http://www.netfilter.org/patch-o-matic/pom-base.html#pom-base-u32 Inspecting individual bits I'd like to look at the "More Fragments" flag - a flag which has no existing test in iptables (-f matches 2nd and further fragments, I want to match all fragments except the last). Byte 6 contains this, so I'll start with offset 3 and throw away bytes 3-5. Normally this would use a mask of 0x000000FF, but I also want to discard the other bits in that last byte. The only bit I want to keep is the third from the top (0010 0000), so the mask I'll use is 0x00000020 . Now I have two choices; move that bit down to the lowest position and compare, or leave it in its current position and compare. To move it down, we'll right shift 5 bits. The final test is: iptables -m u32 --u32 "3&0x20>>5=1" If I take the other approach of leaving the bit where it is, I need to be careful about the compare value on the right. If that bit is turned on, the compare value needs to be 0x20 as well. iptables -m u32 --u32 "3&0x20=0x20" Both approaches return true if the More Fragments flag is turned on. On Sat, 2005-01-08 at 15:53 +0000, Andy Furniss wrote: > Piszcz, Justin Michael wrote: > > Yes, if you use NAT, you cannot block fragmented packets. > > Assuming my testing isn't too lame then you can drop with a policer. It > will still let the last packet through though, as the match is on the > more fragments flag. I suppose using the next field could do them all - > but I don't know how to say not with u32. > > tc qdisc add dev eth0 handle ffff: ingress > > tc filter add dev eth0 parent ffff: prio 1 protocol ip u32 \ > match ip protocol 17 0xff \ > match u8 0x20 0x20 at 6 \ > police rate 1kbit burst 10 drop \ > flowid :1 > > The rate is irrelevant here, it's the burst 10 that means that only > packets <= 10 bytes will ever pass. > > To delete it do > > tc qdisc del dev eth0 handle ffff: ingress > > To see stats - > > tc -s qdisc ls dev eth0 > > Andy. > > PS > > I had to remove jason from the cc as my isps mailserver threw a domain > not found. > > > > > > > -----Original Message----- > > From: netfilter-bounces@xxxxxxxxxxxxxxxxxxx [mailto:netfilter-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Bruno Wallace > > Sent: Monday, January 03, 2005 7:39 AM > > To: Jason Opperisano; netfilter@xxxxxxxxxxxxxxxxxxx > > Subject: Re: how to block udp frag? > > > > the iptables dont see this traffic.. > > > > > > On Sat, 1 Jan 2005 19:08:45 -0500, Jason Opperisano <opie@xxxxxxxxxxx> wrote: > > > >>On Sat, Jan 01, 2005 at 09:58:41PM -0200, Bruno Wallace wrote: > >> > >>>hello, > >>>how to block this????? > >>> > >>>20:53:44.628586 83.102.166.15 > xxx.xxx.151.35: udp (frag 1720:25@512) > >>>(ttl 53, len 45) > >>>0x0000 4500 002d 06b8 0040 3511 2599 5366 a60f E..-...@5.%.Sf.. > >>>0x0010 c896 9723 11ef 0035 0019 1e70 71f7 0100 ...#...5...pq... > >>>0x0020 0001 0000 0000 0000 0000 0200 0100 .............. > >>>20:53:47.197264 83.102.166.24 > xxx.xxx.151.34: udp (frag > >>>48577:25@512) (ttl 53, len 45) > >>>0x0000 4500 002d bdc1 0040 3511 6e87 5366 a618 E..-...@xxxxxxxx > >>>0x0010 c896 9722 11ef 0035 0019 1e68 71f7 0100 ..."...5...hq... > >>>0x0020 0001 0000 0000 0000 0000 0200 0100 .............. > >>>20:53:49.306206 83.102.166.76 > xxx.xxx.145.115: udp (frag > >>>21990:25@512) (ttl 53, len 45) > >>>0x0000 4500 002d 55e6 0040 3511 dbdd 5366 a64c E..-U..@xxxxxxxx > >>>0x0010 c896 9173 11ef 0035 0019 23e3 71f7 0100 ...s...5..#.q... > >>>0x0020 0001 0000 0000 0000 0000 0200 0100 .............. > >>>20:53:49.529603 83.102.166.7 > xxx.xxx.146.119: udp (frag > >>>26427:25@512) (ttl 53, len 45) > >>>0x0000 4500 002d 673b 0040 3511 c9c9 5366 a607 E..-g;.@xxxxxxxx > >>>0x0010 c896 9277 11ef 0035 0019 2324 71f7 0100 ...w...5..#$q... > >>>0x0020 0001 0000 0000 0000 0000 0200 0100 > >>> > >>>thanks > >>>Bruno Wallace > >> > >>either (a) use a default deny policy that doesn't allow UDP traffic or > >>(b) in your rules where you accept UDP traffic, specify "! -f" which, > >>according to the man page: > >> > >> When the "!" argument precedes the "-f" flag, the rule will only match > >> head fragments, or unfragmented packets. > >> > >>-j > >> > >> > > > > > > > > > >
