Running into errors reattaching to a GNU screen session that tunnels X over SHH

Question

After tunneling X over an SSH connection using screen on the remote, how can I reattach the screen session from another machine and tunnel X to that other machine?

I either get "cannot open display" type errors, or the app tunnels to the first computer. I assume it has something to to with ENV vars in the screen session, but I'm not sure how to fix the error.

Answer 1

When using screen, you will have to copy the required environment variables manually.

Detach from the screen session, run echo $DISPLAY and copy the value.

Now reattach and run export DISPLAY="copied value".

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When you create a Screen session, the screen server automatically inherits a copy of the current environment, which is further inherited by the shells or other processes that you start in new Screen windows inside the session.

xterm                        (DISPLAY=":0")
└── bash                     (DISPLAY=":0")
    └── screen (client)      (DISPLAY=":0")
        └── SCREEN (server)  (DISPLAY=":0")
            ├── irssi        (DISPLAY=":0")
            ├── mutt         (DISPLAY=":0")
            └── bash         (DISPLAY=":0")

When you connect over SSH with X11 forwarding enabled, the SSH server sets the $DISPLAY environment variable pointing to your X11 server.

However, when you reattach to a Screen session, the processes running inside that session will not see it; they are still holding copies of the old environment.

sshd                         (DISPLAY="localhost:12")
└── bash                     (DISPLAY="localhost:12")
    └── screen (client)      (DISPLAY="localhost:12")

SCREEN (server)              (DISPLAY=":0")
├── irssi                    (DISPLAY=":0")
├── mutt                     (DISPLAY=":0")
└── bash                     (DISPLAY=":0")

This is because a process is not allowed to update environments of other processes; only of itself. Even though the screen client process has the new environment, it cannot give it to the screen server; the protocol used by Screen simply does not have such a function.

Some other multiplexers, such as tmux, allow clients to send a copy of its environment to the server upon reattaching. This is not completely effective; it only allows newly opened tmux windows to inherit the new environment, but still cannot do anything about already running windows.