netfilter/iptables synproxy; need help deciding

Wed Oct 8 15:21:33 UTC 2014

Try the examples given here: https://r00t-services.net/knowledgebase/14/Homemade-DDoS-Protection-Using-IPTables-SYNPROXY.html

Your line make no sense to me:

"""
-A PREROUTING -d 172.16.20.98/32 -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j CT --notrack 
""" 

The first entry should register it in the raw table and this is for SYN only, RST, ACK etc'  should be handled by an INVALID filter together with "sysctl -w net/netfilter/nf_conntrack_tcp_loose=0".

So only for the SYN packets you configure the SYNPROXY as such:

iptables -t raw -I PREROUTING -p tcp -m tcp --syn -j CT --notrack
iptables -I INPUT -p tcp -m tcp -m state --state INVALID,UNTRACKED -j SYNPROXY --sack-perm --timestamp --wscale 7 --mss 1460

In my case I believe I adjusted the INPUT on the second line to FORWARD as my box functions as a bridge. I'll check and mail you when I get home.

Kind regards / Vriendelijke groet,
IS Group
Thijs Stuurman

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-----Oorspronkelijk bericht-----
Van: Paige Thompson [mailto:paigeadele at gmail.com] 
Verzonden: Wednesday, October 8, 2014 5:14 PM
Aan: Thijs Stuurman; Nanog
Onderwerp: Re: netfilter/iptables synproxy; need help deciding

On 10/08/14 18:06, Thijs Stuurman wrote:
> I set up a bridge at home to filter traffic using iptables with 
> synproxy. I tried to adjust the lines so that it would log hits but that wouldn't work It gave me a message to read dmesg why it didn't work but dmesg had no information in it.
> However, when I turned on the lines in my iptables configuration file 
> (bash script to load in the rules basicly) it did filter out a SYN 
> attack and the output of "cat /proc/net/stat/synproxy" showed the 
> syn_received go up. (see 
> https://r00t-services.net/knowledgebase/14/Homemade-DDoS-Protection-Us
> ing-IPTables-SYNPROXY.html)
>
> A tcpdump on the bridge confirmed the packets coming in and on my server behind it they didn't so that worked while I would perfectly fine access the apache service.
>
>
> I haven't done any further testing, just got the setup to work late last night.
>
>
> Kind regards / Vriendelijke groet,
> IS Group
> Thijs Stuurman
>
> Powered by results.
>
> Wielingenstraat 8 | T +31 (0)299 476 185
> 1441 ZR Purmerend | F +31 (0)299 476 288 http://www.is.nl | KvK Hoorn 
> 36049256
>
> IS Group is ISO 9001:2008, ISO/IEC 27001:2005, ISO 20.000-1:2005, ISAE 
> 3402 en PCI DSS certified.
>
> -----Oorspronkelijk bericht-----
> Van: NANOG [mailto:nanog-bounces at nanog.org] Namens Paige Thompson
> Verzonden: Wednesday, October 8, 2014 4:51 PM
> Aan: Nanog
> Onderwerp: netfilter/iptables synproxy; need help deciding
>
> Hi,
>
> I guess syncookies wasn't enough and the SYNPROXY target is a relatively new addition to netfilter. If I remember correctly this has been a part of BSD PF for quite some time and is pretty easy to get up and working.
> I recently tried to set this up on one of my gateways considering that it's just one less uncovered means for somebody to be a dick that I have to deal with in the future. But, after spending some time researching and asking on Freenode I have been unable to determine whether or not it works, or even makes any sense. I'm starting to think it's a moot point.
> pastie.org/private/gjsypxkpjs8kuev0tlbxrw#22 (iptables rules, plenty 
> of things to pick at but please try to focus on the subject of 
> synproxy for the purpose of this e-mail.)
>
> based on the following table I want to say its not working because it seems to never change:
> http://pastie.org/private/xwct5opbb0aajcko2tnpw
>
> more info on 
> /proc/stat/synproxy:http://www.spinics.net/lists/netdev/msg264350.html
>
> My only guess is that you can't do this at all with NAT because it relies on conntrack or maybe it will only work with SNAT? I don't understand this well enough to say; are proper firewall rules really a science that need to be understood that far in depth? Why is this not documented? This tutorial seems to indicate that you could use this with a NAT'd network:
> http://www.academia.edu/6773989/Homemade_DDoS_Protection_Using_IPTable
> s_SYNPROXY
>
> I really would like to come to some closure on this subject. Whether 
> it needs to be done right or not done at all, I'm tired of it looming 
> over me. I really want to believe I should do the very best to have 
> all mitigation techniques already in place, but I'm having a hard time 
> understanding why this is next to impossible to figure out if it's so 
> important. #netfilter on freenode is next to no help, the mailing list 
> seems to be unavailable.... the things people are saying about how I 
> should "just switch" back to using pf seem like a drastic solution 
> when people in #netfilter are so content (yet many of them have never 
> heard of synproxy before.)
>
>
> Any thoughts on this are appreciated,
>
> -Paige

Yeah, I have no way to test for sure but I can tell you this which I forgot to mention:

All of my services still work with these rules -A PREROUTING -d 172.16.20.98/32 -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j CT --notrack -A PREROUTING -d 172.16.40.98/32 -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j CT --notrack -A PREROUTING -d 172.16.80.98/32 -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j CT --notrack

None of my services worked with this rule:
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j CT --notrack

I sort of get it, but I totally don't get it. I'm not sure what traffic that second rule is matching (or if the -d even works in the raw table maybe thats bunk too.) I don't think the first set are working, but I have no way to test it either. 