On 27.11.2016 19:03, Jaaz Portal wrote:
2016-11-27 18:30 GMT+01:00 André Warnier (tomcat) <a...@ice-sa.com>:
On 27.11.2016 14:26, Jaaz Portal wrote:
hi,
everything i know so far is just this single log line that appeared in
apache error.log
[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
there was nothing else, just this strange line
This is not a "strange" line. It is telling you something.
One problem is that you seem convinced in advance, without serious proof,
that there is a "bug" or a vulnerability in httpd or tomcat.
Read the explanation of the httpd parameter, here :
http://httpd.apache.org/docs/2.4/mod/mpm_common.html#maxrequestworkers
and try to understand it.
I understand it very well. We are serving no more that 500 clients per day
so there was no other option that some kind of attack.
About the "bug" or "vulnerability" : a webserver is supposed to serve HTTP
requests from clients. That is what you expect of it. It has no choice but
to accept any client connection and request, up to the maximum it can
handle considering its configuration and the capacity of the system on
which it runs. That is not a bug, it is a feature.
We have some weeks ago come under attack from some Polish group. It was
first bind that was DoS'ed, we was running on stable Debian so i updated
bind to latest version. It did not helped. They has dos'ed it so we
switched to other dns provider. That has helped.
Then they exploited some well know vulnerability in mod_proxy. We have
updated apache to the latest but again they has exploited it, so we have
switched to mod_jk. And then guess what. They exploited it too so i decided
to write to this list looking for help before trying jetty.
The normal Apache httpd access log, will log a request only when it is
finished. If the request never finishes, it will not get logged.
That may be why you do not see these requests in the log.
But have a look at this Apache httpd module :
http://httpd.apache.org/docs/2.4/mod/mod_log_forensic.html
ok, thanks, will take care
Note : that is also why I was telling you to enable the mod_jk log, and to
examine it.
Because mod_jk will also log information before the request produces a
response.
and server hanged.
Again, /what/ is "hanged" ? Apache httpd, or tomcat ?
Apache was accepting connection but not processed it. After restart of
tomcat server it worked again.
Also in
access logs there are no clues that it was under any heavy load.
after around hour after discovering that our server hanged-out we have
restarted tomcat server
and it worked again
Yes, because that will close all connections between Apache httpd and
tomcat, and abort all requests that are in the process of being processed
by tomcat. So mod_jk will get an error from tomcat, and will report an
error to httpd, and httpd will communicate that error to the clients, and
close their connection.
It still does not tell you what the problem was.
The only thing that it suggests, is that the "bad" requests actually make
it all the way to tomcat.
correct
i will enable logs that you has pointed out and we will look what i will
catch
however i think we have only one chance, as if the solution we has found
out (connection_timeout + mod_bn)
will work they will stop exploiting it
thank you very much for all the help and explanations
i will report to the list new facts once they will attack us again
best regards,
artur
Ok, but also read this e.g. :
https://www.corero.com/blog/695-going-after-the-people-behind-ddos-attacks.html
Attempts to bring down a site by DoS attacks is a crime, in most places.
You can report it, while at the same time trying to defend yourself against it.
It is also relatively easy, and quite inexpensive in terms of system resources, to run a
small shell script which takes a list every few seconds of the connections to the port of
your webserver, and which IPs they are coming *from*.
E.g.
First try the netstat command, to see what it lists, like :
# netstat -n --tcp | more
Then you will want to filter this a bit, to only consider established connections to your
webserver, for example :
# netstat -n --tcp | grep ":80" | grep "ESTABLISHED"
Then you will want to send this to a logfile, regularly, like this :
# date >> some_file.log
# netstat -n --tcp | grep ":80" | grep "ESTABLISHED" >> some_file.log
(repeat every 3 seconds)
This will not generate GB of logfiles, and it will tell you, when the problem happens, how
many connections there are exactly to your webserver, and where they are coming from.
Then later you can further analyse this information..
i think that setting connection-timeout and limiting the number of clients
by mod_bd i will
have effect that next time somebody will try this exploit it will block
his
access to the site
for minute or two, pretty good holistic solution i would say
still, it seems that there is serious vulnerability somewhere in apache,
mod_jk or tomcat
i would like to help find it out but need some hints which debug options
enable to catch the bad guys
when they will try next time
best regards,
artur
2016-11-27 13:58 GMT+01:00 André Warnier (tomcat) <a...@ice-sa.com>:
On 27.11.2016 13:23, Jaaz Portal wrote:
hi Andre,
thank you very much this was very educative but in my case it is little
bit
different.
The server is no flooded, there is maybe dozen of very sophisticated
connections that somehow hangs apache workers threads
Can you be a bit more specific ?
When you say "apache workers threads", do you mean threads in Apache
httpd, or threads in Apache Tomcat ? (both are Apache webservers, so it
is
difficult to tell what you are talking about, unless you are very
precise).
Let me give you some additional explanations, and maybe you can figure
out
exactly where it "hangs".
From the Apache httpd front-end point of view, mod_jk (the connector to
Apache Tomcat) is basically one among other "response generators".
Apache
httpd does not "know" that behind mod_jk, there is one or more Tomcat
servers. Apache httpd receives the original client request, and
depending
on the URL of the request, it will pass it to mod_jk or to another
response
generator, to generate the response to the request.
That mod_jk in the background is using a Tomcat server to actually
generate the response, is none of the business of Apache httpd, and it
does
not care. All it cares about, is to actually receive the response from
mod_jk, and pass it back to the client.
If httpd passes a request to mod_jk, it is because you have specified in
the Apache configuration, the type of URL that it should pass to mod_jk..
That happens via your "JkMount (URL pattern)" directives in Apache httpd.
Of course Apache httpd will not pass a request to mod_jk, before it has
received at least the URL of the request, and analysed it to determine
*if*
it should pass it to mod_jk (*).
If the mod_jk logging is enabled, you can see in it, exactly *which*
requests are passed to mod_jk and to Tomcat.
Do you know *which* requests, from which clients, cause this "thread
hanging" symptom ?
Once you would know this, maybe you can design a strategy to block
specifically these requests.
and the effect is permanent. Quickly the pool is exhausted
which pool exactly ?
and the only
solution that works is to restart tomcat.
I think it is a bug similar to this one from mod_proxy:
https://tools.cisco.com/security/center/viewAlert.x?alertId=34971
Maybe, maybe not. As long as we do not know what the requests are, that
block things, we do not know this.
I think also that your solution with setting connectionTimeout will solve
the problem, at least partially. THANK YOU.
Same thing, we do not know this yet. It was only giving this
explanation,
to help you think about where the problem may be.
I would like to help you further investigate this issue, as our server
comes under such attack once or twice in a week.
Other than giving you hints, there is not much I or anyone else can do
to
help. You are the one with control of your servers and logfiles, so you
have to investigate and provide more precise information.
(*) actually, to be precise, Apache httpd passes *all* requests to
mod_jk,
to ask it "if it wants that request". mod_jk then accepts or declines,
depending on the JkMount instructions. If mod_jk declines, then Apache
httpd will ask the next response generator if it wants this request,
etc...
best regards,
artur
2016-11-27 12:46 GMT+01:00 André Warnier (tomcat) <a...@ice-sa.com>:
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/tomc
at-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/tomc
at-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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