Re: rate limit SIP INVITES

[sean darcy <seandarcy2@gmail.com>]

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
		udp dport 5060 @th,184,64 5928222864759342418 counter packets 0 bytes 0
	}
}

I've gotten over 100 INVITEs or REGISTERs .

Also nft changes the hex to decimal. Here's the input:

# INVITE ascii = 0x494e56495445 hex , 48 (6 * 8 ) bits long
# REGISTER ascii = 52 45 47 49 53 54 45 52 ,   64 ( 8 * 8 ) bits long
         chain raw {
                 type filter hook prerouting priority raw; policy accept;
                 udp dport 5060 @th,184,48 0x494e56495445 counter 
packets 0 bytes 0
                 udp dport 5060 @th,184,64 0x5245474953544552 counter
         }

I'd suggest nft should leave it in hex.

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?

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.)

udp dport 5060 @th,216,48 0x494e56495445 counter

Sorry for being so long-winded. I appreciate your help in sorting this out.

sean