Attach a tmux session to a remote machine

I am using tmux on my local machine and usually have several sessions simultaneously.

What I usually do is I have a session with different windows to work locally and the other sessions in which I connect several windows to one host per session.

A session / window tree would look like this in the daily use I make of tmux:

(TMUX on my local machine)
 |
 +- session 1: local
 |  \_ window 1: local shell
 |  \_ window 2: local shell
 |  \_ ...
 |
 + session 2: somehost
 |  \_ window1: ssh user@somehost
 |  \_ window2: ssh user@somehost
 |  \_ ...
 |
 + session 3: someotherhost
    \_ window1: ssh user@someotherhost
    \_ window2: ssh user@someotherhost
    \_ ...

Is there a way to make session 2 & session 3 some sort of remote sessions connecting to a tmux session created on somehost & someotherhost?

The above tree would look like this:

(TMUX on my local machine)
 |
 +- session 1: local
 |  \_ window 1: local shell
 |  \_ window 2: local shell
 |  \_ ...
 |
 + session 2 linked to an existing session on somehost
 |  \_ window1: shell on somehost
 |  \_ window2: shell on somehost
 |  \_ ...
 |
 + session 3 linked to an existing session on someotherhost
    \_ window1: shell on someotherhost
    \_ window2: shell on someotherhost
    \_ ...

I found this topic but I am not sure this is what I want to do: Is sharing a tmux sockets between hosts possible?

I suppose what I am looking for would require me to have the same tmux configuration on my local machine, somehost & someotherhost but that would not be a problem.

You can pass a command to ssh when connecting to a remote host. Include the -t option with tmux attach-session to connect to the remote tmux session:

ssh <remote host> -t tmux attach-session

This post on attaching to a tmux session via ssh explains it in more detail.

If your main problem is that you have to type ssh remotehost everytime you open new window, the default-command session option may be a workaround.

It automatically execute the command you specify when you open new windows in the sessions you set it to.

A simple setting in .tmux.conf:

bind-key C-r new-session ssh remotehost \; set-option default-command "ssh remotehost"

This key binding Ctrl-r creates a new session where every newly opened window firstly execute ssh remotehost on behalf of you.

You can choose hosts to connnect at session creation time with the settings below:

bind-key C-r command-prompt "new-session -s %1 ssh %1 \; set-option default-command \"ssh %1\""

This one asks you the host you want to do ssh at each session creation (not at each window creation) and starts the session dedicated to that host.

It takes little time to execute many ssh if you enable SSH multiplexing in your ssh config. The .ssh/config example is below:

Host remotehost
    ControlMaster auto
    ControlPath ~/.ssh/mux-%r@%h:%p
    ControlPersist 10

Note that this tmux setting does not create tmux sessions in remote hosts. If ssh connection is lost, affected tmux windows never return.

You could use a tool like socat to tunnel the unix domain socket used by tmux to your local machine. But you are really just swapping a tunnel (ssh) for another (socat) so it is not really better, and also insecure.