autossh - Online in the Cloud

PROGRAM:

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 a 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.

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.

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