78

I have a development server, which is only accessible from 127.0.0.1:8000, not 192.168.1.x:8000. As a quick hack, is there a way to set up something to listen on another port (say, 8001) so that from the local network I could connect 192.168.1.x:8001 and it would tunnel the traffic between the client and 127.0.0.1:8000?

  • 4
    netcat can do this. – Andy Apr 1 '11 at 5:25
45

Using ssh is the easiest solution.

ssh -g -L 8001:localhost:8000 -f -N user@remote-server.com

This forwards the local port 8001 on your workstation to the localhost address on remote-server.com port 8000.
-g means allow other clients on my network to connect to port 8001 on my workstation. Otherwise only local clients on your workstation can connect to the forwarded port.
-N means all I am doing is forwarding ports, don't start a shell.
-f means fork into background after a successful SSH connection and log-in.
Port 8001 will stay open for many connections, until ssh dies or is killed. If you happen to be on Windows, the excellent SSH client PuTTY can do this as well. Use 8001 as the local port and localhost:8000 and the destination and add a local port forwarding in settings. You can add it after a successful connect with PuTTY.

  • 4
    What does the user@remote-server.com do? It's definitely unneeded for port forwarding, however ssh mandates having this argument, more over, it tries to connect there. And upon setting this pesky option to hostname it outputs …port 22: Connection refused (no, I didn't use the 22 port). Unless I'm missing something, the command plainly doesn't work. – Hi-Angel Dec 1 '16 at 13:43
  • @Hi-Angel user@remote-server.com is just an example and you should not take it literally. You have to replace this with a name of computer you want to connect to and your username on this computer. This information is needed to establish ssh connection. Only after ssh connection is established ports can be forwarded through this connection. – Piotr Dobrogost Feb 1 '17 at 23:45
  • If you want the port to be available from boot then see "autossh" in a systemd service using the above method - everythingcli.org/ssh-tunnelling-for-fun-and-profit-autossh – Richard Hollis Oct 27 '17 at 13:03
  • I also get "connection refused". And I still don't understand why the user@remote-server.com argument is needed when there is no SSH connection involved (according to -N). Should just forward packets. – Alexander Taylor Dec 10 '17 at 7:42
  • 2
    @AlexanderTaylor -N does not mean there is no SSH connection. It simply means do not execute a remote command (see the man page). The <user>@<host> argument is necessary, because this does open an SSH connection to <host> (which for OP's case would be localhost), and forwards the desired port through that SSH tunnel. It is one solution for OP's problem, but not the simplest. To forward to localhost without using ssh, you can use socat or netcat as in StephaneChazelas and not-a-user 's answers – user143943 Apr 18 '18 at 11:35
97

With socat on the server:

socat tcp-listen:8001,reuseaddr,fork tcp:localhost:8000

By default, socat will listen on TCP port 8001 on any IPv4 or IPv6 address (if supported) on the machine. You can restrict it to IPv4/6 by replacing tcp-listen with tcp4-listen or tcp6-listen, or to a specific local address by adding a ,bind=that-address.

Same for the connecting socket you're proxying to, you can use any address in place of localhost, and replace tcp with tcp4 or tcp6 if you want to restrict the address resolution to IPv4 or IPv6 addresses.

Note that for the server listening on port 8000, the connection will appear as coming from the proxy (in the case of localhost, that will be localhost), not the original client. You'd need to use DNAT approaches (but which requires superuser privileges) for the server to be able to tell who's the client.

46

Using the traditional nc is the easiest solution:

nc -l -p 8001 -c "nc 127.0.0.1 8000"

This version of nc is in the netcat-traditional package on Ubuntu. (You have to update-alternatives or call it nc.traditional.)

Note that in contrast to ssh this is not encrypted. Keep that in mind if you use it outside one host.

