Date: Thu, 13 May 1999 11:35:43 -0700
From: Richard Steenbergen <humble@LIGHTNING.NET.>
To: [email protected]Subject: SYN floods against FreeBSD
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Here's a quickie for the people who have been plagued with high bandwidth
syn flood attacks, a kernel patch for FreeBSD 3.1-STABLE which rate limits
SYN processing. Its messy but functional and I don't have time to make it
better (thats the fbsd developers job, not mine :P), cd /usr/src/sys,
patch < synlim, add "options SYN_RATELIM" (I highly recommend ICMP_BANDLIM
as well) to your kernel, recompile, and sysctl net.inet.tcp.synlim will be
available (default to 100). This is the maximium number of SYNs per second
that will be processed, the rest will be silently discarded. On my test
system (P2 450 running 3.1-stable being hit w/15,000 packets per sec),
this has successfully brought CPU usage from 100% to ~20% (against an open
port which is replying with unacknowledged ACKs).
Which brings us to the more sticky topic of kernel panics when under SYN
flood (which I believe to be the cause of some earlier posts from certain
people at Exodus Communications *cough*). Lord knows I found enough of
them when doing this testing, but the one that seems to be the biggie for
crashing when under syn flood is as follows (heh just turned off the
synlim and panic'd within 8 seconds while writing this):
panic: free: multiple frees
(kgdb) bt
#0 boot (howto=256) at ../../kern/kern_shutdown.c:285
#1 0xc0138c09 in panic (fmt=0xc02192b7 "free: multiple frees")
at ../../kern/kern_shutdown.c:446
#2 0xc0135aaf in free (addr=0xc0cdd600, type=0xc0239330)
at ../../kern/kern_malloc.c:333
#3 0xc01768f4 in ifafree (ifa=0xc0cdd600) at ../../net/route.c:262
#4 0xc0176876 in rtfree (rt=0xc34ce700) at ../../net/route.c:236
#5 0xc0176c84 in rtrequest (req=2, dst=0xc34cbac0, gateway=0xc34cbad0,
netmask=0x0, flags=393223, ret_nrt=0x0) at ../../net/route.c:536
#6 0xc017b34d in in_rtqkill (rn=0xc34ce700, rock=0xc0231610)
at ../../netinet/in_rmx.c:242
#7 0xc0176064 in rn_walktree (h=0xc0cd9e00, f=0xc017b2fc <in_rtqkill>,
w=0xc0231610) at ../../net/radix.c:956
#8 0xc017b3ec in in_rtqtimo (rock=0xc0cd9e00) at ../../netinet/in_rmx.c:283
#9 0xc013d19b in softclock () at ../../kern/kern_timeout.c:124
Which after a quick examination seems to be a perioditic routing table
cleanup. It seems that in_rtqtimo is scheduled to run every
net.inet.ip.rtexpire seconds (which is dynamicly adjusted and can never go
lower then net.inet.ip.rtminexpire). When the system is under heavy load
from processing lots of small packets (they don't even have to be SYNs,
anything which can get routed will do the trick, though the packet kiddies
would get very little gain from just sending an ip header since its going
to be padded to 64 bytes for the eth frame anyhow), this route cleanup
code will go wacking at routes it shouldn't and free some memory twice. In
the course of testing I've gotten my rtq_reallyold to -3 and seen lots of
"tvotohz: negative time difference -2 sec 0 usec". Perhaps someone with
free time or more specific knowledge of this area would like to FIX IT? =)
Perhaps when I get more free time I'll test some other *nix's. I would
really recommend putting all this rate limiting code at an ipfw level.
If you would like to contact me regarding this please use
[email protected] (at least if you want a quick reply), thanks.
--
Richard Steenbergen <humble@lightning.net.> humble@EFNet PGP ID: 0x741D0374
PGP Key Fingerprint: C6EF EFA0 83B2 071F 1AB6 B879 1F70 4303 741D 0374
http://users.quadrunner.com/humble
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Content-Description: SYN rate limit patch for fbsd 3.1
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