Saturday, July 23, 2005

Internal DNS


I just noticed something interesting.  Since we use DHCP in out network, I was curious what ip address were allocated to the devices on my network.  So I ran the following command:



nmap -sP 192.168.1.0/24


I noticed something interesting.  A lot of the machines have fully qualified domain names.  I was curious why this was happening, especially since I didn't create these entries.




About a week ago, I updated to the latest version of the sveasoft firmware for my Linksys WRT54GS.  It looks like when this version allocates an ip address, it takes note of the host name and then creates a dns entry for it. This works perfectly if the client using dhcp does not specify dns servers, then the computers will default to using the router as the dns server.


This is great because now I can use a host name to access any machine that gets it's ip address from dhcp.


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1 comment:

  1. I was doing some troubleshooting on a friends small home/office network, and noticed a similar thing.
    He just has a Mac, a PC and an ADSL Modem/router, that does DHCP. Because he didn't have static IP addresses, the computers were unable to hold onto SMB connections for longer than a couple of days (which I later worked out to be the expiry time for the DHCP lease).
    What I didn't realise, until after working for about 1 1/2 hours, was that the modem also did DNS, and automatically added an entry for a machine that asked for a DHCP address with whatever that machine thinks it should be called.
    So, instead of having to try 10.0.0.x a heap of times, or use a static IP address, he just needed to remember that the PC was called 'Stephen' , and the Mac was called 'Robert', or something similar.
    So, I could have saved heaps of time, just by trying a hostname (not fully qualified, but) rather than screwing around with DNS settings, DCHP leases and hostname files.

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