k4start - Obtain and optionally keep active a Kerberos v4 ticket

k4start [-bnqstv] [-f srvtab] [-H minutes] [-I service instance] [-i client instance] [-K minutes] [-k ticket file] [-l minutes] [-p pid file] [-r service realm] [-S service name] [-u client principal] [username [command ...]]

k4start obtains and caches an initial Kerberos v4 ticket-granting ticket for a principal. k4start can be used as an alternative to kinit, but it is primarily intended to be used by programs that want to use a srvtab to obtain Kerberos credentials, such as a web server that needs to authenticate to another service such as an LDAP server or to AFS.

Normally, the principal for which to give tickets should be specified as the first argument. The -u and -i options can be used as an alternative mechanism for specifying the principal, but generally aren't as convenient. If no username 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 k4start in the default local realm.

Optionally, a command may be given on the command line of k4start. 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. k4start 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 be specified as a regular command-line argument rather than using the -u and -i options and a srvtab must be specified with -f.

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 CWsh -c CIcommandCW as the command to run and quote command.

If the command contains command-line options (like CW-c), put on the command line before the beginning of the command to tell k4start to not parse those options as its own.

-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 k4start will be running and can only be used if a srvtab is specified with -f. k4start will not background itself until after it does the initial authentication, 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 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.

-f srvtab

Authenticate using the srvtab srvtab rather than asking for a password.

-H minutes

Check for a happy ticket, one that has a remaining lifetime of at least minutes minutes. If such a ticket is found, exit immediately with status 0. Otherwise, try to obtain a new ticket.

-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 period 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 the next wakeup, and gets a new ticket if needed.

-k ticket file

Use ticket file as the ticket cache rather than the contents of the environment variable KRBTKFILE or the library default.

-l minutes

Set the ticket lifetime in minutes. Note that if the ticket lifetime will only be accurate to within five minutes, may be approximate if the ticket lifetime is over ten hours, and cannot be more than about 21 hours unless your Kerberos libraries support long ticket lifetimes.

-n

Regardless of the setting of the environment variable KINIT_PROG, don't run the program.

-p pid file

Save the process ID (PID) of the running k4start 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 k4start daemon. Note that, when used with -b the PID file is written out after k4start 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 k4start is getting a service ticket. The default value is CWkrbtgt, 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 srvtab and the -f option intead.

-t

Run an external program after getting a TGT. The default use of this is to run aklog to get a token. If the environment variable KINIT_PROG is set, the program is run regardless of the -t flag (unless the -n flag is given).

-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.

The program exits with status 0 if it successfully gets a ticket or has a happy ticket (see -H). If k4start runs aklog or some other program k4start returns the exit status of that program.

Use the /etc/srvtab srvtab to obtain a ticket granting ticket for the principal rcmd.example, putting the ticket cache in /tmp/service.tgt. The lifetime is 600 minutes and the program wakes up every 10 minutes to check if the ticket is about to expire.

    k4start -k /tmp/service.tgt -f /etc/srvtab -K 10 -l 600 \
        rcmd.example

Do the same, but using the default ticket cache and run the command /usr/local/bin/auth-backup. k4start will continue running until the command finishes.

    k4start -f /etc/srvtab -K 10 -l 600 rcmd.example \
        /usr/local/bin/auth-backup

Shows the permissions of the temporary cache file created by k4start:

    k4start -f /etc/srvtab rcmd.example \
        -- sh -c 'ls -l $KRBTKFILE'

Notice the CW-- before the command to keep k4start from parsing the CW-c as its own option.

Starts k4start 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/k4start.pid \
        --exec /usr/local/bin/k4start -- -b -p /var/run/k4start.pid \
        -f /etc/srvtab rcmd.example

This uses /var/run/k4start.pid as the PID file and obtains rcmd.example tickets from the system srvtab file. k4start would then be stopped with:

    start-stop-daemon --stop --pidfile /var/run/k4start.pid
    rm -f /var/run/k4start.pid

This code could be added to an init script for Apache, for example, to start a k4start process alongside Apache to manage its Kerberos credentials.

If the environment variable KINIT_PROG is set to a program (such as aklog) then this program will automatically be executed after the ticket granting ticket has been retrieved.

If no ticket file (with -k) or command is specified on the command line, k4start will use the environment variable KRBTKTFILE to determine the location of the the ticket granting ticket. If either a command is specified or the -k option is used, KRBTKFILE will be set to point to that file before running the aklog program or any command given on the command line.

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, k4start will create a temporary ticket cache file of the form CW/tmp/tkt%d_%s where CW%d is the UID k4start is running as and CW%s is a random string.

k5start(1), kinit(1)

The kstart web page at <http://www.eyrie.org/~eagle/software/kstart/> will have the current version of k4start and k5start.

k4start was originally written by Robert Morgan. Modifications and additional features were added by Booker C. Bense. Additional cleanup and current maintenance are done by Russ Allbery <rra@stanford.edu>.

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