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Editorial Team
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Asked: 2026-06-08T18:24:03+00:00

When using telnetlib I noticed some interesting variables: AUTHENTICATION = chr(37) # Authenticate XAUTH

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When using telnetlib I noticed some interesting variables:

AUTHENTICATION = chr(37) # Authenticate
XAUTH = chr(41) # XAUTH
PRAGMA_LOGON = chr(138) # TELOPT PRAGMA LOGON
SSPI_LOGON = chr(139) # TELOPT SSPI LOGON

Also, this page lists some RFCs which describe AUTHENTICATION option in detail.

If I understand correctly, if a telnet server supports authentication I should be receiving IAC DO AUTHENTICATION when connecting to it. However, when trying to connect to telnetd on a Linux machine I do not receive this option and so I can’t send IAC WILL AUTHENTICATION.

Is my understandng correct? Is there a way I can ask the server to perform authentication? Is it even possible to authenticate telnet session automatically? (without listening to “Login:”, “Username:”, “Password:” and similar prompts)

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1 Answer

  1. Editorial Team

    Yes, you are understanding RFC 1416 correctly. The server should send DO first, if it supports authentication. You can try sending WILL anyway but that’ll be against the spec and probably won’t work anyway.

    By Linux telnetd, you probably mean the netkit telnetd. And as far as I can see on its manpage:

    -a authmode This option may be used for specifying what mode should be used for
    authentication. Note that this option is only useful if telnetd has
    been compiled with support for authentication, which is not available
    in the current version    . […]

    So I’d simply say it can’t do that. I’m using the OpenBSD telnetd (telnet-bsd package in Gentoo); it seems to have some auth bits but I wasn’t able to get it to work at all. I suspect it only supports Kerberos-based auth.

    Looking at the authentication types list in RFC 2941, it seems that most authentication types were never even specified. Looking at those which are, DSS/DSA is mostly interesting (it is used in SSH as well) but I guess it will be really hard to find a compliant server and client. Sadly, it seems that there were no proposals for simple (plaintext) authentications there.

    So, to sum up: yes, it would be possible if someone implemented it. But since telnet is practically dead, I doubt you’ll find server implementations for that. And even if you did, you’d have to write the client-side yourself which won’t be that easy with the supported schemes. So I doubt it’s worth the effort.

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