Thanks Hal!

[Gerald Combs]

Brian Kelsay wrote:
> Gerald Combs wrote:
> 
>> bam:/home/gerald> telnet kclug.org 25
>> Trying 139.146.133.42...
>> Connected to kclug.org.
>> Escape character is '^]'.
>> 220 kclug.org ESMTP Postfix (Debian/GNU)
>> quit
>> 221 Bye
>> Connection closed by foreign host.
> 
> 
> Umm, why is telnet running/responding on the server?

I'm not sure if telnet is running on the server, but Postfix is
responding on port 25.  Most telnet clients allow you to specify a port
number after the host name.  If you specify a non-standard port from
Telnet's perspective (not 23), the telnet client just opens a direct
("raw") connection (normally it tries to exchange environment
information upon connecting).  At this point you're just pushing bytes
at the server, like any other TCP client.  Since a bunch of popular TCP
protocols are text-based, this means you can interact with the server
manually.

This has several uses, including the ability to apply Occam's razor in a
troubleshooting situation.  If someone is having trouble connecting to a
web server, you can run

----
bam:/home/gerald> telnet www.cnn.com 80
Trying 64.236.16.52...
Connected to www.cnn.com.
Escape character is '^]'.
HEAD / HTTP/1.0
Host: www.cnn.com

HTTP/1.1 200 OK
Server: Netscape-Enterprise/6.1 AOL
Date: Fri, 27 Aug 2004 04:13:04 GMT
Last-modified: Fri, 27 Aug 2004 04:13:05 GMT
Expires: Fri, 27 Aug 2004 04:14:05 GMT
Cache-control: private,max-age=60
Content-type: text/html
Content-length: 0
Connection: close

Connection closed by foreign host.
----

In this case, we know that CNN is up, so we might want to start looking
for problems on the browser end.

Other examples:

POP
----
xxxx:/home/gerald> telnet xxxxxxx 110
Trying xxxxxxxxxx...
Connected to xxxxxxx.
Escape character is '^]'.
+OK POP3 xxxxxx 2003.85 server ready
user xxxxxx
+OK User name accepted, password please
pass xxxxxx
+OK Mailbox open, 1188 messages
quit
+OK Sayonara
Connection closed by foreign host.
----

NNTP
----
bam:/home/gerald> telnet news-server.kc.rr.com 119
Trying 24.94.170.87...
Connected to news-server.kc.rr.com.
Escape character is '^]'.
200 Powered By Twister - Road Runner - Kansas City, MO (Twister v2.0.6.342)
group rec.bicycles.tech
211 2322 335670 338086 rec.bicycles.tech
quit
205 GoodBye
Connection closed by foreign host.
----