netfilter/iptables synproxy; need help deciding

Wed Oct 8 15:06:26 UTC 2014

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-Using-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_IPTables_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