How does dhcp(d) choose an IP address?

Gregory P. Ennis PoMec at PoMec.Net
Sat Oct 13 22:53:07 UTC 2007


On Sat, 2007-10-13 at 22:15 +0100, Timothy Murphy wrote:
> I was reading the thread on "Determining IP information for lo"
> and it reminded of a strange phenomena I noticed on my system.
> 
> The laptop I am using now, which should be 192.168.2.11
> (according to /etc/hosts)
> has been given the address 192.168.2.101,
> although I don't see any mention of this address
> anywhere on the desktop running dhcpd,
> or on the laptop,
> except in /var/lib/dhclient/dhclient-eth0.leases .
> (The correct MAC address for the laptop, and its desired IP address,
> is given in /etc/dhcpd.conf , but for some reason this is ignored.)
> 
> I should say that WiFi works perfectly well with this wrong address.
> But it has made me realize that I am very hazy about how dhcp works,
> and how exactly dhcpd determines the addresses it will give out.
> Is there a reasonably clear account of this anywhere?
> 
> I saw incidentally that something or someone
> (I assume a yum update)
> had moved /etc/dhclient.conf to dhclient.conf.bak ,
> just leaving what looked like a fairly useless dhclient.conf.sample ,
> confirming my view that the people who write sample files for Fedora
> are somewhat lacking in common sense.
> 
> 
> 
My answer may be too simple but trying checking ifconfig on the root
account of your laptop to see what the mac address is on your ethernet
lan card as opposed to your wireless ethernet card.  Make sure your
dhcpd server has both mac addressed listed.  You can assign the same ip
address to each connection as long as you do not use both at the same
time.  If you need to use both at the same time you can use the dhcpd
server to assign different ip addresses to each connection.

Greg Ennis




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