From: John Beckett 

Randall Parker

wrote in message news::
> [...too many questions!...]

I've noticed some DHCP servers in modem/routers etc use short periods like
30 minutes for the lease time. Seems ultra stupid to me. A normal DHCP
server offers a lease of something like 8 days.

The DHCP server can provide T1 and T2 options when giving the settings to
the client. You can use Wireshark to capture the network traffic to see if
that is happening, although I wouldn't think it's really worthwhile.

The default T1 is half the lease time, and T2 is 87.5% (? guessing from
memory). After T1, the client will unicast a Request to the DHCP server
that issued the lease, and the server should Ack to extend the lease.

Lease renewal is when the client sends a Request (DHCPREQUEST).

A Windows computer broadcasts a Request when restarting. If there is no Ack
and if the DHCP server previously issued a default gateway (router), the
client pings the router. If there is a reply, the client assumes everything
is ok and continues using its IP settings. It will retry to contact the
DHCP server.

> Are the rules for the above any different when using ethernet versus, say,
> RNDIS over a USB bus?

No idea, but you can be pretty sure that DHCP works in a fixed way and
won't have any idea what weird kind of network of interface is being used.
It's also quite possible that a USB gadget could give all sorts of
problems.

You should attack this sort of thing from a different angle. At the Windows
client, at command prompt:
  ipconfig /all

If that shows IP settings that are sensible, then the client should work.
If it doesn't (e.g. if it shows APIPA like 169.154.x.x) then it won't work.

Only worry about DHCP if 'ipconfig /all' shows junk settings.

John

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