is available in $smtp_command
.
These variables can be tested using a condition
condition. For example,
here is an ACL for use with AUTH, which insists that either the session is
encrypted, or the CRAM-MD5 authentication method is used. In other words, it
does not permit authentication methods that use cleartext passwords on
unencrypted connections.
acl_check_auth: accept encrypted = * accept condition = ${if eq{${uc:$smtp_command_argument}}\ {CRAM-MD5}} deny message = TLS encryption or CRAM-MD5 required |
(Another way of applying this restriction is to arrange for the authenticators
that use cleartext passwords not to be advertised when the connection is not
encrypted. You can use the generic server_advertise_condition
authenticator
option to do this.)
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An individual ACL consists of a number of statements. Each statement starts with a verb, optionally followed by a number of conditions and "modifiers". Modifiers can change the way the verb operates, define error and log messages, set variables, insert delays, and vary the processing of accepted messages.
If all the conditions are met, the verb is obeyed. The same condition may be used (with different arguments) more than once in the same statement. This provides a means of specifying an "and" conjunction between conditions. For example:
deny dnslists = list1.example dnslists = list2.example |
If there are no conditions, the verb is always obeyed. Exim stops evaluating the conditions and modifiers when it reaches a condition that fails. What happens then depends on the verb (and in one case, on a special modifier). Not all the conditions make sense at every testing point. For example, you cannot test a sender address in the ACL that is run for a VRFY command.
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The ACL verbs are as follows:
accept
: If all the conditions are met, the ACL returns "accept". If any
of the conditions are not met, what happens depends on whether endpass
appears among the conditions (for syntax see below). If the failing condition
is before endpass
, control is passed to the next ACL statement; if it is
after endpass
, the ACL returns "deny". Consider this statement, used to
check a RCPT command:
accept domains = +local_domains endpass verify = recipient |
If the recipient domain does not match the domains
condition, control
passes to the next statement. If it does match, the recipient is verified, and
the command is accepted if verification succeeds. However, if verification
fails, the ACL yields "deny", because the failing condition is after
endpass
.
The endpass
feature has turned out to be confusing to many people, so its
use is not recommended nowadays. It is always possible to rewrite an ACL so
that endpass
is not needed, and it is no longer used in the default
configuration.
If a message
modifier appears on an accept
statement, its action
depends on whether or not endpass
is present. In the absence of endpass
(when an accept
verb either accepts or passes control to the next
statement), message
can be used to vary the message that is sent when an
SMTP command is accepted. For example, in a RCPT ACL you could have:
accept <some conditions> message = OK, I will allow you through today |
You can specify an SMTP response code, optionally followed by an "extended
response code" at the start of the message, but the first digit must be the
same as would be sent by default, which is 2 for an accept
verb.
If endpass
is present in an accept
statement, message
specifies
an error message that is used when access is denied. This behaviour is retained
for backward compatibility, but current "best practice" is to avoid the use
of endpass
.
defer
: If all the conditions are true, the ACL returns "defer" which, in
an SMTP session, causes a 4xx response to be given. For a non-SMTP ACL,
defer
is the same as deny
, because there is no way of sending a
temporary error. For a RCPT command, defer
is much the same as using a
redirect
router and ‘:defer:’ while verifying, but the defer
verb can
be used in any ACL, and even for a recipient it might be a simpler approach.
deny
: If all the conditions are met, the ACL returns "deny". If any of
the conditions are not met, control is passed to the next ACL statement. For
example,
deny dnslists = blackholes.mail-abuse.org |
rejects commands from hosts that are on a DNS black list.
discard
: This verb behaves like accept
, except that it returns
"discard" from the ACL instead of "accept". It is permitted only on ACLs
that are concerned with receiving messages. When all the conditions are true,
the sending entity receives a "success" response. However, discard
causes
recipients to be discarded. If it is used in an ACL for RCPT, just the one
recipient is discarded; if used for MAIL, DATA or in the non-SMTP ACL, all the
message's recipients are discarded. Recipients that are discarded before DATA
do not appear in the log line when the log_recipients
log selector is set.
If the log_message
modifier is set when discard
operates,
its contents are added to the line that is automatically written to the log.
The message
modifier operates exactly as it does for accept
.
drop
: This verb behaves like deny
, except that an SMTP connection is
forcibly closed after the 5xx error message has been sent. For example:
drop message = I don't take more than 20 RCPTs condition = ${if > {$rcpt_count}{20}} |
There is no difference between deny
and drop
for the connect-time ACL.
The connection is always dropped after sending a 550 response.
require
: If all the conditions are met, control is passed to the next ACL
statement. If any of the conditions are not met, the ACL returns "deny". For
example, when checking a RCPT command,
require message = Sender did not verify verify = sender |
passes control to subsequent statements only if the message's sender can be
verified. Otherwise, it rejects the command. Note the positioning of the
message
modifier, before the verify
condition. The reason for this is
discussed in section Condition and modifier processing.
warn
: If all the conditions are true, a line specified by the
log_message
modifier is written to Exim's main log. Control always passes
to the next ACL statement. If any condition is false, the log line is not
written. If an identical log line is requested several times in the same
message, only one copy is actually written to the log. If you want to force
duplicates to be written, use the logwrite
modifier instead.
If log_message
is not present, a warn
verb just checks its conditions
and obeys any "immediate" modifiers (such as control
, set
,
logwrite
, and add_header
) that appear before the first failing
condition. There is more about adding header lines in section
Adding header lines in ACLs.
If any condition on a warn
statement cannot be completed (that is, there is
some sort of defer), the log line specified by log_message
is not written.
This does not include the case of a forced failure from a lookup, which
is considered to be a successful completion. After a defer, no further
conditions or modifiers in the warn
statement are processed. The incident
is logged, and the ACL continues to be processed, from the next statement
onwards.
When one of the warn
conditions is an address verification that fails, the
text of the verification failure message is in $acl_verify_message
. If you
want this logged, you must set it up explicitly. For example:
warn !verify = sender log_message = sender verify failed: $acl_verify_message |
At the end of each ACL there is an implicit unconditional deny
.
As you can see from the examples above, the conditions and modifiers are written one to a line, with the first one on the same line as the verb, and subsequent ones on following lines. If you have a very long condition, you can continue it onto several physical lines by the usual backslash continuation mechanism. It is conventional to align the conditions vertically.
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There are some special variables that can be set during ACL processing. They
can be used to pass information between different ACLs, different invocations
of the same ACL in the same SMTP connection, and between ACLs and the routers,
transports, and filters that are used to deliver a message. The names of these
variables must begin with $acl_c
or $acl_m
, followed either by a digit or
an underscore, but the remainder of the name can be any sequence of
alphanumeric characters and underscores that you choose. There is no limit on
the number of ACL variables. The two sets act as follows:
$acl_c
persist
throughout an SMTP connection. They are never reset. Thus, a value that is set
while receiving one message is still available when receiving the next message
on the same SMTP connection.
$acl_m
persist only
while a message is being received. They are reset afterwards. They are also
reset by MAIL, RSET, EHLO, HELO, and after starting up a TLS session.
When a message is accepted, the current values of all the ACL variables are
preserved with the message and are subsequently made available at delivery
time. The ACL variables are set by a modifier called set
. For example:
accept hosts = whatever set acl_m4 = some value accept authenticated = * set acl_c_auth = yes |
Note: A leading dollar sign is not used when naming a variable that is to
be set. If you want to set a variable without taking any action, you can use a
warn
verb without any other modifiers or conditions.
What happens if a syntactically valid but undefined ACL variable is
referenced depends on the setting of the strict_acl_vars
option. If it is
false (the default), an empty string is substituted; if it is true, an
error is generated.
Versions of Exim before 4.64 have a limited set of numbered variables, but their names are compatible, so there is no problem with upgrading.
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An exclamation mark preceding a condition negates its result. For example:
deny domains = *.dom.example !verify = recipient |
causes the ACL to return "deny" if the recipient domain ends in dom.example and the recipient address cannot be verified. Sometimes negation can be used on the right-hand side of a condition. For example, these two statements are equivalent:
deny hosts = !192.168.3.4 deny !hosts = 192.168.3.4 |
However, for many conditions (verify
being a good example), only left-hand
side negation of the whole condition is possible.
The arguments of conditions and modifiers are expanded. A forced failure of an expansion causes a condition to be ignored, that is, it behaves as if the condition is true. Consider these two statements:
accept senders = ${lookup{$host_name}lsearch\ {/some/file}{$value}fail} accept senders = ${lookup{$host_name}lsearch\ {/some/file}{$value}{}} |
Each attempts to look up a list of acceptable senders. If the lookup succeeds,
the returned list is searched, but if the lookup fails the behaviour is
different in the two cases. The fail
in the first statement causes the
condition to be ignored, leaving no further conditions. The accept
verb
therefore succeeds. The second statement, however, generates an empty list when
the lookup fails. No sender can match an empty list, so the condition fails,
and therefore the accept
also fails.
ACL modifiers appear mixed in with conditions in ACL statements. Some of them
specify actions that are taken as the conditions for a statement are checked;
others specify text for messages that are used when access is denied or a
warning is generated. The control
modifier affects the way an incoming
message is handled.
The positioning of the modifiers in an ACL statement important, because the
processing of a verb ceases as soon as its outcome is known. Only those
modifiers that have already been encountered will take effect. For example,
consider this use of the message
modifier:
require message = Can't verify sender verify = sender message = Can't verify recipient verify = recipient message = This message cannot be used |
If sender verification fails, Exim knows that the result of the statement is
"deny", so it goes no further. The first message
modifier has been seen,
so its text is used as the error message. If sender verification succeeds, but
recipient verification fails, the second message is used. If recipient
verification succeeds, the third message becomes "current", but is never used
because there are no more conditions to cause failure.
For the deny
verb, on the other hand, it is always the last message
modifier that is used, because all the conditions must be true for rejection to
happen. Specifying more than one message
modifier does not make sense, and
the message can even be specified after all the conditions. For example:
deny hosts = ... !senders = *@my.domain.example message = Invalid sender from client host |
The "deny" result does not happen until the end of the statement is reached, by which time Exim has set up the message.
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The ACL modifiers are as follows:
This modifier specifies one or more header lines that are to be added to an incoming message, assuming, of course, that the message is ultimately accepted. For details, see section Adding header lines in ACLs.
