This manual is last updated $Date: 2008-01-29 09:00:00 +0000 (Tis, 29 Jan 2008) $ for version 1.2.1 of Heimdal.
--- The Detailed Node Listing ---
Setting up a realm
Applications
Authentication modules
Kerberos 4 issues
Windows 2000 compatability
Programming with Kerberos
Heimdal is a free implementation of Kerberos 5. The goals are to:
Heimdal has the following features (this does not mean any of this works):
libkrb5 library that should be possible to get to work with
simple applications
If you find bugs in this software, make sure it is a genuine bug and not just a part of the code that isn't implemented.
Bug reports should be sent to heimdal-bugs@h5l.org. Please
include information on what machine and operating system (including
version) you are running, what you are trying to do, what happens, what
you think should have happened, an example for us to repeat, the output
you get when trying the example, and a patch for the problem if you have
one. Please make any patches with diff -u or diff -c.
Suggestions, comments and other non bug reports are also welcome.
There are two mailing lists with talk about Heimdal. heimdal-announce@sics.se is a low-volume announcement list, while heimdal-discuss@sics.se is for general discussion. Send a message to majordomo@sics.se to subscribe.
The source code for heimdal, links to binaries and the manual (this document) can be found on our web-page at http://www.pdc.kth.se/heimdal/.
Now this Cerberus had three heads of dogs, the tail of a dragon, and on his back the heads of all sorts of snakes. — Pseudo-Apollodorus Library 2.5.12
Kerberos is a system for authenticating users and services on a network. It is built upon the assumption that the network is “unsafe”. For example, data sent over the network can be eavesdropped and altered, and addresses can also be faked. Therefore they cannot be used for authentication purposes. Kerberos is a trusted third-party service. That means that there is a third party (the kerberos server) that is trusted by all the entities on the network (users and services, usually called principals). All principals share a secret password (or key) with the kerberos server and this enables principals to verify that the messages from the kerberos server are authentic. Thus trusting the kerberos server, users and services can authenticate each other.
Note This discussion is about Kerberos version 4, but version 5 works similarly.
In Kerberos, principals use tickets to prove that they are who they claim to be. In the following example, A is the initiator of the authentication exchange, usually a user, and B is the service that A wishes to use.
To obtain a ticket for a specific service, A sends a ticket request to the kerberos server. The request contains A's and B's names (along with some other fields). The kerberos server checks that both A and B are valid principals.
Having verified the validity of the principals, it creates a packet containing A's and B's names, A's network address (Aaddr), the current time (tissue), the lifetime of the ticket (life), and a secret session key (KAB). This packet is encrypted with B's secret key (KB). The actual ticket (TAB) looks like this: ({A, B, Aaddr, tissue, life, KAB}KB).
The reply to A consists of the ticket (TAB), B's name, the current time, the lifetime of the ticket, and the session key, all encrypted in A's secret key ({B, tissue, life, KAB, TAB}KA). A decrypts the reply and retains it for later use.
Before sending a message to B, A creates an authenticator consisting of A's name, A's address, the current time, and a “checksum” chosen by A, all encrypted with the secret session key ({A, Aaddr, tcurrent, checksum}KAB). This is sent together with the ticket received from the kerberos server to B. Upon reception, B decrypts the ticket using B's secret key. Since the ticket contains the session key that the authenticator was encrypted with, B can now also decrypt the authenticator. To verify that A really is A, B now has to compare the contents of the ticket with that of the authenticator. If everything matches, B now considers A as properly authenticated.
An impostor, C could steal the authenticator and the ticket as it is transmitted across the network, and use them to impersonate A. The address in the ticket and the authenticator was added to make it more difficult to perform this attack. To succeed C will have to either use the same machine as A or fake the source addresses of the packets. By including the time stamp in the authenticator, C does not have much time in which to mount the attack.
C can hijack B's network address, and when A sends her credentials, C just pretend to verify them. C can't be sure that she is talking to A.
It would be possible to add a replay cache to the server side. The idea is to save the authenticators sent during the last few minutes, so that B can detect when someone is trying to retransmit an already used message. This is somewhat impractical (mostly regarding efficiency), and is not part of Kerberos 4; MIT Kerberos 5 contains it.
To authenticate B, A might request that B sends something back that proves that B has access to the session key. An example of this is the checksum that A sent as part of the authenticator. One typical procedure is to add one to the checksum, encrypt it with the session key and send it back to A. This is called mutual authentication.
The session key can also be used to add cryptographic checksums to the messages sent between A and B (known as message integrity). Encryption can also be added (message confidentiality). This is probably the best approach in all cases.
The original paper on Kerberos from 1988 is Kerberos: An Authentication Service for Open Network Systems, by Jennifer Steiner, Clifford Neuman and Jeffrey I. Schiller.
A less technical description can be found in Designing an Authentication System: a Dialogue in Four Scenes by Bill Bryant, also from 1988.
These documents can be found on our web-page at http://www.pdc.kth.se/kth-krb/.
Heimdal uses GNU Autoconf to configure for specific hosts, and GNU
Automake to manage makefiles. If this is new to you, the short
instruction is to run the configure script in the top level
directory, and when that finishes make.
If you want to build the distribution in a different directory from the source directory, you will need a make that implements VPATH correctly, such as GNU make.
You will need to build the distribution:
gcc.
When everything is built, you can install by doing make
install. The default location for installation is /usr/heimdal,
but this can be changed by running configure with
‘--prefix=/some/other/place’.
If you need to change the default behaviour, configure understands the following options:
You will need a fairly recent version of our Kerberos 4 distribution for
rshd and popper to support version 4 clients.
A realm is an administrative domain. The name of a Kerberos realm is usually the Internet domain name in uppercase. Call your realm the same as your Internet domain name if you do not have strong reasons for not doing so. It will make life easier for you and everyone else.
To setup a realm you will first have to create a configuration file: /etc/krb5.conf. The krb5.conf file can contain many configuration options, some of which are described here.
There is a sample krb5.conf supplied with the distribution.
The configuration file is a hierarchical structure consisting of sections, each containing a list of bindings (either variable assignments or subsections). A section starts with ‘[‘section-name’]’. A binding consists of a left hand side, an equal sign (‘=’) and a right hand side (the left hand side tag must be separated from the equal sign with some whitespace). Subsections have a ‘{’ as the first non-whitespace character after the equal sign. All other bindings are treated as variable assignments. The value of a variable extends to the end of the line.
[section1]
a-subsection = {
var = value1
other-var = value with {}
sub-sub-section = {
var = 123
}
}
var = some other value
[section2]
var = yet another value
In this manual, names of sections and bindings will be given as strings separated by slashes (‘/’). The ‘other-var’ variable will thus be ‘section1/a-subsection/other-var’.
For in-depth information about the contents of the configuration file, refer to the krb5.conf manual page. Some of the more important sections are briefly described here.
The ‘libdefaults’ section contains a list of library configuration parameters, such as the default realm and the timeout for KDC responses. The ‘realms’ section contains information about specific realms, such as where they hide their KDC. This section serves the same purpose as the Kerberos 4 krb.conf file, but can contain more information. Finally the ‘domain_realm’ section contains a list of mappings from domains to realms, equivalent to the Kerberos 4 krb.realms file.
To continue with the realm setup, you will have to create a configuration file, with contents similar to the following.
[libdefaults]
default_realm = MY.REALM
[realms]
MY.REALM = {
kdc = my.kdc my.slave.kdc
kdc = my.third.kdc
}
[domain_realm]
.my.domain = MY.REALM
If you use a realm name equal to your domain name, you can omit the ‘libdefaults’, and ‘domain_realm’, sections. If you have a DNS SRV-record for your realm, or your Kerberos server has DNS CNAME ‘kerberos.my.realm’, you can omit the ‘realms’ section too.
The database library will look for the database in the directory /var, so you should probably create that directory. Make sure the directory has restrictive permissions.
