Too many TIME_WAIT connections
alfred at freebsd.org
Mon Oct 1 17:07:55 PDT 2007
* Jamie Ostrowski <jamie.ostrowski at gmail.com> [071001 16:02] wrote:
> Hello -
> I've got a mailserver running FreeBSD 4.11 and Sendmail 8.13 that has
> been running as a mailserver for a couple of years without any
> load/connection problems. Here are my memory stats:
> Mem: 71M Active, 265M Inact, 96M Wired, 24M Cache, 60M Buf, 36M Free
> Swap: 2048M Total, 760K Used, 2047M Free
> Then all of a sudden we started experiencing dropped connections even though
> the load average is generally around 2.0 or less.
> I found the problem today: there are currently 1300 socket connections
> suspended at status TIME_WAIT on the incoming smtp port.
> I checked some of my kernel settings:
> kern.ipc.somaxconn = 128
> net.inet.tcp.msl: 30000
> I suspect this is a dos attack: they're just opening these connections,
> and then let them hang there and they don't close them, so they just build
> up and the machine rejects new connections.
> Based on my configuration, does anyone have some suggestions on how I
> might tweak the system to overcome this (apparent?) DOS attack?
You can tweak msl, but it probably makes more sense to use some form
of firewall, ipfw, ipfilter, pf, etc on the box.
you can use netstat to see the remote addresses, just block them.
- Alfred Perlstein
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