  • 2
    anyone know the equivalent on netcat-openbsd? – Sridhar Sarnobat Jul 23 '17 at 18:20
  • 2
    Analog for netcat version that is included in busybox: nc -v -lk -p 8001 -e /usr/bin/nc 127.0.0.1 8000. (Description of params) – Ivan Kolmychek Mar 16 '18 at 13:30
  • 2
    Working, but the nc command ends after the first remote connection. Add -k if you need to keep it running. – Sopalajo de Arrierez Jun 20 '18 at 23:49
  • I'm getting this error: nc: cannot use -p and -l on CentOS 6.4. Is there a work around? – Nick Predey Jul 16 '18 at 15:52
  • I prefer this solution over the ssh one because it makes it easier to use as root, when one needs to locally forward a privileged port. – Christian Aug 15 '18 at 0:58
25

OpenBSD netcat is available by default on Linux and also on OS X.

OSX:

mkfifo a
mkfifo b
nc 127.0.0.1 8000 < b > a &
nc -l 8001 < a > b &

Linux:

mkfifo backpipe
nc -l 12345 0<backpipe | nc www.google.com 80 1>backpipe

An alternative that works on OS X bash is to use a bidirectional pipe. It may work on other Unixes:

nc 127.0.0.1 8000 <&1 | nc -l 8001 >&0
  • I didn't notice at first that you were using openbsd netcat. This is better than having to install another netcat from an Ubuntu package. – RobertR Feb 26 '15 at 15:24
  • OpenBSD example failed on Ubuntu 15.04. With the shell redirects, netcat fails to open the port for listening as seen by ss -tan or netstat -tan. – Justin C Sep 3 '15 at 0:12
  • ⁺¹. FTR: the alternative way works on Ubuntu – Hi-Angel Nov 30 '16 at 10:41
  • I don't understand your solution. Can you explain it? – Trismegistos Sep 19 '17 at 8:21
  • @trismegistos In these examples the netcat listener and client redirect input into some shared files (mkfifo pipes..first in first out), and use those shared files as their source/destination of input/output, effectively creating a tunnel. Usually client/listener are used, but some techniques use client+client/listener+listener- wiki.securityweekly.com/… and slideshare.net/amiable_indian/secrets-of-top-pentesters are must reads. – Info5ek Oct 4 '17 at 18:02
4

Quoting a David Spillett's answer on ServerFault

rinetd should do the job, and a Windows binary for it can be had from http://www.boutell.com/rinetd/ (for anyone looking for the same thing under Linux, rinetd is in the standard repositories of just about every distro so can be installed with "apt-get install rinetd" or "yum install rinetd" or similar)

It is a simple binary that takes a configuration file in the format

bindaddress bindport connectaddress connectport

For example:

192.168.1.1 8001 127.0.0.1 8000

or

0.0.0.0 8001 127.0.0.1 8000

if you want to bind the incoming port to all the interfaces.

3
iptables -t nat -A PREROUTING -p tcp --dport <origin-port> -j REDIRECT --to-port <destination-port>

service iptables save
service iptables restart
  • Upon trying to connect to dport, as in nc -v localhost 2345, I'm getting Connection refused. I'm not very good in iptables, but I guess the dport has to have a listening app. – Hi-Angel Dec 1 '16 at 12:49
  • What if origin-port is on different interface than destination port? – Trismegistos Sep 19 '17 at 8:19
0

Based on Mark A.'s answer, I had to make a small tweak to get it to work for my Mac (at least on macOS Mojave Version 10.14.4)

mkfifo a
mkfifo b
nc 127.0.0.1 8000 < b > a &
nc -l 8001 < a > b &
printf "" > a

That printf statement seems to be crucial. Otherwise the netcat command to connect to port 8000 will never actually try to connect, and the netcat command to listen on port 8001 will never actually listen on port 8001. Without the printf, every time I would try to connect to port 8001 I would get connection refused.

My assumption is that netcat must somehow block on stdin (maybe it's trying to read it for some reason) before actually doing any Socket operations. As such, without the printf statement writing to fifo a, the netcat command will never start listening on port 8001.

Note: I would have left an answer on Mark's post, but I don't have reputation yet.

0

This is a new way to tunnel two udp ports on server: https://github.com/9crk/udpeer

udpeer 8001 8002

To test:

nc -u xxxx.com 8001
nc -u xxxx.com 8002

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.