# mkdir /var/heimdal
The keys of all the principals are stored in the database. If you choose to, these can be encrypted with a master key. You do not have to remember this key (or password), but just to enter it once and it will be stored in a file (/var/heimdal/m-key). If you want to have a master key, run ‘kstash’ to create this master key:
# kstash
Master key:
Verifying password - Master key:
If you want to generate a random master key you can use the --random-key flag to kstash. This will make sure you have a good key on which attackers can't do a dictionary attack.
If you have a master key, make sure you make a backup of your master key file; without it backups of the database are of no use.
To initialise the database use the kadmin program, with the -l option (to enable local database mode). First issue a init MY.REALM command. This will create the database and insert default principals for that realm. You can have more than one realm in one database, so ‘init’ does not destroy any old database.
Before creating the database, ‘init’ will ask you some questions about maximum ticket lifetimes.
After creating the database you should probably add yourself to it. You do this with the ‘add’ command. It takes as argument the name of a principal. The principal should contain a realm, so if you haven't set up a default realm, you will need to explicitly include the realm.
# kadmin -l
kadmin> init MY.REALM
Realm max ticket life [unlimited]:
Realm max renewable ticket life [unlimited]:
kadmin> add me
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
Password:
Verifying password - Password:
Now start the KDC and try getting a ticket.
# kdc &
# kinit me
me@MY.REALMS's Password:
# klist
Credentials cache: /tmp/krb5cc_0
Principal: me@MY.REALM
Issued Expires Principal
Aug 25 07:25:55 Aug 25 17:25:55 krbtgt/MY.REALM@MY.REALM
If you are curious you can use the ‘dump’ command to list all the entries in the database. It should look something similar to the following example (note that the entries here are truncated for typographical reasons):
kadmin> dump
me@MY.REALM 1:0:1:0b01d3cb7c293b57:-:0:7:8aec316b9d1629e3baf8 ...
kadmin/admin@MY.REALM 1:0:1:e5c8a2675b37a443:-:0:7:cb913ebf85 ...
krbtgt/MY.REALM@MY.REALM 1:0:1:52b53b61c875ce16:-:0:7:c8943be ...
kadmin/changepw@MY.REALM 1:0:1:f48c8af2b340e9fb:-:0:7:e3e6088 ...
All modifications of principals are done with with kadmin.
A principal has several attributes and lifetimes associated with it.
Principals are added, renamed, modified, and deleted with the kadmin commands ‘add’, ‘rename’, ‘modify’, ‘delete’. Both interactive editing and command line flags can be used (use –help to list the available options).
There are different kinds of types for the fields in the database; attributes, absolute time times and relative times.
When doing interactive editing, attributes are listed with ‘?’.
The attributes are given in a comma (‘,’) separated list. Attributes are removed from the list by prefixing them with ‘-’.
kadmin> modify me
Max ticket life [1 day]:
Max renewable life [1 week]:
Principal expiration time [never]:
Password expiration time [never]:
Attributes [disallow-renewable]: requires-pre-auth,-disallow-renewable
kadmin> get me
Principal: me@MY.REALM
[...]
Attributes: requires-pre-auth
The format for absolute times are any of the following:
never
now
YYYY-mm-dd
YYYY-mm-dd HH:MM:SS
The format for relative times are any of the following combined:
N year
M month
O day
P hour
Q minute
R second
There are two tools that can check the consistency of the Kerberos configuration file and the Kerberos database.
The Kerberos configuration file is checked using verify_krb5_conf. The tool checks for common errors, but commonly there are several uncommon configuration entries that are never added to the tool and thus generates “unknown entry” warnings. This is usually nothing to worry about.
The database check is built into the kadmin tool. It will check for common configuration error that will cause problems later. Common check are for existence and flags on important principals. The database check by run by the following command :
kadmin check REALM.EXAMPLE.ORG
To extract a service ticket from the database and put it in a keytab, you need to first create the principal in the database with ‘ank’ (using the --random-key flag to get a random key) and then extract it with ‘ext_keytab’.
kadmin> add --random-key host/my.host.name
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
kadmin> ext host/my.host.name
kadmin> exit
# ktutil list
Version Type Principal
1 des-cbc-md5 host/my.host.name@MY.REALM
1 des-cbc-md4 host/my.host.name@MY.REALM
1 des-cbc-crc host/my.host.name@MY.REALM
1 des3-cbc-sha1 host/my.host.name@MY.REALM
Heimdal can be configured to support 524, Kerberos 4 or kaserver. All these services are turned off by default. Kerberos 4 is always supported by the KDC, but the Kerberos 4 client support also depends on Kerberos 4 support having been included at compile-time, using --with-krb4=dir.
524 is a service that allows the KDC to convert Kerberos 5 tickets to Kerberos 4 tickets for backward compatibility. See also Using 2b tokens with AFS in See AFS.
524 can be turned on by adding this to the configuration file
[kdc]
enable-524 = yes
Kerberos 4 is the predecessor to to Kerberos 5. It only supports single DES. You should only enable Kerberos 4 support if you have needs for compatibility with an installed base of Kerberos 4 clients/servers.
Kerberos 4 can be turned on by adding this to the configuration file
[kdc]
enable-kerberos4 = yes
Kaserver is a Kerberos 4 that is used in AFS. The protocol has some extra features over plain Kerberos 4, but like Kerberos 4, only uses single DES.
You should only enable Kaserver support if you have needs for compatibility with an installed base of AFS machines.
Kaserver can be turned on by adding this to the configuration file
[kdc]
enable-kaserver = yes
The administration server, kadmind, can be started by inetd (which isn't recommended) or run as a normal daemon. If you want to start it from inetd you should add a line similar to the one below to your /etc/inetd.conf.
kerberos-adm stream tcp nowait root /usr/heimdal/libexec/kadmind kadmind
You might need to add ‘kerberos-adm’ to your /etc/services as ‘749/tcp’.
Access to the administration server is controlled by an ACL file, (default /var/heimdal/kadmind.acl.) The file has the following syntax:
principal [priv1,priv2,...] [glob-pattern]
The matching is from top to bottom for matching principals (and if given, glob-pattern). When there is a match, the access rights of that line are applied.
The privileges you can assign to a principal are: ‘add’, ‘change-password’ (or ‘cpw’ for short), ‘delete’, ‘get’, ‘list’, and ‘modify’, or the special privilege ‘all’. All of these roughly correspond to the different commands in kadmin.
If a glob-pattern is given on a line, it restricts the access rights for the principal to only apply for subjects that match the pattern. The patterns are of the same type as those used in shell globbing, see fnmatch(3).
In the example below ‘lha/admin’ can change every principal in the database. ‘jimmy/admin’ can only modify principals that belong to the realm ‘E.KTH.SE’. ‘mille/admin’ is working at the help desk, so he should only be able to change the passwords for single component principals (ordinary users). He will not be able to change any ‘/admin’ principal.
lha/admin@E.KTH.SE all
jimmy/admin@E.KTH.SE all *@E.KTH.SE
jimmy/admin@E.KTH.SE all */*@E.KTH.SE
mille/admin@E.KTH.SE change-password *@E.KTH.SE
To allow users to change their passwords, you should run kpasswdd. It is not run from inetd.
You might need to add ‘kpasswd’ to your /etc/services as ‘464/udp’.
It is important that users have good passwords, both to make it harder to guess them and to avoid off-line attacks (although pre-authentication provides some defence against off-line attacks). To ensure that the users choose good passwords, you can enable password quality controls in kpasswdd and kadmind. The controls themselves are done in a shared library or an external program that is used by kpasswdd. To configure in these controls, add lines similar to the following to your /etc/krb5.conf:
[password_quality]
policies = external-check builtin:minimum-length module:policyname
external_program = /bin/false
policy_libraries = library1.so library2.so
In ‘[password_quality]policies’ the module name is optional if the policy name is unique in all modules (members of ‘policy_libraries’).
The built-in polices are
Executes the program specified by ‘[password_quality]external_program’.
A number of key/value pairs are passed as input to the program, one per line, ending with the string ‘end’. The key/value lines are of the form
principal: principal
new-password: password
where password is the password to check for the previous principal.
If the external application approves the password, it should return ‘APPROVED’ on standard out and exit with exit code 0. If it doesn't approve the password, an one line error message explaining the problem should be returned on standard error and the application should exit with exit code 0. In case of a fatal error, the application should, if possible, print an error message on standard error and exit with a non-zero error code.
