Need help with while loop/ARP script in bash

wolverene13 · 06-13-2011, 10:24 PM

Hi guys,

I am trying to write a script wherein bash reads from a list of IP's, pings them to generate an ARP entry for any nodes with those IP addresses, and then finds a particular entry based on MAC address. Why, you ask? Well, I have a Cisco WAP that pulls its address via DHCP and I'd like to be able to update its DNS entry in BIND dynamically, but it's not capable of that as far as I know. So, I figure I can obtain its IP via ARP (by using the script) for use in another script that will dynamically update my DNS files. Here's what I have so far, but it doesn't seem to work right. "localips.txt" is the list of possible DHCP addresses, "arpentries.txt" is going to be the file that is generated that contains the ARP entries that are generated, and "00:0b:5f:5f:03:bb" is the MAC of the WAP. The output will go to "tsunamisip.txt", which should ultimately be the IP of the WAP. Any ideas? Thanks in advance!

Code:

#!/bin/bash

cat localips.txt | while read host
do ping -c 1 $host && sudo arp $host | tee arpentries.txt
cat arpentries.txt | grep 00:0b:5f:5f:03:bb | cut -c 1-11 | tee tsunamisip.txt
done

binary_pearl · 06-13-2011, 11:32 PM

My knowledge of IOS is very limited, but do you have the capability of using perl or python? The complexity of what you are trying to do scripting-wise might be easier in one of those languages, assuming you have the time to learn one of them if your not already familiar.

Another thought, check out:
http://dag.wieers.com/howto/bits/bind-ddns.php

I believe `nsupdate` is used to dynamically update BIND. If that works, you might be able to avoid having to script your own solution.

--Shaun

grail · 06-14-2011, 12:11 AM

ummm ... I do not use arp but from examples I have found, it seems to say the ip address would be in the second column and not the first, as indicated by your cut?? I will use this format for my example, but you can easily change $2 to $1 in the awk:

Code:

while read -r host
do
    ping -c1 -q $host &>/dev/null && sudo arp $host | awk '{print $2}' >> tsunamisip.txt
done<localips.txt