Bash script to automatically proxy SSH connections through a proxy host when off-network

ssh_proxy.sh

#!/bin/bash

# Put the following in the `~/.ssh/config` for each host to
# conditionally proxy:
#   ProxyCommand ssh_proxy.sh %h %p bastion.example.net

if [ $# -ne 3 ]; then
    echo "USAGE: $0 HOST PORT PROXYHOST" >&2
    exit 99
fi

if host "${1}" >/dev/null 2>&1 && \
        echo | nc -w1 "${1}" "${2}" 2>&1 | \ 
        egrep '^SSH-[0-9]\.[0-9]' >/dev/null 2>&1; then
    # echo "Using netcat" >&2
    nc -w35 "${1}" "${2}"
    exit $?
else
    # echo "Using SSH" >&2
    ssh -W "${1}:${2}" "${3}"
    exit $?
fi

exit 99