ssh-cron(1)

NAME
ssh-cron - cron-like daemon able to use ssh-connections

SYNOPSIS
ssh-cron [OPTIONS] [crontab-file]
[OPTIONS] - cf. section OPTIONS
[crontab-file] - file containing jobs to run.

DESCRIPTION

Consider the situation where a computer every now and then must access a remote computer to do some useful things at that remote computer (like running a stealth(1) file integrity scan). In order to do so the computer must be allowed to make ssh(1) connections to the remote computer. But since the commands are not executed by the user but by cron(1), the ssh-keys which are required to access the remote computer cannot use passphrases.

This is an undesirable situation: if the computer running the ssh commands gets compromised, then the remote computers are compromised as well, since the attacker may access these remote systems using ssh keys not requiring passphrases.

Ssh-Cron offers a way out of this undesirable situation, while still allowing commands to be executed on remote computers. Here’s how this is realized:

o   Normally, ssh-cron runs as a daemon program. When ssh-cron starts it first reads and parses a crontab-like specification file. Following this, ssh-cron spawns a child process, and terminates.

o   Ssh-Cron’s daemon process itself spawns an ssh-agent(1) child process, executing all scheduled commands.

o   In addition, ssh-cron’s daemon defines communication channels between itself and its ssh-agent(1) child process;

o   Ssh-Cron’s daemon sends the command ssh-add(1) to its child process as its first command to execute, and using normal user-interaction means (e.g., using ssh-askpass(1)) ssh-agent is provided with the required passphrase for the ssh key(s).

o   Ssh-Cron’s daemon now monitors the time, firing off scheduled commands at their required moments in time. If these commands require access to remote computers, then this access is granted, as ssh-agent is able to provide the passphrase.

o   If an ssh-cron daemon process is already running, then the --reload option (see below), can be used to load the ssh-cron daemon with the commands and environment variable settings from a specified crontab-file, replacing the currently stored commands and environment settings by the ones provided in the reloaded file.

When shell control characters (like redirection symbols) must be used in command specifications, they should be escaped. E.g., as in echo hello world \> /dev/null.

Users sharing a computer each define their own ssh-cron specification file. When a user logs out and leaves the system the daemon process continues to run, executing its scheduled commands at their scheduled times, using ssh-keys whenever required.

If the accounts for which ssh-cron jobs are running are ever compromised, the remote computers remain safe, as the passphrases of the available ssh-keys remain unavailable.

To prevent unauthorized modifications of the commands scheduled by the ssh-cron daemon themselves a passphrase is required when starting ssh-cron’s daemon process. The passphrase itself is not stored in the daemon (instead, it stores a sha256(1) hash value), which avoids access to the ssh-cron daemon’s passphrase by browsing the computer’s memory. The passphrase must be at least 10 characters long and must be provided when reloading or listing a running ssh-cron program’s crontab file.

The above mentioned facilities are not supported by crontab(1) itself. Cron(1), which is responsible for executing scheduled crontab commands, has no access to the passphrases of ssh-keys (which are otherwise provided ssh-agent).

RETURN VALUE

Ssh-Cron returns 0 if the daemon was successfully started. Otherwise 1 is returned.

OPTIONS

Where available, single letter options are listed between parentheses following their associated long-option variants. Single letter options require arguments if their associated long options also require arguments.

Several options have default values. Run ssh-cron --help for an overview of the implemented default option values. Also, several options can be specified in a configuration file (where this doesn’t hold true, it is explicitly mentioned at the relevant options).

The configuration file (not to be confused with the file containing the scheduled commands, which is provided as ssh-cron command-line file argument) ignores empty lines and all information on lines starting at a hash-mark (#, optionally preceded by blanks and/or tabs). The configuration file is used to specify ssh-cron’s options using their long variants. However, in the configuration file the initial hyphens of command-line options must be omitted, and optionally a colon may be appended to these long options names. Note that multi-word option arguments should not be surrounded by quotes. Examples:

    stdout
    syslog-facility: LOCAL0
    mailer: /usr/bin/mail -s "some subject" me@c-plusplus.nl

Command-line options always override configuration file options.

o   --agent=agent
    absolute path to the agent program (plus its argument(s)) providing the ssh-keys.

    By default /usr/bin/ssh-agent /bin/bash is used.

o   --config=path (-c)
    config file containing long option specifications.

    By default ~/.ssh-cron is used.

    This option cannot be specified in the configuration file.

o   --forground
    ssh-cron is not run as a daemon. To properly end ssh-cron when it’s not running as a daemon, execute ssh-cron --terminate in a separate process or end it using ctrl-C or by sending the process a SIGTERM signal.

