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Sunday, March 31, 2013

Realtime Chat between Cisco Routers

You might probably know that it's possible to send messages from one vty line to another on a single Cisco router.

R1#send ?
  *        All tty lines
  <0-17>   Send a message to a specific line
  aux      Auxiliary line
  console  Primary terminal line
  log      Logging destinations
  qdm      Send a message to QDM client
  vty      Virtual terminal
  xsm      Send a message to XSM client

R1#send 1
Enter message, end with CTRL/Z; abort with CTRL/C:
Send message? [confirm]


*** Message from tty0 to tty1:

There is a way to send automatically some custom-made udp packets from a Cisco router to a specific destination, in order to emulate the heartbeat mechanism of SixXS. Tcl seemed like a nice option, but as far as i know its implementation in Cisco IOS doesn't support extensions (Tcl doesn't have a builtin command for udp channels, so we must use an extension to enable it).

Asynchronous Serial Traffic Over User Datagram Protocol or UDPTN (UDP Telnet) is an IOS feature that provides the ability to encapsulate asynchronous data into UDP packets, and then unreliably transmit this data without needing to establish a connection with a receiving device. UDPTN is similar to Telnet in that both are used to transmit data, but UDPTN is unique in that it does not require that a connection be established with a receiving device.

Its usage is quite simple. You just have to enable udptn as an output transport under your vtys and then open a connection to the remote end.

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#line vty 0 15
R1(config-line)#transport output ssh udptn

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#line vty 0 15
R2(config-line)#transport output ssh udptn

You have various options regarding the role of each device, but usually one end is transmitting and the other end is receiving. If you need 2-way communication, then you need to enable both directions. You can use any port above 1024 or just the default 57.

R1#udptn 3740 /transmit /receive
Trying, 3740 ... Open

R2#udptn 3740 /transmit /receive
Trying, 3740 ... Open

It becomes more interesting if you send packets to a multicast/broadcast address, so everyone having an open connection there will see the data.

There are some annoying things, like that you can't see locally the entered chars, or that you get chars on top of the previous chars (you can use spaces in order to clear the line), but you can't expect the full thing.

R2#udptn 3740 /transmit /receive
Trying, 3740 ... Open
How are you doing today?     ! This was typed on R1

R1#udptn 3740 /transmit /receive
Trying, 3740 ... Open
Fine, thanks                 ! This was typed on R2

Voila! You just made it possible to have a chat with your friend at a remote Cisco router! If you want to stop the session, you can use Ctrl-Shift-6 + x and then enter the "disconnect" command.

There are 2 terminal options that can be configured under source vtys and can change the behavior of text output:

dispatch-timeout 10000 : This one makes the packets be transmitted every 10 secs
dispatch-character 13 : This one causes the current number of typed chars to be sent after you press Enter (ASCII 13). By default each char is sent immediately.

Note : Because of its ability to send raw UDP datagrams that might conflict with other protocols, UDPTN has an implicit access list that only allows UDPTN connections to UDP port 57 (default) or UDP ports greater than 1024.

If only now i could find a way to send such messages automatically, i would probably solve my initial issue. EEM doesn't provide a mechanism to feed chars into a remote session and the Tcl "typeahead/exec" solution makes the process get stuck (and i can't find a way to clear it). Any idea how to send Ctrl-Shift-6 + x?

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