The minimum length password quality check reads the configuration file stanza ‘[password_quality]min_length’ and requires the password to be at least this length.
The character-class password quality check reads the configuration file stanza ‘[password_quality]min_classes’. The policy requires the password to have characters from at least that many character classes. Default value if not given is 3.
The four different characters classes are, uppercase, lowercase, number, special characters.
If you want to write your own shared object to check password policies, see the manual page kadm5_pwcheck(3).
Code for a password quality checking function that uses the cracklib library can be found in lib/kadm5/sample_password_check.c in the source code distribution. It requires that the cracklib library be built with the patch available at ftp://ftp.pdc.kth.se/pub/krb/src/cracklib.patch.
A sample policy external program is included in lib/kadm5/check-cracklib.pl.
If no password quality checking function is configured, the only check performed is that the password is at least six characters long.
To check the password policy settings, use the command password-quality in kadmin program. The password verification is only performed locally, on the client. It may be convenient to set the environment variable ‘KRB5_CONFIG’ to point to a test version of krb5.conf while you're testing the ‘[password_quality]’ stanza that way.
Now you should be able to run all the clients and servers. Refer to the appropriate man pages for information on how to use them.
It is desirable to have at least one backup (slave) server in case the master server fails. It is possible to have any number of such slave servers but more than three usually doesn't buy much more redundancy.
All Kerberos servers for a realm must have the same database so that they present the same service to the users. The hprop program, running on the master, will propagate the database to the slaves, running hpropd processes.
Every slave needs a database directory, the master key (if it was used for the database) and a keytab with the principal ‘hprop/hostname’. Add the principal with the ktutil command and start hpropd, as follows:
slave# ktutil get -p foo/admin hprop/`hostname`
slave# mkdir /var/heimdal
slave# hpropd
The master will use the principal ‘kadmin/hprop’ to authenticate to the slaves. This principal should be added when running kadmin -l init but if you do not have it in your database for whatever reason, please add it with kadmin -l add.
master# hprop slave
This was just an hands-on example to make sure that everything was working properly. Doing it manually is of course the wrong way, and to automate this you will want to start hpropd from inetd on the slave(s) and regularly run hprop on the master to regularly propagate the database. Starting the propagation once an hour from cron is probably a good idea.
There is also a newer, and still somewhat experimental, mechanism for doing incremental propagation in Heimdal. Instead of sending the whole database regularly, it sends the changes as they happen on the master to the slaves. The master keeps track of all the changes by assigning a version number to every change to the database. The slaves know which was the latest version they saw and in this way it can be determined if they are in sync or not. A log of all the changes is kept on the master, and when a slave is at an older version than the oldest one in the log, the whole database has to be sent.
Protocol-wise, all the slaves connect to the master and as a greeting tell it the latest version that they have (‘IHAVE’ message). The master then responds by sending all the changes between that version and the current version at the master (a series of ‘FORYOU’ messages) or the whole database in a ‘TELLYOUEVERYTHING’ message. There is also a keep-alive protocol that makes sure all slaves are up and running.
The program that runs on the master is ipropd-master and all clients run ipropd-slave.
Create the file /var/heimdal/slaves on the master containing all the slaves that the database should be propagated to. Each line contains the full name of the principal (for example ‘iprop/hemligare.foo.se@FOO.SE’).
You should already have ‘iprop/tcp’ defined as 2121, in your /etc/services. Otherwise, or if you need to use a different port for some peculiar reason, you can use the --port option. This is useful when you have multiple realms to distribute from one server.
Then you need to create those principals that you added in the configuration file. Create one ‘iprop/hostname’ for the master and for every slave.
master# /usr/heimdal/sbin/ktutil get iprop/`hostname`
The next step is to start the ipropd-master process on the master server. The ipropd-master listens on the UNIX domain socket /var/heimdal/signal to know when changes have been made to the database so they can be propagated to the slaves. There is also a safety feature of testing the version number regularly (every 30 seconds) to see if it has been modified by some means that do not raise this signal. Then, start ipropd-slave on all the slaves:
master# /usr/heimdal/libexec/ipropd-master &
slave# /usr/heimdal/libexec/ipropd-slave master &
To manage the iprop log file you should use the iprop-log command. With it you can dump, truncate and replay the logfile.
The encryption types that the KDC is going to assign by default is possible to change. Since the keys used for user authentication is salted the encryption types are described together with the salt strings.
Salting is used to make it harder to pre-calculate all possible keys. Using a salt increases the search space to make it almost impossible to pre-calculate all keys. Salting is the process of mixing a public string (the salt) with the password, then sending it through an encryption type specific string-to-key function that will output the fixed size encryption key.
In Kerberos 5 the salt is determined by the encryption type, except in some special cases.
In des there is the Kerberos 4 salt
(none at all) or the afs-salt (using the cell (realm in
AFS lingo)).
In arcfour (the encryption type that Microsoft Windows 2000 uses)
there is no salt. This is to be compatible with NTLM keys in Windows
NT 4.
[kadmin]default_keys in krb5.conf controls
what salting to use.
The syntax of [kadmin]default_keys is
‘[etype:]salt-type[:salt-string]’. ‘etype’ is the encryption
type (des-cbc-crc, arcfour-hmac-md5, aes256-cts-hmac-sha1-96),
salt-type is the type of salt (pw-salt or afs3-salt), and the
salt-string is the string that will be used as salt (remember that if
the salt is appended/prepended, the empty salt "" is the same thing as
no salt at all).
Common types of salting include
v4 (or des:pw-salt:)
The Kerberos 4 salting is using no salt at all. Reason there is colon at the end of the salt string is that it makes the salt the empty string (same as no salt).
v5 (or pw-salt)
pw-salt uses the default salt for each encryption type is
specified for. If the encryption type ‘etype’ isn't given, all
default encryption will be used.
afs3-salt
afs3-salt is the salt that is used with Transarc kaserver. It's
the cell name appended to the password.
Suppose you reside in the realm ‘MY.REALM’, how do you authenticate to a server in ‘OTHER.REALM’? Having valid tickets in ‘MY.REALM’ allows you to communicate with Kerberised services in that realm. However, the computer in the other realm does not have a secret key shared with the Kerberos server in your realm.
It is possible to share keys between two realms that trust each other. When a client program, such as telnet or ssh, finds that the other computer is in a different realm, it will try to get a ticket granting ticket for that other realm, but from the local Kerberos server. With that ticket granting ticket, it will then obtain service tickets from the Kerberos server in the other realm.
For a two way trust between ‘MY.REALM’ and ‘OTHER.REALM’ add the following principals to each realm. The principals should be ‘krbtgt/OTHER.REALM@MY.REALM’ and ‘krbtgt/MY.REALM@OTHER.REALM’ in ‘MY.REALM’, and ‘krbtgt/MY.REALM@OTHER.REALM’ and ‘krbtgt/OTHER.REALM@MY.REALM’in ‘OTHER.REALM’.
In Kerberos 5 the trust can be configured to be one way. So that users from ‘MY.REALM’ can authenticate to services in ‘OTHER.REALM’, but not the opposite. In the example above, the ‘krbtgt/MY.REALM@OTHER.REALM’ then should be removed.
The two principals must have the same key, key version number, and the same set of encryption types. Remember to transfer the two keys in a safe manner.
vr$ klist
Credentials cache: FILE:/tmp/krb5cc_913.console
Principal: lha@E.KTH.SE
Issued Expires Principal
May 3 13:55:52 May 3 23:55:54 krbtgt/E.KTH.SE@E.KTH.SE
vr$ telnet -l lha hummel.it.su.se
Trying 2001:6b0:5:1095:250:fcff:fe24:dbf...
Connected to hummel.it.su.se.
Escape character is '^]'.
Waiting for encryption to be negotiated...
[ Trying mutual KERBEROS5 (host/hummel.it.su.se@SU.SE)... ]
[ Kerberos V5 accepts you as ``lha@E.KTH.SE'' ]
Encryption negotiated.