This modifier does nothing of itself, and processing of the ACL always
continues with the next condition or modifier. The value of continue
is in
the side effects of expanding its argument. Typically this could be used to
update a database. It is really just a syntactic tidiness, to avoid having to
write rather ugly lines like this:
condition = ${if eq{0}{<some expansion>}{true}{true}} |
Instead, all you need is
continue = <some expansion> |
This modifier affects the subsequent processing of the SMTP connection or of an
incoming message that is accepted. The effect of the first type of control
lasts for the duration of the connection, whereas the effect of the second type
lasts only until the current message has been received. The message-specific
controls always apply to the whole message, not to individual recipients,
even if the control
modifier appears in a RCPT ACL.
As there are now quite a few controls that can be applied, they are described
separately in section Use of the control modifier. The control
modifier can be used
in several different ways. For example:
accept
statement:
accept ...some conditions control = queue_only |
In this case, the control is applied when this statement yields "accept", in other words, when the conditions are all true.
accept
statement:
accept ...some conditions... control = queue_only ...some more conditions... |
If the first set of conditions are true, the control is applied, even if the statement does not accept because one of the second set of conditions is false. In this case, some subsequent statement must yield "accept" for the control to be relevant.
warn
to apply the control, leaving the
decision about accepting or denying to a subsequent verb. For
example:
warn ...some conditions... control = freeze accept ... |
This example of warn
does not contain message
, log_message
, or
logwrite
, so it does not add anything to the message and does not write a
log entry.
require
verb. For example:
require control = no_multiline_responses |
This modifier may appear in any ACL. It causes Exim to wait for the time
interval before proceeding. However, when testing Exim using the -bh
option, the delay is not actually imposed (an appropriate message is output
instead). The time is given in the usual Exim notation, and the delay happens
as soon as the modifier is processed. In an SMTP session, pending output is
flushed before the delay is imposed.
Like control
, delay
can be used with accept
or deny
, for
example:
deny ...some conditions... delay = 30s |
The delay happens if all the conditions are true, before the statement returns "deny". Compare this with:
deny delay = 30s ...some conditions... |
which waits for 30s before processing the conditions. The delay
modifier
can also be used with warn
and together with control
:
warn ...some conditions... delay = 2m control = freeze accept ... |
If delay
is encountered when the SMTP PIPELINING extension is in use,
responses to several commands are no longer buffered and sent in one packet (as
they would normally be) because all output is flushed before imposing the
delay. This optimization is disabled so that a number of small delays do not
appear to the client as one large aggregated delay that might provoke an
unwanted timeout. You can, however, disable output flushing for delay
by
using a control
modifier to set no_delay_flush
.
This modifier, which has no argument, is recognized only in accept
and
discard
statements. It marks the boundary between the conditions whose
failure causes control to pass to the next statement, and the conditions whose
failure causes the ACL to return "deny". This concept has proved to be
confusing to some people, so the use of endpass
is no longer recommended as
"best practice". See the description of accept
above for more details.
This modifier sets up a message that is used as part of the log message if the
ACL denies access or a warn
statement's conditions are true. For example:
require log_message = wrong cipher suite $tls_cipher encrypted = DES-CBC3-SHA |
log_message
is also used when recipients are discarded by discard
. For
example:
discard <some conditions> log_message = Discarded $local_part@$domain because... |
When access is denied, log_message
adds to any underlying error message
that may exist because of a condition failure. For example, while verifying a
recipient address, a :fail: redirection might have already set up a
message.
The message may be defined before the conditions to which it applies, because
the string expansion does not happen until Exim decides that access is to be
denied. This means that any variables that are set by the condition are
available for inclusion in the message. For example, the $dnslist_
<xxx>
variables are set after a DNS black list lookup succeeds. If the expansion of
log_message
fails, or if the result is an empty string, the modifier is
ignored.
If you want to use a warn
statement to log the result of an address
verification, you can use $acl_verify_message
to include the verification
error message.
If log_message
is used with a warn
statement, "Warning:" is added to
the start of the logged message. If the same warning log message is requested
more than once while receiving a single email message, only one copy is
actually logged. If you want to log multiple copies, use logwrite
instead
of log_message
. In the absence of log_message
and logwrite
, nothing
is logged for a successful warn
statement.
If log_message
is not present and there is no underlying error message (for
example, from the failure of address verification), but message
is present,
the message
text is used for logging rejections. However, if any text for
logging contains newlines, only the first line is logged. In the absence of
both log_message
and message
, a default built-in message is used for
logging rejections.
This modifier makes it possible to specify which logs are used for messages about ACL rejections. Its argument is a colon-separated list of words that can be "main", "reject", or "panic". The default is ‘main:reject’. The list may be empty, in which case a rejection is not logged at all. For example, this ACL fragment writes no logging information when access is denied:
deny <some conditions> log_reject_target = |
This modifier can be used in SMTP and non-SMTP ACLs. It applies to both permanent and temporary rejections.
This modifier writes a message to a log file as soon as it is encountered when
processing an ACL. (Compare log_message
, which, except in the case of
warn
and discard
, is used only if the ACL statement denies
access.) The logwrite
modifier can be used to log special incidents in
ACLs. For example:
accept <some special conditions> control = freeze logwrite = froze message because ... |
By default, the message is written to the main log. However, it may begin with a colon, followed by a comma-separated list of log names, and then another colon, to specify exactly which logs are to be written. For example:
logwrite = :main,reject: text for main and reject logs logwrite = :panic: text for panic log only |
This modifier sets up a text string that is expanded and used as a response
message when an ACL statement terminates the ACL with an "accept", "deny",
or "defer" response. (In the case of the accept
and discard
verbs,
there is some complication if endpass
is involved; see the description of
accept
for details.)
The expansion of the message happens at the time Exim decides that the ACL is
to end, not at the time it processes message
. If the expansion fails, or
generates an empty string, the modifier is ignored. Here is an example where
message
must be specified first, because the ACL ends with a rejection if
the hosts
condition fails:
require message = Host not recognized hosts = 10.0.0.0/8 |
(Once a condition has failed, no further conditions or modifiers are processed.)
For ACLs that are triggered by SMTP commands, the message is returned as part
of the SMTP response. The use of message
with accept
(or discard
)
is meaningful only for SMTP, as no message is returned when a non-SMTP message
is accepted. In the case of the connect ACL, accepting with a message modifier
overrides the value of smtp_banner
. For the EHLO/HELO ACL, a customized
accept message may not contain more than one line (otherwise it will be
truncated at the first newline and a panic logged), and it cannot affect the
EHLO options.
When SMTP is involved, the message may begin with an overriding response code, consisting of three digits optionally followed by an "extended response code" of the form n.n.n, each code being followed by a space. For example:
deny message = 599 1.2.3 Host not welcome hosts = 192.168.34.0/24 |
The first digit of the supplied response code must be the same as would be sent by default. A panic occurs if it is not. Exim uses a 550 code when it denies access, but for the predata ACL, note that the default success code is 354, not 2xx.
Notwithstanding the previous paragraph, for the QUIT ACL, unlike the others, the message modifier cannot override the 221 response code.
The text in a message
modifier is literal; any quotes are taken as
literals, but because the string is expanded, backslash escapes are processed
anyway. If the message contains newlines, this gives rise to a multi-line SMTP
response.
If message
is used on a statement that verifies an address, the message
specified overrides any message that is generated by the verification process.
However, the original message is available in the variable
$acl_verify_message
, so you can incorporate it into your message if you
wish. In particular, if you want the text from :fail:
items in redirect
routers to be passed back as part of the SMTP response, you should either not
use a message
modifier, or make use of $acl_verify_message
.
For compatibility with previous releases of Exim, a message
modifier that
is used with a warn
verb behaves in a similar way to the add_header
modifier, but this usage is now deprecated. However, message
acts only when
all the conditions are true, wherever it appears in an ACL command, whereas
add_header
acts as soon as it is encountered. If message
is used with
warn
in an ACL that is not concerned with receiving a message, it has no
effect.
This modifier puts a value into one of the ACL variables (see section ACL variables).
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The control
modifier supports the following settings:
This modifier allows a client host to use the SMTP AUTH command even when it has not been advertised in response to EHLO. Furthermore, because there are apparently some really broken clients that do this, Exim will accept AUTH after HELO (rather than EHLO) when this control is set. It should be used only if you really need it, and you should limit its use to those broken clients that do not work without it. For example:
warn hosts = 192.168.34.25 control = allow_auth_unadvertised |
Normally, when an Exim server receives an AUTH command, it checks the name of the authentication mechanism that is given in the command to ensure that it matches an advertised mechanism. When this control is set, the check that a mechanism has been advertised is bypassed. Any configured mechanism can be used by the client. This control is permitted only in the connection and HELO ACLs.
These two controls are permitted only in the ACL specified by acl_smtp_rcpt
(that is, during RCPT processing). By default, the contents of $local_part
are lower cased before ACL processing. If "caseful_local_part" is specified,
any uppercase letters in the original local part are restored in $local_part
for the rest of the ACL, or until a control that sets "caselower_local_part"
is encountered.
These controls affect only the current recipient. Moreover, they apply only to
local part handling that takes place directly in the ACL (for example, as a key
in lookups). If a test to verify the recipient is obeyed, the case-related
handling of the local part during the verification is controlled by the router
configuration (see the caseful_local_part
generic router option).
This facility could be used, for example, to add a spam score to local parts
containing upper case letters. For example, using $acl_m4
to accumulate the
spam score:
warn control = caseful_local_part set acl_m4 = ${eval:\ $acl_m4 + \ ${if match{$local_part}{[A-Z]}{1}{0}}\ } control = caselower_local_part |
Notice that we put back the lower cased version afterwards, assuming that is what is wanted for subsequent tests.
These controls make it possible to be selective about when SMTP synchronization
is enforced. The global option smtp_enforce_sync
specifies the initial
state of the switch (it is true by default). See the description of this option
in chapter Main configuration for details of SMTP synchronization checking.
The effect of these two controls lasts for the remainder of the SMTP
connection. They can appear in any ACL except the one for the non-SMTP
messages. The most straightforward place to put them is in the ACL defined by
acl_smtp_connect
, which is run at the start of an incoming SMTP connection,
before the first synchronization check. The expected use is to turn off the
synchronization checks for badly-behaved hosts that you nevertheless need to
work with.
This control works in exactly the same way as fakereject
(described below)
except that it causes an SMTP 450 response after the message data instead of a
550 response. You must take care when using fakedefer
because it causes the
messages to be duplicated when the sender retries. Therefore, you should not
use fakedefer
if the message is to be delivered normally.
