Small servers

calendar Mar 26, 2011 · 2 min read · networking  ·
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A quick note about tcp small servers.

DOC-CD says:

The TCP small servers consist of three services: 
Discard (port 9), Echo (port 7), and Chargen (port 19).

If we do a portscan to a router before and after enabling tcp-small-server with the command:

R(config)#service tcp-small-servers

We can see that these ports are opened:

Starting nmap V. 3.00 ( www.insecure.org/nmap/ ) Interesting ports on (10.1.0.254): 
Port State Service 
7/tcp open echo 
9/tcp open discard 
13/tcp open daytime 
19/tcp open chargen

The DOC-CD misses port 13 corresponding to Daytime Protocol. As Fox Mulder used to say:

trust no one

Let's check if it works:

telnet 10.1.0.254 13 

Trying 10.1.0.254... 
Connected to 10.1.0.254. 
Escape character is '^]'.

Saturday, March 26, 2011 18:09:30-ROME 

Connection closed by foreign host.

Great! IOS tells us the time.

The chargen port:

telnet 10.1.0.254 19

Trying 10.1.0.254...
Connected to 10.1.0.254.
Escape character is '^]'.

!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefg !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefgh "#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghi #$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghij $%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijk %&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijkl

The echo port is quite funny, it send us back whatever we send it:

telnet 10.1.0.254 7 

Trying 10.1.0.254... 
Connected to 10.1.0.254.
Escape character is '^]'. 

hi!
hi!
hello!
hello!

The traffic sent to the discard port is simply discarded, used for testing:

telnet 10.1.0.254 9 

Trying 10.1.0.254... 
Connected to 10.1.0.254. 
Escape character is '^]'. 

junk junk trash

UDP small servers work as TCP, just over a different transport protocol.

HTH