Ubuntu Manpage: k5start - Obtain and optionally keep active a Kerberos ticket

NAME

       k5start - Obtain and optionally keep active a Kerberos ticket

SYNOPSIS

       k5start [-abFhLnPqstvx] [-c child pid file] [-f keytab]
           [-g group] [-H minutes] [-I service instance]
           [-i client instance] [-K minutes] [-k ticket cache]
           [-l time string] [-m mode] [-o owner]
           [-p pid file] [-r service realm] [-S service name]
           [-u client principal] [principal [command ...]]

       k5start -U -f keytab [-abFhLnPqstvx] [-c child pid file]
           [-g group] [-H minutes] [-I service instance]
           [-K minutes] [-k ticket cache] [-l time string]
           [-m mode] [-o owner] [-p pid file]
           [-r service realm] [-S service name] [command ...]

DESCRIPTION

k5start obtains and caches an initial Kerberos ticket-granting ticket for a principal. k5start can be
used as an alternative to kinit, but it is primarily intended to be used by programs that want to use a
keytab to obtain Kerberos credentials, such as a web server that needs to authenticate to another service
such as an LDAP server.

Normally, the principal for which to give tickets should be specified as the first argument. principal
may be either just a principal name (including the optional instance) or a full principal and realm
string. The -u and -i options can be used as an alternative mechanism for specifying the principal, but
generally aren't as convenient. If no principal is given as either the first argument or the argument to
the -u option, the client principal defaults to the Unix username of the user running k5start in the
default local realm.

Optionally, a command may be given on the command line of k5start. If so, that command is run after
Kerberos authentication (and running aklog if desired), with the appropriate environment variables set to
point it to the right ticket cache. k5start will then continue running, waking up periodically to
refresh credentials slightly before they would expire, until the command completes. (The frequency with
which it wakes up to refresh credentials can still be controlled with the -K option.) To run in this
mode, the principal must either be specified as a regular command-line argument or via the -U option; the
-u and -i options may not be used. Also, a keytab must be specified with -f to run a specific command.

The command will not be run using the shell, so if you want to use shell metacharacters in the command
with their special meaning, give "sh -c command" as the command to run and quote command.

If the command contains command-line options (like "-c"), put -- on the command line before the beginning
of the command to tell k5start to not parse those options as its own.

When running a command, k5start propagates HUP, TERM, INT, and QUIT signals to the child process and does
not exit when those signals are received. (If the propagated signal causes the child process to exit,
k5start will then exit.) This allows k5start to react properly when run under a command supervision
system such as runit(8) or svscan(8) that uses signals to control supervised commands, and to run
interactive commands that should receive Ctrl-C.

If a running k5start receives an ALRM signal, it immediately refreshes the ticket cache regardless of
whether it is in danger of expiring.

If k5start is run with a command or the -K flag and the -x flag is not given, it will keep trying even if
the initial authentication fails. It will retry the initial authentication immediately and then with
exponential backoff to once per minute, and keep trying until authentication succeeds or it is killed.
The command, if any, will not be started until authentication succeeds.

OPTIONS

-a When run with either the -K flag or a command, always renew tickets each time k5start wakes up.
Without this option, k5start will only try to renew a ticket as often as necessary to prevent the
ticket from expiring. With this option, k5start will renew tickets according to the interval
specified with the -K flag.

This behavior probably should have been the default behavior of -K. The default was not changed to
avoid changes for existing users, but for new applications, consider always using -a with -K.

This option is important if another program is manipulating the ticket cache that k5start is using.
For example, if another program is automatically renewing a ticket more frequently than k5start, then
k5start will never see a ticket that is close to expiring and will therefore, by default, never try
to renew the ticket. This means that k5start will also never renew AFS tokens, even if the -t option
was given, since k5start only renews AFS tokens after it successfully renews a ticket. If this
option is specified in such a situation, k5start will renew its ticket every time it checks the
ticket, so AFS tokens will be renewed.

This argument is only valid in combination with either -K or a command to run.

