Strong Authentication at Fermilab

Chapter 17: The Kerberos Configuration File: krb5.conf

Chapter 17: The Kerberos Configuration File: krb5.conf

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In this chapter we describe the Kerberos configuration file krb5.conf.

A krb5.conf file must exist in the /etc directory on each UNIX node that is running Kerberos. We provide a template for this file in the krb5conf product in KITS (under ftp://ftp.fnal.gov:8021/products/krb5conf/ ).

If you install Fermi kerberos from KITS using UPS/UPD or RPM for Linux, the krb5conf product (and file) gets installed automatically for you. If you obtain Kerberos from another source, you must obtain this file yourself, edit it as necessary, and copy it into the /etc directory of your machine.

You may need to update your krb5.conf file from time to time as the template in KITS gets updated. New versions are announced on the kerberos-users@fnal.gov mailing list.

If you need to change a setting in krb5.conf but cannot or don't want to change the file in /etc, you can copy /etc/krb5.conf to a new file and edit this copy. Then set the environment variable $KRB5_CONFIG to the full name of your copy. Your copy will be honored by client programs such as kinit or rlogin, but not by programs that need a trusted configuration file, e.g., ksu and the service daemons.

17.1 What does krb5.conf Control?

The file consists of several stanzas, each of which controls certain aspects of the installation:

• [libdefaults] sets defaults for Kerberos on your system, e.g., default realm, default ticket lifetime
• [realms] tells where to find the KDCs for each realm
• [instancemapping] maps client principal properly (for things like cron jobs which require a special principal)
• [domain_realm] maps domains to realms
• [logging] tells Kerberos where and how to log errors
• [appdefaults] lists default settings for outgoing Kerberized network connection applications and for incoming portal mode connections

In section 17.4 krb5.conf.template we list the template krb5.conf file (current as of November `01) with annotations.

17.2 Reinstall krb5conf Using UPD

To reinstall krb5conf and thus update your /etc/krb5.conf file using UPS/UPD, log in as root (or any login id with permissions to write in /etc), and run:

% upd install krb5conf -G -c
 

Then on all nodes in the cluster (including the original node), run the command:

% ups installAsRoot krb5conf
 

Or instead of the ups installAsRoot command, after running upd install, you may manually set the SOURCE_FILE environmental variable to point to the krb5.conf.template script:

% SOURCE_FILE=/path/to/krb5/ups/krb5.conf.template
 

and then invoke the install script

% /path/to/krb5/ups/install
 

17.3 Obtain krb5conf without Using UPD

If you're not running UPS/UPD, go to ftp://ftp.fnal.gov:8021/products/krb5conf/vx_y/NULL/krb5conf_vx_y_NULL.tar (where x_y is 1_9 as of January 2005). Download and untar the file. Look at the top of the installAsRoot script for instructions on how to install it without UPS. If you're not running AFS, check to be sure that the installAsRoot script changes the following line in /etc/krb5.conf to "false":

krb5_run_aklog = false
 

17.4 krb5.conf.template

For reference, we provide the krb5.conf.template file contents for version v1_9, with some explanations inserted. If you install the krb5conf product using UPD, the necessary name substitutions will be made as part of the installation; otherwise, you need to edit this file manually.

[libdefaults]

This section sets defaults for Kerberos on your system.

ticket_lifetime = 1560

There are some implementations of Kerberos that read the above number as seconds, and is equivalent to 26 hours. In MIT-derived code (which Fermi's is), it's read as minutes.

default_realm = xMYREALMx

The UPD installation process changes xMYREALMx to FNAL.GOV. (In Kerberos transactions, this default_realm is assumed when you mention any principal without its "@REALM" part.)

checksum_type = 1

ccache_type = 3

default_tgs_enctypes = des-cbc-crc

default_tkt_enctypes = des-cbc-crc

[realms]

This section lists the realms, and for each the KDCs, admin server (master KDC), the default_domain for converting between Kerberos v4 and Kerberos v5 service names, and principal-to-account name matching info.

