How to attach to a tmux session on a remote machine that has been started locally on that machine?

Question

There are two physical machines in front of me: nixos and mac.

1. Physically typing in a terminal on nixos:

   nixos$ tmux ls
   0: 5 windows (created Thu Sep 16 19:21:38 2021) (attached)
   1: 1 windows (created Fri Sep 17 00:50:45 2021)
   2: 5 windows (created Wed Sep 22 05:37:28 2021) (attached)
   4: 1 windows (created Thu Sep 23 21:51:01 2021)
   

2. Then physically typing on mac:

   mac$ ssh nixos
   Last login: Sun Sep 26 13:24:58 2021 from 192.168.0.27
   
   nixos$ tmux ls
   no server running on /run/user/1000/tmux-1000/default
   

Is there are a way to attach to the already existing tmux sessions when connecting from a remote machine?

Answer 1

You have to connect using the socket name.

In nixos tmux session, check the value of TMUX_TMPDIR (if undefined, use /tmp) and look if a socket file is there. If not, send SIGUSR1 to that tmux server and recheck.

When you have got the socket file, try from mac: Connect to nixos, Ensure that TMUX_TMPDIR is exactly matching & Execute tmux with that socket using -S socket-name and tmux should be able to list the existing session.
tmux -S socket-name ls

In case that does not work for some weird reason, restart the tmux session on nixos with -L socket-name and connect from mac and use -S socket-name to ensure that both are using the exact same TMUX_TMPDIR and the exact same socket file.