Last login: Sat May 3 14:11:47 from vr.l.nxs.se
hummel$ exit
vr$ klist
Credentials cache: FILE:/tmp/krb5cc_913.console
Principal: lha@E.KTH.SE
Issued Expires Principal
May 3 13:55:52 May 3 23:55:54 krbtgt/E.KTH.SE@E.KTH.SE
May 3 13:55:56 May 3 23:55:54 krbtgt/SU.SE@E.KTH.SE
May 3 14:10:54 May 3 23:55:54 host/hummel.it.su.se@SU.SE
If you want to use cross realm authentication through an intermediate
realm, it must be explicitly allowed by either the KDCs or the server
receiving the request. This is done in krb5.conf in the
[capaths] section.
When the ticket transits through a realm to another realm, the destination realm adds its peer to the "transited-realms" field in the ticket. The field is unordered, since there is no way to know if know if one of the transited-realms changed the order of the list.
The syntax for [capaths] section:
[capaths]
CLIENT-REALM = {
SERVER-REALM = PERMITTED-CROSS-REALMS ...
}
The realm STACKEN.KTH.SE allows clients from SU.SE and
DSV.SU.SE to cross it. Since STACKEN.KTH.SE only has
direct cross realm setup with KTH.SE, and DSV.SU.SE only
has direct cross realm setup with SU.SE they need to use both
SU.SE and KTH.SE as transit realms.
[capaths]
SU.SE = {
STACKEN.KTH.SE = KTH.SE
}
DSV.SU.SE = {
STACKEN.KTH.SE = SU.SE KTH.SE
}
The order of the PERMITTED-CROSS-REALMS is not important when
doing transit cross realm verification.
However, the order is important when the [capaths] section is used
to figure out the intermediate realm to go to when doing multi-realm
transit. When figuring out the next realm, the first realm of the list
of PERMITTED-CROSS-REALMS is chosen. This is done in both the
client kerberos library and the KDC.
If there is information about where to find the KDC or kadmind for a realm in the krb5.conf for a realm, that information will be preferred, and DNS will not be queried.
Heimdal will try to use DNS to find the KDCs for a realm. First it
will try to find a SRV resource record (RR) for the realm. If no
SRV RRs are found, it will fall back to looking for an A RR for
a machine named kerberos.REALM, and then kerberos-1.REALM, etc
Adding this information to DNS minimises the client configuration (in the common case, resulting in no configuration needed) and allows the system administrator to change the number of KDCs and on what machines they are running without caring about clients.
The downside of using DNS is that the client might be fooled to use the wrong server if someone fakes DNS replies/data, but storing the IP addresses of the KDC on all the clients makes it very hard to change the infrastructure.
An example of the configuration for the realm EXAMPLE.COM:
$ORIGIN example.com.
_kerberos._tcp SRV 10 1 88 kerberos.example.com.
_kerberos._udp SRV 10 1 88 kerberos.example.com.
_kerberos._tcp SRV 10 1 88 kerberos-1.example.com.
_kerberos._udp SRV 10 1 88 kerberos-1.example.com.
_kpasswd._udp SRV 10 1 464 kerberos.example.com.
_kerberos-adm._tcp SRV 10 1 749 kerberos.example.com.
More information about DNS SRV resource records can be found in RFC-2782 (A DNS RR for specifying the location of services (DNS SRV)).
Heimdal also supports a way to lookup a realm from a hostname. This to minimise configuration needed on clients. Using this has the drawback that clients can be redirected by an attacker to realms within the same cross realm trust and made to believe they are talking to the right server (since Kerberos authentication will succeed).
An example configuration that informs clients that for the realms it.example.com and srv.example.com, they should use the realm EXAMPLE.COM:
$ORIGIN example.com.
_kerberos.it TXT "EXAMPLE.COM"
_kerberos.srv TXT "EXAMPLE.COM"
This document describes how to install the LDAP backend for Heimdal. Note that before attempting to configure such an installation, you should be aware of the implications of storing private information (such as users' keys) in a directory service primarily designed for public information. Nonetheless, with a suitable authorisation policy, it is possible to set this up in a secure fashion. A knowledge of LDAP, Kerberos, and C is necessary to install this backend. The HDB schema was devised by Leif Johansson.
This assumes, OpenLDAP 2.3 or later.
Requirements:
--with-openldap=/usr/local (adjust according to where you have
installed OpenLDAP).
You can verify that you manage to configure LDAP support by running kdc --builtin-hdb, and checking that ‘ldap:’ is one entry in the list.
Its also possible to configure the ldap backend as a shared module, see option –hdb-openldap-module to configure.
include /usr/local/etc/openldap/schema/hdb.schema
access to *
by dn.exact="uid=heimdal,dc=services,dc=example,dc=com" write
...
authz-regexp "gidNumber=.*\\\+uidNumber=0,cn=peercred,cn=external,cn=auth''
"uid=heimdal,dc=services,dc=example,dc=com"
The sasl-regexp is for mapping between the SASL/EXTERNAL and a user in a tree. The user that the key is mapped to should be have a krb5Principal aux object with krb5PrincipalName set so that the “creator” and “modifier” is right in kadmin.
Another option is to create an admins group and add the dn to that group.
Since Heimdal talks to the LDAP server over a UNIX domain socket, and uses external sasl authentication, it's not possible to require security layer quality (ssf in cyrus-sasl lingo). So that requirement has to be turned off in OpenLDAP slapd configuration file slapd.conf.
sasl-secprops minssf=0
slapd -h "ldapi:/// ldap:///"
Note: These is a bug in slapd where it appears to corrupt the krb5Key binary attribute on shutdown. This may be related to our use of the V3 schema definition syntax instead of the old UMich-style, V2 syntax.
[kdc]
database = {
dbname = ldap:ou=KerberosPrincipals,dc=example,dc=com
hdb-ldap-structural-object = inetOrgPerson
acl_file = /path/to/kadmind.acl
mkey_file = /path/to/mkey
}
‘mkey_file’ can be excluded if you feel that you trust your ldap directory to have the raw keys inside it. The hdb-ldap-structural-object is not necessary if you do not need Samba comatibility.
kdc# kadmin -l
kadmin> init EXAMPLE.COM
Realm max ticket life [unlimited]:
Realm max renewable ticket life [unlimited]:
kadmin> ank lukeh
Max ticket life [1 day]:
Max renewable life [1 week]:
Principal expiration time [never]:
Password expiration time [never]:
Attributes []:
lukeh@EXAMPLE.COM's Password:
Verifying password - lukeh@EXAMPLE.COM's Password:
kadmin> exit
Verify that the principal database has indeed been stored in the directory with the following command:
kdc# ldapsearch -L -h localhost -D cn=manager \
-w secret -b ou=KerberosPrincipals,dc=example,dc=com \
'objectclass=krb5KDCEntry'
index objectClass eq
index cn eq,sub,pres
index uid eq,sub,pres
index displayName eq,sub,pres
index krb5PrincipalName eq
The smbk5pwd overlay, updates the krb5Key and krb5KeyVersionNumber appropriately when it receives an LDAP Password change Extended Operation:
https://sec.miljovern.no/bin/view/Info/TroubleshootingGuide
The Samba domain and the Kerberos realm can have different names since arcfour's string to key functions principal/realm independent. So now will be your first and only chance name your Kerberos realm without needing to deal with old configuration files.
First, you should set up Samba and get that working with LDAP backend.
Now you can proceed as in See Using LDAP to store the database. Heimdal will pick up the Samba LDAP entries if they are in the same search space as the Kerberos entries.
Some services require Kerberos credentials when they start to make connections to other services or need to use them when they have started.
