Ubuntu Manpage: krenew - Renew a Kerberos ticket

NAME

       krenew - Renew a Kerberos ticket

SYNOPSIS

       krenew [-abhiLstvx] [-c child pid file] [-H minutes]
           [-K minutes] [-k ticket cache] [-p pid file]
           [command ...]

DESCRIPTION

       krenew renews an existing renewable ticket.  When run without any arguments, it just
       attempts to renew the existing ticket-granting ticket in the current ticket cache,
       equivalent to "kinit -R", but it can optionally run a program like aklog to refresh AFS
       tokens, can run as a daemon and wake up periodically to renew the ticket cache, or can run
       a specified command and keep renewing the ticket cache until the command finishes (or
       renewal is no longer possible).  If a command is specified, krenew by default wakes up
       every 60 minutes (1 hour) to check the ticket cache.

       If a command is given, krenew makes a copy of the ticket cache and creates a private
       ticket cache just for that command, thus isolating it from later destruction of the
       original ticket cache.  This allows krenew to maintain authentication for a command even
       if, for example, the user running the command logs out and OpenSSH destroys their original
       ticket cache.

       If a command is given, it 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 krenew to
       not parse those options as its own.

       If krenew is built with setpag() support and AFS tokens are requested with the -t option,
       it will put the command in a separate PAG before obtaining AFS tokens so that they don't
       interfere with other processes on the system.

       When running a command, krenew 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, krenew will then exit.)  This allows krenew 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 krenew receives an ALRM signal, it immediately refreshes the ticket cache
       regardless of whether it is in danger of expiring.

OPTIONS

-a When run with either the -K flag or a command, always renew tickets each time krenew
wakes up. Without this option, krenew will only try to renew a ticket as often as
necessary to prevent the ticket from expiring. With this option, krenew 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
krenew is using. For example, if another program is automatically renewing a ticket
more frequently than krenew, then krenew will never see a ticket that is close to
expiring and will therefore, by default, never try to renew the ticket. This means
that krenew will also never renew AFS tokens, even if the -t option was given, since
krenew only renews AFS tokens after it successfully renews a ticket. If this option
is specified in such a situation, krenew 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 krenew will be
running. krenew will not background itself until after it does the initial ticket
renewal, 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, krenew 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.

Use of this flag on Mac OS X without specifying a file-based ticket cache by either
using -k or setting KRB5CCNAME will probably not do what you want. Ticket caches on
Mac OS X are, by default, per-session and with -b krenew will detach itself from your
existing ticket cache. Instead, to renew the default ticket cache on Mac OS X, try
something like:

(krenew -K 60 &)

to run krenew in the background but within the current session.

When using this option, consider also using -L to report krenew 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 krenew is backgrounded
and changes its working directory to /, so relative paths for the PID file will be
relative to / (probably not what you want).

-H minutes
Only renew the ticket if it has a remaining lifetime of less than minutes minutes. If
either the ticket already has a sufficiently long remaining lifetime or renewal was
successful, run the command (if one was specified) or exit immediately with status 0
(if none was). Otherwise, try to renew the ticket so that it will have a remaining
lifetime of at least minutes, exit with an error if unsuccessful, and then run the
command, if any.

If -H is used with -K, krenew will not exit immediately. Instead, the specified
remaining lifetime will replace the default value of two minutes, meaning that krenew
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 Ignore errors in renewing the ticket and keep running. Normally, krenew exits as soon
as the ticket cache either disappears or the tickets run out of renewable lifetime.
If this flag is given, it will complain about the failure to standard error (unless -b
was given) but continue running, waking up to try again after the next check interval
(see -K). This is useful if some other process may recreate an expired ticket cache
and krenew should stay around and act on that recreated ticket cache once it's
present.

If the initial ticket cache renew fails, krenew will retry the renewal 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 cache
renewal succeeds.

This flag is only useful in daemon mode or when a command was given.

-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
before the next scheduled check, and renews the 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 that doesn't cause krenew to exit,
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.

-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 krenew
to terminate when run with -i are logged with level LOG_WARNING. Fatal errors are
logged with level LOG_ERR.

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

-p pid file
Save the process ID (PID) of the running krenew 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 krenew daemon.

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

-s Normally, when krenew exits abnormally while running a command (if, for example, the
ticket's renewable lifetime has expired), it leaves the command running. If -s is
given, it will send a SIGHUP signal to the command before exiting. This can be useful
if it's pointless for the command to keep running without Kerberos tickets.

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

Otherwise, if either krenew was not built with AFS setpag() support or AFS is not
available, but the Linux kafs module is available and krenew 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, krenew will run the external program without
doing any credential isolation, which may also affect the credentials of the invoking
process.

-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, krenew keeps running even if it fails to renew the ticket cache as long as
the ticket cache still exists and appears to be renewable. It tries again at the next
check interval. With this option, krenew will instead exit.

EXIT STATUS

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

EXAMPLES

Renew the current ticket-granting ticket.

krenew

Wake up every ten minutes and check to see if the ticket cache needs renewing. If it
does, re-run aklog as well.

krenew -K 10 -t

Run the program /usr/local/bin/compute-job in the background, checking every hour to see
if the ticket needs to be renewed (the default). Put the PID of the krenew job in
/var/run/compute.pid. Obtain a new AFS token each time the ticket has to be renewed.

krenew -b -t -p /var/run/compute.pid /usr/local/bin/compute-job

If you wanted to pass options to /usr/local/bin/compute-job, putting a "--" argument
before it would be necessary to keep krenew from interpreting those options as its own.

If you want to redirect output to a file that requires authentication to write to, you
will need to do that redirection in a sub-shell. In other words, the following command:

krenew -t compute-job > /afs/local/data/output

won't work if /afs/local/data/output requires an AFS token to write to. The job, while
running, will have an AFS token, but the output redirection is done in the parent shell
and doesn't benefit from krenew. The above should instead be written as:

krenew -t -- sh -c 'compute-job > /afs/local/data/output'

With this command, the shell doing the redirection will also be run under krenew and have
the benefit of the AFS token it obtains.

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 krenew.  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, krenew will use
       the environment variable KRB5CCNAME to determine the location of the the ticket granting
       ticket.  If 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.

AUTHORS

       krenew was written by Russ Allbery <eagle@eyrie.org>.  It was based heavily on k5start by
       Booker C. Bense, which in turn was based on the k4start code written by Robert Morgan.

COPYRIGHT AND LICENSE

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

       Copyright 2006, 2008-2012, 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