This control is permitted only for the MAIL, RCPT, and DATA ACLs, in other words, only when an SMTP message is being received. If Exim accepts the message, instead the final 250 response, a 550 rejection message is sent. However, Exim proceeds to deliver the message as normal. The control applies only to the current message, not to any subsequent ones that may be received in the same SMTP connection.
The text for the 550 response is taken from the control
modifier. If no
message is supplied, the following is used:
550-Your message has been rejected but is being 550-kept for evaluation. 550-If it was a legitimate message, it may still be 550 delivered to the target recipient(s). |
This facility should be used with extreme caution.
This control is permitted only for the MAIL, RCPT, DATA, and non-SMTP ACLs, in other words, only when a message is being received. If the message is accepted, it is placed on Exim's queue and frozen. The control applies only to the current message, not to any subsequent ones that may be received in the same SMTP connection.
This modifier can optionally be followed by ‘/no_tell’. If the global option
freeze_tell
is set, it is ignored for the current message (that is, nobody
is told about the freezing), provided all the control=freeze modifiers that
are obeyed for the current message have the ‘/no_tell’ option.
Exim normally flushes SMTP output before implementing a delay in an ACL, to
avoid unexpected timeouts in clients when the SMTP PIPELINING extension is in
use. This control, as long as it is encountered before the delay
modifier,
disables such output flushing.
Exim normally flushes SMTP output before performing a callout in an ACL, to
avoid unexpected timeouts in clients when the SMTP PIPELINING extension is in
use. This control, as long as it is encountered before the verify
condition
that causes the callout, disables such output flushing.
This control is available when Exim is compiled with the content scanning extension. Content scanning may require a copy of the current message, or parts of it, to be written in "mbox format" to a spool file, for passing to a virus or spam scanner. Normally, such copies are deleted when they are no longer needed. If this control is set, the copies are not deleted. The control applies only to the current message, not to any subsequent ones that may be received in the same SMTP connection. It is provided for debugging purposes and is unlikely to be useful in production.
This control is permitted for any ACL except the one for non-SMTP messages. It seems that there are broken clients in use that cannot handle multiline SMTP responses, despite the fact that RFC 821 defined them over 20 years ago.
If this control is set, multiline SMTP responses from ACL rejections are suppressed. One way of doing this would have been to put out these responses as one long line. However, RFC 2821 specifies a maximum of 512 bytes per response ("use multiline responses for more" it says - ha!), and some of the responses might get close to that. So this facility, which is after all only a sop to broken clients, is implemented by doing two very easy things:
message
modifier supplies a multiline response, only the first
line is output.
The setting of the switch can, of course, be made conditional on the calling host. Its effect lasts until the end of the SMTP connection.
This control turns off the advertising of the PIPELINING extension to SMTP in
the current session. To be useful, it must be obeyed before Exim sends its
response to an EHLO command. Therefore, it should normally appear in an ACL
controlled by acl_smtp_connect
or acl_smtp_helo
. See also
pipelining_advertise_hosts
.
This control is permitted only for the MAIL, RCPT, DATA, and non-SMTP ACLs, in
other words, only when a message is being received. If the message is accepted,
it is placed on Exim's queue and left there for delivery by a subsequent queue
runner. No immediate delivery process is started. In other words, it has the
effect as the queue_only
global option. However, the control applies only
to the current message, not to any subsequent ones that may be received in the
same SMTP connection.
This control is permitted only for the MAIL, RCPT, and start of data ACLs (the
latter is the one defined by acl_smtp_predata
). Setting it tells Exim that
the current message is a submission from a local MUA. In this case, Exim
operates in "submission mode", and applies certain fixups to the message if
necessary. For example, it add a Date: header line if one is not present.
This control is not permitted in the acl_smtp_data
ACL, because that is too
late (the message has already been created).
Chapter Message processing describes the processing that Exim applies to messages. Section Submission mode for non-local messages covers the processing that happens in submission mode; the available options for this control are described there. The control applies only to the current message, not to any subsequent ones that may be received in the same SMTP connection.
This control applies to locally submitted (non TCP/IP) messages, and is the complement of ‘control’ ‘=’ ‘submission’. It disables the fixups that are normally applied to locally-submitted messages. Specifically:
local_sender_retain
).
This control may be useful when a remotely-originated message is accepted,
passed to some scanning program, and then re-submitted for delivery. It can be
used only in the acl_smtp_mail
, acl_smtp_rcpt
, acl_smtp_predata
,
and acl_not_smtp_start
ACLs, because it has to be set before the message's
data is read.
Note: This control applies only to the current message, not to any others
that are being submitted at the same time using -bs
or -bS
.
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All four possibilities for message fixups can be specified:
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The add_header
modifier can be used to add one or more extra header lines
to an incoming message, as in this example:
warn dnslists = sbl.spamhaus.org : \ dialup.mail-abuse.org add_header = X-blacklisted-at: $dnslist_domain |
The add_header
modifier is permitted in the MAIL, RCPT, PREDATA, DATA,
MIME, and non-SMTP ACLs (in other words, those that are concerned with
receiving a message). The message must ultimately be accepted for
add_header
to have any significant effect. You can use add_header
with
any ACL verb, including deny
(though this is potentially useful only in a
RCPT ACL).
If the data for the add_header
modifier contains one or more newlines that
are not followed by a space or a tab, it is assumed to contain multiple header
lines. Each one is checked for valid syntax; ‘X-ACL-Warn:’ is added to the
front of any line that is not a valid header line.
Added header lines are accumulated during the MAIL, RCPT, and predata ACLs. They are added to the message before processing the DATA and MIME ACLs. However, if an identical header line is requested more than once, only one copy is actually added to the message. Further header lines may be accumulated during the DATA and MIME ACLs, after which they are added to the message, again with duplicates suppressed. Thus, it is possible to add two identical header lines to an SMTP message, but only if one is added before DATA and one after. In the case of non-SMTP messages, new headers are accumulated during the non-SMTP ACLs, and are added to the message after all the ACLs have run. If a message is rejected after DATA or by the non-SMTP ACL, all added header lines are included in the entry that is written to the reject log.
Header lines are not visible in string expansions until they are added to the message. It follows that header lines defined in the MAIL, RCPT, and predata ACLs are not visible until the DATA ACL and MIME ACLs are run. Similarly, header lines that are added by the DATA or MIME ACLs are not visible in those ACLs. Because of this restriction, you cannot use header lines as a way of passing data between (for example) the MAIL and RCPT ACLs. If you want to do this, you can use ACL variables, as described in section ACL variables.
The add_header
modifier acts immediately it is encountered during the
processing of an ACL. Notice the difference between these two cases:
accept add_header = ADDED: some text <some condition> accept <some condition> add_header = ADDED: some text |
In the first case, the header line is always added, whether or not the
condition is true. In the second case, the header line is added only if the
condition is true. Multiple occurrences of add_header
may occur in the same
ACL statement. All those that are encountered before a condition fails are
honoured.
For compatibility with previous versions of Exim, a message
modifier for a
warn
verb acts in the same way as add_header
, except that it takes
effect only if all the conditions are true, even if it appears before some of
them. Furthermore, only the last occurrence of message
is honoured. This
usage of message
is now deprecated. If both add_header
and message
are present on a warn
verb, both are processed according to their
specifications.
By default, new header lines are added to a message at the end of the existing header lines. However, you can specify that any particular header line should be added right at the start (before all the Received: lines), immediately after the first block of Received: lines, or immediately before any line that is not a Received: or Resent-something: header.
This is done by specifying ":at_start:", ":after_received:", or ":at_start_rfc:" (or, for completeness, ":at_end:") before the text of the header line, respectively. (Header text cannot start with a colon, as there has to be a header name first.) For example:
warn add_header = \ :after_received:X-My-Header: something or other... |
If more than one header line is supplied in a single add_header
modifier,
each one is treated independently and can therefore be placed differently. If
you add more than one line at the start, or after the Received: block, they end
up in reverse order.
Warning: This facility currently applies only to header lines that are added in an ACL. It does NOT work for header lines that are added in a system filter or in a router or transport.
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Some of conditions listed in this section are available only when Exim is compiled with the content-scanning extension. They are included here briefly for completeness. More detailed descriptions can be found in the discussion on content scanning in chapter Content scanning at ACL time.
Not all conditions are relevant in all circumstances. For example, testing
senders and recipients does not make sense in an ACL that is being run as the
result of the arrival of an ETRN command, and checks on message headers can be
done only in the ACLs specified by acl_smtp_data
and acl_not_smtp
. You
can use the same condition (with different parameters) more than once in the
same ACL statement. This provides a way of specifying an "and" conjunction.
The conditions are as follows:
The possible values of the argument are the same as for the
acl_smtp_
xxx options. The named or inline ACL is run. If it returns
"accept" the condition is true; if it returns "deny" the condition is
false. If it returns "defer", the current ACL returns "defer" unless the
condition is on a warn
verb. In that case, a "defer" return makes the
condition false. This means that further processing of the warn
verb
ceases, but processing of the ACL continues.
If the nested acl
returns "drop" and the outer condition denies access,
the connection is dropped. If it returns "discard", the verb must be
accept
or discard
, and the action is taken immediately - no further
conditions are tested.
ACLs may be nested up to 20 deep; the limit exists purely to catch runaway loops. This condition allows you to use different ACLs in different circumstances. For example, different ACLs can be used to handle RCPT commands for different local users or different local domains.
If the SMTP connection is not authenticated, the condition is false. Otherwise, the name of the authenticator is tested against the list. To test for authentication by any authenticator, you can set
authenticated = * |
This feature allows you to make up custom conditions. If the result of expanding the string is an empty string, the number zero, or one of the strings "no" or "false", the condition is false. If the result is any non-zero number, or one of the strings "yes" or "true", the condition is true. For any other value, some error is assumed to have occurred, and the ACL returns "defer". However, if the expansion is forced to fail, the condition is ignored. The effect is to treat it as true, whether it is positive or negative.
This condition is available only when Exim is compiled with the
content-scanning extension, and it is allowed only in the ACL defined by
acl_smtp_mime
. It causes the current MIME part to be decoded into a file.
If all goes well, the condition is true. It is false only if there are
problems such as a syntax error or a memory shortage. For more details, see
chapter Content scanning at ACL time.
This condition is available only when Exim is compiled with the content-scanning extension. Its use is described in section The demime condition.