The easiest way to get tickets for a service is to store the key in a keytab. Both ktutil get and kadmin ext can be used to get a keytab. ktutil get is better in that way it changes the key/password for the user. This is also the problem with ktutil. If ktutil is used for the same service principal on several hosts, they keytab will only be useful on the last host. In that case, run the extract command on one host and then securely copy the keytab around to all other hosts that need it.
host# ktutil -k /etc/krb5-service.keytab \
get -p lha/admin@EXAMPLE.ORG service-principal@EXAMPLE.ORG
lha/admin@EXAMPLE.ORG's Password:
To get a Kerberos credential file for the service, use kinit in the --keytab mode. This will not ask for a password but instead fetch the key from the keytab.
service@host$ kinit --cache=/var/run/service_krb5_cache \
--keytab=/etc/krb5-service.keytab \
service-principal@EXAMPLE.ORG
Long running services might need credentials longer then the expiration time of the tickets. kinit can run in a mode that refreshes the tickets before they expire. This is useful for services that write into AFS and other distributed file systems using Kerberos. To run the long running script, just append the program and arguments (if any) after the principal. kinit will stop refreshing credentials and remove the credentials when the script-to-start-service exits.
service@host$ kinit --cache=/var/run/service_krb5_cache \
--keytab=/etc/krb5-service.keytab \
service-principal@EXAMPLE.ORG \
script-to-start-service argument1 argument2
PK-INIT is levering the existing PKI infrastructure to use certificates to get the initial ticket, that is usually the krbtgt.
To use PK-INIT you must first have a PKI, so if you don't have one, it is time to create it. Note that you should read the whole chapter of the document to see the requirements on the CA software.
There needs to exist a mapping between the certificate and what principals that certificate is allowed to use. There are several ways to do this. The administrator can use a configuration file, storing the principal in the SubjectAltName extension of the certificate, or store the mapping in the principals entry in the kerberos database.
This section documents the requirements on the KDC and client certificates and the format used in the id-pkinit-san OtherName extention.
The certificate for the KDC have serveral requirements.
First the certificate should have an Extended Key Usage (EKU) id-pkkdcekuoid (1.3.6.1.5.2.3.5) set. Second there must be a subjectAltName otherName using oid id-pkinit-san (1.3.6.1.5.2.2) in the type field and a DER encoded KRB5PrincipalName that matches the name of the TGS of the target realm.
Both of these two requirements are not required by the standard to be checked by the client if it have external information what the certificate the KDC is supposed to be used. So it's in the interest of minimum amount of configuration on the clients they should be included.
Remember that if the client would accept any certificate as the KDC's certificate, the client could be fooled into trusting something that isn't a KDC and thus expose the user to giving away information (like password or other private information) that it is supposed to secret.
Also, if the certificate has a nameConstraints extention with a Generalname with dNSName or iPAdress it must match the hostname or adress of the KDC.
The client certificate may need to have a EKU id-pkekuoid (1.3.6.1.5.2.3.4) set depending on the certifiate on the KDC.
It possible to store the principal (if allowed by the KDC) in the certificate and thus delegate responsibility to do the mapping between certificates and principals to the CA.
OtherName extention in the GeneralName is used to do the mapping between certifiate and principal in the certifiate or storing the krbtgt principal in the KDC certificate.
The principal is stored in a SubjectAltName in the certificate using OtherName. The oid in the type is id-pkinit-san.
id-pkinit-san OBJECT IDENTIFIER ::= { iso (1) org (3) dod (6)
internet (1) security (5) kerberosv5 (2) 2 }
The data part of the OtherName is filled with the following DER encoded ASN.1 structure:
KRB5PrincipalName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
where Realm and PrincipalName is defined by the Kerberos ASN.1 specification.
hx509 is the X.509 software used in Heimdal to handle certificates. hx509 uses different syntaxes to specify the different formats the certificates are stored in and what formats they exist in.
There are several formats that can be used, PEM, embedded into PKCS12 files, embedded into PKCS11 devices and raw DER encoded certificates. Below is a list of types to use.
The main feature of DIR is that the directory is read on demand when iterating over certificates, that way applictions can for some cases avoid to store all certificates in memory. It's very useful for tests that iterate over larger amount of certificates.
Syntax is:
DIR:/path/to/der/files
Its useful to have one PEM file that contains all the trust anchors.
Syntax is:
FILE:certificate.pem,private-key.key,other-cert.pem,....
Syntax is:
PKCS11:shared-object.so
Syntax is:
PKCS12:/path/to/file.pfx
First configure the client's trust anchors and what parameters to verify, see subsection below how to do that. Now you can use kinit to get yourself tickets. One example how that can look like is:
$ kinit -C FILE:$HOME/.certs/lha.crt,$HOME/.certs/lha.key lha@EXAMPLE.ORG
Enter your private key passphrase:
: lha@nutcracker ; klist
Credentials cache: FILE:/tmp/krb5cc_19100a
Principal: lha@EXAMPLE.ORG
Issued Expires Principal
Apr 20 02:08:08 Apr 20 12:08:08 krbtgt/EXAMPLE.ORG@EXAMPLE.ORG
Using PKCS11 it can look like this instead:
$ kinit -C PKCS11:/usr/heimdal/lib/hx509.so lha@EXAMPLE.ORG
PIN code for SoftToken (slot):
$ klist
Credentials cache: API:4
Principal: lha@EXAMPLE.ORG
Issued Expires Principal
Mar 26 23:40:10 Mar 27 09:40:10 krbtgt/EXAMPLE.ORG@EXAMPLE.ORG
Write about the kdc.
[appdefaults]
pkinit_anchors = FILE:/path/to/trust-anchors.pem
[realms]
EXAMPLE.COM = {
pkinit_require_eku = true
pkinit_require_krbtgt_otherName = true
pkinit_win2k = no
pkinit_win2k_require_binding = yes
}
[kdc]
enable-pkinit = yes
pkinit_identity = FILE:/secure/kdc.crt,/secure/kdc.key
pkinit_anchors = FILE:/path/to/trust-anchors.pem
pkinit_pool = PKCS12:/path/to/useful-intermediate-certs.pfx
pkinit_pool = FILE:/path/to/other-useful-intermediate-certs.pem
pkinit_allow_proxy_certificate = false
pkinit_win2k_require_binding = yes
Note that the file name is space sensitive.
# cat /var/heimdal/pki-mapping
# comments starts with #
lha@EXAMPLE.ORG:C=SE,O=Stockholm universitet,CN=Love,UID=lha
lha@EXAMPLE.ORG:CN=Love,UID=lha
First you need to generate a CA certificate, change the –subject to something appropriate, the CA certificate will be valid for 10 years.
You need to change –subject in the command below.
hxtool issue-certificate \
--self-signed \
--issue-ca \
--generate-key=rsa \
--subject="CN=CA,DC=test,DC=h5l,DC=se" \
--lifetime=10years \
--certificate="FILE:ca.pem"
The KDC needs to have a certificate, so generate a certificate of the type “pkinit-kdc” and set the PK-INIT specifial SubjectAltName to the name of the krbtgt of the realm.
You need to change –subject and –pk-init-principal in the command below.
hxtool issue-certificate \
--ca-certificate=FILE:ca.pem \
--generate-key=rsa \
--type="pkinit-kdc" \
--pk-init-principal="krbtgt/TEST.H5L.SE@TEST.H5L.SE" \
--subject="uid=kdc,DC=test,DC=h5l,DC=se" \
--certificate="FILE:kdc.pem"
The users also needs to have a certificate, so generate a certificate of the type “pkinit-client”. The client doesn't need to have the PK-INIT SubjectAltName set, you can have the Subject DN in the ACL file (pki-mapping) instead.
You need to change –subject and –pk-init-principal in the command below.
hxtool issue-certificate \
--ca-certificate=FILE:ca.pem \
--generate-key=rsa \
--type="pkinit-client" \
--pk-init-principal="lha@TEST.H5L.SE" \
--subject="uid=lha,DC=test,DC=h5l,DC=se" \
--certificate="FILE:user.pem"
hxtool also contains a tool that will validate certificates according to rules from the PKIX document. These checks are not complete, but a good test to check if you got all of the basic bits right in your certificates.
hxtool validate FILE:user.pem
This section tries to give the CA owners hints how to create certificates using OpenSSL (or CA software based on OpenSSL).
To make OpenSSL create certificates with krb5PrincipalName use openssl.cnf as described below. To see a complete example of creating client and KDC certificates, see the test-data generation script lib/hx509/data/gen-req.sh in the source-tree. The certicates it creates are used to test the PK-INIT functionality in tests/kdc/check-kdc.in.
