I frequently use SSH for accessing one of the many development workstations I use for work, which includes developing network services among other things. A couple of years ago I wrote about this hidden gem in ssh which allows dynamocaily forwarding ports. This handy little feature allows dynamocailly adding local port forwards from within an already running SSH session. Recently however this feature has stopped working properly, emitting commandline disabled.

It turns out that this is due to a backwards incompatible change which OpenSSH released in 9.2 earlier this year:

ssh(1): add a new EnableEscapeCommandline ssh_config(5) option that controls whether the client-side ~C escape sequence that provides a command-line is available. Among other things, the ~C command-line could be used to add additional port-forwards at runtime.

The reason for this change is to support some sandboxing use-case which I don’t entirely understand but also don’t need, so I needed to add the following option to my host entries in ~/.ssh/config:

Host foobar
    EnableEscapeCommandLine yes

This can also be configured on the command line with -o EnableEscapeCommandline=yes. Happy port forwarding!