This condition checks for entries in DNS black lists. These are also known as "RBL lists", after the original Realtime Blackhole List, but note that the use of the lists at mail-abuse.org now carries a charge. There are too many different variants of this condition to describe briefly here. See sections Using DNS lists-DNS lists and IPv6 for details.
This condition is relevant only after a RCPT command. It checks that the domain
of the recipient address is in the domain list. If percent-hack processing is
enabled, it is done before this test is done. If the check succeeds with a
lookup, the result of the lookup is placed in $domain_data
until the next
domains
test.
Note carefully (because many people seem to fall foul of this): you cannot
use domains
in a DATA ACL.
If the SMTP connection is not encrypted, the condition is false. Otherwise, the name of the cipher suite in use is tested against the list. To test for encryption without testing for any specific cipher suite(s), set
encrypted = * |
This condition tests that the calling host matches the host list. If you have name lookups or wildcarded host names and IP addresses in the same host list, you should normally put the IP addresses first. For example, you could have:
accept hosts = 10.9.8.7 : dbm;/etc/friendly/hosts |
The lookup in this example uses the host name for its key. This is implied by the lookup type "dbm". (For a host address lookup you would use "net-dbm" and it wouldn't matter which way round you had these two items.)
The reason for the problem with host names lies in the left-to-right way that
Exim processes lists. It can test IP addresses without doing any DNS lookups,
but when it reaches an item that requires a host name, it fails if it cannot
find a host name to compare with the pattern. If the above list is given in the
opposite order, the accept
statement fails for a host whose name cannot be
found, even if its IP address is 10.9.8.7.
If you really do want to do the name check first, and still recognize the IP address even if the name lookup fails, you can rewrite the ACL like this:
accept hosts = dbm;/etc/friendly/hosts accept hosts = 10.9.8.7 |
The default action on failing to find the host name is to assume that the host
is not in the list, so the first accept
statement fails. The second
statement can then check the IP address.
If a hosts
condition is satisfied by means of a lookup, the result
of the lookup is made available in the $host_data
variable. This
allows you, for example, to set up a statement like this:
deny hosts = net-lsearch;/some/file message = $host_data |
which gives a custom error message for each denied host.
This condition is relevant only after a RCPT command. It checks that the local
part of the recipient address is in the list. If percent-hack processing is
enabled, it is done before this test. If the check succeeds with a lookup, the
result of the lookup is placed in $local_part_data
, which remains set until
the next local_parts
test.
This condition is available only when Exim is compiled with the content-scanning extension. It causes the incoming message to be scanned for viruses. For details, see chapter Content scanning at ACL time.
This condition is available only when Exim is compiled with the
content-scanning extension, and it is allowed only in the ACL defined by
acl_smtp_mime
. It causes the current MIME part to be scanned for a match
with any of the regular expressions. For details, see chapter
Content scanning at ACL time.
This condition can be used to limit the rate at which a user or host submits messages. Details are given in section Rate limiting incoming messages.
This condition is relevant only after a RCPT command. It checks the entire recipient address against a list of recipients.
This condition is available only when Exim is compiled with the content-scanning extension, and is available only in the DATA, MIME, and non-SMTP ACLs. It causes the incoming message to be scanned for a match with any of the regular expressions. For details, see chapter Content scanning at ACL time.
This condition tests the domain of the sender of the message against the given
domain list. Note: The domain of the sender address is in
$sender_address_domain
. It is not put in $domain
during the testing
of this condition. This is an exception to the general rule for testing domain
lists. It is done this way so that, if this condition is used in an ACL for a
RCPT command, the recipient's domain (which is in $domain
) can be used to
influence the sender checking.
Warning: It is a bad idea to use this condition on its own as a control on relaying, because sender addresses are easily, and commonly, forged.
This condition tests the sender of the message against the given list. To test for a bounce message, which has an empty sender, set
senders = : |
Warning: It is a bad idea to use this condition on its own as a control on relaying, because sender addresses are easily, and commonly, forged.
This condition is available only when Exim is compiled with the content-scanning extension. It causes the incoming message to be scanned by SpamAssassin. For details, see chapter Content scanning at ACL time.
This condition is true in an SMTP session if the session is encrypted, and a
certificate was received from the client, and the certificate was verified. The
server requests a certificate only if the client matches tls_verify_hosts
or tls_try_verify_hosts
(see chapter Encrypted SMTP connections using TLS/SSL).
This condition checks whether the sending host (the client) is authorized to send email. Details of how this works are given in section Client SMTP authorization (CSA).
This condition is relevant only in an ACL that is run after a message has been
received, that is, in an ACL specified by acl_smtp_data
or
acl_not_smtp
. It checks that there is a verifiable address in at least one
of the Sender:, Reply-To:, or From: header lines. Such an address
is loosely thought of as a "sender" address (hence the name of the test).
However, an address that appears in one of these headers need not be an address
that accepts bounce messages; only sender addresses in envelopes are required
to accept bounces. Therefore, if you use the callout option on this check, you
might want to arrange for a non-empty address in the MAIL command.
Details of address verification and the options are given later, starting at
section Address verification (callouts are described in section
Callout verification). You can combine this condition with the senders
condition to restrict it to bounce messages only:
deny senders = : message = A valid sender header is required for bounces !verify = header_sender |
This condition is relevant only in an ACL that is run after a message has been
received, that is, in an ACL specified by acl_smtp_data
or
acl_not_smtp
. It checks the syntax of all header lines that can contain
lists of addresses (Sender:, From:, Reply-To:, To:, Cc:,
and Bcc:). Unqualified addresses (local parts without domains) are
permitted only in locally generated messages and from hosts that match
sender_unqualified_hosts
or recipient_unqualified_hosts
, as
appropriate.
Note that this condition is a syntax check only. However, a common spamming ploy used to be to send syntactically invalid headers such as
To: @ |
and this condition can be used to reject such messages, though they are not as common as they used to be.
This condition is true if a HELO or EHLO command has been received from the
client host, and its contents have been verified. If there has been no previous
attempt to verify the HELO/EHLO contents, it is carried out when this
condition is encountered. See the description of the helo_verify_hosts
and
helo_try_verify_hosts
options for details of how to request verification
independently of this condition.
For SMTP input that does not come over TCP/IP (the -bs
command line
option), this condition is always true.
This condition checks that there are no blind (bcc) recipients in the message. Every envelope recipient must appear either in a To: header line or in a Cc: header line for this condition to be true. Local parts are checked case-sensitively; domains are checked case-insensitively. If Resent-To: or Resent-Cc: header lines exist, they are also checked. This condition can be used only in a DATA or non-SMTP ACL.
There are, of course, many legitimate messages that make use of blind (bcc) recipients. This check should not be used on its own for blocking messages.
This condition is relevant only after a RCPT command. It verifies the current
recipient. Details of address verification are given later, starting at section
Address verification. After a recipient has been verified, the value
of $address_data
is the last value that was set while routing the address.
This applies even if the verification fails. When an address that is being
verified is redirected to a single address, verification continues with the new
address, and in that case, the subsequent value of $address_data
is the
value for the child address.
This condition ensures that a verified host name has been looked up from the IP
address of the client host. (This may have happened already if the host name
was needed for checking a host list, or if the host matched host_lookup
.)
Verification ensures that the host name obtained from a reverse DNS lookup, or
one of its aliases, does, when it is itself looked up in the DNS, yield the
original IP address.
If this condition is used for a locally generated message (that is, when there is no client host involved), it always succeeds.
This condition is relevant only after a MAIL or RCPT command, or after a
message has been received (the acl_smtp_data
or acl_not_smtp
ACLs). If
the message's sender is empty (that is, this is a bounce message), the
condition is true. Otherwise, the sender address is verified.
If there is data in the $address_data
variable at the end of routing, its
value is placed in $sender_address_data
at the end of verification. This
value can be used in subsequent conditions and modifiers in the same ACL
statement. It does not persist after the end of the current statement. If you
want to preserve the value for longer, you can save it in an ACL variable.
Details of verification are given later, starting at section Address verification. Exim caches the result of sender verification, to avoid doing it more than once per message.
This is a variation of the previous option, in which a modified address is verified as a sender.
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In its simplest form, the dnslists
condition tests whether the calling host
is on at least one of a number of DNS lists by looking up the inverted IP
address in one or more DNS domains. For example, if the calling host's IP
address is 192.168.62.43, and the ACL statement is
deny dnslists = blackholes.mail-abuse.org : \ dialups.mail-abuse.org |
the following records are looked up:
43.62.168.192.blackholes.mail-abuse.org 43.62.168.192.dialups.mail-abuse.org |
As soon as Exim finds an existing DNS record, processing of the list stops. Thus, multiple entries on the list provide an "or" conjunction. If you want to test that a host is on more than one list (an "and" conjunction), you can use two separate conditions:
deny dnslists = blackholes.mail-abuse.org dnslists = dialups.mail-abuse.org |
If a DNS lookup times out or otherwise fails to give a decisive answer, Exim behaves as if the host does not match the list item, that is, as if the DNS record does not exist. If there are further items in the DNS list, they are processed.
This is usually the required action when dnslists
is used with deny
(which is the most common usage), because it prevents a DNS failure from
blocking mail. However, you can change this behaviour by putting one of the
following special items in the list:
+include_unknown behave as if the item is on the list +exclude_unknown behave as if the item is not on the list (default) +defer_unknown give a temporary error |
Each of these applies to any subsequent items on the list. For example:
deny dnslists = +defer_unknown : foo.bar.example |
Testing the list of domains stops as soon as a match is found. If you want to warn for one list and block for another, you can use two different statements:
deny dnslists = blackholes.mail-abuse.org warn message = X-Warn: sending host is on dialups list dnslists = dialups.mail-abuse.org |
DNS list lookups are cached by Exim for the duration of the SMTP session, so a lookup based on the IP address is done at most once for any incoming connection. Exim does not share information between multiple incoming connections (but your local name server cache should be active).
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By default, the IP address that is used in a DNS list lookup is the IP address of the calling host. However, you can specify another IP address by listing it after the domain name, introduced by a slash. For example:
deny dnslists = black.list.tld/192.168.1.2 |
This feature is not very helpful with explicit IP addresses; it is intended for use with IP addresses that are looked up, for example, the IP addresses of the MX hosts or nameservers of an email sender address. For an example, see section Multiple explicit keys for a DNS list below.
