Re: how to block udp frag?

[Andy Furniss <andy.furniss@dsl.pipex.com>]

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@lists.netfilter.org [mailto:netfilter-bounces@lists.netfilter.org] On Behalf Of Bruno Wallace
> Sent: Monday, January 03, 2005 7:39 AM
> To: Jason Opperisano; netfilter@lists.netfilter.org
> 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@817west.com> 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..-...@5.n.Sf..
>>>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..@5...Sf.L
>>>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;.@5...Sf..
>>>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
>>
>>
> 
> 
>