Comcast Business SMC (static IP) and Untangle issue

[SirBC]

I have been successfully running Untangle in router mode behind my Comcast SMC modem (bridged with a static IP) for years but I'm starting to have some issues that I can't pin down.

I have the Comcast modem with DHCP turned off and the firewall turned off. Untangle handles DHCP (192.168.1.x) and DNS. I have a mix of wired and wireless devices (Access point connected via LAN to Untangle, DHCP turned off on the AP).

Just recently I couldn't print to my IP printer which has never been an issue in the past. While troubleshooting I realized my computer IP (wired connection) now had a 10.1.10.x ip. It must have somehow pulled it from the comcast modem, even though I've confirmed that DHCP is turned off on the comcast modem. The only way to get it to connect to the Untangle 192.168.1.x subnet is to assign a static IP. Other devices on the network show an IP from Untangle. When I log into the comcast modem and look at connected devices, it only shows the MAC address of Untangle.

I just purchased a new tablet and had the same issue when connecting to the wireless network; it's pulling a 10.1.10.x ip.

Is there something obvious that could be causing this? I'm at a bit of a loss as to why some of my devices are now getting an IP from the comcast modem when DHCP is turned off there while the Untangle box is turned on.

[dmorris]

I would start by using tcpdump/wireshark to see what device is providing DHCP.

You could also avoid it probably by turning on 'DHCP Authoritative' in config > network > advanced > options.
thats a hack though, as you'll still have a rogue dhcp service on your network.

[SirBC]

I ran Microsoft's Rogue DHCP server checker and it lists both Untangle and the Comcast modem as DHCP servers, so I guess something is flaky with the Comcast modem.

[engine411]

I ran Microsoft's Rogue DHCP server checker and it lists both Untangle and the Comcast modem as DHCP servers, so I guess something is flaky with the Comcast modem.

I have the same Comcast setup as you. In my experience, a "flaky Comcast modem" is a dime a dozen. We've had them replaced often. Usually Comcast doesn't blink an eye if I complain about the type of issue you're experiencing, they just replace it.

We started using a 2nd ISP, wireless, because of this. VOIP is actually running completely on the wireless ISP. That's another stroke against Comcast, they do not prioritize SIP packets unless it's from their SIP service. Would love to leave, but FIOS or other compelling alternative is not available here.

[sky-knight]

I'd just go buy a Motorolla surfboard, or a Cisco DOCSIS modem that doesn't have an internal router and make them assign it to your account. Integrated ISP router / connection doodads don't have a good history. Just look at all the trouble DSL ISPs cause with their all in wonder NAT messes. If you can get them into a bridge mode you're good but why should you have to do all that?

Business class delivery should be a simple layer 2 device from the get go... put the public address on your equipment and turn it on. That's the END of it. But nope... always have to make things hard with these "integrated solutions".

[SirBC]

I'd just go buy a Motorolla surfboard, or a Cisco DOCSIS modem that doesn't have an internal router and make them assign it to your account. Integrated ISP router / connection doodads don't have a good history. Just look at all the trouble DSL ISPs cause with their all in wonder NAT messes. If you can get them into a bridge mode you're good but why should you have to do all that?

Business class delivery should be a simple layer 2 device from the get go... put the public address on your equipment and turn it on. That's the END of it. But nope... always have to make things hard with these "integrated solutions".

Comcast *forces* you rent their modem if you want a static IP. They will not provision a modem, even the exact same SMC modem, unless you get it from them. Same modem sitting in my clost for the last 4 years but they still doubled the montly price of the modem rental.

[sky-knight]

If I remember correctly you can still configure it in bridge mode. Alternately, I believe it can also be configured with static routes. While not ideal, a NAT'less Untangle router behind the thing would carve up the broadcast domain and cure the DHCP problem.

Another brain wave, have you tried to create filter rules to prevent DHCP traffic from exiting external?

[SirBC]

The comcast modem has been in bridge mode since it was installed several years ago. It's still showing as DHCP as being turned off. A tech is going to swap it out tomorrow, hopefully that will address the issue.

I didn't think of the filter rule. I'll take a look at that too.

[sky-knight]

Wait... so Untangle is a router behind the "modem"?

[SirBC]

Yes. The modem is bridged and Untangle is handling DHCP and DNS.