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My network connection is pretty crappy today so I looked at my router (TP-LINK TD-W8950ND) logs and can see these entries a few times every minute:

Date/Time   Facility    Severity    Message
Sep 7 22:26:16  user    notice  igmp[1435]: setsockopt IP_MSFILTER

This router runs on some kind of *nix kernel, so maybe this looks familiar to someone. I wonder if it has anything to do with my poor connection?

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up vote 2 down vote accepted

That message is pretty thin; it doesn't even tell you there is actually a problem.

It has something to do with multicast source filtering.

Normally with multicasting, an IPv4 machine joins a multicast group using the IGMP protocol — this is why you see igmp in the log line — which tells all the network equipment around it that it wants to receive anything sent to that multicast group. RFC3678 specifies a way to modify that, saying things like "...but only if it comes from this set of IPs" or "...but not if it comes from IP 1.2.3.4".

Why you should be seeing this on an Internet router, though, baffles me. Multicasting generally doesn't work over the Internet. It always was intended to, but the ISPs and backbones never really did anything to allow it. (Same reason we're still waiting for IPv6, a few minutes from IPv4's "midnight.")

Therefore, unless you know for a fact that you have applications using multicast across that router, I would see if there is a way to simply turn it off.

Beware, if this router is something like a home WiFi/switch/router combo, you probably don't want to turn this feature off. You are likely to prevent multicast within the LAN. Multicasting on LANs is still relatively rare compared to broadcasting and unicasting, but it tends to get used quietly, so you may not even know you're using it. Whereas you can be pretty sure about whether or not you're using multicasting over the Internet, because you probably would have to ask your ISP whether they could do it and to enable it for you, it's assumed that it always works on LANs, particularly on business-class ones. As a result, a lot of business-focused apps just assume they can do it. Norton Ghost multicasts images when restoring more than one machine to a ghosted image, for example.

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