Cisco: TCP and UDP small servers

Do you like Linux with all this services that you can enable on the fly when you want to test something? I know that I really like Linux boxes. But what about Cisco? Well Cisco supports also some services to be enabled for testing purposes. This are called “TCP and UDP small servers”.

Maybe I should start by telling you that there are never ending discussions about this servers, whenever they should be enabled and how to protect the access to them,  since this is still an open security issue which can attract and attacker. In my opinion you cannot keep them close and also running some tests (that require certain ports to be open) in the same time, but let open access to them is not a solution either. So, what can you do, is to enable TCP and UDP small servers only when you need them for testing and then disable these services. Another solution that I see, is to let this services all the time enabled, but to use some security tools (e.g. access-lists on external ports) to reduce the amount of hosts that can access them. In this way you let them accessible only for the hosts from where you want to be. And the third solution is not to enabled them at all, but then you cannot test anything. It’s like the story when a computer is safe? When you destroy the hard-drive and look the device into a safe. Good, the computer is secure, but you cannot use it anymore, so what’s the point of doing this?

But back to our discussion, TCP and UDP small servers are servers (strange phrase, I know) that run in the router which are useful for diagnostics.

TCP small servers:
Echoes back whatever you type through the telnet x.x.x.x echo command.
Generates a stream of ASCII data. Use the telnet x.x.x.x chargen command.
Throws away whatever you type. Use the telnet x.x.x.x discard command.
Returns system date and time, if it is correct. It is correct if you run Network Time Protocol (NTP), or have set the date and time manually from the exec level. Use the telnet x.x.x.x daytime command.

UDP small servers:
Echoes the payload of the datagram you send.
Silently pitches the datagram you send.
Chargen: Pitches the datagram you send, and responds with a 72-character string of ASCII characters terminated with a CR+LF.

In addition to the one above, the Cisco devices also offers finger service and async line bootp service, which you can independently turn on / off.

For this presentation I will use 2 point-to-point connected routers named RT-TEST-CLIENT ( /30) and RT-SERVERS-ENABLE ( /30). I will enable TCP and UDP small servers on one of them and test from the other one. Please click the image below to see the video presentation:

TCP and UDP small servers

If you cannot see the Flash movie above please consult this text document which explain how to enable TCP / UDP small server and how to test them.

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Calin is a network engineer, with more than 20 years of experience in designing, installing, troubleshooting, and maintaining large enterprise WAN and LAN networks.

2 thoughts on “Cisco: TCP and UDP small servers”

    1. How do u stop the chargen message in order to get back to the Global configuration mode of the router

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