-b After starting, detach from the controlling terminal and run in the background. This option only
makes sense in combination with -K or a command that k5start will be running and can only be used if
a keytab is specified with -f. k5start will not background itself until after it has tried
authenticating once, so that any initial errors will be reported, but it will then redirect output to
/dev/null and no subsequent errors will be reported.

If this flag is given, k5start will also change directories to "/". All paths (such as to a command
to run or a PID file) should therefore be given as absolute, not relative, paths.

If used in conjunction with a command to run, that command will also run in the background and will
also have its input and output redirected to /dev/null. It will have to report any errors via some
other mechanism for the errors to be seen.

Note that on Mac OS X, the default ticket cache type is per-session and using the -b flag will
disassociate k5start from the existing ticket cache. When using -b in conjunction with -K on Mac OS
X, you probably also want to use the -k flag to specify a ticket cache file and force the use of a
file cache.

When using this option, consider also using -L to report k5start errors to syslog.

-c child pid file
Save the process ID (PID) of the child process into child pid file. child pid file is created if it
doesn't exist and overwritten if it does exist. This option is only allowed when a command was given
on the command line and is most useful in conjunction with -b to allow management of the running
child process.

Note that, when used with -b, the PID file is written out after k5start is backgrounded and changes
its working directory to /, so relative paths for the PID file will be relative to / (probably not
what you want).

-F Do not get forwardable tickets even if the local configuration says to get forwardable tickets by
default. Without this flag, k5start does whatever the library default is.

-f keytab
Authenticate using the keytab keytab rather than asking for a password. A key for the client
principal must be present in keytab.

-g group
After creating the ticket cache, change its group ownership to group, which may be either the name of
a group or a numeric group ID. Ticket caches are created with 0600 permissions by default, so this
will have no useful effect unless used with -m.

-H minutes
Check for a happy ticket, defined as one that has a remaining lifetime of at least minutes minutes.
If such a ticket is found, do not attempt authentication. Instead, just run the command (if one was
specified) or exit immediately with status 0 (if none was). Otherwise, try to obtain a new ticket
and then run the command, if any.

If -H is used with -t, the external program will always be run even if a ticket with a sufficient
remaining lifetime was found.

If -H is used with -K, k5start will not exit immediately. Instead, the specified remaining lifetime
will replace the default value of two minutes, meaning that k5start will ensure, each time it wakes
up, that the ticket has a remaining lifetime of the minutes argument. This is an alternative to -a
to ensure that tickets always have a certain minimal amount of lifetime remaining.

-h Display a usage message and exit.

-I service instance
The instance portion of the service principal. The default is the default realm of the machine.
Note that unlike the client principal, a non-default service principal must be specified with -I and
-S; one cannot provide the instance portion as part of the argument to -S.

-i client instance
Specifies the instance portion of the principal. This option doesn't make sense except in
combination with -u. Note that the instance can be specified as part of username through the normal
convention of appending a slash and then the instance, so one never has to use this option.

-K minutes
Run in daemon mode to keep a ticket alive indefinitely. The program reawakens after minutes minutes,
checks if the ticket will expire before or less than two minutes after the next scheduled check, and
gets a new ticket if needed. (In other words, it ensures that the ticket will always have a
remaining lifetime of at least two minutes.) If the -H flag is also given, the lifetime specified by
it replaces the two minute default.

If this option is not given but a command was given on the command line, the default interval is 60
minutes (1 hour).

If an error occurs in refreshing the ticket cache, the wake-up interval will be shortened to one
minute and the operation retried at that interval for as long as the error persists.

-k ticket cache
Use ticket cache as the ticket cache rather than the contents of the environment variable KRB5CCNAME
or the library default. ticket cache may be any ticket cache identifier recognized by the underlying
Kerberos libraries. This generally supports a path to a file, with or without a leading "FILE:"
string, but may also support other ticket cache types.

If any of -o, -g, or -m are given, ticket cache must be either a simple path to a file or start with
"FILE:" or "WRFILE:".