To use this example you have to use OpenSSL 0.9.8a or later.
[user_certificate]
subjectAltName=otherName:1.3.6.1.5.2.2;SEQUENCE:princ_name
[princ_name]
realm = EXP:0, GeneralString:MY.REALM
principal_name = EXP:1, SEQUENCE:principal_seq
[principal_seq]
name_type = EXP:0, INTEGER:1
name_string = EXP:1, SEQUENCE:principals
[principals]
princ1 = GeneralString:userid
Command usage
openssl x509 -extensions user_certificate
openssl ca -extensions user_certificate
Clients using a Windows KDC with PK-INIT need configuration since windows uses pre-standard format and this can't be autodetected.
The pkinit_win2k_require_binding option requires the reply for the KDC to be of the new, secure, type that binds the request to reply. Before clients should fake the reply from the KDC. To use this option you have to apply a fix from Microsoft.
[realms]
MY.MS.REALM = {
pkinit_win2k = yes
pkinit_win2k_require_binding = no
}
The client certificates need to have the extended keyusage “Microsoft Smartcardlogin” (openssl have the oid shortname msSmartcardLogin).
See Microsoft Knowledge Base Article - 281245 “Guidelines for Enabling Smart Card Logon with Third-Party Certification Authorities” for a more extensive description of how set setup an external CA to it includes all information that will make a Windows KDC happy.
To enable Microsoft Smartcardlogin> for certificates in your Windows 2000 CA, you want to look at Microsoft Knowledge Base Article - 313274 “HOW TO: Configure a Certification Authority to Issue Smart Card Certificates in Windows”.
The problem of having different authentication mechanisms has been
recognised by several vendors, and several solutions have appeared. In
most cases these solutions involve some kind of shared modules that are
loaded at run-time. Modules for some of these systems can be found in
lib/auth. Presently there are modules for Digital's SIA,
and IRIX' login and xdm (in
lib/auth/afskauthlib).
How to install the SIA module depends on which OS version you're running. Tru64 5.0 has a new command, siacfg, which makes this process quite simple. If you have this program, you should just be able to run:
siacfg -a KRB5 /usr/athena/lib/libsia_krb5.so
On older versions, or if you want to do it by hand, you have to do the following (not tested by us on Tru64 5.0):
xdm.)
Users with local passwords (like ‘root’) should be able to login safely.
When using Digital's xdm the ‘KRB5CCNAME’ environment variable isn't passed along as it should (since xdm zaps the environment). Instead you have to set ‘KRB5CCNAME’ to the correct value in /usr/lib/X11/xdm/Xsession. Add a line similar to
KRB5CCNAME=FILE:/tmp/krb5cc`id -u`_`ps -o ppid= -p $$`; export KRB5CCNAME
If you use CDE, dtlogin allows you to specify which additional
environment variables it should export. To add ‘KRB5CCNAME’ to this
list, edit /usr/dt/config/Xconfig, and look for the definition of
‘exportList’. You want to add something like:
Dtlogin.exportList: KRB5CCNAME
Digital's ‘ENHANCED’ (C2) security, and Kerberos solve two different problems. C2 deals with local security, adds better control of who can do what, auditing, and similar things. Kerberos deals with network security.
To make C2 security work with Kerberos you will have to do the following.
At present ‘su’ does not accept the vouching flag, so it will not work as expected.
Also, kerberised ftp will not work with C2 passwords. You can solve this by using both Digital's ftpd and our on different ports.
Remember, if you do these changes you will get a system that most certainly does not fulfil the requirements of a C2 system. If C2 is what you want, for instance if someone else is forcing you to use it, you're out of luck. If you use enhanced security because you want a system that is more secure than it would otherwise be, you probably got an even more secure system. Passwords will not be sent in the clear, for instance.
The IRIX support is a module that is compatible with Transarc's afskauthlib.so. It should work with all programs that use this library. This should include login and xdm.
The interface is not very documented but it seems that you have to copy libkafs.so, libkrb.so, and libdes.so to /usr/lib, or build your afskauthlib.so statically.
The afskauthlib.so itself is able to reside in /usr/vice/etc, /usr/afsws/lib, or the current directory (wherever that is).
IRIX 6.4 and newer seem to have all programs (including xdm and login) in the N32 object format, whereas in older versions they were O32. For it to work, the afskauthlib.so library has to be in the same object format as the program that tries to load it. This might require that you have to configure and build for O32 in addition to the default N32.
Apart from this it should “just work”; there are no configuration files.
Note that recent Irix 6.5 versions (at least 6.5.22) have PAM, including a pam_krb5.so module. Not all relevant programs use PAM, though, e.g. ssh. In particular, for console graphical login you need to turn off ‘visuallogin’ and turn on ‘xdm’ with chkconfig.
AFS is a distributed filesystem that uses Kerberos for authentication.
For more information about AFS see OpenAFS http://www.openafs.org/ and Arla http://www.stacken.kth.se/projekt/arla/.
ktutil -k AFSKEYFILE:KeyFile get afs@MY.REALM
or you can extract it with kadmin
kadmin> ext -k AFSKEYFILE:/usr/afs/etc/KeyFile afs@My.CELL.NAME
You have to make sure you have a des-cbc-md5 encryption type since that
is the enctype that will be converted.
You need a /usr/vice/etc/ThisCell containing the cellname of your AFS-cell.
ktutil copy krb4:/root/afs-srvtab AFSKEYFILE:/usr/afs/etc/KeyFile.
If keyfile already exists, this will add the new key in afs-srvtab to KeyFile.
2b is the name of the proposal that was implemented to give basic Kerberos 5 support to AFS in rxkad. It's not real Kerberos 5 support since it still uses fcrypt for data encryption and not Kerberos encryption types.
Its only possible (in all cases) to do this for DES encryption types because only then the token (the AFS equivalent of a ticket) will be smaller than the maximum size that can fit in the token cache in the OpenAFS/Transarc client. It is a so tight fit that some extra wrapping on the ASN1/DER encoding is removed from the Kerberos ticket.
2b uses a Kerberos 5 EncTicketPart instead of a Kerberos 4 ditto for the part of the ticket that is encrypted with the service's key. The client doesn't know what's inside the encrypted data so to the client it doesn't matter.
To differentiate between Kerberos 4 tickets and Kerberos 5 tickets, 2b uses a special kvno, 213 for 2b tokens and 255 for Kerberos 5 tokens.
Its a requirement that all AFS servers that support 2b also support native Kerberos 5 in rxkad.
Support for 2b tokens in the kdc are turned on for specific principals
by adding them to the string list option [kdc]use_2b in the
kdc's krb5.conf file.
[kdc]
use_2b = {
afs@SU.SE = yes
afs/it.su.se@SU.SE = yes
}
There is no need to configure AFS clients for 2b support. The only software that needs to be installed/upgrade is a Kerberos 5 enabled afslog.
Modern versions of Cisco IOS has some support for authenticating via Kerberos 5. This can be used both by having the router get a ticket when you login (boring), and by using Kerberos authenticated telnet to access your router (less boring). The following has been tested on IOS 11.2(12), things might be different with other versions. Old versions are known to have bugs.
To make this work, you will first have to configure your router to use Kerberos (this is explained in the documentation). A sample configuration looks like the following:
aaa new-model
aaa authentication login default krb5-telnet krb5 enable
aaa authorization exec krb5-instance
kerberos local-realm FOO.SE
kerberos srvtab entry host/router.foo.se 0 891725446 4 1 8 012345678901234567
kerberos server FOO.SE 10.0.0.1
kerberos instance map admin 15
This tells you (among other things) that when logging in, the router should try to authenticate with kerberised telnet, and if that fails try to verify a plain text password via a Kerberos ticket exchange (as opposed to a local database, RADIUS or something similar), and if that fails try the local enable password. If you're not careful when you specify the `login default' authentication mechanism, you might not be able to login at all. The `instance map' and `authorization exec' lines says that people with `admin' instances should be given `enabled' shells when logging in.
The numbers after the principal on the `srvtab' line are principal type, time stamp (in seconds since 1970), key version number (4), keytype (1 == des), key length (always 8 with des), and then the key.
To make the Heimdal KDC produce tickets that the Cisco can decode you might have to turn on the ‘encode_as_rep_as_tgs_rep’ flag in the KDC. You will also have to specify that the router can't handle anything but ‘des-cbc-crc’. This can be done with the ‘del_enctype’ command of ‘kadmin’.
This all fine and so, but unless you have an IOS version with encryption (available only in the U.S) it doesn't really solve any problems. Sure you don't have to send your password over the wire, but since the telnet connection isn't protected it's still possible for someone to steal your session. This won't be fixed until someone adds integrity to the telnet protocol.
A working solution would be to hook up a machine with a real operating system to the console of the Cisco and then use it as a backwards terminal server.
The KDC has built-in version 4 support. It is not enabled by default, see setup how to set it up.
The KDC will also have kaserver emulation and be able to handle
AFS-clients that use klog.
For more about AFS, see the section See AFS.
First, Kerberos 4 and Kerberos 5 principals are different. A version 4 principal consists of a name, an instance, and a realm. A version 5 principal has one or more components, and a realm (the terms “name” and “instance” are still used, for the first and second component, respectively). Also, in some cases the name of a version 4 principal differs from the first component of the corresponding version 5 principal. One notable example is the “host” type principals, where the version 4 name is ‘rcmd’ (for “remote command”), and the version 5 name is ‘host’. For the class of principals that has a hostname as instance, there is an other major difference, Kerberos 4 uses only the first component of the hostname, whereas Kerberos 5 uses the fully qualified hostname.
Because of this it can be hard or impossible to correctly convert a version 4 principal to a version 5 principal 1. The biggest problem is to know if the conversion resulted in a valid principal. To give an example, suppose you want to convert the principal ‘rcmd.foo’.
The ‘rcmd’ name suggests that the instance is a hostname (even if there are exceptions to this rule). To correctly convert the instance ‘foo’ to a hostname, you have to know which host it is referring to. You can to this by either guessing (from the realm) which domain name to append, or you have to have a list of possible hostnames. In the simplest cases you can cover most principals with the first rule. If you have several domains sharing a single realm this will not usually work. If the exceptions are few you can probably come by with a lookup table for the exceptions.
In a complex scenario you will need some kind of host lookup mechanism. Using DNS for this is tempting, but DNS is error prone, slow and unsafe 2.
Fortunately, the KDC has a trump on hand: it can easily tell if a
principal exists in the database. The KDC will use
krb5_425_conv_principal_ext to convert principals when handling
to version 4 requests.
If you want to convert an existing version 4 database, the principal conversion issue arises too.
If you decide to convert your database once and for all, you will only have to do this conversion once. It is also possible to run a version 5 KDC as a slave to a version 4 KDC. In this case this conversion will happen every time the database is propagated. When doing this conversion, there are a few things to look out for. If you have stale entries in the database, these entries will not be converted. This might be because these principals are not used anymore, or it might be just because the principal couldn't be converted.
You might also see problems with a many-to-one mapping of principals. For instance, if you are using DNS lookups and you have two principals ‘rcmd.foo’ and ‘rcmd.bar’, where `foo' is a CNAME for `bar', the resulting principals will be the same. Since the conversion function can't tell which is correct, these conflicts will have to be resolved manually.
Given the following set of hosts and services:
foo.se rcmd
mail.foo.se rcmd, pop
ftp.bar.se rcmd, ftp
you have a database that consists of the following principals:
‘rcmd.foo’, ‘rcmd.mail’, ‘pop.mail’, ‘rcmd.ftp’, and ‘ftp.ftp’.
lets say you also got these extra principals: ‘rcmd.gone’, ‘rcmd.old-mail’, where ‘gone.foo.se’ was a machine that has now passed away, and ‘old-mail.foo.se’ was an old mail machine that is now a CNAME for ‘mail.foo.se’.
When you convert this database you want the following conversions to be done:
rcmd.foo host/foo.se
rcmd.mail host/mail.foo.se
pop.mail pop/mail.foo.se
rcmd.ftp host/ftp.bar.se
ftp.ftp ftp/ftp.bar.se
rcmd.gone removed
rcmd.old-mail removed
A krb5.conf that does this looks like:
[realms]
FOO.SE = {
v4_name_convert = {
host = {
ftp = ftp
pop = pop
rcmd = host
}
}
v4_instance_convert = {
foo = foo.se
ftp = ftp.bar.se
}
default_domain = foo.se
}
The ‘v4_name_convert’ section says which names should be considered having an instance consisting of a hostname, and it also says how the names should be converted (for instance ‘rcmd’ should be converted to ‘host’). The ‘v4_instance_convert’ section says how a hostname should be qualified (this is just a hosts-file in disguise). Host-instances that aren't covered by ‘v4_instance_convert’ are qualified by appending the contents of the ‘default_domain’.
Actually, this example doesn't work. Or rather, it works to well. Since it has no way of knowing which hostnames are valid and which are not, it will happily convert ‘rcmd.gone’ to ‘host/gone.foo.se’. This isn't a big problem, but if you have run your kerberos realm for a few years, chances are big that you have quite a few `junk' principals.
If you don't want this you can remove the ‘default_domain’ statement, but then you will have to add entries for all your hosts in the ‘v4_instance_convert’ section.
Instead of doing this you can use DNS to convert instances. This is not a solution without problems, but it is probably easier than adding lots of static host entries.
To enable DNS lookup you should turn on ‘v4_instance_resolve’ in the ‘[libdefaults]’ section.
The database conversion is done with ‘hprop’. You can run this command to propagate the database to the machine called ‘slave-server’ (which should be running a ‘hpropd’).
hprop --source=krb4-db --master-key=/.m slave-server
This command can also be to use for converting the v4 database on the server:
hprop -n --source=krb4-db -d /var/kerberos/principal --master-key=/.m | hpropd -n
‘kadmind’ can act as a version 4 kadmind, and you can do most operations, but with some restrictions (since the version 4 kadmin protocol is, lets say, very ad hoc.) One example is that it only passes des keys when creating principals and changing passwords (modern kpasswd clients do send the password, so it's possible to to password quality checks). Because of this you can only create principals with des keys, and you can't set any flags or do any other fancy stuff.
To get this to work, you have to add another entry to inetd (since version 4 uses port 751, not 749).
And then there are a many more things you can do; more on this in a later version of this manual. Until then, UTSL.
The Heimdal kdc can emulate a kaserver. The kaserver is a Kerberos 4 server with pre-authentication using Rx as the on-wire protocol. The kdc contains a minimalistic Rx implementation.
There are three parts of the kaserver; KAA (Authentication), KAT (Ticket
Granting), and KAM (Maintenance). The KAA interface and KAT interface
both passes over DES encrypted data-blobs (just like the
Kerberos-protocol) and thus do not need any other protection. The KAM
interface uses rxkad (Kerberos authentication layer for Rx) for
security and data protection, and is used for example for changing
passwords. This part is not implemented in the kdc.
Another difference between the ka-protocol and the Kerberos 4 protocol
is that the pass-phrase is salted with the cellname in the string to
key function in the ka-protocol, while in the Kerberos 4 protocol there
is no salting of the password at all. To make sure AFS-compatible keys
are added to each principals when they are created or their password are
changed, ‘afs3-salt’ should be added to
‘[kadmin]default_keys’.
For more about AFS, see the section See AFS.
The Transarc Windows client uses Kerberos 4 to obtain tokens, and thus
does not need a kaserver. The Windows client assumes that the Kerberos
server is on the same machine as the AFS-database server. If you do not
like to do that you can add a small program that runs on the database
servers that forward all kerberos requests to the real kerberos
server. A program that does this is krb-forward
(ftp://ftp.stacken.kth.se/pub/projekts/krb-forward).
Windows 2000 (formerly known as Windows NT 5) from Microsoft implements Kerberos 5. Their implementation, however, has some quirks, peculiarities, and bugs. This chapter is a short summary of the things that we have found out while trying to test Heimdal against Windows 2000. Another big problem with the Kerberos implementation in Windows 2000 is that the available documentation is more focused on getting things to work rather than how they work, and not that useful in figuring out how things really work.
