Man page of AUTOSSH
Section: User Commands (1)
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- monitor and restart ssh sessions
is a program to start a copy of ssh and monitor it, restarting it as
necessary should it die or stop passing traffic.
The original idea and the mechanism were from rstunnel (Reliable SSH
Tunnel). With version 1.2 of
the method changed:
uses ssh to construct a loop of ssh forwardings (one from local to
remote, one from remote to local), and then sends test data that it
expects to get back. (The idea is thanks to Terrence Martin.)
With version 1.3, a new method is added (thanks to Ron Yorston): a
port may be specified for a remote echo service that will echo back
the test data. This avoids the congestion and the aggravation of
making sure all the port numbers on the remote machine do not
collide. The loop-of-forwardings method remains available for
situations where using an echo service may not be possible.
tries to distinguish the manner of death of the ssh process it
is monitoring and act appropriately. The rules are:
If the ssh process exited normally (for example, someone typed
"exit" in an interactive session),
exits rather than restarting;
itself receives a SIGTERM, SIGINT, or a SIGKILL signal, it assumes
that it was deliberately signalled, and exits after killing the child
itself receives a SIGUSR1 signal, it kills the child ssh process and starts
a new one;
Periodically (by default every 10 minutes),
attempts to pass traffic on the monitor forwarded port. If this fails,
will kill the child ssh process (if it is still running) and start a new one;
If the child ssh process dies for any other reason,
will attempt to start a new one.
If the ssh session fails with an exit status of 1 on the very first
will assume that there is some problem with syntax or the connection
setup, and will exit rather than retrying;
There is a "starting gate" time. If the first ssh process fails within
the first few seconds of being started,
it never made it "out of the starting gate", and exits. This is to handle
initial failed authentication, connection, etc. This time is 30 seconds
by default, and can be adjusted (see the AUTOSSH_GATETIME environment
variable below). If AUTOSSH_GATETIME is set to 0, then both behaviours
are disabled: there is no "starting gate", and autossh will restart even
if ssh fails on the first run with an exit status of 1.
If the ssh connection fails and attempts to restart it fail in
will start delaying its attempts to
restart, gradually backing farther and farther off up to a
maximum interval of the
poll time (usually 10 minutes).
can be "prodded" to retry by signalling it, perhaps with
SIGHUP ("kill -HUP").
As connections must be established unattended, the use of
requires that some form of automatic authentication be set up. The use
of RSAAuthentication with ssh-agent is the recommended method. The
example wrapper script attempts to check if there is an agent running
for the current environment, and to start one if there isn't.
It cannot be stressed enough that you must make sure ssh works on its
own, that you can set up the session you want before you try to
run it under
If you are tunnelling and using an older version of ssh that does not
flag, you should upgrade (your version has security
flaws). If you can't upgrade, you may wish to do as rstunnel does, and
give ssh a command to run, such as "sleep 99999999999".
- -M port[:echo_port]
specifies the base monitoring port to use. Without the echo port,
this port and the port
immediately above it (
+ 1) should be something nothing else is
will send test data on the base monitoring port, and
receive it back on the port above. For example, if you specify "-M
will set up forwards so that it can send data on port
20000 and receive it back on 20001.
Alternatively, a port for a remote echo service may be specified. This
should be port 7 if you wish to use the standard inetd echo service.
When an echo port is specified, only the specified monitor port is
used, and it carries the monitor message in both directions.
Many people disable the echo service, or even disable inetd, so check
that this service is available on the remote machine. Some operating
systems allow one to specify that the service only listen on the
localhost (loopback interface), which would suffice for this use.
The echo service may also be something more complicated: perhaps
a daemon that monitors a group of ssh tunnels.
Setting the monitor port to 0 turns the monitoring function off, and
autossh will only restart ssh upon ssh's exit. For example, if you are
using a recent version of OpenSSH, you may wish to explore using the
options to have the SSH client exit if it finds itself no longer
connected to the server. In many ways this may be a better solution than
the monitoring port.
causes autossh to drop to the background before running ssh. The
flag is stripped from arguments passed to ssh. Note that there is a crucial
a difference between
with autossh, and
with ssh: when used with
ssh will be unable to ask for passwords or passphrases.
to display its version number and exit.
Other than the flag to set the connection monitoring port,
uses environment variables to control features. ssh seems to be
still collecting letters for options, and this seems the easiest
way to avoid collisions.
If this variable is set, the logging level is set to to LOG_DEBUG, and
if the operating system supports it, syslog is set to duplicate log
entries to stderr.
Specifies the time to wait before the first connection test. Thereafter
the general poll time is used (see AUTOSSH_POLL below).
Specifies how long ssh must be up before we consider it a successful
connection. The default is 30 seconds. Note that if AUTOSSH_GATETIME
is set to 0, then not only is the gatetime behaviour turned off, but
autossh also ignores the first run failure of ssh. This may be useful
when running autossh at boot.
Specifies the log level, corresponding to the levels used by syslog;
so 0-7 with 7 being the chattiest.
should use the named log file, rather than syslog.
Sets the maximum number of seconds that the program should run. Once
the number of seconds has been passed, the ssh child will be killed
and the program will exit.
Specifies how many times ssh should be started. A negative number
means no limit on the number of times ssh is started. The default
value is -1.
Append message to echo message sent when testing connections.
(Cygwin only.) When set to "yes" , autossh sets up to run as an NT
service under cygrunsrv. This adds the -N flag for ssh if not already
set, sets the log output to stdout, and changes the behaviour on ssh
exit so that it will restart even on a normal exit.
Specifies the path to the ssh executable, in case it is
different than the path compiled in.
Write autossh pid to specified file.
Specifies the connection poll time in seconds; default is 600 seconds.
If the poll time is less than twice the network timeouts (default 15
seconds) the network timeouts will be adjusted downward to 1/2 the
Sets the connection monitoring port. Mostly in case ssh appropriates
-M at some time. But because of this possible use, AUTOSSH_PORT
overrides the -M flag. A value of 0 turns the monitoring function off.
was written by Carson Harding.
- CONTROLLING SSH
- SSH exits
- Startup behaviour
- Continued failures
- Connection setup
- SEE ALSO
This document was created by
using the manual pages.
Time: 03:41:09 GMT, September 24, 2010