-L Report messages to syslog as well as to standard output or standard error. All messages will be
logged with facility LOG_DAEMON. Regular messages that are displayed on standard output are logged
with level LOG_NOTICE. Errors that don't cause k5start to terminate are logged with level
LOG_WARNING. Fatal errors are logged with level LOG_ERR.

This is useful when debugging problems in combination with -b.

-l time string
Set the ticket lifetime. time string should be in a format recognized by the Kerberos libraries for
specifying times, such as "10h" (ten hours) or "10m" (ten minutes). Known units are "s", "m", "h",
and "d". For more information, see kinit(1).

-m mode
After creating the ticket cache, change its file permissions to mode, which must be a file mode in
octal (640 or 444, for example).

Setting a mode that does not allow k5start to read or write to the ticket cache will cause k5start to
fail and exit when using the -K option or running a command.

-n Ignored, present for option compatibility with the now-obsolete k4start.

-o owner
After creating the ticket cache, change its ownership to owner, which may be either the name of a
user or a numeric user ID. If owner is the name of a user and -g was not also given, also change the
group ownership of the ticket cache to the default group for that user.

-P Do not get proxiable tickets even if the local configuration says to get proxiable tickets by
default. Without this flag, k5start does whatever the library default is.

-p pid file
Save the process ID (PID) of the running k5start process into pid file. pid file is created if it
doesn't exist and overwritten if it does exist. This option is most useful in conjunction with -b to
allow management of the running k5start daemon.

Note that, when used with -b the PID file is written out after k5start is backgrounded and changes
its working directory to /, so relative paths for the PID file will be relative to / (probably not
what you want).

-q Quiet. Suppresses the printing of the initial banner message saying what Kerberos principal tickets
are being obtained for, and also suppresses the password prompt when the -s option is given.

-r service realm
The realm for the service principal. This defaults to the default local realm.

-S service name
Specifies the principal for which k5start is getting a service ticket. The default value is
"krbtgt", to obtain a ticket-granting ticket. This option (along with -I) may be used if one only
needs access to a single service. Note that unlike the client principal, a non-default service
principal must be specified with both -S and -I; one cannot provide the instance portion as part of
the argument to -S.

-s Read the password from standard input. This bypasses the normal password prompt, which means echo
isn't suppressed and input isn't forced to be from the controlling terminal. Most uses of this
option are a security risk. You normally want to use a keytab and the -f option instead.

-t Run an external program after getting a ticket. The intended use of this is to run aklog to get a
token. If the environment variable AKLOG (or KINIT_PROG for backward compatibility) is set, it
overrides the compiled-in default.

If a command was given on the command line, k5start will attempt to isolate the AFS credentials for
that command from the invoking process. There are two possible ways in which this is done.

First, if k5start has been built with AFS setpag() support and AFS is available, k5start will create
a new PAG before running the external program.

Otherwise, if either k5start was not built with AFS setpag() support or AFS is not available, but the
Linux kafs module is available and k5start was built with libkeyutils support, it will create a new
session keyring and link it to the current user keyring before running the external program.

If neither of these conditions are true, k5start will run the external program without doing any
credential isolation, which may also affect the credentials of the invoking process.

-U Rather than requiring the authentication principal be given on the command line, read it from the
keytab specified with -f. The principal will be taken from the first entry in the keytab. -f must
be specified if this option is used.

When -U is given, k5start will not expect a principal name to be given on the command line, and any
arguments after the options will be taken as a command to run.

-u client principal
This specifies the principal to obtain credentials as. The entire principal may be specified here,
or alternatively just the first portion may be specified with this flag and the instance specified
with -i.

Note that there's normally no reason to use this flag rather than simply giving the principal on the
command line as the first regular argument.

-v Be verbose. This will print out a bit of additional information about what is being attempted and
what the results are.

-x Exit immediately on any error. Normally, when running a command or when run with the -K option,
k5start keeps running even if it fails to refresh the ticket cache and will try again at the next
check interval. With this option, k5start will instead exit.

EXIT STATUS

       The program exits with status 0 if it successfully gets a ticket or has a  happy  ticket  (see  -H).   If
       k5start  runs  aklog  or  some  other program k5start returns the exit status of that program if it exits
       normally.  If the program exits abnormally due to a signal, k5start will exit with a status of  128  plus
       the signal number.  (This matches the behavior of bash.)

