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?
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 with TCP port-forward:
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 upper 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 it now, 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:
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 '
The same UDP/DNS access is made with,
socat tcp4-listen:5353,reuseaddr,fork UDP:nameserver:53
socat udp4-listen:53,reuseaddr,fork tcp:localhost:5353
Refer socat examples for more.
SSH (at least OpenSSH) has support for simple VPNs. Using the
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.
Or you could simply use ssf (which was designed to handle this use case), with a simple command:
#>./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):
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: https://securesocketfunneling.github.io/ssf/
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:
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”.
client$ sudo socat UDP4-LISTEN:161,fork TCP4:localhost:10000
That sets up socat on the client. That should do it.
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.