The Day the Telnet Died

Lots of text games - MUDs - still play over telnet using dedicated MUD clients that implement their own telnet stack. Outright blocking the port has an outsized side efffe on them, this is simply not right.

Or Notepad, for a change!

If MUDs and other games were indeed using port 23/tcp for player access, they were not only incorrect but rather dangerous.

Since 23/tcp is a well-known IANA-registered port for the Telnet service, it is an RFC violation to use it for a service that is not telnetd/remote logins via TELNET protocol.

Any port below 1024 signifies that it is a "privileged port". This is an archaic distinction that developed in high-trust R&E networks, but it did signify that the listener on the port had administrative/root access to spawn a service there, so it was kind of a signal that you could "trust" the remote server with your login credentials.

The privileged ports were also priority, because if the unprivileged ones were "first come, first served" for unprivileged users, the administrator would have the ability to enforce the uniqueness of "privileged ports", and disable or kill any process that shouldn't be using one. A MUD Wizard who finds their port in-use (bound) on start is on their own.

Typically there were no MUDs running with, or needing, root privileges. They were run under user accounts, or specific unprivileged role accounts. They had no need of a privileged port, and many were clandestine or unauthorized, and forced to use a higher port number. That's why the 4-digit ports became so popular.

Anyway, the custom has already developed of blocking port 23 to protect users from unwittingly opening a management or login interface. Most shrewd admins would choose a port that isn't routinely blocked and filtered... and port-scanned.

If your favorite MUD runs on port 23 today, such as nethack or something, then I am glad for this change, which will force the administrator to select a unique port that does not imply privilege, TELNET protocol, or shell login credentials. It is totally RFC-compliant to select an unassigned port above 1023, and MUD conventions have popularized several numbers that are still recognizable to players today.

in particular it is very rare I find a running diku or lp that does not at least use the telnet ECHO option to attempt to control password display.

I worked for an ISP in the mid-90s and had been on the Internet since 1989 or so. I recall the progression for me was something like this:

We used telnet in college no problem. It was a fairly well-accepted method of remote access. The heterogeneous network had many different modes, but a major dialup point was the Annex box, which supported telnet into the Unix or VMS machines.

Between Unix machines, we would often prefer "rlogin" instead. There were several horrific iterations of other remote-access protocols such as "remsh". rlogin was notorious for its "/etc/hosts.equiv" authorization method which trusted DNS and should've been perceived as Swiss Cheese from the outset. rlogin was, IIRC, directly related to rsh and rcp and used the same frameworks. rlogin was no more secure than telnet, but probably less secure because of its conveniences.

We also used port 23/tcp for remote management, for example Cisco routers. They weren't running telnetd, but it was the port where you connected remotely and logged in with or without credentials.

rlogin persisted alongside telnet, until encryption came into fashion and ssh was distributed. Once ssh was available and working well, everyone knew that telnetd and rlogind were on borrowed time. The services were shut down and disabled in inetd. The ports were sometimes blocked. Security advisories went out.

I suppose it took a long, long time for ssh to finally dominate, and for people to abandon telnetd mostly, but it was fairly thorough. We all recognized the superiority of sshd's authentication and encrypted channels.

There were mitigations for people to extend their legacy use of telnetd and rlogind. For example, tcp wrappers and fail2ban could be implemented. Firewall filters could select only authorized networks. VPNs could tunnel through an Intranet that still used them. So, the services lived on wherever they didn't need to be exposed on the public Internet. But I think most Unix admins got the picture by the end of the dot-com bubble.