Hi.
Have a look that the indicated parameters in the two pages below.
You may be the target of such a variant of DDoS attack : many clients open a TCP
connection to your server (front-end), but then never sends a HTTP request on that
connection. In the meantime, the server accepts the TCP connection, and passes it on to a
"child" process or thread for processing. The child then waits for the HTTP request line
to arrive on the connection (during a certain time), but it never arrives. After a while,
this triggers a timeout (see below), but the standard value of that timeout may be such
that in the meantime, a lot of other connections have been established by other such
nefarious clients, so a lot of resources of the webserver are tied up, waiting for
something that will never come.
Since there is never any real request sent on the connection, you would (probably) not see
this in the logs either.
The above is the basic mechanism of such an attack. There may be variations, such as the
client not "not sending" a request line, but sending it extremely slowly, thus achieving
perhaps similar kinds of effects.
As someone pointed out, it is quite difficult to do something about this at the level of
the webserver itself, because by the time you would do something about it, the resources
have already been consumed and your server is probably already overloaded.
There are specialised front-end devices and software available, to detect and protect
against this kind of attack.
You may want to have a look at the following parameters, but make sure to read the caveats
(side-effects, interlocking timeouts etc.), otherwise you may do more harm than good.
Another thing : the settings below are for Apache Tomcat, which in your case is the
back-end. It would of course be much better to detect and eliminate this at the front-end,
or even before. I had a look at the Apache httpd documentation, and could not find a
corresponding parameter. But I am sure that it must exist. You may want to post this same
question on the Apache httpd user's list for a better response.
Tomcat configuration settings :
AJP Connector :
(http://tomcat.apache.org/tomcat-8.5-doc/config/ajp.html#Standard_Implementations)
connectionTimeout
The number of milliseconds this Connector will wait, after accepting a connection, for the
request URI line to be presented. The default value for AJP protocol connectors is -1
(i.e. infinite).
(You could for example try to set this to 3000 (milliseconds) or even lower. That should
be more than enough for any legitimate client so send the HTTP request line. Note however
that by doing this at the Tomcat level, you will probably move the problem to the Apache
httpd/mod_jk level. But at least it might confirm that this is the problem that you are
seeing. The mod_jk logfile at the httpd level may give you some hints there.)
HTTP Connector :
(http://tomcat.apache.org/tomcat-8.5-doc/config/http.html#Standard_Implementation)
connectionTimeout
The number of milliseconds this Connector will wait, after accepting a connection, for the
request URI line to be presented. Use a value of -1 to indicate no (i.e. infinite)
timeout. The default value is 60000 (i.e. 60 seconds) but note that the standard
server.xml that ships with Tomcat sets this to 20000 (i.e. 20 seconds). Unless
disableUploadTimeout is set to false, this timeout will also be used when reading the
request body (if any).
On 26.11.2016 09:57, Jaaz Portal wrote:
hi,
sorry, its mod_jk no jk2, my typo. All at latest versions. We tried with
mod proxy too.
There is no flood of the server. Nobody is flooding us, they use some
specific connections after which pool of apache workers is exhausted and
blocked
and we need to restart tomcat server.
It is some kind of exploit but do not know how to log it to obtain details.
i had put a limit on connections per client with hope that this will help
but once again, it is not a flood.
They open several connections that are not dropped by apache when they
disconnect. This way whole pool is quickly exhausted and the server broken.
i would like to help you to figure details of this attack but this is
production server so it is impossible to much debugging options
best,
artur
2016-11-25 23:44 GMT+01:00 Niranjan Babu Bommu <niranjan.bo...@gmail.com>:
you can find who is flooding site in apache access.log and block them in
firewall.
ex to find the IP:
cat /var/log/apache2/access.log |cut -d' ' -f1 |sort |uniq -c|sort -gr
On Fri, Nov 25, 2016 at 8:42 AM, Jaaz Portal <jaazpor...@gmail.com> wrote:
hi,
we are from some weeks struggling with some Polish hackers that are
bringing our server down. After updating apache to latest version
(2.4.23)
and tomcat (8.0.38) available for debian systems we still cannot secure
our
server.
Today it has stopped to respond again and we needed to restart tomcat
process to get it back alive.
There is no too much clues in the logs. The apache error.log gives just
this line:
[Fri Nov 25 13:08:00.647835 2016] [mpm_event:error] [pid 13385:tid
1397934896385
92] AH00484: server reached MaxRequestWorkers setting, consider raising
the
MaxR
equestWorkers setting
seems that somehow tomcat, mod-jk2 or even apache is vulnerable to some
new
exploit, as we certainly does not have such traffic that would block our
server otherwise
for now we have increased MaxRequestWorkers and we have limited number of
connections from one client to 5 by mod_bw and limited number of
simultaneous connections from one ip by iptables but does not know if
this
will help
best regards,
artur
--
*Thanks*
*Niranjan*
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