iptables drop packet by hex string match
12,774
The hex string needs to be surrounded by |
symbols. The spaces are optional
iptables --append INPUT --match string --algo kmp --hex-string '|f4 6d 04 25 b2 02 00 0a|' --jump ACCEPT
Note that string matching should be a last resort. It's intensive, and unreliable because it works on packets not connections. It also only starts working on the third packet in a TCP connection which limits what actions you can use (you can't NAT the connection for example).
Related videos on Youtube
Author by
Flint
Updated on September 18, 2022Comments
-
Flint almost 2 years
I got this packet captured with tcpdump but I'm not sure how to use the --hex-string param to match the packet. Can someone show me how to do it?
11:18:26.614537 IP (tos 0x0, ttl 17, id 19245, offset 0, flags [DF], proto UDP (17), length 37) x.x.187.207.1234 > x.x.152.202.6543: [no cksum] UDP, length 9 0x0000: f46d 0425 b202 000a b853 22cc 0800 4500 .m.%.....S"...E. 0x0010: 0025 4b2d 4000 1111 0442 5ebe bbcf 6701 .%[email protected]^...g. 0x0020: 98ca 697d 6989 0011 0000 ffff ffff 5630 ..i}i.........V0 0x0030: 3230 3300 0000 0000 0000 0000 203.........
-
Flint almost 12 yearsThanks! Another thing, if I want to limit the packet matching to only UDP packet of length in the tcpdump capture with --length param, which length should I use? 17 or 9?
-
mgorven almost 12 years@Flint It probably searches the entire IP packet, so 37. The UDP payload is 9, the IP payload is 17, and the entire IP packet is 37.