Talkd and 'mesg n': a story from the old Unix days

February 6, 2021

I tweeted:

An ancient Unix habit: I often still reflexively run 'mesg n' on my (single-user) workstation before starting screen, even though it's been a very long time since I ran talkd and so had any worries about that.

Back in the old days of Unix there was a program called talk, which actually made it into the POSIX standard (which I was surprised to discover just now). Talk enabled live two way communication, instead of the one way communication of write (which is also a POSIX standard command), but the relevant thing about it was that it generally worked through a daemon, talkd. Your Unix server ran talkd, and when you ran talk it communicated with talkd to notify the person you wanted to talk to and then let them connect with you.

Back in the days, this communication was done over IP, not (say) Unix domain sockets. Since it was the old days of a trusting network environment, your talkd would accept requests from everyone, not just the local machine (and talk would chat across the network), letting anyone on your local network or often the entire Internet try to start up a talk session with you. This meant that even on a single user workstation, there was a reason to run 'mesg n' to avoid having random talk notifications overwrite screen's output and interfere with it.

(On a multi-user machine, other people on the same machine might try to write to you and you'd want to keep that from interfering with your screen session. This isn't an issue on a single user workstation.)

It's been a very long time since my workstation ran talkd, even for requests from the local network. But my reflexes still want to run that 'mesg n' before I start screen.

(I didn't put 'mesg n' in my shell .profile for what is ultimately fuzzy reasons.)

Written on 06 February 2021.
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Last modified: Sat Feb 6 23:41:38 2021
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