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hardware bum

join:2004-01-26
State College, PA

DHCP troubles

I'm on Comcast and have a small home network. Router is an Asus RT-N66U behind a Motorola SB6121. The router DHCP server is configured to give out IP addresses in the 192.168.1.54 to 192.168.1.254 range. I have a few devices configured for static IP addresses between 192.168.1.2 and 192.168.1.20. The router is at 192.168.1.1. The modem is at 192.168.100.1

Lately, when any device not configured for static IP pulls an address, it's getting an assignment in 192.168.100.xxx range. Finally it hit me that the modem is also a DHCP server so, of course, this is causing problems.

I think this is a configuration problem in the router but I can't seem to figure out what might have changed. Any ideas?


Bink
Villains... knock off all that evil

join:2006-05-14
Castle Rock, CO
kudos:4

So long as the modem is plugged into the WAN port of the router, clients should never get an IP from the modem.



NetFixer
Freedom is NOT Free
Premium
join:2004-06-24
The Boro
Reviews:
·Cingular Wireless
·Comcast Business..
·Vonage
reply to hardware bum

I am not familiar with the Asus RT-N66U, but some SOHO routers have the ability to be setup as DHCP relay servers. You might look at the RT-N66U's setup to see if you see anything resembling a DHCP relay setting checked.
--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.


bdnhsv

join:2012-01-20
Huntsville, AL
reply to hardware bum

Are your modem, router and these other devices all plugged into a switch?


hardware bum

join:2004-01-26
State College, PA
reply to hardware bum

"So long as the modem is plugged into the WAN port of the router, clients should never get an IP from the modem."

That's what I thought, too.

"You might look at the RT-N66U's setup to see if you see anything resembling a DHCP relay setting checked."

I don't see anything like this in the RT-N66U configuration. (Not saying it's not there)

"Are your modem, router and these other devices all plugged into a switch?"

There is a downstream switch.
cable >> modem >> NT-R66U (wan port) >> fan out in several branches
Wiring has been stable for 6 months or more. This problem started about a month ago.


Bink
Villains... knock off all that evil

join:2006-05-14
Castle Rock, CO
kudos:4
reply to hardware bum

Are you certain there are no other DHCP servers on the network (assigning a range similar to that of the modem)?


tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1
reply to hardware bum

my comcast modem ip is 192.168.100.1 but it only has 1 network port on it and that plugs directly into my router WAN port which does DHCP.

i have only used the 192.168.100.1 address to web into the cable modem to check signal strength, i have never seen network/router options, i dont think it works that way with the device i have (it is just a gateway).

on my network router, i use 192.168.10 - .30 for anything static and DHCP starts at .100 and goes up to .150 and i have never had any of the issues you are describing.

i have worked on comcast cable installs where comcast issues the home their own router, but it doesnt sound like you have that.

in business environments the device they have been giving out (lately) is a cable modem/router, but most companies disable DHCP on that device and turn it into a gateway (sometimes a call to comcast is needed to confirm it is in gateway mode). in all of the business scenarios i have been involved in, the IP from the comcast device is 10.0.10.1, i forget what it is in the residential setups (if it isnt 192.168.100.1).



shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
reply to hardware bum

Did you clone a MAC address on one of the PC's for the modem WAN port?

If, so, i wonder if the router is getting confused with the same MAC on both sides.



NetFixer
Freedom is NOT Free
Premium
join:2004-06-24
The Boro
Reviews:
·Cingular Wireless
·Comcast Business..
·Vonage
reply to tomdlgns

said by tomdlgns:

in business environments the device they have been giving out (lately) is a cable modem/router, but most companies disable DHCP on that device and turn it into a gateway (sometimes a call to comcast is needed to confirm it is in gateway mode). in all of the business scenarios i have been involved in, the IP from the comcast device is 10.0.10.1

Actually, the SMC gateway boxes are in gateway mode by default, and it requires a Comcast tech with the proper access credentials to put it into bridge mode.

