sshswitch

#!/usr/bin/env bash

sshswitch_usage() {
	mdformat <<EOusage | ${PAGER:-cat}

**NAME**
	${0##*/} - select ssh connection method based on availability

**SYNOPSIS**
	${0##*/} [**-dn**] _hostname_
	${0##*/} [**-dn**] _host_ ["_command line_"]

**DESCRIPTION**
	This is a ProxyCommand helper.

	In the first usage, _hostname_ is a name found in /etc/ethers,
	whose MAC address will be looked up in the local ARP cache
	(after 1 ping if required). This allows you to ssh to hosts
	whose IP addresses change, but whose (static) ethernet address
	is known, without configuring dynamic DNS in your DHCP server.

	In the second usage, the first option is the name or IP address
	you're trying to reach. It may be an IP wildcard pattern (for
	example "192.168.1.*").  If the first option is a hostname, then
	the IP is looked up in /etc/ethers. If the IP is local (i.e. in
	the ARP cache after 1 ping), we connect to it directly, as with
	the first usage. However, if that fails, we execute the
	remaining options as a command line to establish the connection.

	The **-d** option causes extra debugging information to be sent to
	stderr. The **-n** option causes ${0##*/} to print the
	commands it would run instead of actually running them.

**EXAMPLES**
	The following might be used if you sometimes connect to the
	192.168.2.0/24 network, which has a gateway on its .1 IP, and
	that gateway is perhaps accessible from some other host via an
	SSH tunnel or VPN.

	**host gw 192.168.2.1**
	    **ProxyCommand ~/bin/sshswitch 192.168.2.1 ssh -xaqW10.0.0.2:22 foo**

	Note that quotes aroung the command are optional.

	**host bar 192.168.2.***
	    **ProxyCommand ~/bin/sshswitch %h "ssh -xaqW%h:22 gw"**

	The following could be used for any host that might get a
	dynamic IP address, for example, from a DHCP server.

	The following might be used to connect to hosts with dynamic
	(DHCP-assigned) IP addresses on the local network, without the
	need to use a DHCP-controlled DNS zone. In this case, the only
	thing static is the MAC address.

	~/.ssh/config:

	    **host flarn spoo rpi**
	        **ProxyCommand ~/bin/sshswitch %h**

	/etc/ethers:

	    **dc:a9:04:03:02:01       flarn**
	    **dc:a9:04:03:02:03       spoo**
	    **b8:27:eb:03:02:01       rpi**

	$ arp -an
	? (192.168.1.2) at dc:a9:04:03:02:01 on em0 ifscope [ethernet]

EOusage
}

mdformat() {
	local md so us mr me
	if $topt; then
		md=$(tput md)	# bold
		so=$(tput so)	# standout (inverse?)
		us=$(tput us)	# underline
		mr=$(tput mr)	# reverse
		me=$(tput me)	# exit attributes
	fi
	sed -E \
		-e "s/\*\*([^*]+)\*\*/$md\1$me/g" \
		-e "s/\*([^*]+)\*/$us\1$me/g" \
		-e "s/_([^_]+)_/$us\1$me/g" \
		-e "s/^#+ *(.+)/$so\1$me/g" \
		-e "s/#([^#]+)#/$mr\1$me/g"
}

if [ -t 1 ]; then
	topt=true
else
	topt=false
fi

dopt=false; nopt=false
while getopts dn opt; do
	case "$opt" in
		d) dopt=true ;;
		n) nopt=true ;;
		*) sshswitch_usage; exit ;;
	esac
done
shift $((OPTIND - 1))

# Populate ARP cache
ping -t 1 -c 1 "$1" >/dev/null 2>/dev/null &
sleep 0.25 2>/dev/null || sleep 1

if [ $# -eq 0 ]; then
	sshswitch_usage
elif [ $# -eq 1 ] && grep -qw "$1" /etc/ethers; then
	$dopt && echo nc "$(arp -an | awk -v h="$1" 'NR==FNR{a[$1]=$2;next} a[$4]==h{gsub(/[()]/,"",$2);print $2;exit}' /etc/ethers -)" 22
	$nopt ||      nc "$(arp -an | awk -v h="$1" 'NR==FNR{a[$1]=$2;next} a[$4]==h{gsub(/[()]/,"",$2);print $2;exit}' /etc/ethers -)" 22
elif [ $# -eq 1 ]; then
	printf 'ERROR: host not found\n' >&2
	exit 68
elif arp -an | awk -v h="$1" '$2=="("h")" && $4~/[0-9a-f]+:/{r=0} END{exit r}' r=1 -; then
	# If the IP in $1 is local, connect directly
	case "$1" in
		*:*)	$dopt && echo nc "${1%:*}" "${1#*}" ;
			$nopt ||      nc "${1%:*}" "${1#*}" ;;
		*)	$dopt && echo nc "$1" 22 ;
			$nopt ||      nc "$1" 22 ;;
	esac
else
	# If the IP is not local, run a command.
	shift
	$*
fi