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 From:  Fred Wright <fw at well dot com>
 To:  m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] Cable modem issue with m0n0wall
 Date:  Wed, 4 Aug 2004 23:01:17 -0700 (PDT)
On Wed, 4 Aug 2004, Justin Ellison wrote:
> 
> Just getting in on this thread - forgive me if I missed something...
> 
> If your cable modem is not coming online, then it has nothing to do with
> m0n0wall, or any other box behind it.  T3 timeouts mean that the modem
> has lost communication from the CMTS, and it basically reboots and tries
> to re-register.  

True, and to further confuse things, some cable modems (including this
one) have a built-in DHCP server to provide a *local* IP to access the
modem itself.  This normally defers to the "real" DHCP server once the
link is established, but in the meantime it can give the illusion that you
have ISP connectivity when you really don't.

> On Tue, 2004-08-03 at 18:45, CygnusX1 wrote:
> > I just came across some info that this may be a problem with the DHCP 
> > client in FreeBSD/m0n0wall...
> > 
> > http://www.chetnet.co.uk/portal/forum/archive/index.php/t-2650

This looks like someone else having the same misconecption.

On Wed, 4 Aug 2004, Justin Ellison wrote:

> From what I can tell, your internet connection is dropping - correct? It
> works for awhile and then stops?  When your connection drops, is your
> modem online?  Look at the LED's on your modem when the connection is
> down, and see what the manual says about the modem's status at that
> point.  Your modem's log indicates that it is loosing sync with the CMTS
> - nothing m0n0 or any other computer can do to cause or fix that.  That
> said, there are a few things to try.
> 
> 1)  The ICMP packets are just the DHCP server pinging the address to see
> if it's available - no worries there.  However, you are spoofing a MAC
> address on the WAN.  The problems you are seeing could happen if someone
> else has statically assigned the IP address to their PC/Router that is
> being handed out to you by the DHCP server.  If Comcast is doing things
> right, they stop this, but many cable providers don't.  The simplest

Comcast doesn't block much of anything.  Here I get several packets per
second of ARP and DHCP traffic related to other customers.  I'd be very
surprised if answering one of those ARPs didn't work. :-)

> solution if this is the problem?  Spoof the mac address of the XP
> machine - this will give you the same IP as your XP machine had - verify
> it to make sure.

Better yet, don't spoof anything.  Comcast doesn't care what MAC address
you use, and they don't guarantee a static IP even if you keep the same
MAC address.

> 2)  You mention storms.  Many times, water can get into the splices
> between the pedestal and your modem, which causes a lot of ingress. 
> What are the signal levels on the modem?  I'm 99% sure that the problem
> is with your signals, as those T3 time-outs are warnings about a loss of
> signal from the coax port on your modem to the upstream port on the
> CMTS.  Log the time from which your connection drops, and the times that
> it comes back.  Compare those times to the T3 timeouts (modem goes down)
> and the "registration complete" messages on the modem.  If they match
> up, call Comcast and have them dig into why your modem is going offline
> so much.

And around here they have lots of AC power leakage onto the cable, though
I've seen no evidence that it actually screws up communication.  The
open-circuit voltage on my incoming cable shield is about 40VAC, with a
source impedance of about 18k.  Not enough to get electrocuted, but enough
to give a nasty surprise.  Though they didn't bother fixing it even after
their own tech got zapped. :-) I decided to write them off and installed a
ground isolator in series with the incoming cable, and then grounded *my*
side of the isolator.  I've heard reports of some Comcast customers
getting better TV reception after installing ground isolators, although I
didn't really notice a difference here.

Also beware that an RG-59 center conductor sometimes makes marginal
contact in an F connector, especially when RG-6 has been previously
used.  It's better to stick to RG-6, although it's stiffer.

					Fred Wright