    This option cannot be specified in the configuration file.

o   --help (-h)
    ssh-cron ends after writing basic usage information to the standard output stream.

    This option cannot be specified in the configuration file.

o   --list (-l)
    list the currently defined environment settings and cron-commands (the crontab-file argument must be omitted). This option is incompatible with --reload and --terminate.

    This option cannot be specified in the configuration file.

o   --log=path (-L)
    log messages are appended to path. If path does not exist, it is first created.

o   --mailer=command (-m)
    information written to the standard output or standard error streams of the commands executed by ssh-cron is sent by e-mail to the current user. Use --mailer to redefine (or to suppress sending e-mail by specifying an empty mailer command (i.e., --mailer "")).

    By default /usr/bin/mail -s \"Ssh-cron $*\" $USER@localhost is used, with $* replaced by the exected command as specified in the crontab file argument.

o   --reload (-r)
    reload the running ssh-cron program with de cron-commands defined in the crontab-file argument (which must be provided). This option is incompatible with --list and --terminate.

    This option cannot be specified in the configuration file.

o   --stdout (-s)
    in addition to using a log file and/or writing syslog messages send all messages to the standard output. This option is not available if ssh-cron runs as a daemon process.

    This option cannot be specified in the configuration file.

o   --syslog
    use this option to send messages to the syslog daemon. By default syslog messages are written to the syslog’s DAEMON facility with priority NOTICE.

o   --syslog-facility=facility
    the syslog facility used when writing messages to the syslog daemon. By default this is DAEMON. For an overview of facilities and their meanings, see, e.g., syslog(3). With ssh-cron the facilities DAEMON, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7, and USER can be used.

    By default facility DAEMON is used.

o   --syslog-priority=priority
    the syslog priority used when writing messages to the syslog daemon. By default this is NOTICE. For an overview of priorities and their meanings, see, e.g., syslog(3). With ssh-cron all defined priorities can be used. E.g., EMERG, ALERT, CRIT, ERR, WARNING, NOTICE, INFO and DEBUG.

    By default priority NOTICE is used.

o   --syslog-tag=tag
    syslog messages can be provided with a tag, which simplifies locating them in the syslog log-files. See also section RSYSLOG FILTERING below.

    By default the tag SSH-CRON is used.

o   --terminate (-t)
    terminate a running ssh-cron daemon. The crontab-file argument must be omitted. This option is incompatible tt with the (--list) and --reload options.

    This option cannot be specified in the configuration file.

o   --uds=path (-u)
    when ssh-cron runs as a daemon or in the foreground, then path specifies the location of its Unix domain socket. When ssh-cron is called with the options --list, --reload, or --terminate then it uses the uds file to communicate with the running ssh-cron proram. When the running ssh-cron program ends it removes the uds file. If the uds file couldn’t be removed (e.g., ssh-cron received a SIGKILL signal) then the uds file must be removed `by hand’ before ssh-cron can start again.

    By default ~/.ssh-cron.uds is used.

o   --verbose
    additional messages about ssh-cron’s mode of operation are sent to ssh-cron’s log facilities (specified by --log, --syslog, and/or --stdout).

o   --version (-v)
    ssh-cron’s version number is written to the standard output stream.

    This option cannot be specified in the configuration file.

RSYSLOG FILTERING

When using rsyslogd(1) property based filters may be used to filter syslog messages and write them to a file of your choice. E.g., to filter messages starting with the syslog message tag (e.g., SSH-CRON) use

:syslogtag, isequal, "SSH-CRON:"   /var/log/ssh-cron.log
:syslogtag, isequal, "SSH-CRON:"   stop

Note that the colon is part of the tag, but is not specified with the syslog-tag option.

This results in writing all messages having the SSH-CRON: tag to be written on /var/log/ssh-cron.log. More extensive filtering is also supported, see, e.g., http://www.rsyslog.com/doc/rsyslog_conf_filter.html and http://www.rsyslog.com/doc/property_replacer.html

SEE ALSO

cron(1), crontab(1), crontab(5), , rsyslogd(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-askpass(1), stealth(1), syslog(3)

BUGS
None reported.

COPYRIGHT
This is free software, distributed under the terms of the `GNU General Public License’. Copyright remains with the author. ssh-cron is available at https://fbb-git.gitlab.io/ssh-cron/.

ORGANIZATION
University of Groningen.

AUTHOR
Frank B. Brokken (f.b.brokken@rug.nl).