My answer is related to the answer of related, but more complicate question and not tested.
You need the iproute2
package installed.
Add to /etc/iproute2/rt_tables
the line
200 vpn-route
and then write a script that you call after VPN is initialized:
# set default gateway of vpn-route
ip route add default via $VPNGATEWAY dev $VPNINTERFACE table vpn-route
# use this for marked packages
ip rule add fwmark 0x1 table vpn-route
# mark outgoing ssh packages
iptables -t mangle -A OUTPUT -o $WANINTERFACE -p tcp --dport 22 -j MARK --set-mark 1
# rewrite source address
iptables -t nat -A POSTROUTING -o $VPNINTERFACE -j MASQUERADE
Of course, you need to replace the $...
variables with their actual values.
PS: If your IP on the WAN-interface is fix, you can replace the last line with
iptables -A POSTROUTING -t nat -o $WANINTERFACE -p tcp --dport 22 -j SNAT --to $WANIP