Reverse ssh tunnel using openssh 8.6

BerzinTehvs · 07-15-2021, 01:45 PM

Some time ago when I used openSSH 7.x and older kernels, creation of reverse tunnels or tunnels as such was an easy exercise. Now, with 8.6, I can't create any, all I get is the message:
Tunnel device open failed.
Could not request tunnel forwarding.

tunneling is allowed in both ssh and sshd configs at both ends.

there is additional message:
debug1: sys_tun_open: failed to open tunnel control device "/dev/net/tun": Permission denied

I understand that I have to allow regular user to open tun device, but how?
(Slackware64-current, openSSH 8.6)

pan64 · 07-15-2021, 01:56 PM

probably it is a simple permission issue, the user has no right to access that device. Who is the owner/group of it?

Turbocapitalist · 07-15-2021, 01:57 PM

Please show the full line of what you are writing to invoke the SSH client so we can see what you are attempting. tun is not needed in a standard reverse tunnel.

BerzinTehvs · 07-18-2021, 02:47 PM

/usr/bin/ssh -NTq -o ServerAliveInterval=60 -o ExitOnForwardFailure=yes -R *:10103:localhost:22 -i /home/some_user/.ssh/id_rsa -l some_user $remotehost

does not matter with *: or w/o

BerzinTehvs · 07-26-2021, 12:00 PM

So, seems - no idea why its not working?

(owner of /dev/net/tun is root.root, permissions - 666)

Turbocapitalist · 07-26-2021, 12:07 PM

No, there is far too little information so far. Please try the following,

Code:

ssh -v -v -o ServerAliveInterval=60 -o ExitOnForwardFailure=yes \
-R 10103:localhost:22 -i /home/some_user/.ssh/id_rsa -l some_user $remotehost

You can use either one, two, or three -v options there. If you post the output here, feel free to obfuscate addresses and accounts but regardless of that be sure to enclose it between [code] [/code] tags.

BerzinTehvs · 08-01-2021, 04:17 PM

Code:

... skipped a lot of msg related to auth
debug1: Authentication succeeded (publickey).
Authenticated to RHOST ([RHOST]:22).
debug1: Remote connections from *:10122 forwarded to local address localhost:443
debug1: Requesting tun unit 2147483647 in mode 1
debug1: sys_tun_open: failed to configure tunnel (mode 1): Operation not permitted
Tunnel device open failed.
Could not request tunnel forwarding.

Turbocapitalist · 08-01-2021, 10:44 PM

Can you show the line you are using when trying to establish a connection? There should be no -w only -R for the tunnel.

Edit: PS Which distros, including versions, do you have for the client and the server?