How to make SSH traffic go through a second routing table

Question

I am basically trying to SSH into a RPi that is always connected to a VPN, which makes SSH fails. I received few ideas on how to solve the issue, and the best IMHO is to make the SSH traffic go through a 2nd routing table.

The problem is this is rather complicated for a network noob like me. So I was wondering whether I could get some pointers to get started, as I am completely lost. Especially on how to mark SSH traffic to go through the 2nd table.

Thanks

-a

EDIT: Thanks again for the answer Marius. I finally found some time to work on this, and here is what I have done.

1. sudo modprobe ip_conntrack
2. sudo iptables -A INPUT -m state --state NEW -p tcp --dport 22 -j CONNMARK --set-mark 1
3. then as root:
4. echo 200 hma >> /etc/iproute2/rt_tables
5. ip route add 10.0.1.0/24 dev eth0 table hma
6. ip rule add fwmark 1 table hma

but it does not seem to work. As I am really not sure as to what to do, this is not really a surprise.

More specifically, I am really unsure about 2) and 4). Is 2) correct at all? And for 4), where do I find the right IP to add, what does the /24 mean, and should it go through eth0 (my vpn appears to be tun0)?

Thanks again

EDIT2: After a reboot, I have actually lost SSH access to the pi :(

Answer 1

It is actually very easy, luckily for all of us: let me call IP2 the IP address of the localhost in the second routing table. Then all you have to do is:

      ssh -b IP2 me@some_remote_machine 

The -b option instructs ssh to bind (i.e., use as source address) to the IP address IP2. If you have already setup the second routing table, with a rule like

      ip rule from IP2 table2

where table2 is the second routing table, you are good to go.