openssh unix dev - Mar 2023 - Uniquely Identifying the Local TTY of an SSH Connection

Packing the data in TERM is a great idea! I?ll see what I can do with that.
>> Finally, some administrative notes: I wasn't able to sign up for
this
>> mailing list at
>> https://lists.mindrot.org/mailman/subscribe/openssh-unix-dev because
>> attempts to do so were met by a "Bug in Mailman version
2.1.39" page.
>
> I just tried that and didn't get that error, although I also
haven't
> got the confirmation email yet. Could you please try again and if it
> happens again. mail me the error message off-list?
yeah I got that worked out and forgot to delete that part of my message when I
resent it. Sorry about that.
>

I've managed to figure out a scheme using SendEnv. The way it works is that
I add the following to my local .bashrc

```
export LC__LOCAL_TTY_NAME="ssh-$(basename $(tty))"
```

then on the same local machine add an entry to my .ssh/config

```
Host = remote
  Hostname = my.remote.host
  SendEnv LC__LOCAL_TTY_NAME
  ControlPath ~/.ssh/cm-%r@%h:%p
  ControlMaster auto
  ControlPersist 10m
```

and finally in my .bashrc on my remote host I have an entry

```
if [[ $- =~ i ]] && [[ -n "$LC__LOCAL_TTY_NAME" ]]; then
  exec shpool attach "$LC__LOCAL_TTY_NAME"
fi
```

I did try packing the tty name in TERM, but using that with SendEnv would have
required mutating my local TERM in a way that could have messed up programs
running locally.

This approach works great, but I would also like to be able to set things up so
that I can explicitly name the session I want to connect to and do `ssh main` or
`ssh edit` to go directly to my remote host and attach to a session named `main`
or `edit`. The SetEnv ssh config option seems to be just what the doctor ordered
in order to do this, so I've added a few blocks to my local .ssh/config

```
Host = main
  Hostname = my.remote.host
  SetEnv LC__LOCAL_TTY_NAME=main
  ControlPath ~/.ssh/cm-%r@%h:%p
  ControlMaster auto
  ControlPersist 10m

Host = edit
  Hostname = my.remote.host
  SetEnv LC__LOCAL_TTY_NAME=edit
  ControlPath ~/.ssh/cm-%r@%h:%p
  ControlMaster auto
  ControlPersist 10m
```

this sorta works, but there seems to be a strange interaction between the
Control{Path,Master,Persist} options and SetEnv. If I try to immediately ssh to
my remote host from two different terminals using `ssh main` then `ssh edit`,
LC__LOCAL_TTY_NAME will get set to `main` in both, presumably because of some
sticky state on the shared connection. Oddly, if I first do `ssh remote` in a
separate terminal to use the SendEnv mechanism, then do `ssh main` and `ssh
edit` in terminals two and three, this approach works as I was hoping and
terminal two has LC__LOCAL_TTY_NAME set to `main` while terminal three has it
set to `edit`. This feels like a bug in ssh to me, but maybe it is actually
working as intended and I'm just holding it wrong.

On the assumption that this is a bug, I've been trying to repro locally by
cloning and building https://github.com/openssh/openssh-portable.git, then
running the sshd it builds locally and logging in with the ssh binary it builds.
Would those two components contain everything that could be in play here, or do
I need to worry about using the ssh-agent from HEAD as well? (I know my setup is
using my system ssh-agent because I configured an authorized_keys file with my
id_rsa.pub in it, then was able to log in to the locally running sshd with no
password after running ssh-add).