Telnetd fails to handle some of TIOCPKT control bytes.

Takashi Yano takashi.yano@nifty.ne.jp
Sat Feb 28 12:15:00 GMT 2015


Package: inetutils-server
Version: 1.9.1-2 or older

Telnetd in inetutils-server package lacks handling of some of
TIOCPKT control bytes. The most influential thing is a lack of
handling of TIOCPKT_DATA. TIOCPKT_DATAs i.e. '\0's frequently
appear in the stream of network side.

In most cases, '\0' is invisible on a terminal, therefore it is
hardly noticed. However, sometimes multibyte characters are
displayed incorrectly by interference of inserted '\0'.

To reproduce this problem:
1) Install intetutils-server package.
2) Make a typescript file by:
      script -qc 'telnet localhost'
3) Do something to produce some text outputs.
4) Logout from the telnet session.
5) Look into the typescript file, then you will find
   many extra '\0's (^@) in it.

To fix this problem, I have made following patch.

In this patch, the first byte from read() is always treated as
a TIOCPKT control byte. Therefore, it is simply read out instead
of being peeked. This is based on description in man tty_ioctl,
which says the first byte returned by read() is always a TIOCPKT
control byte.


--- telnetd.c.orig      2012-01-06 22:58:30.000000000 +0900
+++ telnetd.c   2015-02-20 19:18:55.808004100 +0900
@@ -601,11 +601,10 @@ telnetd_run (void)
          /* Something to read from the pty... */
          if (pty_read () < 0)
            break;
-         c = pty_get_char (1);
+         c = pty_get_char (0); /* read TIOCPKT control byte */
 #if defined TIOCPKT_IOCTL
          if (c & TIOCPKT_IOCTL)
            {
-             pty_get_char (0);
              copy_termbuf ();
              localstat ();
            }
@@ -613,7 +612,6 @@ telnetd_run (void)
          if (c & TIOCPKT_FLUSHWRITE)
            {
              static char flushdata[] = { IAC, DM };
-             pty_get_char (0);
              netclear ();      /* clear buffer back */
              net_output_datalen (flushdata, sizeof (flushdata));
              set_neturg ();
@@ -630,7 +628,6 @@ telnetd_run (void)
                                   IAC, SB, TELOPT_LFLOW,
                                   flowmode ? LFLOW_ON : LFLOW_OFF, IAC, SE);
                }
-             pty_get_char (0);
            }

        }


-- 
Takashi Yano <takashi.yano@nifty.ne.jp>
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