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There are some lists that are keyed on domain names rather than inverted IP addresses (see for example the domain based zones link at http://www.rfc-ignorant.org/). No reversing of components is used with these lists. You can change the name that is looked up in a DNS list by listing it after the domain name, introduced by a slash. For example,
deny message = Sender's domain is listed at $dnslist_domain dnslists = dsn.rfc-ignorant.org/$sender_address_domain |
This particular example is useful only in ACLs that are obeyed after the RCPT or DATA commands, when a sender address is available. If (for example) the message's sender is user@tld.example the name that is looked up by this example is
tld.example.dsn.rfc-ignorant.org |
A single dnslists
condition can contain entries for both names and IP
addresses. For example:
deny dnslists = sbl.spamhaus.org : \ dsn.rfc-ignorant.org/$sender_address_domain |
The first item checks the sending host's IP address; the second checks a domain name. The whole condition is true if either of the DNS lookups succeeds.
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The syntax described above for looking up explicitly-defined values (either names or IP addresses) in a DNS blacklist is a simplification. After the domain name for the DNS list, what follows the slash can in fact be a list of items. As with all lists in Exim, the default separator is a colon. However, because this is a sublist within the list of DNS blacklist domains, it is necessary either to double the separators like this:
dnslists = black.list.tld/name.1::name.2 |
or to change the separator character, like this:
dnslists = black.list.tld/<;name.1;name.2 |
If an item in the list is an IP address, it is inverted before the DNS blacklist domain is appended. If it is not an IP address, no inversion occurs. Consider this condition:
dnslists = black.list.tld/<;192.168.1.2;a.domain |
The DNS lookups that occur are:
2.1.168.192.black.list.tld a.domain.black.list.tld |
Once a DNS record has been found (that matches a specific IP return address, if specified - see section Additional matching conditions for DNS lists), no further lookups are done. If there is a temporary DNS error, the rest of the sublist of domains or IP addresses is tried. A temporary error for the whole dnslists item occurs only if no other DNS lookup in this sublist succeeds. In other words, a successful lookup for any of the items in the sublist overrides a temporary error for a previous item.
The ability to supply a list of items after the slash is in some sense just a syntactic convenience. These two examples have the same effect:
dnslists = black.list.tld/a.domain : black.list.tld/b.domain dnslists = black.list.tld/a.domain::b.domain |
However, when the data for the list is obtained from a lookup, the second form is usually much more convenient. Consider this example:
deny message = The mail servers for the domain \ $sender_address_domain \ are listed at $dnslist_domain ($dnslist_value); \ see $dnslist_text. dnslists = sbl.spamhaus.org/<|${lookup dnsdb {>|a=<|\ ${lookup dnsdb {>|mxh=\ $sender_address_domain} }} } |
Note the use of ‘>|’ in the dnsdb lookup to specify the separator for multiple DNS records. The inner dnsdb lookup produces a list of MX hosts and the outer dnsdb lookup finds the IP addresses for these hosts. The result of expanding the condition might be something like this:
dnslists = sbl.spahmaus.org/<|192.168.2.3|192.168.5.6|... |
Thus, this example checks whether or not the IP addresses of the sender domain's mail servers are on the Spamhaus black list.
The key that was used for a successful DNS list lookup is put into the variable
$dnslist_matched
(see section Variables set from DNS lists).
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DNS lists are constructed using address records in the DNS. The original RBL just used the address 127.0.0.1 on the right hand side of each record, but the RBL+ list and some other lists use a number of values with different meanings. The values used on the RBL+ list are:
127.1.0.1 RBL 127.1.0.2 DUL 127.1.0.3 DUL and RBL 127.1.0.4 RSS 127.1.0.5 RSS and RBL 127.1.0.6 RSS and DUL 127.1.0.7 RSS and DUL and RBL |
Section Additional matching conditions for DNS lists below describes how you can distinguish between different values. Some DNS lists may return more than one address record; see section Handling multiple DNS records from a DNS list for details of how they are checked.
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When an entry is found in a DNS list, the variable $dnslist_domain
contains
the name of the overall domain that matched (for example,
‘spamhaus.example’), $dnslist_matched
contains the key within that domain
(for example, ‘192.168.5.3’), and $dnslist_value
contains the data from
the DNS record. When the key is an IP address, it is not reversed in
$dnslist_matched
(though it is, of course, in the actual lookup). In simple
cases, for example:
deny dnslists = spamhaus.example |
the key is also available in another variable (in this case,
$sender_host_address
). In more complicated cases, however, this is not true.
For example, using a data lookup (as described in section Multiple explicit keys for a DNS list)
might generate a dnslists lookup like this:
deny dnslists = spamhaus.example/<|192.168.1.2|192.168.6.7|... |
If this condition succeeds, the value in $dnslist_matched
might be
‘192.168.6.7’ (for example).
If more than one address record is returned by the DNS lookup, all the IP
addresses are included in $dnslist_value
, separated by commas and spaces.
The variable $dnslist_text
contains the contents of any associated TXT
record. For lists such as RBL+ the TXT record for a merged entry is often not
very meaningful. See section Detailed information from merged DNS lists for a way of obtaining more
information.
You can use the DNS list variables in message
or log_message
modifiers
- although these appear before the condition in the ACL, they are not
expanded until after it has failed. For example:
deny hosts = !+local_networks message = $sender_host_address is listed \ at $dnslist_domain dnslists = rbl-plus.mail-abuse.example |
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You can add an equals sign and an IP address after a dnslists
domain name
in order to restrict its action to DNS records with a matching right hand side.
For example,
deny dnslists = rblplus.mail-abuse.org=127.0.0.2 |
rejects only those hosts that yield 127.0.0.2. Without this additional data, any address record is considered to be a match. For the moment, we assume that the DNS lookup returns just one record. Section Handling multiple DNS records from a DNS list describes how multiple records are handled.
More than one IP address may be given for checking, using a comma as a
separator. These are alternatives - if any one of them matches, the
dnslists
condition is true. For example:
deny dnslists = a.b.c=127.0.0.2,127.0.0.3 |
If you want to specify a constraining address list and also specify names or IP addresses to be looked up, the constraining address list must be specified first. For example:
deny dnslists = dsn.rfc-ignorant.org\ =127.0.0.2/$sender_address_domain |
If the character ‘&’ is used instead of ‘=’, the comparison for each listed IP address is done by a bitwise "and" instead of by an equality test. In other words, the listed addresses are used as bit masks. The comparison is true if all the bits in the mask are present in the address that is being tested. For example:
dnslists = a.b.c&0.0.0.3 |
matches if the address is x.x.x.3, x.x.x.7, x.x.x.11, etc. If you want to test whether one bit or another bit is present (as opposed to both being present), you must use multiple values. For example:
dnslists = a.b.c&0.0.0.1,0.0.0.2 |
matches if the final component of the address is an odd number or two times an odd number.
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You can supply a negative list of IP addresses as part of a dnslists
condition. Whereas
deny dnslists = a.b.c=127.0.0.2,127.0.0.3 |
means "deny if the host is in the black list at the domain a.b.c and the IP address yielded by the list is either 127.0.0.2 or 127.0.0.3",
deny dnslists = a.b.c!=127.0.0.2,127.0.0.3 |
means "deny if the host is in the black list at the domain a.b.c and the IP address yielded by the list is not 127.0.0.2 and not 127.0.0.3". In other words, the result of the test is inverted if an exclamation mark appears before the ‘=’ (or the ‘&’) sign.
Note: This kind of negation is not the same as negation in a domain, host, or address list (which is why the syntax is different).
If you are using just one list, the negation syntax does not gain you much. The previous example is precisely equivalent to
deny dnslists = a.b.c !dnslists = a.b.c=127.0.0.2,127.0.0.3 |
However, if you are using multiple lists, the negation syntax is clearer. Consider this example:
deny dnslists = sbl.spamhaus.org : \ list.dsbl.org : \ dnsbl.njabl.org!=127.0.0.3 : \ relays.ordb.org |
Using only positive lists, this would have to be:
deny dnslists = sbl.spamhaus.org : \ list.dsbl.org deny dnslists = dnsbl.njabl.org !dnslists = dnsbl.njabl.org=127.0.0.3 deny dnslists = relays.ordb.org |
which is less clear, and harder to maintain.
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A DNS lookup for a dnslists
condition may return more than one DNS record,
thereby providing more than one IP address. When an item in a dnslists
list
is followed by ‘=’ or ‘&’ and a list of IP addresses, in order to restrict
the match to specific results from the DNS lookup, there are two ways in which
the checking can be handled. For example, consider the condition:
dnslists = a.b.c=127.0.0.1 |
What happens if the DNS lookup for the incoming IP address yields both 127.0.0.1 and 127.0.0.2 by means of two separate DNS records? Is the condition true because at least one given value was found, or is it false because at least one of the found values was not listed? And how does this affect negated conditions? Both possibilities are provided for with the help of additional separators ‘==’ and ‘=&’.
dnslists = a.b.c==127.0.0.1 |
and the DNS lookup yields both 127.0.0.1 and 127.0.0.2, the condition is false because 127.0.0.2 is not listed. You would need to have:
dnslists = a.b.c==127.0.0.1,127.0.0.2 |
for the condition to be true.
When ‘!’ is used to negate IP address matching, it inverts the result, giving the precise opposite of the behaviour above. Thus:
dnslists = a.b.c!&0.0.0.1 |
If the DNS lookup yields both 127.0.0.1 and 127.0.0.2, the condition is false because 127.0.0.1 matches.
dnslists = a.b.c!=&0.0.0.1 |
If the DNS lookup yields both 127.0.0.1 and 127.0.0.2, the condition is true, because 127.0.0.2 does not match. You would need to have:
dnslists = a.b.c!=&0.0.0.1,0.0.0.2 |
for the condition to be false.
When the DNS lookup yields only a single IP address, there is no difference between ‘=’ and ‘==’ and between ‘&’ and ‘=&’.
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When the facility for restricting the matching IP values in a DNS list is used,
the text from the TXT record that is set in $dnslist_text
may not reflect
the true reason for rejection. This happens when lists are merged and the IP
address in the A record is used to distinguish them; unfortunately there is
only one TXT record. One way round this is not to use merged lists, but that
can be inefficient because it requires multiple DNS lookups where one would do
in the vast majority of cases when the host of interest is not on any of the
lists.
A less inefficient way of solving this problem is available. If
two domain names, comma-separated, are given, the second is used first to
do an initial check, making use of any IP value restrictions that are set.
