Re: wierd firewall log entries: interpretation?

James Miller wrote:
> Hello all:
>
> I run a Freesco router/firewall here and check the logs from time to
> time. The typical entry looks something like this:
>
> Apr 29 17:39:35 - kernel: IP fw-in deny eth0 TCP 218.85.135.54:2109
> my.router.ip.addy:80 L=60 S=0x00 I=19787 F=0x4000 T=48
>
> I interpret it to mean that someone from the internet, using address
> 218.85.135.54 is making a request to port 80 on my router/firewall, and
> that they are issuing the request from port 2109 on their machine. Is
> this pretty much on target? I guess they're checking to see if I'm
> running a web server or something.

Yes. Spot on.

>
> Anyway, given these suppositions, I occassionally get some entries that
> confuse me. They confuse me because, in place of my.router.ip.addy,
> there is a different IP address. It's not one from my LAN, and it's not
> one from the range of university addresses on which my router/firewall
> is located (close range WAN?). In instances I give below, one address is
> 224.0.0.251. So, it's as if the firewall is telling my that someone is
> sending a request to my router as though its address were 224.0.0.251,
> and that the kernel is blocking the request. Here are some examples that
> appeared recently:
>
> Apr 29 18:15:42 - kernel: IP fw-in deny eth0 UDP 134.48.206.176:49243
> 224.0.0.251:53 L=59 S=0x00 I=9282 F=0x0000 T=1
> Apr 29 18:15:42 - kernel: IP fw-in deny eth0 UDP 134.48.206.176:49243
> 224.0.0.251:53 L=59 S=0x00 I=9284 F=0x0000 T=1 Apr 29 23:59:07 - kernel:
> IP fw-in deny eth0 UDP 134.48.206.176:49268 224.0.0.251:53 L=59 S=0x00
> I=40040 F=0x0000 T=1
> Apr 29 23:59:08 - kernel: IP fw-in deny eth0 UDP 134.48.206.176:49268
> 224.0.0.251:53 L=59 S=0x00 I=40042 F=0x0000 T=1
>
> Apr 30 09:10:00 - kernel: IP fw-in deny eth0 TCP 134.48.206.92:62718
> 128.11.250.3:80 L=52 S=0x00 I=29969 F=0x4000 T=63
> Apr 30 09:10:00 - kernel: IP fw-in deny eth0 TCP 134.48.206.92:62718
> 128.11.250.3:80 L=52 S=0x00 I=29969 F=0x4000 T=62
> Apr 30 12:48:58 - kernel: IP fw-in deny eth0 UDP 134.48.206.176:49192
> 224.0.0.251:53 L=59 S=0x00 I=1839 F=0x0000 T=1
> Apr 30 12:48:58 - kernel: IP fw-in deny eth0 UDP 134.48.206.176:49192
> 224.0.0.251:53 L=59 S=0x00 I=1841 F=0x0000 T=1
>
> The addresses from which the request is being made in these cases are
> addresses local to the WAN I'm on (university network). But their last 3
> digits are different than my router/firewall's. I get a pretty much
> static IP from the university, btw (changes maybe once a year).
>
> So, what is the explanation for this? Of course it could be something
> simple and inocuous, and it's just my sketchy understanding of
> firewalls, routing, and networking that makes them seem suspicious. Any
> cause for concern here? Clarifications on fundamental aspects of what's
> involved?

IP addresses with A-quad values 224-239 are "multicast" addresses.
intended for use with protocols where a single source sends traffic
simultaneously to multiple recipients. Multicast servers are not common,
and I don't have any experience with them. In the context of your
problem, I bet that whatever is sending the multicasts is causing them
to use the broadcast Ethernet (MAC) address FF:FF:FF:FF:FF:FF, which
causes all devices on the Ethernet to process the packet (like an ARP
query packet). (If Freesco can be set to include the MAC address in its
logging, you can verify this guess.)

To learn more, you'll need to read up on IP multicast addresses. (Try
Google; I found some superficial stuff very quickly, but didn't take the
time to track down a good reference.)

But I wouldn't worry about the security implications; your firewall
should be successful in blocking all this traffic, and since it DENYs
(rather than REJECTs) these packets, it provides no information to the
other end when doing the blocking.

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