Slowloris and You
UPDATE: 20090826 – Corrected typo in “Slowloris and You.” It used to say “Slowlaris and You.” I keep getting slowloris confused with my nickname for “Solaris.” =D
Back in July, http://ha.ckers.org/slowloris/ published an exploit against Apache and other web servers (go to the link for further) that takes advantage of multi-threaded applications. It works by tying up web server threads with partial HTTP requests, then sends TCP handshakes to keep the socket open. In general, multi-threaded web servers such as httpd, apache, and apache2 are vulnerable. IIS and most proxies are not vulnerable
CERT suggested using iptables to rate limit incoming port 80 requests. In general, this should be fine for many applications, though CERT has warned that some large clients behind NAT’s may be affected and thus the hitcount/time ratio should be adjusted according to your needs.
http://www.funtoo.org/en/security/slowloris/ offers tips on mitigating this attack by enabling delayed binding on hardware load balancers.
In short, it appears as though the consensus mitigation method involves connection restrictions in the form of iptables or apache modules (most are of limited value, frankly), or shielding the web servers behind load balancers (such as HA-Proxy).
Dynamic iptables – “Flexible (and fun)”
Have you ever said to yourself that there should be a tool to do x, start building a tool to do it, then about halfway through your little project, somebody glances over your shoulder and says to you “hey, I use a tool like that, it’s called y, you should check it out,” so you do, and that tool is far more comprehensive and well built than the one you were working on?
Well this isn’t one of those times, because this tool hit me from left field while I was researching ways to mitigate a DDoS attack. Though there are many, many ways to do it, if all you have is a Linux box facing the world with nothing to hide its private parts except iptables, then this “flexible (and fun)” toolset is another weapon you can deploy when you get that 2:30AM call saying “our website’s down and I think it’s being DDoS’d.”
The tool is a simple set of scripts that make adding and removing specific IP’s quick and simple. The main site of the author is at http://www.ibm.com/developerworks/library/l-fw/, or is available (hosted locally) here.
Once installed, you can simply ban/unban an IP by typing ipdrop {IP ADDRESS} {on|off}
While perusing this thread at webhostingtalk.com, member dynamicnet mentioned grep-ing for ridiculous levels of SYN_RECV ‘d connections (this is indicative of a TCP SYN Flood attack) and generating ipdrop commands for quick banning of a SYN Flood-ing IP’s. Though you may accidentally drop one or two legitimate IP’s (have a rule already in place so you don’t ban yourself out of a remote box), you’ll likely get the bulk of the attacking IP’s.
Use netstat -n -p|grep SYN_REC | wc -l to count how many SYN_RECV connections you have.
Use netstat -n -p | grep SYN_REC | awk '{print $5}' | sort -u | awk -F: '{print "ipdrop "$1 " on"}' to generate code to ban IP’s in SYN_RECV status.
Use cat /root/.dynfw-ipdrop |awk -F: '{print "ipdrop "$1" off"}' to generates code to “undrop” those IP’s.
