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.
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 a difference between -f with autossh, and -f with
ssh: when used with autossh ssh will be unable to ask for
passwords or passphrases.
-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. 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.
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.
SEE ALSO
ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), cygrunsrv(1).