[Bug 196902] [PATCH] telnet: bad terminal setup in character + local echo mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196902

            Bug ID: 196902
           Summary: [PATCH] telnet: bad terminal setup in character +
                    local echo mode
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: sbrabec at suse.cz

Created attachment 151861
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=151861&action=edit
Proposed fix

If the telnet client switches to the character + local echo mode (a
non-standard combination) and it runs in a smart terminal capable of doing
echo, telnet client configures the terminal to perform the echo. Telnet client
itself does no echo.

There is a problem that telnet uses an incorrect combination of flags, which
causes End Of Line being displayed as "^M" instead of performing CR+LF.


How to reproduce:

1. Run telnet client in a smart virtual terminal capable of local echo (for
example "xterm" or "gnome-terminal")

2. Connect to a telnetd capable and properly implementing WONT ECHO.

3. Login to a shell that is capable to work in no echo mode:
- For example "bash --noediting" or call "set +o vi ; set +o emacs" before
changing telnet mode.
- You can also reproduce it with "cat" in place of login shell.

4. Set the mode to the local echo using "send dont echo".

Now your telnet session should be in the character local echo mode. (Character
mode is the default and we did not change it before switching to the local echo
mode.)

Without the fix:
telnet> send dont echo
linux> #hello^Mlinux>

With the fix:
telnet> send dont echo
linux> #hello
linux>

Note: Characters "^M" are generated by mis-configured terminal on the client
side. They are not passed to the telnet session. Telnet session correctly gets
EOL (LF or CRLF).


Technical details:

Current implementation uses following set of flags:
onlcr -icrnl echo

In CRLF mode it first correctly sets 
onlcr icrnl echo
but later in the code it changes flags to
onlcr -icrnl echo

The -icrnl causes echoing of control characters as "^{letter}", including LF
displayed as "^M".

According to Werner Fink <werner at suse.com>, this is a correct behavior of
terminal, just the combination of flags used is incorrect.

However the problem was discovered on the Linux system, I believe, that the
problem is system independent, and affects all systems. Attached patch seems to
fix this issue without breaking any other telnet modes.

As FreeBSD seems to inherit the original BSD code, I am sending this report
here as well.

This problem probably exists from the very first implementation of character
echo mode in the BSD telnet (1987), and affects probably all implementations
derived from it:
- NetBSD telnet (the original code)
  Reported there as http://gnats.netbsd.org/49392
- OpenBSD telnet (branched 1995 from NetBSD)
  Reported as
http://lists.openbsd.org/cgi-bin/mj_wwwusr?list=bugs&brief=on&func=archive-get-part&extra=201501/57
(browsing archives needs registration)
- netkit-telnet (now in Debian, branched between 1990 and 1993)
- telnet-bsd (Linux port from 1999 created by Thorsten Kukuk <kukuk at suse.de>)
- FreeBSD telnet (significantly rewritten, branched between 1993 and 1997)

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