The title pretty much sums it up. I would like to send UDP traffic through a SSH tunnel. Specifically, I need to be able to send UDP packets through the tunnel and have the server be able to send them back to me on the other side. I know how to do it for TCP connections. Is this it possible with UDP?

9 Answers

This small guide tells you how to send UDP traffic via SSH using tools that come standard (ssh,nc,mkfifo) with most UNIX-like operating systems.

Performing UDP tunneling through an SSH connection

Step by step

Open a TCP forward port with your SSH connection

On your local machine (local), connect to the distant machine (server) by SSH, with the additional -L option so that SSH will do TCP port-forwarding:

local# ssh -L 6667:localhost:6667 server.foo.com

This will allow TCP connections on the port number 6667 of your local machine to be forwarded to the port number 6667 on server.foo.com through the secure channel.

Setup the TCP to UDP forward on the server

On the server, we open a listener on the TCP port 6667 which will forward data to UDP port 53 of a specified IP. If you want to do DNS forwarding like me, you can take the first nameserver's IP you will find in /etc/resolv.conf.

But first, we need to create a fifo. The fifo is necessary to have two-way communications between the two channels. A simple shell pipe would only communicate left process' standard output to right process' standard input.

server# mkfifo /tmp/fifo
server# nc -l -p 6667 < /tmp/fifo | nc -u 192.168.1.1 53 > /tmp/fifo

This will allow TCP traffic on server's port 6667 to be forwarded to UDP traffic on 192.168.1.1's port 53, and responses to come back.

Setup the UDP to TCP forward on your machine

Now, we need to do the opposite of what was done above on the local machine. You need priviledged access to bind the UDP port 53.

local# mkfifo /tmp/fifo
local# sudo nc -l -u -p 53 < /tmp/fifo | nc localhost 6667 > /tmp/fifo

This will allow UDP traffic on local machine's port 53 to be forwarded to TCP traffic on local machine's port 6667. Enjoy your local DNS server :)

As you've probably guessed, when a DNS query will be performed on the local machine, e.g. on local UDP port 53, it will be forwarded to local TCP port 6667, then to server's TCP port 6667, then to server's DNS server, UDP port 53 of 192.168.1.1. To enjoy DNS services on your local machine, put the following line as first nameserver in your /etc/resolv.conf:

nameserver 127.0.0.1

7

This example (I think John's answer points the the same thing at a different place), describes how to access another machine's UDP/DNS services over an TCP/SSH connection.

We will forward local UDP/53 traffic to TCP, then TCP traffic with the port-forwarding mechanism of SSH to the other machine, then TCP to UDP/53 on the other end.
Typically, you can do it with openvpn.
But here, we'll do it with simpler tools, only openssh and netcat.

At the end of that page, is another comment with a reference to 'socat',
The same UDP/DNS access is made with,

Server side: socat tcp4-listen:5353,reuseaddr,fork UDP:nameserver:53
Client side: socat udp4-listen:53,reuseaddr,fork tcp:localhost:5353

Refer socat examples for more.

2

SSH (at least OpenSSH) has support for simple VPNs. Using the -w or Tunnel option in the ssh client, you can create a tun device at both ends, which can be used to forward any kind of IP traffic. (See also Tunnel in the manual page of ssh_config(5).) Note that this requires OpenSSH (and probably root privileges) at both ends.

6

Or you could simply use ssf (which was designed to handle this use case), with a simple command:

---

Client side:

#>./ssfc -U 53:192.168.1.1:53 server.foo.com

This command redirects local port 53 (dns) to 192.168.1.1 port 53, through a secure tunnel between localhost and server.foo.com.

---

You will need a ssf server (instead of - or next to - your ssh server):

#>./ssfs

By the way, both client and server side of ssf work on Windows / Linux / Mac. This is a userland application, so you don't need tun/tap or VPN.

To redirect port 53, you will need administrative privileges - regardless of the tool you're using.

For more info, details, use case, or download:

7

I couldn't get nc to work for SNMP, because SNMP clients keep choosing a new source UDP port, and several can be active at once.

Instead, I've written a post describing how to do it with socat in this blog post, using SNMP as an example. Essentially, using two terminals, starting with an overview:

Terminal one:

client$ ssh -L 10000:localhost:10000 server
server$ socat -T10 TCP4-LISTEN:10000,fork UDP4:switch:161

This creates the SSH forwarding of TCP port 10000 and runs socat on the server. Notice how the switch’s IP address is mentioned in the socat command line as “switch”.

Terminal two:

client$ sudo socat UDP4-LISTEN:161,fork TCP4:localhost:10000

That sets up socat on the client. That should do it.

5

A VPN is a better solution if you have access to an UDP port.

If you only have access to the TCP SSH port, then an SSH tunnel is as good as a VPN, at least for ping and packet backtracking.

1

on ssh server:

sudo ip tuntap add dev tun7 mode tun user SSHUSER
sudo ip addr add 192.168.7.1/30 dev tun7
sudo iptables -t nat -A PREROUTING -p udp -i eth0 --dport PUBLIC_PORT -j DNAT --to-destination 192.168.7.2:LOCAL_PORT
sudo iptables -t filter -A FORWARD -i eth0 -p udp -d 192.168.7.2 --dport LOCAL_PORT -j ACCEPT
sudo iptables -t filter -A FORWARD -i tun7 -o eth0 -j ACCEPT
sudo iptables -t nat -A POSTROUTING -o tun7 -j MASQUERADE
# set "PermitTunnel point-to-point" for user SSHUSER in /etc/ssh/sshd_config
sudo sshd -t && sudo systemctl restart sshd

replace eth0 with the interface connected to the internet

on ssh client running udp server:

sudo ip tuntap add dev tun8 mode tun user LOCALUSER
sudo ip addr add 192.168.7.2/30 dev tun8
ssh -v -w 8:7 SSHUSER@REMOTEHOST

you might also need sudo ip link set tun7 up on the remote host and sudo ip link set tun8 up on the local host.

inspired by comment UDP traffic through SSH tunnel

Use sshuttle to set up a VPN over SSH without the hassle of manually configuring tunnel mode. It supports UDP.

An experienced red teamer got this to work and proven by wireshark, but had to change the UDP packets into TCP packets and then forward them into the ssh tunnel and then convert them from TCP packets back into UDP packets to the listener UDP port of the application.

2

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