If and when we cross-authenticate with some other site, each host that wants to initiate connections to the other site will have to list that site's realm information here. (We think it won't be necessary for accepting connections from that site.)

FNAL.GOV = {

kdc = krb-fnal-1.fnal.gov:88

kdc = krb-fnal-2.fnal.gov:88

kdc = krb-fnal-3.fnal.gov:88

kdc = krb-fnal-4.fnal.gov:88

kdc = krb-fnal-5.fnal.gov:88

kdc = krb-fnal-6.fnal.gov:88

admin_server = krb-fnal-admin.fnal.gov

default_domain = fnal.gov

WIN.FNAL.GOV = {

kdc = littlebird.win.fnal.gov:88

kdc = bigbird.win.fnal.gov:88

default_domain = fnal.gov

}

...

[instancemapping]

This deals with the instance portion of a principal (see principal in the Glossary). The lines that follow instruct Kerberos to strip a trailing /cron/* or /cms/* portion of the client principal when generating a Kerberos v4 ticket for the service called afs.

afs = {

cron/* = ""

cms/* = ""

afs/* = ""

}

[logging]

This section tells Kerberos where and how to log errors; through syslog or directly to file.

kdc = SYSLOG:info:local1

admin-server = SYSLOG:info:local2

default = SYSLOG:err:auth

[domain_realm]

In this section the domains get mapped to the realms. (This determines the realm in which you need to get a service ticket to log into a Kerberized host in a particular domain.) For individual machines in a domain that need to be mapped to a different realm than the domain as a whole, list each machine separately, mapped to the correct realm. Make your changes in the lower part of this section as noted below.

.fnal.gov = FNAL.GOV

.cdms-soudan.org = FNAL.GOV

.minos-soudan.org = FNAL.GOV

.dhcp.fnal.gov = FNAL.GOV

...

.cs.ttu.edu = FNAL.GOV

...

.harvard.edu = FNAL.GOV

... (other "friends and family" by request)

# The whole "top half" is replaced during "ups installAsRoot krb5conf", so:

# It would probably be a bad idea to change anything on or above this line

# If you need to add any .domains or hosts, put them here

[domain_realm]

mojo.lunet.edu = FNAL.GOV

[appdefaults]

This section lists default application settings (ticket attributes, login parameters, etc.). Each of the applications listed may have additional attributes set (e.g., ticket lifetime, and so on). All of these defaults (or nearly all) can be overridden by a command-line flag. The krb5.conf file just sets the defaults for the host. (The ftp client does not look for defaults here; the ftpd ticket lifetime set in the file is invoked for CRYPTOCard FTP access and kcron.)

default_lifetime = 7d

retain_ccache = false

retain_ccache determines whether tickets in a user's ticket cache on a particular host get saved (true) or destroyed (false) when the user closes his session on that host.

autologin = true

forward = true

forwardable = true

forward should in most cases be set to true, in order to forward tickets obtained as "forwardable" to remote hosts by default.

renewable = true

encrypt = true

krb5_aklog_path = /usr/krb5/bin/aklog

The initial list is for common settings. These values are used by all the applications except when an overriding value is listed for a particular application; see below.

telnet = {

}

Telnet uses the common settings; no overrides.

rcp = {

forward = false

encrypt = false

allow_fallback = true

}

Whereas rcp sets two overrides (the first of which is unnecessary) and one additional parameter.

rsh = {

allow_fallback = true

}

rlogin = {

allow_fallback = false

}

login = {

forwardable = true

krb5_run_aklog = true

krb5_get_tickets = true

krb4_get_tickets = false

krb4_convert = false

}

login is invoked by telnetd (not telnet) and sshd (not ssh), and may be invoked by the OS for a local (console) login. CRYPTOCard logins use these settings.

kinit = {

forwardable = true

krb5_run_aklog = true

}

pam = {

forwardable = true

}

rshd = {

krb5_run_aklog = true

}

ftpd = {

krb5_run_aklog = true

default_lifetime = 10h

}

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