If there is a match, the first domain is used, without any IP value
restrictions, to get the TXT record. As a byproduct of this, there is also
a check that the IP being tested is indeed on the first list. The first
domain is the one that is put in $dnslist_domain
. For example:
reject message = \ rejected because $sender_host_address is blacklisted \ at $dnslist_domain\n$dnslist_text dnslists = \ sbl.spamhaus.org,sbl-xbl.spamhaus.org=127.0.0.2 : \ dul.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.10 |
For the first blacklist item, this starts by doing a lookup in sbl-xbl.spamhaus.org and testing for a 127.0.0.2 return. If there is a match, it then looks in sbl.spamhaus.org, without checking the return value, and as long as something is found, it looks for the corresponding TXT record. If there is no match in sbl-xbl.spamhaus.org, nothing more is done. The second blacklist item is processed similarly.
If you are interested in more than one merged list, the same list must be given several times, but because the results of the DNS lookups are cached, the DNS calls themselves are not repeated. For example:
reject dnslists = \ http.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.2 : \ socks.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.3 : \ misc.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.4 : \ dul.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.10 |
In this case there is one lookup in dnsbl.sorbs.net, and if none of the IP values matches (or if no record is found), this is the only lookup that is done. Only if there is a match is one of the more specific lists consulted.
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If Exim is asked to do a dnslist lookup for an IPv6 address, it inverts it nibble by nibble. For example, if the calling host's IP address is 3ffe:ffff:836f:0a00:000a:0800:200a:c031, Exim might look up
1.3.0.c.a.0.0.2.0.0.8.0.a.0.0.0.0.0.a.0.f.6.3.8. f.f.f.f.e.f.f.3.blackholes.mail-abuse.org |
(split over two lines here to fit on the page). Unfortunately, some of the DNS lists contain wildcard records, intended for IPv4, that interact badly with IPv6. For example, the DNS entry
*.3.some.list.example. A 127.0.0.1 |
is probably intended to put the entire 3.0.0.0/8 IPv4 network on the list. Unfortunately, it also matches the entire 3::/4 IPv6 network.
You can exclude IPv6 addresses from DNS lookups by making use of a suitable
condition
condition, as in this example:
deny condition = ${if isip4{$sender_host_address}} dnslists = some.list.example |
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The ratelimit
ACL condition can be used to measure and control the rate at
which clients can send email. This is more powerful than the
smtp_ratelimit_*
options, because those options control the rate of
commands in a single SMTP session only, whereas the ratelimit
condition
works across all connections (concurrent and sequential) from the same client
host. The syntax of the ratelimit
condition is:
ratelimit = <m> / <p> / <options> / <key> |
If the average client sending rate is less than m messages per time period p then the condition is false; otherwise it is true.
As a side-effect, the ratelimit
condition sets the expansion variable
$sender_rate
to the client's computed rate, $sender_rate_limit
to the
configured value of m, and $sender_rate_period
to the configured value
of p.
The parameter p is the smoothing time constant, in the form of an Exim time interval, for example, ‘8h’ for eight hours. A larger time constant means that it takes Exim longer to forget a client's past behaviour. The parameter m is the maximum number of messages that a client is permitted to send in each time interval. It also specifies the number of messages permitted in a fast burst. By increasing both m and p but keeping m/p constant, you can allow a client to send more messages in a burst without changing its overall sending rate limit. Conversely, if m and p are both small, messages must be sent at an even rate.
There is a script in ‘util/ratelimit.pl’ which extracts sending rates from
log files, to assist with choosing appropriate settings for m and p
when deploying the ratelimit
ACL condition. The script prints usage
instructions when it is run with no arguments.
The key is used to look up the data for calculating the client's average
sending rate. This data is stored in a database maintained by Exim in its spool
directory, alongside the retry and other hints databases. The default key is
$sender_host_address
, which applies the limit to each client host IP address.
By changing the key you can change how Exim identifies clients for the purpose
of ratelimiting. For example, to limit the sending rate of each authenticated
user, independent of the computer they are sending from, set the key to
$authenticated_id
. You must ensure that the lookup key is meaningful; for
example, $authenticated_id
is only meaningful if the client has
authenticated, and you can check with the authenticated
ACL condition.
If you want to limit the rate at which a recipient receives messages, you can
use the key ‘$local_part@$domain’ with the per_rcpt
option (see below) in
a RCPT ACL.
Internally, Exim includes the smoothing constant p and the options in the lookup key because they alter the meaning of the stored data. This is not true for the limit m, so you can alter the configured maximum rate and Exim will still remember clients' past behaviour, but if you alter the other ratelimit parameters Exim forgets past behaviour.
Each ratelimit
condition can have up to three options. One option
specifies what Exim measures the rate of, and the second specifies how Exim
handles excessively fast clients. The third option can be ‘noupdate’, to
disable updating of the ratelimiting database (see section Reading ratelimit data without updating).
The options are separated by a slash, like the other parameters. They may
appear in any order.
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The per_conn
option limits the client's connection rate.
The per_mail
option limits the client's rate of sending messages. This is
the default if none of the per_*
options is specified.
The per_byte
option limits the sender's email bandwidth. Note that it is
best to use this option in the DATA ACL; if it is used in an earlier ACL it
relies on the SIZE parameter on the MAIL command, which may be inaccurate or
completely missing. You can follow the limit m in the configuration with K,
M, or G to specify limits in kilobytes, megabytes, or gigabytes, respectively.
The per_rcpt
option causes Exim to limit the rate at which
recipients are accepted. To be effective, it would need to be used in
either the acl_smtp_rcpt
or the acl_not_smtp
ACL. In the
acl_smtp_rcpt
ACL, the number of recipients is incremented by one.
In the case of a locally submitted message in the acl_not_smtp
ACL,
the number of recipients incremented is equal to $recipients_count
for the entire message. Note that in either case the rate limiting
engine will see a message with many recipients as a large high-speed
burst.
The per_cmd
option causes Exim to recompute the rate every time the
condition is processed. This can be used to limit the SMTP command rate.
This command is essentially an alias of per_rcpt
to make it clear
that the effect is to limit the rate at which individual commands,
rather than recipients, are accepted.
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If a client's average rate is greater than the maximum, the rate limiting
engine can react in two possible ways, depending on the presence of the
strict
or leaky
options. This is independent of the other
counter-measures (such as rejecting the message) that may be specified by the
rest of the ACL. The default mode is leaky, which avoids a sender's
over-aggressive retry rate preventing it from getting any email through.
The strict
option means that the client's recorded rate is always updated.
The effect of this is that Exim measures the client's average rate of attempts
to send email, which can be much higher than the maximum it is actually
allowed. If the client is over the limit it may be subjected to
counter-measures in the ACL until it slows down below the maximum rate. The
smoothing period determines the time it takes for a high sending rate to decay
exponentially to 37% of its peak value, which means that you can work out the
time (the number of smoothing periods) that a client is subjected to
counter-measures after an over-limit burst with this formula:
ln(peakrate/maxrate) |
The leaky
(default) option means that the client's recorded rate is not
updated if it is above the limit. The effect of this is that Exim measures the
client's average rate of successfully sent email, which cannot be greater than
the maximum allowed. If the client is over the limit it may suffer some
counter-measures (as specified in the ACL), but it will still be able to send
email at the configured maximum rate, whatever the rate of its attempts. This
is generally the better choice if you have clients that retry automatically.
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Exim's other ACL facilities are used to define what counter-measures are taken when the rate limit is exceeded. This might be anything from logging a warning (for example, while measuring existing sending rates in order to define policy), through time delays to slow down fast senders, up to rejecting the message. For example:
# Log all senders' rates warn ratelimit = 0 / 1h / strict log_message = Sender rate $sender_rate / $sender_rate_period # Slow down fast senders; note the need to truncate $sender_rate # at the decimal point. warn ratelimit = 100 / 1h / per_rcpt / strict delay = ${eval: ${sg{$sender_rate}{[.].*}{}} - \ $sender_rate_limit }s # Keep authenticated users under control deny authenticated = * ratelimit = 100 / 1d / strict / $authenticated_id # System-wide rate limit defer message = Sorry, too busy. Try again later. ratelimit = 10 / 1s / $primary_hostname # Restrict incoming rate from each host, with a default # set using a macro and special cases looked up in a table. defer message = Sender rate exceeds $sender_rate_limit \ messages per $sender_rate_period ratelimit = ${lookup {$sender_host_address} \ cdb {DB/ratelimits.cdb} \ {$value} {RATELIMIT} } |
Warning: If you have a busy server with a lot of ratelimit
tests,
especially with the per_rcpt
option, you may suffer from a performance
bottleneck caused by locking on the ratelimit hints database. Apart from
making your ACLs less complicated, you can reduce the problem by using a
RAM disk for Exim's hints directory (usually ‘/var/spool/exim/db/’). However
this means that Exim will lose its hints data after a reboot (including retry
hints, the callout cache, and ratelimit data).
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If the noupdate
option is present on a ratelimit
ACL condition, Exim
computes the rate and checks the limit as normal, but it does not update the
saved data. This means that, in relevant ACLs, it is possible to lookup the
existence of a specified (or auto-generated) ratelimit key without incrementing
the ratelimit counter for that key. In order for this to be useful, another ACL
entry must set the rate for the same key (otherwise it will always be zero).
For example:
acl_check_connect: deny ratelimit = 100 / 5m / strict / noupdate log_message = RATE: $sender_rate/$sender_rate_period \ (max $sender_rate_limit) |
... some other logic and tests... |
acl_check_mail: warn ratelimit = 100 / 5m / strict / per_cmd condition = ${if le{$sender_rate}{$sender_rate_limit}} logwrite = RATE UPDATE: $sender_rate/$sender_rate_period \ (max $sender_rate_limit) |
In this example, the rate is tested and used to deny access (when it is too high) in the connect ACL, but the actual computation of the remembered rate happens later, on a per-command basis, in another ACL.
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Several of the verify
conditions described in section
ACL conditions cause addresses to be verified. Section
Sender address verification reporting discusses the reporting of sender verification failures.
The verification conditions can be followed by options that modify the
verification process. The options are separated from the keyword and from each
other by slashes, and some of them contain parameters. For example:
verify = sender/callout verify = recipient/defer_ok/callout=10s,defer_ok |
The first stage of address verification, which always happens, is to run the
address through the routers, in "verify mode". Routers can detect the
difference between verification and routing for delivery, and their actions can
be varied by a number of generic options such as verify
and verify_only
(see chapter Generic options for routers). If routing fails, verification fails.
