Discussion:
0xFF in Telnet Protocol
(too old to reply)
Alexander Voropay
2001-02-21 16:17:03 UTC
Permalink
Another problem is 0xFF codepoint (not for Linux console, but for
some software and Telnet protocol).
Where do you see a problem with telnet and 0xFF?
The 0xFF code in Telnet is IAC "Interpret As Command" escape-like
character. See RFC-854 "Telnet Protocol Specification".
In telnet from Linux to Linux or Solaris I can, after having set the
options "inbinary" and "outbinary", enter and output the 0xFF character.
Can you point out a
place in the telnet protocol specification which mishandles 0xFF?
You should double this code 0xFF 0xFF to send exactly one 0xFF
or to go into special "binary mode" : RFC-856. Anyway, binary mode
will break all "telnet nogotiations" : sreen size, TERMACAP, e.t.c.
controlled by IAC sequences (and even our favorite RFC-2066
"Telnet Charset Options" ;))


P.S. DO NOT USE TELNET, USE SSH ! ;-)
Jeffrey Altman
2001-02-21 17:03:46 UTC
Permalink
Post by Alexander Voropay
The 0xFF code in Telnet is IAC "Interpret As Command" escape-like
character. See RFC-854 "Telnet Protocol Specification".
As per rfc 854 the IAC character is used to quote all instances of
0xFF that appear in the application data stream.
Post by Alexander Voropay
In telnet from Linux to Linux or Solaris I can, after having set the
options "inbinary" and "outbinary", enter and output the 0xFF character.
Can you point out a
place in the telnet protocol specification which mishandles 0xFF?
You should double this code 0xFF 0xFF to send exactly one 0xFF
or to go into special "binary mode" : RFC-856. Anyway, binary mode
will break all "telnet nogotiations" : sreen size, TERMACAP, e.t.c.
controlled by IAC sequences (and even our favorite RFC-2066
"Telnet Charset Options" ;))
Telnet Binary mode does not affect the use of the IAC character; nor
does Binary mode break any other Telnet Option. Binary mode only does
two things:

. it ensures that both application end points can accept data with
the 8th bit set

. it changes the handling of CR controls. A raw CR no longer needs
to be sent as CR NUL to distinguish it from the end of line
indicator CR LF.
Post by Alexander Voropay
P.S. DO NOT USE TELNET, USE SSH ! ;-)
Telnet implementations that support the Telnet START_TLS option and/or
the Telnet Authentication option are usually more secure than SSH. If
you believe it to be insecure than you had better stop using your web
browser since the same level of security is provided.

A list of secure telnet implementations from various sources can be
found at

http://www.kermit-project.org/telnetd.html


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