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The Day the Telnet Died

labs.greynoise.io

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Re: The Day the Telnet Died

#11

Post by Twisol »

gerdesj wrote: post #9

telnet isn't just for ... telnet. $ telnet smtp.example.co.uk 25 HELO me MAIL FROM: gerdesj@example2.co.uk RCPT TO: gerdesj@example.co.uk DATA .. or you can use SWAKS! For some odd reason telnet is becoming rare as an installed binary.

The difference between "telnet" the program and "telnet" the protocol is especially important in this discussion, I think.

A more "proper" tool for that is netcat -- I doubt SMTP supports the Telnet option negotiations subsystem. (I also doubt SMTP servers can interpret the full suite of Network Virtual Terminal (NVT) commands that the Telnet protocol supports.) There's clearly enough similarity between the two protocols that if you're just using it to transfer plaintext it will probably work out fine, but they are distinct protocols.

Re: The Day the Telnet Died

#15

Post by Mixtape »

MBCook wrote: post #7
Twisol wrote: post #5

> Someone upstream of a significant chunk of the internet’s transit infrastructure apparently decided telnet traffic isn’t worth carrying anymore. That’s probably the right call. Does this impact traffic for MUDs at all? I know several MUDs operate on nonstandard Telnet ports, but many still allow connection on port 23. Does this block end-to-end Telnet traffic, or does it only block attempts to access Telnet service…

It wasn’t clear from the article but I assumed they were filtering for the attack specifically. Since Telnet is totally plain text that would absolutely be easy to do right?

Wouldn't that imply that >80% of all monitored telnet sessions were exploit attempts for the specific CVE in question? Even with the scale of modern botnets, that seems unrealistic for a single vuln that was undisclosed at the time.

Re: The Day the Telnet Died

#16

Post by quotemstr »

gerdesj wrote: post #9

telnet isn't just for ... telnet. $ telnet smtp.example.co.uk 25 HELO me MAIL FROM: gerdesj@example2.co.uk RCPT TO: gerdesj@example.co.uk DATA .. or you can use SWAKS! For some odd reason telnet is becoming rare as an installed binary.

You want nc (usually with -v) or socat. telnet is muscle memory for a lot of people (myself included sometimes) but it's a strictly inferior choice these days for poking arbitrary plaintext services.

Re: The Day the Telnet Died

#18

Post by ktpsns »

gerdesj wrote: post #9

telnet isn't just for ... telnet. $ telnet smtp.example.co.uk 25 HELO me MAIL FROM: gerdesj@example2.co.uk RCPT TO: gerdesj@example.co.uk DATA .. or you can use SWAKS! For some odd reason telnet is becoming rare as an installed binary.

I used telnet(1) as a generic TCP text client for many years before switching to GNU/BSD netcat. Nowadays, netcat is more prominent then telnet, and telnet had its corner cases with control characters.

Never heard about https://jetmore.org/john/code/swaks/, thanks for the tip.

Re: The Day the Telnet Died

#19

Post by RupertSalt »

Twisol wrote: post #5

> Someone upstream of a significant chunk of the internet’s transit infrastructure apparently decided telnet traffic isn’t worth carrying anymore. That’s probably the right call. Does this impact traffic for MUDs at all? I know several MUDs operate on nonstandard Telnet ports, but many still allow connection on port 23. Does this block end-to-end Telnet traffic, or does it only block attempts to access Telnet service…

Most MUDs do not use Telnet.

MUDs use plaintext TCP protocols that are accessible to a wide range of clients.

The Telnet protocol is well-defined and not completely plaintext. There are in-band signaling methods and negotiations. Telnet is defined to live on 23/tcp as an IANA well-known, privileged, reserved port.

MUDs do none of this. You can usually connect to a MUD using a Telnet client, but most players hate the experience and often deride this method in favor of a dedicated, programmable client.

The fact that MUDs inhabit higher 4-digit ports is an artifact from their beginnings as unprivileged, user-run servers without a standardized protocol or an assigned “well-known port” presence. If you want your MUD to be particularly inaccessible, you could certainly run on port 23 now!

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