Re: DHCP
- From: "d" <kevindu28@xxxxxxxxx>
- Date: 18 Dec 2005 21:18:54 -0800
Using ethereal has help me discover the source and destination of these
errand DHCP packets. The source address is a laptop machine on the
network that has an address of 192.168.0.100. The destination address
is 255.255.255.255. ethereal also reports that the MAC address of the
destination is [ ff : ff : ff : ff : ff : ff ]. The laptop is a
WIndows XPSP1 machine. Lots of other information in the output
sections of ethereal. Nothing that jumps out to me as a problem. So I
guess I have to keep on digging.
prg wrote:
> [snipped/moved top post to bottom]
> > prg wrote:
> > > d wrote:
> > > > I expect to see these messages.
> > > >
> > > > dhcpd: DHCPINFORM from 192.168.0.111 via eth0
> > >
> > > Why do you expect this? DHCPINFORM is used only if the client has
> > > already obtained an IP address by some other (non-dhcp) means.
> > >
> > > > dhcpd: DHCPACK to 192.168.0.111
> > > >
> > > > The messages that appear like below make no sence.
> > > >
> > > > dhcpd: DHCPINFORM from 4.0.0.0 via eth0: unknown subnet 0.0.0.0
> > >
> > > This indicates that the client _has_, by some mysterious means,
> > > obtained the IP 4.0.0.0. The dhcpd server, if it made sense, would
> > > return a unicast (since client has an IP) with other condig params. It
> > > does _not_ make sense, thus the error message.
> > >
> > > >
> > > > Please help me resolve them if there is a problem with my dhcpd
> > > > configuration.
> > >
> > > I don't think it's a dhcpd.conf problem -- looks OK to me, though why
> > > use dhcp for just two hosts is beyond me.
> > >
> > > See RFC2131 for info on DHCPINFORM:
> > > http://www.faqs.org/rfcs/rfc2131.html
> > >
> > > I would run ethereal on the server and catch all the packets exchanged
> > > between the client and server. Also check the client for a clue as to
> > > why/how it is obtaining this IP _before_ sending its dhcp request.
>
>
> d wrote:
> > First, I'm just trying to get an understanding about how DHCPD works
> > within Linux. Thus, I have two Windows XP clients to play with and
> > therefore configured to obtain addresses from the server. (Start small
> > and work up)
>
> I understand. But beware that Windows has a few wrinkles compared to
> the way Linux boxes will behave. To really understand what's going on
> -- especially with dhcp -- using ethereal to examine the packet
> exchange will be invaluable help. Combined with the RFC 2131 you
> should catch on pretty quickly to how it works.
>
> > When I saw the 1st message, I thought it was just the client
> > (192.168.01.111) confirming to the server about its present on the
> > network and that its IP is valid.
> >
> > Please the explain the term "return a unicast"?
>
> When the client first "contacts" the dhcpd server, it does not know
> (normally) just where the server is, ie., it does not know the IP
> address of the server, so it _broadcasts_ its initial packet. If the
> client does not have an IP address (yet) the server would respond with
> a _broadcast_ also (it works because the server has learned the
> client's MAC/nic hardware address).
>
> A unicast packet is directed to a single ( ie., uni-) host and it
> implies that the host has a "working" IP address. Most all normal
> packets are unicast -- ie., directed from one host to another single
> host.
>
> > Thanks for the FAQ, I'll give it a read.
>
> You will almost certainly need some MS docs or XP how-tos to work
> through any MS wrinkles.
>
> http://www.microsoft.com/resources/documentation/Windows/XP/all/reskit/en-us/Default.asp?url=/resources/documentation/Windows/XP/all/reskit/en-us/prjj_ipa_xhku.asp
> http://www.google.com/search?q=windows%20xp%20dhcp
>
> The question remains how the client acquired an IP of 4.0.0.0 (a class
> A network's address -- not normally assigned to _any_ host). Something
> very wrong here ;-)
>
> hth,
> prg
.
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