The available options are as follows:
callout
option is specified, successful routing to one or more
remote hosts is followed by a "callout" to those hosts as an additional
check. Callouts and their sub-options are discussed in the next section.
defer_ok
in the
options, the condition is forced to be true instead. Note that this is a main
verification option as well as a suboption for callouts.
no_details
option is covered in section Sender address verification reporting, which
discusses the reporting of sender address verification failures.
success_on_redirect
option causes verification always to succeed
immediately after a successful redirection. By default, if a redirection
generates just one address, that address is also verified. See further
discussion in section Redirection while verifying.
After an address verification failure, $acl_verify_message
contains the
error message that is associated with the failure. It can be preserved by
coding like this:
warn !verify = sender set acl_m0 = $acl_verify_message |
If you are writing your own custom rejection message or log message when denying access, you can use this variable to include information about the verification failure.
In addition, $sender_verify_failure
or $recipient_verify_failure
(as
appropriate) contains one of the following words:
qualify
: The address was unqualified (no domain), and the message
was neither local nor came from an exempted host.
route
: Routing failed.
mail
: Routing succeeded, and a callout was attempted; rejection
occurred at or before the MAIL command (that is, on initial
connection, HELO, or MAIL).
recipient
: The RCPT command in a callout was rejected.
postmaster
: The postmaster check in a callout was rejected.
The main use of these variables is expected to be to distinguish between rejections of MAIL and rejections of RCPT in callouts.
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For non-local addresses, routing verifies the domain, but is unable to do any checking of the local part. There are situations where some means of verifying the local part is desirable. One way this can be done is to make an SMTP callback to a delivery host for the sender address or a callforward to a subsequent host for a recipient address, to see if the host accepts the address. We use the term callout to cover both cases. Note that for a sender address, the callback is not to the client host that is trying to deliver the message, but to one of the hosts that accepts incoming mail for the sender's domain.
Exim does not do callouts by default. If you want them to happen, you must
request them by setting appropriate options on the verify
condition, as
described below. This facility should be used with care, because it can add a
lot of resource usage to the cost of verifying an address. However, Exim does
cache the results of callouts, which helps to reduce the cost. Details of
caching are in section Callout caching.
Recipient callouts are usually used only between hosts that are controlled by the same administration. For example, a corporate gateway host could use callouts to check for valid recipients on an internal mailserver. A successful callout does not guarantee that a real delivery to the address would succeed; on the other hand, a failing callout does guarantee that a delivery would fail.
If the callout
option is present on a condition that verifies an address, a
second stage of verification occurs if the address is successfully routed to
one or more remote hosts. The usual case is routing by a dnslookup
or a
manualroute
router, where the router specifies the hosts. However, if a
router that does not set up hosts routes to an smtp
transport with a
hosts
setting, the transport's hosts are used. If an smtp
transport has
hosts_override
set, its hosts are always used, whether or not the router
supplies a host list.
The port that is used is taken from the transport, if it is specified and is a
remote transport. (For routers that do verification only, no transport need be
specified.) Otherwise, the default SMTP port is used. If a remote transport
specifies an outgoing interface, this is used; otherwise the interface is not
specified. Likewise, the text that is used for the HELO command is taken from
the transport's helo_data
option; if there is no transport, the value of
$smtp_active_hostname
is used.
For a sender callout check, Exim makes SMTP connections to the remote hosts, to test whether a bounce message could be delivered to the sender address. The following SMTP commands are sent:
HELO <local host name> MAIL FROM:<> RCPT TO:<the address to be tested> QUIT |
LHLO is used instead of HELO if the transport's protocol
option is
set to "lmtp".
A recipient callout check is similar. By default, it also uses an empty address
for the sender. This default is chosen because most hosts do not make use of
the sender address when verifying a recipient. Using the same address means
that a single cache entry can be used for each recipient. Some sites, however,
do make use of the sender address when verifying. These are catered for by the
use_sender
and use_postmaster
options, described in the next section.
If the response to the RCPT command is a 2xx code, the verification
succeeds. If it is 5xx, the verification fails. For any other condition,
Exim tries the next host, if any. If there is a problem with all the remote
hosts, the ACL yields "defer", unless the defer_ok
parameter of the
callout
option is given, in which case the condition is forced to succeed.
A callout may take a little time. For this reason, Exim normally flushes SMTP
output before performing a callout in an ACL, to avoid unexpected timeouts in
clients when the SMTP PIPELINING extension is in use. The flushing can be
disabled by using a control
modifier to set no_callout_flush
.
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The callout
option can be followed by an equals sign and a number of
optional parameters, separated by commas. For example:
verify = recipient/callout=10s,defer_ok |
The old syntax, which had callout_defer_ok
and check_postmaster
as
separate verify options, is retained for backwards compatibility, but is now
deprecated. The additional parameters for callout
are as follows:
This specifies the timeout that applies for the callout attempt to each host. For example:
verify = sender/callout=5s |
The default is 30 seconds. The timeout is used for each response from the
remote host. It is also used for the initial connection, unless overridden by
the connect
parameter.
This parameter makes it possible to set a different (usually smaller) timeout for making the SMTP connection. For example:
verify = sender/callout=5s,connect=1s |
If not specified, this timeout defaults to the general timeout value.
When this parameter is present, failure to contact any host, or any other kind of temporary error, is treated as success by the ACL. However, the cache is not updated in this circumstance.
This operates like the postmaster
option (see below), but if the check for
postmaster@domain fails, it tries just postmaster, without a domain, in
accordance with the specification in RFC 2821. The RFC states that the
unqualified address postmaster should be accepted.
When verifying addresses in header lines using the header_sender
verification option, Exim behaves by default as if the addresses are envelope
sender addresses from a message. Callout verification therefore tests to see
whether a bounce message could be delivered, by using an empty address in the
MAIL command. However, it is arguable that these addresses might never be used
as envelope senders, and could therefore justifiably reject bounce messages
(empty senders). The mailfrom
callout parameter allows you to specify what
address to use in the MAIL command. For example:
require verify = header_sender/callout=mailfrom=abcd@x.y.z |
This parameter is available only for the header_sender
verification option.
This parameter sets an overall timeout for performing a callout verification. For example:
verify = sender/callout=5s,maxwait=30s |
This timeout defaults to four times the callout timeout for individual SMTP commands. The overall timeout applies when there is more than one host that can be tried. The timeout is checked before trying the next host. This prevents very long delays if there are a large number of hosts and all are timing out (for example, when network connections are timing out).
When this parameter is given, the callout cache is neither read nor updated.
When this parameter is set, a successful callout check is followed by a similar
check for the local part postmaster at the same domain. If this address is
rejected, the callout fails (but see fullpostmaster
above). The result of
the postmaster check is recorded in a cache record; if it is a failure, this is
used to fail subsequent callouts for the domain without a connection being
made, until the cache record expires.
The postmaster check uses an empty sender in the MAIL command by default. You can use this parameter to do a postmaster check using a different address. For example:
require verify = sender/callout=postmaster_mailfrom=abc@x.y.z |
If both postmaster
and postmaster_mailfrom
are present, the rightmost
one overrides. The postmaster
parameter is equivalent to this example:
require verify = sender/callout=postmaster_mailfrom= |
Warning: The caching arrangements for postmaster checking do not take account of the sender address. It is assumed that either the empty address or a fixed non-empty address will be used. All that Exim remembers is that the postmaster check for the domain succeeded or failed.
When this parameter is set, before doing the normal callout check, Exim does a
check for a "random" local part at the same domain. The local part is not
really random - it is defined by the expansion of the option
callout_random_local_part
, which defaults to
$primary_host_name-$tod_epoch-testing |
The idea here is to try to determine whether the remote host accepts all local parts without checking. If it does, there is no point in doing callouts for specific local parts. If the "random" check succeeds, the result is saved in a cache record, and used to force the current and subsequent callout checks to succeed without a connection being made, until the cache record expires.
This parameter applies to recipient callouts only. For example:
deny !verify = recipient/callout=use_postmaster |
It causes a non-empty postmaster address to be used in the MAIL command when
performing the callout for the recipient, and also for a "random" check if
that is configured. The local part of the address is ‘postmaster’ and the
domain is the contents of $qualify_domain
.
This option applies to recipient callouts only. For example:
require verify = recipient/callout=use_sender |
It causes the message's actual sender address to be used in the MAIL command when performing the callout, instead of an empty address. There is no need to use this option unless you know that the called hosts make use of the sender when checking recipients. If used indiscriminately, it reduces the usefulness of callout caching.
If you use any of the parameters that set a non-empty sender for the MAIL
command (mailfrom
, postmaster_mailfrom
, use_postmaster
, or
use_sender
), you should think about possible loops. Recipient checking is
usually done between two hosts that are under the same management, and the host
that receives the callouts is not normally configured to do callouts itself.
Therefore, it is normally safe to use use_postmaster
or use_sender
in
these circumstances.
However, if you use a non-empty sender address for a callout to an arbitrary host, there is the likelihood that the remote host will itself initiate a callout check back to your host. As it is checking what appears to be a message sender, it is likely to use an empty address in MAIL, thus avoiding a callout loop. However, to be on the safe side it would be best to set up your own ACLs so that they do not do sender verification checks when the recipient is the address you use for header sender or postmaster callout checking.
Another issue to think about when using non-empty senders for callouts is
caching. When you set mailfrom
or use_sender
, the cache record is keyed
by the sender/recipient combination; thus, for any given recipient, many more
actual callouts are performed than when an empty sender or postmaster is used.
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Exim caches the results of callouts in order to reduce the amount of resources
used, unless you specify the no_cache
parameter with the callout
option. A hints database called "callout" is used for the cache. Two
different record types are used: one records the result of a callout check for
a specific address, and the other records information that applies to the
entire domain (for example, that it accepts the local part postmaster).
When an original callout fails, a detailed SMTP error message is given about the failure. However, for subsequent failures use the cache data, this message is not available.
The expiry times for negative and positive address cache records are
independent, and can be set by the global options callout_negative_expire
(default 2h) and callout_positive_expire
(default 24h), respectively.
