[ On Monday, August 5, 2002 at 10:19:11 (-0700), Johnny C. Lam wrote: ]
> Subject: Re: Cable Modem and Firewall/IPNAT
>
> On Mon, Aug 05, 2002 at 11:55:55AM -0400, Steve Bell wrote:
> > 
> > Just looking for more information if possible. I am running into some 
> > problems with my cable modem, it's not picking up an IP from my service 
> > provider via DHCP. I have tried both empty/non exsiting and configured 
> > dhcp.conf files, but after running dhclient ex0  I get six attempts, and 
> > a final message saying that no DHCPOFFERS were received.  Any general 
> > tips about getting the modem to drop it's current lease, and actively 
> > look for a new one?
> 
> Have you tried power cycling the cable modem?  That usually does it for
> my cable modem.

That's what sometimes worked with my Terayon (proprietery) model.
However not always.

However note that it's not the modem which has a lease.

With most cable modem systems it's only your machine which has the lease
(and the DHCP server which records it).

With DOCSIS modems (1.x anyway), the modem might also have a lease, but
you don't know about it and you don't care about it -- it's for the
WAN-side interface only and will probably (hopefully) be a private IP#
that you can't talk to (from either the CPE port(s), or the Internet)
even if you know what it is (and not even if you can arrange routing to
it -- it should be filtered so only the operators equipment can reach
it and vice versa).

The same goes for the older proprietary LanCITY modems (from which the
DOCSIS standard was originally derived).  They're really just a bridge.
Their management agent is like an independent computer that sits inside
the modem and what it does is independent of what the customer computer
does.

On the other hand the terayon proprietary protocol does some nasty DHCP
proxying in the head-end gateway.  Depending on how the head-end is
configured to operate the lease given to the customer equipment will be
"locked" into a table (and associated with an ARP table entry).  The
last time my modem locked up it took over 24 hours before the head-end
tables were cleared, and I don't know if that happened automatically or
if there was manual intervention.  I probably could have escalated a
support call to get them to speed that up, but I didn't care at the time
(and I no longer have a cable modem).  On the other hand the terayon
gateway can also be configured to allow the customer equipment to get an
automatic lease and ARP entry in the gateway for any arbitrary address
(or even multiple addresses), and that one normally lasts for two
minutes IIRC.  Power cycling the modem won't always clear the ARP and
DHCP proxy entries associated with it in the gateway.  Finally the
gateway can also be configured to give a specified modem one or more
static addresses, and no amount of, power cycling, DHCPing, or waiting
will bypass those.

-- 
								Greg A. Woods

+1 416 218-0098;            <g.a.woods@ieee.org>;           <woods@robohack.ca>
Planix, Inc. <woods@planix.com>; VE3TCP; Secrets of the Weird <woods@weird.com>