Tunneling UDP through SSH


Say you need to forward UDP packets between two remote networks securely. E.g : dns queries from your home machine to your dns servers at work. You should use a VPN between the networks in order to do so (see : http://blog.wains.be/post/routed-openvpn-between-two-subnets-behind-nat-gateways/)

Otherwise, you can use the following way :

1. Connect to the remote server and set up TCP forward client$ ssh -L 22222: remote.server.be

Any request sent to your local tcp/22222 port will be tunneled securely to tcp/22222 on the remote server.

We will then use netcat to forward the TCP queries to the UDP server..

2. TCP to UDP forward with netcat on the server server$ mkfifo /tmp/fifo server$ iptables -A INPUT -p tcp --dport 22222 -j ACCEPT server$ nc -l -p 22222 /tmp/fifo

3. UDP to TCP forward with netcat on the client client$ mkfifo /tmp/fifo client$ sudo nc -l -u -p 53 /tmp/fifo

Use sudo if you are not root, you need root access for binding services to ports under 1024.

4. Query nslookup sub.domain.be

Schema :

client –> request to udp/53 –> netcat forwarding from udp/53 to tcp/22222 –> tcp/22222 request tunneled through SSH –> server receives requests on tcp/22222 –> netcat forwarding from tcp/22222 to the specified IP address on udp/53 –> server

Based on : http://zarb.org/~gc/html/udp-in-ssh-tunneling.html

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