Ubuntu Manpage: autossh — monitor and restart ssh sessions

NAME

       autossh — monitor and restart ssh sessions

SYNOPSIS

       autossh [-V] [-M port[:echo_port]] [-f] [SSH_OPTIONS]

DESCRIPTION

       autossh  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 autossh
       the method changed: autossh 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.

CONTROLLING SSH

SSH exits
autossh tries to distinguish the manner of death of the ssh process it is monitoring and act
appropriately. The rules are:

1. If the ssh process exited normally (for example, someone typed "exit" in an interactive session),
autossh exits rather than restarting;

2. If autossh itself receives a SIGTERM, SIGINT, or a SIGKILL signal, it assumes that it was
deliberately signalled, and exits after killing the child ssh process;

3. If autossh itself receives a SIGUSR1 signal, it kills the child ssh process and starts a new one;

4. Periodically (by default every 10 minutes), autossh attempts to pass traffic on the monitor
forwarded port. If this fails, autossh will kill the child ssh process (if it is still running)
and start a new one;

5. If the child ssh process dies for any other reason, autossh will attempt to start a new one.

Startup behaviour
If the ssh session fails with an exit status of 1 on the very first try, autossh

1. will assume that there is some problem with syntax or the connection setup, and will exit rather
than retrying;

2. There is a "starting gate" time. If the first ssh process fails within the first few seconds of
being started, autossh assumes that 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. The "starting gate" time is
also set to 0 when the -f flag to autossh is used.

Continued failures
If the ssh connection fails and attempts to restart it fail in quick succession, autossh will start
delaying its attempts to restart, gradually backing farther and farther off up to a maximum interval of
the autossh poll time (usually 10 minutes). autossh can be "prodded" to retry by signalling it, perhaps
with SIGHUP ("kill -HUP").

Connection setup
As connections must be established unattended, the use of autossh 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 autossh

If you are tunnelling and using an older version of ssh that does not support the -N 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".

OPTIONS

-M port[:echo_port]
specifies the base monitoring port to use. Without the echo port, this port and the port
immediately above it ( port + 1) should be something nothing else is using. autossh will send
test data on the base monitoring port, and receive it back on the port above. For example, if you
specify “-M 20000”, autossh 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 ServerAliveInterval and ServerAliveCountMax 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.

-f causes autossh to drop to the background before running ssh. The -f flag is stripped from
arguments passed to ssh. Note that there is a crucial difference between -f with autossh, and -f
with ssh: when used with autossh ssh will be unable to ask for passwords or passphrases. When -f
is used, the "starting gate" time (see AUTOSSH_GATETIME) is set to 0.

-V causes autossh to display its version number and exit.

ENVIRONMENT

Other than the flag to set the connection monitoring port, autossh uses environment variables to control
features. ssh seems to be still collecting letters for options, and this seems the easiest way to avoid
collisions.

AUTOSSH_DEBUG
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.

AUTOSSH_FIRST_POLL
Specifies the time to wait before the first connection test. Thereafter the general poll time is
used (see AUTOSSH_POLL below).

AUTOSSH_GATETIME
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.

AUTOSSH_LOGLEVEL
Specifies the log level, corresponding to the levels used by syslog; so 0-7 with 7 being the
chattiest.

AUTOSSH_LOGFILE
Specifies that autossh should use the named log file, rather than syslog.

AUTOSSH_MAXLIFETIME
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.

AUTOSSH_MAXSTART
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.

AUTOSSH_MESSAGE
Append message to echo message sent when testing connections.

AUTOSSH_NTSERVICE
(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.

AUTOSSH_PATH
Specifies the path to the ssh executable, in case it is different than the path compiled in.

AUTOSSH_PIDFILE
Write autossh pid to specified file.

AUTOSSH_POLL
Specifies the connection poll time in seconds; default is 600 seconds. Unless AUTOSSH_FIRST_POLL
is used, the first poll time will set to match the poll time. 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 poll time.

AUTOSSH_PORT
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.

ENVIRONMENT

       There are two particular OpenSSH options that are useful when using autossh : ExitOnForwardFailure=yes on
       the  client  side  to  make  sure forwardings have succeeded when autossh assumes the connection is setup
       properly.  ClientAliveInterval on the server side to make sure the listening  socket  is  closed  on  the
       server side if the connection closes on the client side.

DEBIAN-SPECIFIC BEHAVIOR

       The  debian  version  of  autossh  uses  a  wrapper to automatically select a free monitoring port and -M
       overrides AUTOSSH_PORT, see /usr/share/doc/autossh/README.Debian for further information.

AUTHOR

       autossh was written by Carson Harding.

NAME

SYNOPSIS

DESCRIPTION

CONTROLLING SSH

OPTIONS

ENVIRONMENT

ENVIRONMENT

DEBIAN-SPECIFIC BEHAVIOR

AUTHOR

SEE ALSO