How to remove SSH port forwarding

Question

The following command has been executed on our server, Nexus, to forward remote desktop connections to remote machine Lagos-B:

screen -S rdp
ssh -L [*]:886:10.1.100.91:886 root@lagos-b -N

I need to change this bind but I cannot simply overwrite it by entering new command - so how do I remove this existing SSH port forwarder.

Background if needed:

In original setup Nexus would SSH directly into Lagos-B. Lagos-B (a CentOS system) has socat configured to send incoming TCP traffic on port 886 to another machine.

We have changed this and we now have a VPN connection between Nexus and Lagos-B. If I enter the IP address on Nexus into my laptop and unplug the Nexus I can RDP into the remote system without issues. If I try to RDP to Nexus' IP address with the custom port the connection fails.

Answer 1

If you have access to the existing SSH session, you can use OpenSSH's "escape character" to modify your session on-the-fly. When you type "~" plus a character, it escapes the current SSH session, and allows you to execute a command. From the manpage:

     ~#  List forwarded connections.

     ~C  Open command line.  Currently this allows the addition of port
         forwardings using the -L, -R and -D options (see above).  It also
         allows the cancellation of existing remote port-forwardings using
         -KR[bind_address:]port.  !command allows the user to execute a
         local command if the PermitLocalCommand option is enabled in
         ssh_config(5).  Basic help is available, using the -h option.

You can add, change, remove any command line port forwarding magic you've added previously. Look at the manpage for further documentation.