Telnet Auto Negotiation Failure

[rumpadunk]

On certain devices (Fortinet) I experienced telnet auto negation failures for the telnet session.

Sample from this script: attempting connection to 127.0.0.1:12123 connected to 127.0.0.1:12123

DO ECHO DO SUPPRESS GO AHEAD login: waiting for login prompt: login: expect: watching for login: < DO TERMINAL TYPE WONT TERMINAL TYPE < DO TSPEED WONT TSPEED < DO XDISPLOC WONT XDISPLOC < DO NEW-ENVIRON WONT NEW-ENVIRON < WILL ECHO Enabling Remote Echo < WILL SUPPRESS GO AHEAD Enabling Suppress Go Ahead (SGA) on Receive Enabling Suppress Go Ahead (SGA) on Transmit WILL SUPPRESS GO AHEAD < DO SUPPRESS GO AHEAD < DO ECHO Disabling Remote Echo to prevent Echo loop DONT ECHO Enabling Local Network Echo WILL ECHO < DO NAWS WONT NAWS < WILL STATUS < DO LFLOW WONT LFLOW

Sample from PuttyTel which works properly: 2016-03-09 14:02:35 Looking up host "127.0.0.1" 2016-03-09 14:02:35 Connecting to 127.0.0.1 port 12123 2016-03-09 14:02:35 client: WILL NAWS 2016-03-09 14:02:35 client: WILL TSPEED 2016-03-09 14:02:35 client: WILL TTYPE 2016-03-09 14:02:35 client: WILL NEW_ENVIRON 2016-03-09 14:02:35 client: DO ECHO 2016-03-09 14:02:35 client: WILL SGA 2016-03-09 14:02:35 client: DO SGA 2016-03-09 14:02:36 server: DO TTYPE 2016-03-09 14:02:36 server: DO TSPEED 2016-03-09 14:02:36 server: DO XDISPLOC 2016-03-09 14:02:36 client: WONT XDISPLOC 2016-03-09 14:02:36 server: DO NEW_ENVIRON 2016-03-09 14:02:36 server: DO NAWS 2016-03-09 14:02:36 client: SB NAWS 80,24 2016-03-09 14:02:36 server: WILL ECHO 2016-03-09 14:02:36 server: DO SGA 2016-03-09 14:02:36 server: WILL SGA 2016-03-09 14:02:36 server: SB TSPEED SEND 2016-03-09 14:02:36 client: SB TSPEED IS 38400,38400 2016-03-09 14:02:36 server: SB NEW_ENVIRON SEND 2016-03-09 14:02:36 client: SB NEW_ENVIRON IS 2016-03-09 14:02:36 server: SB TTYPE SEND 2016-03-09 14:02:36 client: SB TTYPE IS XTERM 2016-03-09 14:02:36 server: DO ECHO 2016-03-09 14:02:36 client: WONT ECHO 2016-03-09 14:02:36 server: WILL STATUS 2016-03-09 14:02:36 client: DONT STATUS 2016-03-09 14:02:36 server: DO LFLOW 2016-03-09 14:02:36 client: WONT LFLOW

The workaround that @jnorell came up with is as follows:

Edit Net/Telnet.php, line 983 right now has this:

       if ($this->mode['echo_remote']) {
           $this->debug("Disabling Remote Echo to prevent Echo loop");
           $this->mode['echo_remote'] = false;
           $this->send_telcmd(TEL_DONT, $opt);
       }

Change that to:

       if ($this->mode['echo_remote']) {
           $this->debug("Refusing to Echo");
               $this->send_telcmd(TEL_WONT, $opt);
               break;

               $this->debug("Disabling Remote Echo to prevent Echo loop");
      $this->mode['echo_remote'] = false;
      $this->send_telcmd(TEL_DONT, $opt);
   }

Which essentially sends a "TEL_WONT" instead of a "TEL_DONT" on in response to the DO ECHO.

After you change the code above, valid debug output looks as follows and the code worked properly:

attempting connection to 127.0.0.1:12123 connected to 127.0.0.1:12123

DO ECHO DO SUPPRESS GO AHEAD login: waiting for login prompt: login: expect: watching for login: < DO TERMINAL TYPE WONT TERMINAL TYPE < DO TSPEED WONT TSPEED < DO XDISPLOC WONT XDISPLOC < DO NEW-ENVIRON WONT NEW-ENVIRON < WILL ECHO Enabling Remote Echo < WILL SUPPRESS GO AHEAD Enabling Suppress Go Ahead (SGA) on Receive Enabling Suppress Go Ahead (SGA) on Transmit WILL SUPPRESS GO AHEAD < DO SUPPRESS GO AHEAD < DO ECHO Refusing to Echo WONT ECHO < DO NAWS WONT NAWS < WILL STATUS < DO LFLOW WONT LFLOW

Opening a bug just to track it should anyone else encounter a similar issue.