EXAMPLE

       Use  the  /etc/krb5.keytab  keytab to obtain a ticket granting ticket for the principal host/example.com,
       putting the ticket cache in /tmp/service.tkt.  The lifetime is 10 hours and the program wakes up every 10
       minutes to check if the ticket is about to expire.

           k5start -k /tmp/service.tkt -f /etc/krb5.keytab -K 10 -l 10h \
               host/example.com

       Do the same, but using the default ticket cache and run the command /usr/local/bin/auth-backup.   k5start
       will  continue running until the command finishes.  If the initial authentication fails, keep trying, and
       don't start the command until it succeeds.  This could be used during system startup for a  command  that
       must  have  valid  tickets  before  starting,  and  tolerates  having k5start start before the network is
       completely set up.

           k5start -f /etc/krb5.keytab -K 10 -l 10h host/example.com \
               /usr/local/bin/auth-backup

       Shows the permissions of the temporary cache file created by k5start:

           k5start -f /etc/krb5.keytab host/example.com \
               -- sh -c 'ls -l $KRB5CCNAME'

       Notice the "--" before the command to keep k5start from parsing the "-c" as its own option.

       Do the same thing, but determine the principal from the keytab:

           k5start -f /etc/krb5.keytab -U -- sh -c 'ls -l $KRB5CCNAME'

       Note that no principal is given before the command.

       Starts k5start as a daemon using the Debian start-stop-daemon management program.  This is  the  sort  of
       line that one could put into a Debian init script:

           start-stop-daemon --start --pidfile /var/run/k5start.pid \
               --exec /usr/local/bin/k5start -- -b -p /var/run/k5start.pid \
               -f /etc/krb5.keytab host/example.com

       This  uses  /var/run/k5start.pid  as  the  PID  file and obtains host/example.com tickets from the system
       keytab file.  k5start would then be stopped with:

           start-stop-daemon --stop --pidfile /var/run/k5start.pid
           rm -f /var/run/k5start.pid

       This code could be added to an init script for Apache, for example, to start a k5start process  alongside
       Apache to manage its Kerberos credentials.

ENVIRONMENT

       If  the  environment  variable  AKLOG is set, its value will be used as the program to run with -t rather
       than the default complied into k5start.  If AKLOG is not set and KINIT_PROG is set,  its  value  will  be
       used instead.  KINIT_PROG is honored for backward compatibility but its use is not recommended due to its
       confusing name.

       If no ticket file (with -k) or command is specified on the command line, k5start will use the environment
       variable  KRB5CCNAME to determine the location of the the ticket granting ticket.  If either a command is
       specified or the -k option is used, KRB5CCNAME will be set to point to the ticket file before running the
       aklog program or any command given on the command line.

FILES

       The default ticket cache is determined by the underlying Kerberos libraries.  The default path for  aklog
       is  determined  at  build  time, and will normally be whichever of aklog or afslog is found in the user's
       path.

       If a command is specified and -k was not given, k5start will create a temporary ticket cache file of  the
       form "/tmp/krb5cc_%d_%s" where %d is the UID k5start is running as and %s is a random string.

AUTHORS

       k5start  was  based on the k4start code written by Robert Morgan.  It was ported to Kerberos v5 by Booker
       C. Bense.  Additional cleanup and current maintenance are done by Russ Allbery <eagle@eyrie.org>.

       Implementations of -b and -p and the example for a Debian init script are based on  code  contributed  by
       Navid Golpayegani.

COPYRIGHT AND LICENSE

       Copyright 2015, 2021 Russ Allbery <eagle@eyrie.org>

       Copyright 2002, 2004-2012, 2014 The Board of Trustees of the Leland Stanford Junior University

       Copying  and distribution of this file, with or without modification, are permitted in any medium without
       royalty provided the copyright notice and this notice are preserved.  This file is offered as-is, without
       any warranty.

       SPDX-License-Identifier: FSFAP