sean darcy <seandarcy2@xxxxxxxxx> wrote:
> On 9/27/20 10:03 AM, Pablo Neira Ayuso wrote:
> > On Sun, Sep 27, 2020 at 03:54:47PM +0200, Pablo Neira Ayuso wrote:
> > > On Sat, Sep 26, 2020 at 03:10:24PM -0400, sean darcy wrote:
> > > > nftables-0.9.6
> > > >
> > > > I'm running a VOIP server. There are lots of script kiddies who will bang
> > > > away with 10/sec SIP INVITES or REGISTERS .
> > > >
> > > > In iptables you can match on the string:
> > > >
> > > > -A SIP -i eth0 -p udp -m udp --dport 5060 -m string --string "INVITE"
> > > > --algo bm --from 23 --to 28 -m comment --comment "Catch SIP INVITEs" -j
> > > > SIPINVITE
> > > >
> > > > -A SIP -i eth0 -p udp -m udp --dport 5060 -m string --string "REGISTER"
> > > > --algo bm --from 23 --to 30 -m comment --comment "Catch SIP REGISTERs" -j
> > > > SIPREGISTER
> > > >
> > > > I'm looking at RAW to do the same:
> > >
> > > nft add rule x y udp dport 5060 @th,64,48 0x494e56495445 counter
> > >
> > > @th => transport header
> > > 64 => from bit number 64 (8 bytes after the UDP header)
> > > 48 => extract 48 bits (6 bytes for INVITE)
> >
> > @th,offset,length
> >
> > where offset and length are expressed in bits.
> >
> Thanks for the response.
>
> I corrected it , but it didn't work:
>
> nft list chain filter raw
> table ip filter {
> chain raw {
> type filter hook prerouting priority raw; policy accept;
> udp dport 5060 @th,184,48 80600803923013 counter packets 0 bytes 0
^^^^
64,48
[..]
> Here's the tcpdump output
>
> 0x0000: 001f 1249 0acc 5254 00e7 8e30 0800 45a0 ...I..RT...0..E.
> 0x0010: 03e1 0000 4000 4011 01dd 511d d3c4 d461 ....@.@...Q....a
> 0x0020: 3b4c 13c4 13c4 03cd 386e 494e 5649 5445 ;L......8nINVITE
>
> INVITE starts at bit 337 (128 * 2 ) + ( 5 * 16 ) +1, but this is zero based,
> so use 336, correct?
No. You are asking for @th, so address is relative to the transport (UDP) header,
which is 8 bytes. So INIVITE starts at bit 64, just like Pablo said.
> As I read your response, it's not the offset from the beginning, but after
> the UDP header,
> so 336 - 120 , or 216. (BTW, I always thought the UDP header was 160 bits.)
No: @ll - start at link layer
@nh - start at ip/ipv6 header
@th - start at tcp/udp/whatever transport header protocol is used
The offset is relative to that, not the 'start of packet'.
