The telnet URI Scheme
How to write a link that points at a remote Telnet login, such as telnet://host:23/.
Hoffman, P., "The telnet URI Scheme", RFC 4248, DOI 10.17487/RFC4248, October 2005, <https://www.rfc-editor.org/info/rfc4248>.
@misc{rfc4248,
series = {Request for Comments},
number = 4248,
howpublished = {RFC 4248},
publisher = {RFC Editor},
doi = {10.17487/RFC4248},
url = {https://www.rfc-editor.org/info/rfc4248},
author = {P. Hoffman},
title = {{The telnet URI Scheme}},
pagetotal = 4,
year = 2005,
month = oct,
} RFC text
Contents
Network Working Group P. Hoffman
Request for Comments: 4248 VPN Consortium
Obsoletes: 1738 October 2005
Category: Standards Track
The telnet URI Scheme
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies the telnet Uniform Resource Identifier (URI)
scheme that was originally specified in RFC 1738. The purpose of
this document is to allow RFC 1738 to be made obsolete while keeping
the information about the scheme on standards track.
1. Introduction
URIs were previously defined in [RFC2396], which was updated by
[RFC3986]. Those documents also specify how to define schemes for
URIs.
The first definition for many URI schemes appeared in [RFC1738].
Because that document has been made obsolete, this document copies
the telnet URI scheme from it to allow that material to remain on
standards track.
2. Scheme Definition
The Telnet URL scheme is used to designate interactive services that
may be accessed by the Telnet protocol [STD8].
A telnet URL takes the form:
telnet://<user>:<password>@<host>:<port>/
The final "/" character may be omitted. If :<port> is omitted, the
port defaults to 23. The :<password> can be omitted, as well as the
whole <user>:<password> part. Few implementations handle the user
name and password very well, if at all.
This URL does not designate a data object, but rather an interactive
service. Remote interactive services vary widely in the means by
which they allow remote logins; in practice, the <user> and
<password> supplied are advisory only: clients accessing a telnet URL
merely advise the user of the suggested username and password.
Many RFCs have added various services to the Telnet protocol for
better authentication, encryption of traffic, or both. Those RFCs
have not specified new URI schemes for Telnet to invoke those
services (along the lines of "https" being a different URI scheme
from "http"). Some modern telnet clients attempt to invoke those
more-secure versions of Telnet when resolving a "telnet" URL.
3. Security Considerations
There are many security considerations for URI schemes discussed in
[RFC3986].
The Telnet protocol normally uses passwords in the clear for
authentication, and normally offers no privacy. In normal telnet,
both the user's identity and their password are exposed without any
protection; after that, the contents of the entire Telnet session is
exposed without any protection.
Many extensions have been made to Telnet to make it more secure in
different ways. In particular, [RFC2941] gives a framework based on
a telnet option that many other security extensions have leveraged
off of. These extensions are certainly worthwhile methods for
reducing the obvious problems with exposing the user's name,
password, and plaintext of the session in the clear.
Although some modern telnet clients attempt to invoke those more-
secure versions of Telnet when resolving a "telnet" URL, other telnet
clients do not, so a user cannot rely on this type of security unless
it is explicitly enabled and the results of the security negotiation
are checked.
4. Normative References
[STD8] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
5. Informative References
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC2941] Ts'o, T. and J. Altman, "Telnet Authentication Option",
RFC 2941, September 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
Author's Address
Paul Hoffman
VPN Consortium
127 Segre Place
Santa Cruz, CA 95060
US
EMail: paul.hoffman@vpnc.org
Full Copyright Statement
Copyright (C) The Internet Society (2005).
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AI ELI5
On this page
§1Why This Document Exists
URIs were defined in RFC 2396, which was updated by RFC 3986, and those documents also say how to define schemes. The first definition of many schemes appeared in RFC 1738. Since that document has been made obsolete, this one copies the telnet scheme from it so the material can remain on the standards track.
An old rulebook was being retired, so the telnet page was copied into its own small booklet.
§2The telnet Scheme
The scheme designates interactive services that may be accessed by the Telnet protocol. A telnet URL takes the form telnet://<user>:<password>@<host>:<port>/.
The final / may be omitted. If :<port> is omitted, the port defaults to 23. The password can be left out, as can the whole user and password part. Few implementations handle the user name and password well, if at all.
The URL names an interactive service, not a data object. Because remote services differ widely in how they allow logins, the user and password are advisory only: a client merely advises the user of the suggested values.
The link says which computer to call and optionally which door number. If you leave the door number out, it uses 23.
§3Security Considerations
Normal Telnet uses passwords in the clear and offers no privacy, so the identity, the password and the whole session are exposed. Many extensions improve this, and RFC 2941 gives a framework based on a telnet option that many of them use. Those RFCs did not define new URI schemes, unlike https versus http.
Some modern clients try to use the more secure forms when resolving a telnet URL, but others do not. A user cannot rely on that security unless it is explicitly enabled and the result of the security negotiation is checked.
A plain telnet call is like talking on a speakerphone in a crowded room. Some phones can switch to a private line, but you must check that they did.