Red Hat Bugzilla – Bug 164329
ip broadcast filter doesn't check network field
Last modified: 2007-11-30 17:11:10 EST
Using tcpdump-3.9.1-1.src.rpm. Using "ip broadcast" as a filter on an aliased interface fails because the network part of the address is not checked for a match. For instance, here's an invocation using a /24 setup: [root@segw ken]# tcpdump -i eth1:1 ip broadcast -d -O (000) ldh [12] (001) jeq #0x800 jt 2 jf 9 (002) ld [30] (003) and #0xff000000 (004) jeq #0x0 jt 8 jf 5 (005) ld [30] (006) and #0xff000000 (007) jeq #0xff000000 jt 8 jf 9 (008) ret #96 (009) ret #0 Note how the host field is checked for all-zeroes and all-ones, but the network field is not matched against the interface's configured address. This causes lots of false positives on an unswitched network.
In this case, you're using netmask 255.255.255.0. If it isn't the correct one (probably because libpcap can't obtain correct netmask from aliased interface), you have to specify it with the "net" directive (see man tcpdump) instead the "ip broadcast". If locally broadcasted packet goes to other subnets, it's a bug in routing.