If a host gives a negative response to an SMTP connection, or rejects any commands up to and including
MAIL FROM:<> |
(but not including the MAIL command with a non-empty address),
any callout attempt is bound to fail. Exim remembers such failures in a
domain cache record, which it uses to fail callouts for the domain without
making new connections, until the domain record times out. There are two
separate expiry times for domain cache records:
callout_domain_negative_expire
(default 3h) and
callout_domain_positive_expire
(default 7d).
Domain records expire when the negative expiry time is reached if callouts cannot be made for the domain, or if the postmaster check failed. Otherwise, they expire when the positive expiry time is reached. This ensures that, for example, a host that stops accepting "random" local parts will eventually be noticed.
The callout caching mechanism is based on the domain of the address that is being tested. If the domain routes to several hosts, it is assumed that their behaviour will be the same.
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See section Address verification for a general discussion of verification. When sender verification fails in an ACL, the details of the failure are given as additional output lines before the 550 response to the relevant SMTP command (RCPT or DATA). For example, if sender callout is in use, you might see:
MAIL FROM:<xyz@abc.example> 250 OK RCPT TO:<pqr@def.example> 550-Verification failed for <xyz@abc.example> 550-Called: 192.168.34.43 550-Sent: RCPT TO:<xyz@abc.example> 550-Response: 550 Unknown local part xyz in <xyz@abc.example> 550 Sender verification failed |
If more than one RCPT command fails in the same way, the details are given only for the first of them. However, some administrators do not want to send out this much information. You can suppress the details by adding ‘/no_details’ to the ACL statement that requests sender verification. For example:
verify = sender/no_details |
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A dilemma arises when a local address is redirected by aliasing or forwarding during verification: should the generated addresses themselves be verified, or should the successful expansion of the original address be enough to verify it? By default, Exim takes the following pragmatic approach:
This seems the most reasonable behaviour for the common use of aliasing as a way of redirecting different local parts to the same mailbox. It means, for example, that a pair of alias entries of the form
A.Wol: aw123 aw123: :fail: Gone away, no forwarding address |
work as expected, with both local parts causing verification failure. When a redirection generates more than one address, the behaviour is more like a mailing list, where the existence of the alias itself is sufficient for verification to succeed.
It is possible, however, to change the default behaviour so that all successful
redirections count as successful verifications, however many new addresses are
generated. This is specified by the success_on_redirect
verification
option. For example:
require verify = recipient/success_on_redirect/callout=10s |
In this example, verification succeeds if a router generates a new address, and the callout does not occur, because no address was routed to a remote host.
When verification is being tested via the -bv
option, the treatment of
redirections is as just described, unless the -v
or any debugging option is
also specified. In that case, full verification is done for every generated
address and a report is output for each of them.
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Client SMTP Authorization is a system that allows a site to advertise which machines are and are not permitted to send email. This is done by placing special SRV records in the DNS; these are looked up using the client's HELO domain. At the time of writing, CSA is still an Internet Draft. Client SMTP Authorization checks in Exim are performed by the ACL condition:
verify = csa |
This fails if the client is not authorized. If there is a DNS problem, or if no
valid CSA SRV record is found, or if the client is authorized, the condition
succeeds. These three cases can be distinguished using the expansion variable
$csa_status
, which can take one of the values "fail", "defer",
"unknown", or "ok". The condition does not itself defer because that would
be likely to cause problems for legitimate email.
The error messages produced by the CSA code include slightly more
detail. If $csa_status
is "defer", this may be because of problems
looking up the CSA SRV record, or problems looking up the CSA target
address record. There are four reasons for $csa_status
being "fail":
The csa
verification condition can take an argument which is the domain to
use for the DNS query. The default is:
verify = csa/$sender_helo_name |
This implementation includes an extension to CSA. If the query domain is an address literal such as [192.0.2.95], or if it is a bare IP address, Exim searches for CSA SRV records in the reverse DNS as if the HELO domain was (for example) 95.2.0.192.in-addr.arpa. Therefore it is meaningful to say:
verify = csa/$sender_host_address |
In fact, this is the check that Exim performs if the client does not say HELO.
This extension can be turned off by setting the main configuration option
dns_csa_use_reverse
to be false.
If a CSA SRV record is not found for the domain itself, a search
is performed through its parent domains for a record which might be
making assertions about subdomains. The maximum depth of this search is limited
using the main configuration option dns_csa_search_limit
, which is 5 by
default. Exim does not look for CSA SRV records in a top level domain, so the
default settings handle HELO domains as long as seven
(hostname.five.four.three.two.one.com). This encompasses the vast majority
of legitimate HELO domains.
The dnsdb lookup also has support for CSA. Although dnsdb also supports direct SRV lookups, this is not sufficient because of the extra parent domain search behaviour of CSA, and (as with PTR lookups) dnsdb also turns IP addresses into lookups in the reverse DNS space. The result of a successful lookup such as:
${lookup dnsdb {csa=$sender_helo_name}} |
has two space-separated fields: an authorization code and a target host name. The authorization code can be "Y" for yes, "N" for no, "X" for explicit authorization required but absent, or "?" for unknown.
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Bounce address tag validation (BATV) is a scheme whereby the envelope senders of outgoing messages have a cryptographic, timestamped "tag" added to them. Genuine incoming bounce messages should therefore always be addressed to recipients that have a valid tag. This scheme is a way of detecting unwanted bounce messages caused by sender address forgeries (often called "collateral spam"), because the recipients of such messages do not include valid tags.
There are two expansion items to help with the implementation of the BATV
"prvs" (private signature) scheme in an Exim configuration. This scheme signs
the original envelope sender address by using a simple key to add a hash of the
address and some time-based randomizing information. The prvs
expansion
item creates a signed address, and the prvscheck
expansion item checks one.
The syntax of these expansion items is described in section
Expansion items.
As an example, suppose the secret per-address keys are stored in an MySQL database. A query to look up the key for an address could be defined as a macro like this:
PRVSCHECK_SQL = ${lookup mysql{SELECT secret FROM batv_prvs \ WHERE sender='${quote_mysql:$prvscheck_address}'\ }{$value}} |
Suppose also that the senders who make use of BATV are defined by an address
list called batv_senders
. Then, in the ACL for RCPT commands, you could
use this:
# Bounces: drop unsigned addresses for BATV senders deny message = This address does not send an unsigned reverse path senders = : recipients = +batv_senders # Bounces: In case of prvs-signed address, check signature. deny message = Invalid reverse path signature. senders = : condition = ${prvscheck {$local_part@$domain}\ {PRVSCHECK_SQL}{1}} !condition = $prvscheck_result |
The first statement rejects recipients for bounce messages that are addressed to plain BATV sender addresses, because it is known that BATV senders do not send out messages with plain sender addresses. The second statement rejects recipients that are prvs-signed, but with invalid signatures (either because the key is wrong, or the signature has timed out).
A non-prvs-signed address is not rejected by the second statement, because the
prvscheck
expansion yields an empty string if its first argument is not a
prvs-signed address, thus causing the condition
condition to be false. If
the first argument is a syntactically valid prvs-signed address, the yield is
the third string (in this case "1"), whether or not the cryptographic and
timeout checks succeed. The $prvscheck_result
variable contains the result
of the checks (empty for failure, "1" for success).
There are two more issues you must consider when implementing prvs-signing. Firstly, you need to ensure that prvs-signed addresses are not blocked by your ACLs. A prvs-signed address contains a slash character, but the default Exim configuration contains this statement in the RCPT ACL:
deny message = Restricted characters in address domains = +local_domains local_parts = ^[.] : ^.*[@%!/|] |
This is a conservative rule that blocks local parts that contain slashes. You should remove the slash in the last line.
Secondly, you have to ensure that the routers accept prvs-signed addresses and
deliver them correctly. The easiest way to handle this is to use a redirect
router to remove the signature with a configuration along these lines:
batv_redirect: driver = redirect data = ${prvscheck {$local_part@$domain}{PRVSCHECK_SQL}} |
This works because, if the third argument of prvscheck
is empty, the result
of the expansion of a prvs-signed address is the decoded value of the original
address. This router should probably be the first of your routers that handles
local addresses.
To create BATV-signed addresses in the first place, a transport of this form can be used:
external_smtp_batv: driver = smtp return_path = ${prvs {$return_path} \ {${lookup mysql{SELECT \ secret FROM batv_prvs WHERE \ sender='${quote_mysql:$sender_address}'} \ {$value}fail}}} |
If no key can be found for the existing return path, no signing takes place.
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An MTA is said to relay a message if it receives it from some host and delivers it directly to another host as a result of a remote address contained within it. Redirecting a local address via an alias or forward file and then passing the message on to another host is not relaying, but a redirection as a result of the "percent hack" is.
Two kinds of relaying exist, which are termed "incoming" and "outgoing". A host which is acting as a gateway or an MX backup is concerned with incoming relaying from arbitrary hosts to a specific set of domains. On the other hand, a host which is acting as a smart host for a number of clients is concerned with outgoing relaying from those clients to the Internet at large. Often the same host is fulfilling both functions, but in principle these two kinds of relaying are entirely independent. What is not wanted is the transmission of mail from arbitrary remote hosts through your system to arbitrary domains.
You can implement relay control by means of suitable statements in the ACL that runs for each RCPT command. For convenience, it is often easiest to use Exim's named list facility to define the domains and hosts involved. For example, suppose you want to do the following:
In the main part of the configuration, you put the following definitions:
domainlist local_domains = my.dom1.example : my.dom2.example domainlist relay_domains = friend1.example : friend2.example hostlist relay_hosts = 192.168.45.0/24 |
Now you can use these definitions in the ACL that is run for every RCPT command:
acl_check_rcpt: accept domains = +local_domains : +relay_domains accept hosts = +relay_hosts |
The first statement accepts any RCPT command that contains an address in the local or relay domains. For any other domain, control passes to the second statement, which accepts the command only if it comes from one of the relay hosts. In practice, you will probably want to make your ACL more sophisticated than this, for example, by including sender and recipient verification. The default configuration includes a more comprehensive example, which is described in chapter The default configuration file.
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You can check the relay characteristics of your configuration in the same way
that you can test any ACL behaviour for an incoming SMTP connection, by using
the -bh
option to run a fake SMTP session with which you interact.
For specifically testing for unwanted relaying, the host relay-test.mail-abuse.org provides a useful service. If you telnet to this host from the host on which Exim is running, using the normal telnet port, you will see a normal telnet connection message and then quite a long delay. Be patient. The remote host is making an SMTP connection back to your host, and trying a number of common probes to test for open relay vulnerability. The results of the tests will eventually appear on your terminal.
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