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Education => Home and Small Office Networking => Topic started by: icecream-guy on June 09, 2021, 09:01:51 AM

Title: neighborhood cable modem handing out DHCP
Post by: icecream-guy on June 09, 2021, 09:01:51 AM
Someone in my neighborhood has misconfigured a modem, that hands out DHCP address

we had a power failure this morning, and I keep getting IP addresses from this modem,  and I cannot access the internet.

it is a Netgear device

how can I get around this issue,  I do not know who in the neighborhood has the device.
Title: Re: neighborhood cable modem handing out DHCP
Post by: deanwebb on June 09, 2021, 02:35:46 PM
I'd go with static IP that goes with your own network range, with DNS and default gateway configured.

It would take at least 2 receiving stations with the ability to measure signal strength to triangulate where the bad modem happens to be.
Title: Re: neighborhood cable modem handing out DHCP
Post by: Otanx on June 09, 2021, 04:39:36 PM
Do you have your own cable modem, or do you share? It would be really weird for you to get leases from a neighbor unless there is a bridge between the networks besides the cable modem.

-Otanx
Title: Re: neighborhood cable modem handing out DHCP
Post by: deanwebb on June 10, 2021, 11:17:11 AM
Quote from: Otanx on June 09, 2021, 04:39:36 PM
Do you have your own cable modem, or do you share? It would be really weird for you to get leases from a neighbor unless there is a bridge between the networks besides the cable modem.

-Otanx


Or if the neighbor really cranked up the gain on his wireless side.
Title: Re: neighborhood cable modem handing out DHCP
Post by: Otanx on June 10, 2021, 11:23:32 AM
Quote from: deanwebb on June 10, 2021, 11:17:11 AM
Or if the neighbor really cranked up the gain on his wireless side.

That would be a different SSID, and shouldn't cause issues unless you joined that network. If this is a shared SSID then like Dean said good luck triangulating the signal. The easier way would be to setup a system to keep requesting IPs with different MACs from their DHCP server untill you exhaust the configured pool.

-Otanx
Title: Re: neighborhood cable modem handing out DHCP
Post by: deanwebb on June 10, 2021, 03:02:21 PM
Quote from: Otanx on June 10, 2021, 11:23:32 AM
Quote from: deanwebb on June 10, 2021, 11:17:11 AM
Or if the neighbor really cranked up the gain on his wireless side.

That would be a different SSID, and shouldn't cause issues unless you joined that network. If this is a shared SSID then like Dean said good luck triangulating the signal. The easier way would be to setup a system to keep requesting IPs with different MACs from their DHCP server untill you exhaust the configured pool.

-Otanx


Such tools do exist, and you don't even have to speak Russian to use them!
Title: Re: neighborhood cable modem handing out DHCP
Post by: icecream-guy on June 10, 2021, 03:03:36 PM
my firewall is wired to the network. my firewall is connected to modem is picking up IP address.  modem is connected to cable. cable is a shared medium, no wireless.

Title: Re: neighborhood cable modem handing out DHCP
Post by: Dieselboy on June 10, 2021, 09:57:15 PM
I was also thinking like Otanx said but my post was not sent yesterday for some reason.

But also I am thinking along these lines: Cable = DOCSIS (COAX) at your premises? Like: COAX from the street -> DOCSIS modem -> your firewall. Can you check if the docsis modem is netgear?
Title: Re: neighborhood cable modem handing out DHCP
Post by: icecream-guy on June 13, 2021, 12:15:01 PM
Quote from: Dieselboy on June 10, 2021, 09:57:15 PM
I was also thinking like Otanx said but my post was not sent yesterday for some reason.

But also I am thinking along these lines: Cable = DOCSIS (COAX) at your premises? Like: COAX from the street -> DOCSIS modem -> your firewall. Can you check if the docsis modem is netgear?

my modem is netgear, and the offending device is also netgear

Mine ( i pulled this from my modem GUI)
8C:3B:AD:DB:7D:28

offender
outside 192.168.100.1 2c30.33de.ad02

I guess that doesn't really prove anything, since the DOCSIS Modem does not have IP,  probably a wireless Netgear device on other side of remote modem handing out IP.  I can't really tell what DOCSIS Modem is on offender side.



Title: Re: neighborhood cable modem handing out DHCP
Post by: Dieselboy on June 13, 2021, 11:09:44 PM
I think the offending device is your local docsis modem. Does it have a sticker on it that you can confirm the case? Like a name of manufacturer or mac address?
Title: Re: neighborhood cable modem handing out DHCP
Post by: icecream-guy on June 14, 2021, 09:39:41 AM
Quote from: Dieselboy on June 13, 2021, 11:09:44 PM
I think the offending device is your local docsis modem. Does it have a sticker on it that you can confirm the case? Like a name of manufacturer or mac address?


NETGEAR CM1000
the modem is a pass throuhg device, there is not IP assigned

mac provided was not my modem, it was my AP.
Title: Re: neighborhood cable modem handing out DHCP
Post by: icecream-guy on August 11, 2021, 06:18:19 PM

See private area for packet capture to protect my IP