On Thu, 6 Jan 2000, M. Smith wrote:
> Over the past two months or so, I've been experimenting with Linux RH
> 6.0 as a gateway/firewall machine. I permanently switched to the Linux
> firewall about a week ago. One thing I've noticed during
> experimentation and since the switch is that the Linux machine gets a
> different IP address everytime it boots.
I have noticed this too. Here's the explanation I can provide:
OK, first the easy one. Linux asks for a "lease" everytime eth0 (or
whatever) is brought up. DHCP server assigns the next available IP address
(which is very likely different from the last one your machine's got).
Windows machines however, "remember" their last IP address. Upon startup
they send a "renewal" request to the DHCP server (rather than initiating a
new lease).
If the IP is available, it is re-assigned to the machine with a DHCP "ACK"
packet.
If the IP is not available, a "NACK" is sent. Then the windows machine
asks for a new lease, and the server provides an IP.
Using the above method has a couple of distinct advantages:
1) the machine gets the same IP address every time (provided the IP is not
assigned to somebody else).
2) if the server is not available, the Windows machine keeps the IP
(until expiry of course).
The second point above is very improtant for mobile users, because if you
are on the road, services (like web servers) do not hang waiting for their
IP address. I know this because if my laptop is not connected, I don't
even bother booting Linux. RedHat 6.1 startup scripts seems to address
this startup problem.
In addition, if someone reboots his workstation while a DHCP server is
temporarily unavailable, he won't get stuck.
cheers,
Hossein
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