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Is it possible to route a computer's SSH service through a VPN connection but allow ALL other data to exit through the WAN address? What sort of iptables magic needs to be done to accomplish this?

SSH data → goes through VPN
ALL other data → goes through default route

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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

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If you want to connect to a server using ssh through VPN since your company requires a VPN connection to access the server, there is another way to bypass the VPN. First, you need to connect to your server using VPN. Then, start ngrok to open a reverse ssh (is this the correct term?)

ngrok tcp 22

This will give you an address and a port. It usually looks like this: 0.tcp.eu.ngrok.io:12345. Now, you can stop your VPN and ssh to your server using the above address and port.

ssh yourname@0.tcp.eu.ngrok.io -p 12345

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