An example of a little information can be a bad thing. I’d always thought that the ’service dhcp’ command enabled the dhcp server and client abilities of a device. It finally donned on me that that’s why my DHCP relay agent wasn’t working. Oops. Here’s an exerpt of the Cisco Command Reference:

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service dhcp

To enable the Cisco IOS Dynamic Host Configuration Protocol (DHCP) server and relay agent features on your router, use the service dhcp command in global configuration mode. To disable the Cisco IOS DHCP server and relay agent features, use the no form of this command.

service dhcp 

no service dhcp 

Syntax Description

This command has no arguments or keywords.

Defaults

Enabled

Command Modes

Global configuration

Command History

12.0(1)T

This command was introduced.

Usage Guidelines

The BOOTP and DHCP servers in Cisco IOS software both use the ICMP port (port 67) by default. ICMP "port unreachable messages" will only be returned to the sender if both the BOOTP server and DHCP server are disabled. Disabling only one of the servers will not result in ICMP port unreachable messages.

Elect DR/BDR:

Broadcast  (10/40)
Non-broadcast (30/120)
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Don’t Elect DR/BDR:

Point-to-point  (10/40)
Point-to-multipoint  (30/120)
Point-to-multipoint nonbroadcast  (30/120)

I can never remember which of this entries is which:

%SEC-6-IPACCESSLOGDP = ICMP packets have matched an ACL
%SEC-6-IPACCESSLOGNP = IP protocol but non-TCP and non-UDP packet have matched ACL
%SEC-6-IPACCESSLOGP = UDP or TCP packets have matched an ACL
%SEC-6-IPACCESSLOGRP = Routing protocol packets have matched an ACL
%SEC-6-IPACCESSLOGS = Standard ACL was matched
%SEC-6-IPACCESSLOGRL = ACL logging was rate-limited because of volume

I’d run across the TCL interpreter inside a Cisco router a few years ago and thought “Hmm, that’s cool.  I wonder who’d use that”.  Then, just the other day it hit me:  “There’s a freaking programming language interpreter in the router!”  I did a little googling around there’s a couple of good resources for it, but not very many.

• Cisco’s of course
• Peter Welcher’s site on TCL’ing your router
• Richard Bejtlich’s blog (is there anything the guy doesn’t do?)

I plan on firing up dynamips to see what I can do with it.  Maybe I can write an SMTP server or something.  Or netcat.

If there’s anyone out there who has had experience writing TCL scripts, especially on Cisco gear, let me know. I’d love to learn.