In case anyone else runs into the same problem, it turned out that a
convenient fix was to use mod_evasive, which will temporarily firewall
ips based on number of TCP connections. The same Chinese sites are still
downloading material, but now in an orderly and manageable way :-)
Graham
Hamilton Vera wrote:
You can try to use iptables, to limit the number of TCP connections
$IPTABLES -A INPUT -p TCP -i $WAN -s 0/0 --syn --dport 80 -m connlimit
--connlimit-above 10 -j logdropdos
Or implement a Freebsd firewall with QoS, applying shapes to parallel
TCP connections.
I hope this help.
On Thu, 21 Jun 2007, graham wrote:
Hi,
I've just become involved with a system running apache2.0.55 on ubuntu
with linux 2.6.17.
The system is currently unable to run due to repeated downloads of a
large number of pdfs by systems located in China. These are hogging
all sockets and eventually causing apache to die (I'm appending more
details below in case I've got the wrong end of the stick). The ip
address of these systems varies; they are not a single block, although
they are obviously working together (different ip addresses will ask
for sequentially related pdfs). Each ip address will request multiple
files in parallel.
I'm told that the limit_ipconn module would solve my problem by
limiting the simultaneous accesses from any one ip address. There is
no version of this available for apache2 on ubuntu. I'm wondering if
this is because similar abilities have been built into apache2 itself,
but haven't managed to find any.
Does anyone have any suggestions?
Thanks
Graham
-----------------------------------------------
Notes from log:
The system is running ok, not at particularly heavy load (<1.0), and
apache is apparently running ok and not reporting errors [corrected
later].
Tailing the apache log file shows that the only accesses to the system
are GETs of pdfs from two chinese systems, 218.4.152.91 and
222.218.254.221, which are obviously running the same software.
These systems are trying to systematically work their way through
downloading all chinese pdfs. When a pdf is too large and the download
times out, they immediately try again (at any one moment each system
is trying to download 3 or 4 pdfs).
If I restart apache, I immediately get accesses from all over the
place, including the 2 chinese systems. Eventually the Chinese
accesses capture all the apache processes, and nothing else can get
access.
'Solution' found for this: turn apache off for a few minutes. The
chinese systems went away, and all was fine again.
One hour later ΒΆ
The chinese systems, and the problems, returned. A little more data
this time.
Once the chinese systems are established, netstat shows that they
occupy most sockets but are mostly in CLOSE_WAIT state. All other
requests are stuck in SYNC_RECV.
After this continues for a while the apache processes gradually start
to die off with the following sequence:
alert] (11): setuid: unable to change to uid: 33 (33 is www-data)
[alert] Child 691 returned a Fatal error... Apache is exiting!
[emerg] (43): couldn't grab the accept mutex
semop: Invalid argument
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