Is my cable modem hacked?

[Gman737]

I have observed some strange behavior in my home internet service. I am starting to wonder if my network was hacked, or maybe it's just a ISP issue. Here is my equipment:

ISP: Comcast
Modem: Motorola SB6121 (I own it)
Router: Netgear WNDR3700
Laptop: HP DV7t running Windows 7 Pro
Cell Phone: Sony Xperia Z3c running 5.0.2 (same behavior observed on my old HTC M7)
Tablet: Apple iPad Air

The biggest issue that that my cell phone increasingly does not connect to the internet when using my home wifi. If I use another wifi network or cell data it works fine. It seems to be selective - sometimes only certain apps don't work (Facebook and Chrome browser won't connect but I get email), sometimes nothing works. Sometimes it just lasts a few minutes, sometimes hours. Here's the weird thing: if I can get my phone logged into my VPN service (sometimes I can't get enough connectivity to login), everything instantly works fine. Facebook wasn't working a few seconds ago, now no issues at all under VPN. I had a HTC One M7 and it had the exact same behavior.

My laptop usually works ok. Sometimes I have internet connectivity issues when using a browser. If I'm downloading files or playing online games there are no connectivity issues. If I reboot the modem and router it usually starts working again. Again, if I use VPN there are no issues.

My iPad connects with no issues. I have a Roku box for TV and no issues either.

I first thought that my Netgear router was hacked. I looked at the logs and don't see any unknown devices that were logged in. I changed the admin password. I have a long random WPA2 password. Any other suggestions?

Next I plugged an old D-Link DGL-4300 router into the modem. This is when things got really interesting. I can't get internet access with the D-Link. In fact, within a couple minutes of being connected to the cable modem, I can't access the D-Link from my end, wired or wireless. I can't get to the admin screen, nothing. If I unplug the D-Link from the cable modem and reboot, it works fine. As soon as I plug the cable modem back in, the same thing happens.

I tried accessing my cable modem's admin screen. It seems that Comcast has things locked down from their end, I can see settings and such but can't change anything.

I called Comcast, and all they do is make me direct wire my cable modem to my laptop and reboot the modem. As long as that works (and it usually does), it's not their problem as far as they're concerned.

My next step is to buy a new cable modem. But I'm worried that if the old one got hacked, the new one will get hacked too. Before I spend the money I wanted to see if anyone had other ideas.

 

[Alabalcho]

It seems you have some DNS problems, in the router, phone/laptop, or both.
Reset your router' DNS to use Comcast defaults (75.75.75.75), and try again.

 

[Gman737]

Thanks for the quick response. I looked up the DNS settings on my router and they were set up correctly, primary 75.75.75.75, secondary 75.75.76.76

I changed my phone to a static IP with the recommended DNS settings. I'll let you know if I notice a difference.

 

[riahc3]

Yup, sounds like a DNS issues to me.

Another thing, you attached a D-Link router to a modem? That really makes no sense. At BEST, you would attach the modem to the D-Link router, DISABLE THE DHCP SERVER ON THE MODEM, and then continue on your business. You problably made things worst attaching those two together.

It seems you have a DNS issue. Most likely on your end. To confirm you can setup a static IP on your PC and static DNSs 8.8.8.8 and 8.8.4.4

Also, to rule out any malware on your devices (Ive seen someone who was able to infect all their ARM and x86 devices...), run a LiveCD on your PC and test that out. It would also give a thumbs up that your NIC is in working order.