/usr/bin/autossh is in autossh 1.4c-2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 | #!/bin/sh
# little wrapper to choose a random port for autossh, falling back to $fallback_port
fallback_port="21021"
tcpstat="/proc/net/tcp"
# take an hex port and check whether it is in use (i.e. locally bound) in
# $tcpstat
# unix command semantics: if in use return 0 else return 1
port_in_use() {
if egrep -q "^[0-9 ]+: [0-9A-F]{8}:$1" $tcpstat ; then
return 0
else
return 1
fi
}
echo "$@" | egrep -q -- '-f?M ?[0-9]+' # backward compatibility, skip guess if -M is passed
if [ $? -gt 0 ] && [ -z "$AUTOSSH_PORT" ]; then
portguess=""
if [ -r "/dev/urandom" ] && [ -r "$tcpstat" ]; then
for t in $(seq 1 42); do
# get a random hex
randport=$( od -x -N2 -An /dev/urandom | tr -d ' ' )
# increase it a little "bit"
randport=$( /usr/bin/printf "%04x" $(( 0x$randport | 0x8000 )) )
randport_1=$( /usr/bin/printf "%04x" $(( 0x$randport + 1 )) )
# check if port is in use, possibile race condition between here
# and the exec
if ! port_in_use $randport && ! port_in_use $randport_1; then
portguess=$(( 0x$randport ))
break
fi
done
fi
if [ -z "$portguess" ]; then
fallback=$( /usr/bin/printf "%04x" $fallback_port )
fallback_1=$( /usr/bin/printf "%04x" $(( 0x$fallback + 1 )) )
if ! port_in_use $fallback && ! port_in_use $fallback_1; then
portguess=$fallback_port
else
echo "unable to find a suitable tunnel port"
exit 1
fi
fi
export AUTOSSH_PORT="$portguess"
fi
exec /usr/lib/autossh/autossh "$@"
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