More on DDNS

Murray Taylor murraytaylor at
Wed Jun 11 00:22:37 PDT 2003

An understanding of DHCP is in order..
With the dial up, the ip number would most likely be
(a) dynamic - in that you would get a different one each time
(b) static - in that it would not change for the duration of the

On Cable though, the expectation of the ISP is that you may well stay
connected for longer (infinite) durations. So they use DHCP to manage
their address space and the ip number that they are loaning you
(on the external NIC)

Any DHCP allocated address has a lease-time associated with it.
Usually, when your machine detects that the lease is gonna expire
it 'applies' for a renewal. 

_NOTE_ the isp doesnt tell your machine to expire the lease, _your_
machine determines that the lease has expired.

If the ISP's DHCP server allows, you will often get your same 
ip number back. However this _is not_ assured.

So ...
IF the lease expires 
AND you get allocated a new ip number
THEN you need to do clever things with your DDNS server.

(and maybe firewall etc etc.) The normal dhclient processing
will take care of such things as default routes, /etc/resolv.conf
and the like)

The process is fairly transparent in most cases. It is just
us lot who want external visibility via DNS, (or have grunty
firewalls) that need to do anything special on a DHCP renewal 
that actually changes the ip number.


On Wed, 2003-06-11 at 16:52, Jay Buhrt wrote:
> Well see. This is weird. I was on dial-up and my ip
> would change everytime I logged on, but I configured
> DHCP to BSD and got a 168.x I think or some private
> address. The BSD box never changed ip's even on
> dial-up and they are networked xp - bsd. So now if
> comcast sends out a dhcp or whatever to change ip's
> will my bsd box change? or like 5 mins before the
> expiration on dhcp unplug bsd box from the switch so
> comcasts assigned ip's don't touvh bsd box and then
> plug it back in, lol ???
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Murray Taylor
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