This information should apply to Heimdal 1.2.1 and Windows 2000 Professional. It's of course subject to change all the time and mostly consists of our not so inspired guesses. Hopefully it's still somewhat useful.
You need the command line program called ksetup.exe which is available in the file SUPPORT/TOOLS/SUPPORT.CAB on the Windows 2000 Professional CD-ROM. This program is used to configure the Kerberos settings on a Workstation.
Ksetup store the domain information under the registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\LSA\Kerberos\Domains.
Use the kadmin program in Heimdal to create a host principal in the Kerberos realm.
unix% kadmin
kadmin> ank --password=password host/datan.example.com
The name ‘datan.example.com’ should be replaced with DNS name of the workstation.
You must configure the workstation as a member of a workgroup, as opposed to a member in an NT domain, and specify the KDC server of the realm as follows:
C:> ksetup /setdomain EXAMPLE.COM
C:> ksetup /addkdc EXAMPLE.COM kdc.example.com
Set the machine password, i.e. create the local keytab:
C:> ksetup /SetComputerPassword password
The password used in ksetup /setmachpassword must be the same as the password used in the kadmin ank command.
The workstation must now be rebooted.
A mapping between local NT users and Kerberos principals must be specified. You have two choices. First:
C:> ksetup /mapuser user@MY.REALM nt_user
This will map a user to a specific principal; this allows you to have other usernames in the realm than in your NT user database. (Don't ask me why on earth you would want that...)
You can also say:
C:> ksetup /mapuser * *
The Windows machine will now map any user to the corresponding principal, for example ‘nisse’ to the principal ‘nisse@MY.REALM’. (This is most likely what you want.)
See also the Step-by-Step guide from Microsoft, referenced below.
Install Windows 2000, and create a new controller (Active Directory Server) for the domain.
By default the trust will be non-transitive. This means that only users directly from the trusted domain may authenticate. This can be changed to transitive by using the netdom.exe tool. netdom.exe can also be used to add the trust between two realms.
You need to tell Windows 2000 on what hosts to find the KDCs for the non-Windows realm with ksetup, see See Configuring Windows 2000 to use a Heimdal KDC.
This needs to be done on all computers that want enable cross-realm
login with Mapped Names.
Then you need to add the inter-realm keys on the Windows KDC. Start the Domain Tree Management tool (found in Programs, Administrative tools, Active Directory Domains and Trusts).
Right click on Properties of your domain, select the Trust tab. Press Add on the appropriate trust windows and enter domain name and password. When prompted if this is a non-Windows Kerberos realm, press OK.
Do not forget to add trusts in both directions (if that's what you want).
If you want to use netdom.exe instead of the Domain Tree Management tool, you do it like this:
netdom trust NT.REALM.EXAMPLE.COM /Domain:EXAMPLE.COM /add /realm /passwordt:TrustPassword
You also need to add the inter-realm keys to the Heimdal KDC. Make sure you have matching encryption types (DES, Arcfour and AES in case of Longhorn)
Another issue is salting. Since Windows 2000 does not seem to understand Kerberos 4 salted hashes you might need to turn off anything similar to the following if you have it, at least while adding the principals that are going to share keys with Windows 2000.
[kadmin]
default_keys = v5 v4
So remove v4 from default keys.
What you probably want to use is this:
[kadmin]
default_keys = des-cbc-crc:pw-salt arcfour-hmac-md5:pw-salt
Once that is also done, you can add the required inter-realm keys:
kadmin add krbtgt/NT.REALM.EXAMPLE.COM@EXAMPLE.COM
kadmin add krbtgt/REALM.EXAMPLE.COM@NT.EXAMPLE.COM
Use the same passwords for both keys.
Do not forget to reboot before trying the new realm-trust (after running ksetup). It looks like it might work, but packets are never sent to the non-Windows KDC.
Start the Active Directory Users and Computers tool. Select the
View menu, that is in the left corner just below the real menu (or press
Alt-V), and select Advanced Features. Right click on the user that you
are going to do a name mapping for and choose Name mapping.
Click on the Kerberos Names tab and add a new principal from the non-Windows domain.
This adds ‘authorizationNames’ entry to the users LDAP entry to the Active Directory LDAP catalog. When you create users by script you can add this entry instead.
Windows 2000 supports both the standard DES encryptions (‘des-cbc-crc’ and ‘des-cbc-md5’) and its own proprietary encryption that is based on MD4 and RC4 that is documented in and is supposed to be described in draft-brezak-win2k-krb-rc4-hmac-03.txt. New users will get both MD4 and DES keys. Users that are converted from a NT4 database, will only have MD4 passwords and will need a password change to get a DES key.
The Windows 2000 KDC also adds extra authorisation data in tickets. It is at this point unclear what triggers it to do this. The format of this data is only available under a “secret” license from Microsoft, which prohibits you implementing it.
A simple way of getting hold of the data to be able to understand it better is described here.
There are some issues with salts and Windows 2000. Using an empty salt—which is the only one that Kerberos 4 supported, and is therefore known as a Kerberos 4 compatible salt—does not work, as far as we can tell from out experiments and users' reports. Therefore, you have to make sure you keep around keys with all the different types of salts that are required. Microsoft have fixed this issue post Windows 2003.
Microsoft seems also to have forgotten to implement the checksum algorithms ‘rsa-md4-des’ and ‘rsa-md5-des’. This can make Name mapping (see Create account mappings) fail if a ‘des-cbc-md5’ key is used. To make the KDC return only ‘des-cbc-crc’ you must delete the ‘des-cbc-md5’ key from the kdc using the kadmin del_enctype command.
kadmin del_enctype lha des-cbc-md5
You should also add the following entries to the krb5.conf file:
[libdefaults]
default_etypes = des-cbc-crc
default_etypes_des = des-cbc-crc
These configuration options will make sure that no checksums of the unsupported types are generated.
See also our paper presented at the 2001 Usenix Annual Technical Conference, available in the proceedings or at http://www.usenix.org/publications/library/proceedings/usenix01/freenix01/westerlund.html.
There are lots of texts about Kerberos on Microsoft's web site, here is a short list of the interesting documents that we have managed to find.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\Parameters\LogLevel
with value DWORD equal to 1, and then you'll get logging in the Event
Logger.
Other useful programs include these:
See the Kerberos 5 API introduction and documentation on the Heimdal webpage.
When migrating from a Kerberos 4 KDC.
‘hprop -n --source=<NNN>| hpropd -n’
Replace <NNN> with whatever source you have, like krb4-db or krb4-dump.
Make sure that all things that you use works for you.
Find a sample population of your users and check what programs they use, you can also check the kdc-log to check what ticket are checked out.
Things that might be hard to get away is old programs with support for Kerberos 4. Example applications are old Eudora installations using KPOP, and Zephyr. Eudora can use the Kerberos 4 kerberos in the Heimdal kdc.
Eric Young wrote “libdes”. Heimdal used to use libdes, without it kth-krb would never have existed. Since there are no longer any Eric Young code left in the library, we renamed it to libhcrypto.
All functions in libhcrypto have been re-implemented or used available
public domain code. The core AES function where written by Vincent
Rijmen, Antoon Bosselaers and Paulo Barreto. The core DES SBOX
transformation was written by Richard Outerbridge. imath that
is used for public key crypto support is written by Michael
J. Fromberger.
The University of California at Berkeley initially wrote telnet,
and telnetd. The authentication and encryption code of
telnet and telnetd was added by David Borman (then of Cray
Research, Inc). The encryption code was removed when this was exported
and then added back by Juha Eskelinen.
The popper was also a Berkeley program initially.
Some of the functions in libroken also come from Berkeley by way of NetBSD/FreeBSD.
editline was written by Simmule Turner and Rich Salz. Heimdal
contains a modifed copy.
The getifaddrs implementation for Linux was written by Hideaki
YOSHIFUJI for the Usagi project.
The pkcs11.h headerfile was written by the Scute project.
Bugfixes, documentation, encouragement, and code has been contributed by:
All bugs were introduced by ourselves.