That is why I got rid of the SMC gateway I was using. Comcast techs kept resetting it back to the default gateway mode because bridge mode was considered non-standard. Whenever that happened, my network would be crippled until I could find a Comcast CSR who knew how to put it back into bridge mode (and was willing to go against Comcast's policy to force gateway mode). I know how to do it, but I no longer have the required access credentials since Comcast changed them.
--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.

tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1

said by NetFixer:

said by tomdlgns:

in business environments the device they have been giving out (lately) is a cable modem/router, but most companies disable DHCP on that device and turn it into a gateway (sometimes a call to comcast is needed to confirm it is in gateway mode). in all of the business scenarios i have been involved in, the IP from the comcast device is 10.0.10.1

Actually, the SMC gateway boxes are in gateway mode by default, and it requires a Comcast tech with the proper access credentials to put it into bridge mode.

That is why I got rid of the SMC gateway I was using. Comcast techs kept resetting it back to the default gateway mode because bridge mode was considered non-standard. Whenever that happened, my network would be crippled until I could find a Comcast CSR who knew how to put it back into bridge mode (and was willing to go against Comcast's policy to force gateway mode). I know how to do it, but I no longer have the required access credentials since Comcast changed them.

you are right, my mistake.

that is one thing i never understood about comcast, why not have the proper business equipment for business installs?

they should have a business only team. they do for fiber/metro-e, maybe that is their 'true' business team.

i like their fiber team...the tech shows up (after fiber has been ran) and sets up the hardware, confirms internet connectivity with my block of static IPs and then leaves. that is how it should be.

sorry for going off track.

hardware bum

join:2004-01-26
State College, PA
reply to shdesigns

No MAC cloning on my network and no other DHCP servers that I'm aware of. Decided that I'm going to do some troubleshooting tonight when I get home from work. I'll pull the cable between the modem and router and try to isolate the problem. Maybe bring the whole network down and then back up one device at a time while watching IP addresses.

I'll check back in with the results.



cablegeek01

join:2003-05-13
USA
kudos:1

said by hardware bum:

No MAC cloning on my network and no other DHCP servers that I'm aware of. Decided that I'm going to do some troubleshooting tonight when I get home from work. I'll pull the cable between the modem and router and try to isolate the problem. Maybe bring the whole network down and then back up one device at a time while watching IP addresses.

I'll check back in with the results.

Wireshark will be a big help here. Run it on one of your PCs and you'll see the DHCP replies as well as ARP requests from the DHCP server. You should be able to track down the offending device pretty quickly by the MAC address.


cablegeek01

join:2003-05-13
USA
kudos:1

Click for full size
arp or bootp filter applied
You can see that the motorola device (in my case a moto SB6580) is replying to the DHCP request from my laptop, as well as arping for addresses in the 192.168.0.0/24 network.
Comparing the source MAC address(Ethernet II) for the DHCP reply, or the ARP request to the MAC address of the PC NIC or the cable modem/router MAC address should help you find the offender.

cramer
Premium
join:2007-04-10
Raleigh, NC
kudos:9
reply to hardware bum

Simple troubleshooting steps... unplug the modem: does it still happen? unplug the router: does it still happen?

The 6580 is also a router with a wireless AP, btw.



NetFixer
Freedom is NOT Free
Premium
join:2004-06-24
The Boro
Reviews:
·Cingular Wireless
·Comcast Business..
·Vonage

said by cramer:

The 6580 is also a router with a wireless AP, btw.

???
said by hardware bum:

I'm on Comcast and have a small home network. Router is an Asus RT-N66U behind a Motorola SB6121.


--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.

cramer
Premium
join:2007-04-10
Raleigh, NC
kudos:9

cablegeek mentioned the 6580... nevermind.


hardware bum

join:2004-01-26
State College, PA
reply to hardware bum

OP here with an update:

Well, I found the rogue DHCP server! Turned out to be my Comcast RNG-150 cable box. This box doesn't need to be connected to the network so I just pulled the cable. Now I remember when I got the '150 I saw the ethernet port and just plugged it into the switch in my AV cabinet and forgot about it. Funny thing though, it must have been plugged into the network for more than 2 months before it started acting up like this.

I used netdiscover and dhcp_probe to track this down.



dnoyeB
Ferrous Phallus

join:2000-10-09
Southfield, MI
kudos:1
Reviews:
·Comcast
reply to hardware bum

ROFL, nice! I used to do that as well. It has a plug, maybe some magical behavior will start and I'll get free sh!t! Never happened. So eventually I got bored with it plugged in. Looks like it happened to you though
--
dnoyeB

"Then said I, Wisdom [is] better than strength: nevertheless the poor
man's wisdom [is] despised, and his words are not heard. " Ecclesiastes
9:16