port blocked universal media server xfinity

I've been struggling to get my VoIP (with Asterisk) and all my Linux machines to sync time properly since switching to Comcast Xfinity home from DSL a few months ago. The issue was on my end but the Security team wanted to see this issue resolved for me, really going the extra mile to talk it through and troubleshoot the problem. In the end, after a bumpy start with the Tier-1 support agents, Comcast's support has been exemplary. This is where the setting is in the management interface if you have a similar switch: TL DR: My ZyXEL GS1600 managed switch had a 'DoS Protection' mode enabled that dropped UDP packets between my modem and router if the source and destination port was the same. The problem manifested itself when moving to a provider that doesn't encapsulate the internet connection. While the DSL modem and router were connected in the same way my cable modem is, the PPPoE encapsulation likely prevented the 'DoS protection' from seeing the UDP port numbers and dropping those packets. I am quite sure the reason this appeared with the switch to Comcast was my DSL provider uses PPPoE. I have had this managed switch longer than I've had Comcast, and this problem started when I got Comcast. Unfortunately, even though I am sending all the switch logs to my syslog server, this DoS protection does not generate any log events. It is likely the DoS protection blocks all packets that have the same source and destination port as this is a common tactic in UDP reflection attacks, like the NTP ones that were so common last year. Looking through the management interface on the switch, there was a DoS Protection setting, which was enabled.

When I did this, it fixed the issue! Initially I was in disbelief the layer-2 switch could cause such a problem. I then tried moving the laptop and plugging it directly into the modem. I found that when disconnecting the router and plugging the ethernet cable into my laptop directly, the problem persisted. After talking to the Comcast agent yesterday, I decided to do more troubleshooting with my laptop running Ubuntu. The modem and router were configured on an isolated 'External' VLAN. Between my Arris modem and Asus router, I had a ZyXEL GS1600 managed layer-2 switch. The issue was 100% on my end and had nothing to do with Comcast or their equipment. Thanks for a fantastic agent with the Comcast Security Team, this issue is resolved.

Port blocked universal media server xfinity