To be totally honest, I think the whole way in which vlans are managed in the filters is quite nonsense. The underlying problem is that normally a BPF filter is an "or" or "and" combination of disjoint filters, so if I write "filterA" or "filterB" I assume that the two filters are disjoints, so
"filterA or filterB" should be equivalent to "filterB or filterA" This is not true when using the "vlan" keyword. Vlan sticks globally and increments the offset of the L3 header unconditionally of two bytes, no turning back. For example "ip or vlan 14" is different than "vlan 14 or ip" Have a nice day GV -----Original Message----- From: tcpdump-workers-boun...@lists.tcpdump.org [mailto:tcpdump-workers-boun...@lists.tcpdump.org] On Behalf Of Ani Sinha Sent: Thursday, January 31, 2013 3:42 PM To: tcpdump-workers@lists.tcpdump.org Cc: Bill Fenner; Michael Richardson; Francesco Ruggeri Subject: [tcpdump-workers] "not vlan" filter expression broken catastrophically! hello folks : As you guys have been aware, I am hacking libpcap for a while. Me and Bill noticed something seriously broken for any filter expression that has a "not vlan" in it. For example, take a look at the filter code generated by libpcap with an expression like "not vlan and tcp port 80" : BpfExpression '(not vlan and tcp port 80)' { 0x28, 0, 0, 0x0000000c }, //(000) ldh [12] { 0x15, 19, 0, 0x00008100 }, //(001) jeq #0x8100 jt 21 jf 2 { 0x28, 0, 0, 0x00000010 }, //(002) ldh [16] { 0x15, 0, 6, 0x000086dd }, //(003) jeq #0x86dd jt 4 jf 10 { 0x30, 0, 0, 0x00000018 }, //(004) ldb [24] { 0x15, 0, 15, 0x00000006 }, //(005) jeq #0x6 jt 6 jf 21 { 0x28, 0, 0, 0x0000003a }, //(006) ldh [58] { 0x15, 12, 0, 0x00000050 }, //(007) jeq #0x50 jt 20 jf 8 { 0x28, 0, 0, 0x0000003c }, //(008) ldh [60] { 0x15, 10, 11, 0x00000050 }, //(009) jeq #0x50 jt 20 jf 21 { 0x15, 0, 10, 0x00000800 }, //(010) jeq #0x800 jt 11 jf 21 { 0x30, 0, 0, 0x0000001b }, //(011) ldb [27] { 0x15, 0, 8, 0x00000006 }, //(012) jeq #0x6 jt 13 jf 21 { 0x28, 0, 0, 0x00000018 }, //(013) ldh [24] { 0x45, 6, 0, 0x00001fff }, //(014) jset #0x1fff jt 21 jf 15 { 0xb1, 0, 0, 0x00000012 }, //(015) ldxb 4*([18]&0xf) { 0x48, 0, 0, 0x00000012 }, //(016) ldh [x + 18] { 0x15, 2, 0, 0x00000050 }, //(017) jeq #0x50 jt 20 jf 18 { 0x48, 0, 0, 0x00000014 }, //(018) ldh [x + 20] { 0x15, 0, 1, 0x00000050 }, //(019) jeq #0x50 jt 20 jf 21 { 0x6, 0, 0, 0x0000ffff }, //(020) ret #65535 { 0x6, 0, 0, 0x00000000 }, //(021) ret #0 As you can see, it loads offset 12 (ethertype). For vlan packets, it jumps to #21 and returns false right away. However, for packets that are not vlan tagged, it goes to #2 which loads offset 16 in the packet. Notice that this is wrong! The offsets should be incremented by 4 only for vlan tagged packets and not for non-vlan packets. The problem is that in gencode.c, the off_linktype increments by 4 unconditionally whether or not the packet actually contains a vlan tag. We do not want to increment this offset if "not vlan" is true. So the above filter code is generated wrong. I just wanted to point this out to folks who wishes to dig in and fix it. I do not have time right now to think of a proper solution. It would seem using unconditional increments of offsets like off_linktype below the parser is not going to work. How do you know if the parser is going to take your code generated from the "vlan" expression and just negate it? Or may be we can hack another rule in grammar.y. I don't know. cheers, ani _______________________________________________ tcpdump-workers mailing list tcpdump-workers@lists.tcpdump.org https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers _______________________________________________ tcpdump-workers mailing list tcpdump-workers@lists.tcpdump.org https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers
