httpd-users mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Mohit Anchlia" <>
Subject Re: [users@httpd] Block IP
Date Fri, 06 Jun 2008 20:35:55 GMT
On 6/6/08, André Warnier <> wrote:
> Mohit Anchlia wrote:
>> On 6/5/08, André Warnier <> wrote:
>>> Mohit Anchlia wrote:
>>> On 6/5/08, André Warnier <> wrote:
>>>>> Mohit Anchlia wrote:
>>>>> On 6/5/08, André Warnier <> wrote:
>>>>>> Mohit Anchlia wrote:
>>>>>>> On 6/4/08, Dragon <> wrote:
>>>>>>> André Warnier wrote:
>>>>>>>> Mohit Anchlia wrote:
>>>>>>>>> 2. Another question I had was sometimes we don't get
real physical
>>>>>>>>> IP
>>>>>>>>> of
>>>>>>>>>> the
>>>>>>>>>> machine but the IP of something that's in between
like "router",
>>>>>>>>>>> is
>>>>>>>>>>> there
>>>>>>>>>>> a
>>>>>>>>>>> way to get the real IP so that we don't end up
blocking people
>>>>>>>>>>> coming
>>>>>>>>>>> from
>>>>>>>>>>> that "router" or "proxy"
>>>>>>>>>>> In my opinion, you cannot.  The whole point of
such routers and
>>>>>>>>>>> proxies
>>>>>>>>>>> is
>>>>>>>>>> to make the requests look like they are coming from
>>>>>>>>>> router/proxy,
>>>>>>>>>> so
>>>>>>>>>> that is the sender IP address you are seeing at your
server level,
>>>>>>>>>> and
>>>>>>>>>> that's it.  Your server never receives the original
requester IP
>>>>>>>>>> address.
>>>>>>>>>> ---------------- End original message. ---------------------
>>>>>>>>>> There are legitimate reasons for this to be done
as well,
>>>>>>>>> indiscriminately
>>>>>>>>> blocking such access is a bad idea as it will affect
>>>>>>>>> users.
>>>>>>>>> NAT
>>>>>>>>> and IP address sharing are among the reasons. This allows
>>>>>>>>> organization
>>>>>>>>> to
>>>>>>>>> have a router with one public IP address to serve a larger
>>>>>>>>> network
>>>>>>>>> with private IP addresses. Without this, we would have
run out of
>>>>>>>>> IPv4
>>>>>>>>> addresses a long time ago.
>>>>>>>>> Dragon
>>>>>>>>> If there is no way to get the real IP address then how
would router
>>>>>>>> know
>>>>>>>> which machine to direct the response to. It got to have some
>>>>>>>> information
>>>>>>>> in
>>>>>>>> the packet. For eg: If A send to router B and router sends
to C then
>>>>>>>> when
>>>>>>>> C
>>>>>>>> responds how would B know that the response is for A.
>>>>>>>> You are perfectly right : the router knows the real IP address.
>>>>>>>> it
>>>>>>>> will not tell you, haha.
>>>>>>> Seriously, this is how it works :
>>>>>>> the original system sends out an "open session" packet, through
>>>>>>> router,
>>>>>>> to the final destination.
>>>>>>> The router sees this packet, and analyses it.  It extracts the
>>>>>>> address
>>>>>>> and port of the original sender, and keeps it in a table.
>>>>>>> Then it replaces the IP address by it's own, adds some port number,
>>>>>>> and
>>>>>>> also memorises this new port number in the same table entry.
>>>>>>> Then it sends the modified packet to the external server (yours).
>>>>>>> It knows that the server on the other side is going to respond
>>>>>>> this
>>>>>>> same
>>>>>>> IP address and port (the ones of the router).
>>>>>>> When the return packet from the server comes back, the router
>>>>>>> at
>>>>>>> the
>>>>>>> port in it, finds the corresponding entry in it's table, and
now it
>>>>>>> knows
>>>>>>> to
>>>>>>> whom it should send the packet internally.
>>>>>>> And so on.
>>>>>>> So :
>>>>>>> - the router knows everything
>>>>>>> - the internal system thinks it is talking directly to the external
>>>>>>> server
>>>>>>> - the external server (yours) only sees the router IP and port,
so it
>>>>>>> thinks that is where the packet comes from.
>>>>>>> That's NAT for you, in a nutshell.
>>>>>>> Yes ?
>>>>>>> ---
>>>>>>> Thanks for the great explanation. But, I wonder how do people
>>>>>> app
>>>>>> agains Denial of Service attack. Say Computer A uses Cox/Times warner
>>>>>> (cable) Internet connection and starts attacking B, then how would
>>>>>> system be configured in a way that not all the users using Times
>>>>>> Warner/Cox
>>>>>> are affected. Should it be granular enough to give IP and source
>>>>>> in
>>>>>> IP
>>>>>> blocking rules ?
>>>>>> I think that is quite a different case.  Not all users of an ISP
>>>>> the
>>>>> one you mention I suppose) are "behind" a NAT router that hides their
>>>>> IP
>>>>> address.  Instead, these ISP's have a large pool of public IP addresses
>>>>> which they "own", and they attribute them dynamically to users when
>>>>> they
>>>>> connect (and put the address back in the pool when the user
>>>>> disconnects).
>>>>> If a DOS attack came from a router with a fixed IP address, and
>>>>> everyone
>>>>> would know that this IP address belongs to company xyz, I'm sure that
>>>>> it
>>>>> would not be long before company xyz would be facing a big lawsuit.
>>>>> But in the case of an ISP, with tens of thousands of customers, each
>>>>> one
>>>>> of
>>>>> which gets a different IP address each time he turns on his computer
>>>>> (and
>>>>> anyway once per 24 hours in general), finding out who exactly was "
>>>>>" between 17:45 and 17:53
>>>>> yesterday is a bit more time-consuming.
>>>>> But in that case anyway, you do have a real individual sender IP
>>>>> address
>>>>> when the packet reaches your server, so you can decide to block it.
>>>>> And keep blocking all packets from this address for the next 24 hours.
>>>>> And that's exactly what many servers do.
>>>>> And that is also why sometimes you may turn on your PC at home (getting
>>>>> a
>>>>> brand-new IP address) and find out that you cannot connect to some
>>>>> server
>>>>> because it is rejecting your IP address.  Chances are that you are
>>>>> unlucky
>>>>> enough to have received today the IP address that was used yesterday
>>>>> someone else who used it to send out 1M emails.
>>>>> But isn't this getting a bit off-topic ?
>>>>> If you want to know more about this, I suggest you Google a bit on
>>>>> "blacklists", "greylists" and "whitelists" for example.
>>>>> or start here :
>>>>  Thanks did go off-track a little bit and but it helps me
>>>> understand
>>>> what I should expect when doing such a blocking. Thanks for your
>>>> explanation.
>>>> Now coming back on track, out of below 2 approaches which one is better:
>>>> 1. Use "deny from IP" in <LocationMatch>
>>>> 2. Use RewriteCond and call a perl script dynamically. This helps me
>>>> configure IP dynamically without having to stop and start servers
>>>> everytime
>>>> I change httpd.conf
>>>> Is there any performance impact of using 2 over 1 or any other issues.
>>>> There will be a very big difference : in case (1), the IP addresses or
>>> ranges are pre-processed by Apache at startup time, and the comparison
>>> will
>>> be made by an internal (and fast) Apache module, on the base of
>>> information
>>> in memory.  In case (2), not only are you using a rewrite of the URI, but
>>> in
>>> addition you will be executing a script, which itself is going to read an
>>> external file.  That is going to be several hundred times slower, at
>>> least.
>>>  Thousands of times slower if you recompile and execute the script with
>>> perl
>>> each time (if not under mod_perl).
>>> Now wether it matters or not in your case, depends on the load of your
>>> server. If it is doing nothing anyway 90% of the time, it doesn't matter.
>>>  An Apache restart may or may not be such a big problem either, it all
>>> depends on your circumstances.
>>> But rather than using a perl script, I would definitely in that case use
>>> a
>>> mod_perl add-on module written as a PerlAccessHandler.  But that's
>>> another
>>> story, and one more for the mod_perl list.
>>> I would bet that there exists already such a mod_perl module by the way.
>>> Have a look here :
>>> or, there is probably an example in the Mod_perl Cookbook
>> As per your suggestion I looked at PerlAccessHandler, how would this
>> approach be in terms of performance as compared to have "deny from IP", is
>> it still going to be really bad.
>>  <Location /URL>
>>    PerlAccessHandler Example::AccessHandler
>>  </Location>
>> I will try running some test also.
> Well again, it all depends on your circumstances, what you want to achieve,
> how many accesses you expect, why exactly you want to block or allow some
> IPs, how many different IP's or IP ranges you would want to allow/block, how
> often they change, in function of what they change, whether it is a big
> problem or not for you to do an Apache restart, how loaded your system is
> expected to be, etc..
> Even if one solution looks like it is 200 times slower than another, but
> your server is only loaded at 10% (happens more frequently than you would
> think), and it really makes your life easier for the next 3 years, it's
> worth looking at.
> And even if one solution is 200 times slower than another, that can still
> mean 0,1 millisecond, so is it important for you ?
> A simple tip :
> in the Apache configuration file, you can use an "include" directive, I
> believe just about anywhere, to insert at that point another bit of
> configuration file.
> You could have a simple text file containing all your
> Deny from
> Deny from
> ...
> lines, and include it wherever you want.
> Then a simple Apache restart would re-read it.
> A this file could be written and re-written by some external script which
> decides which IPs are allowed or not. Or edited with vi manually, if that is
> how often changes happen.
> If you have a PerlAccessHandler under mod_perl :
> - perl itself is part of the server, so it does not have to be reloaded
> each time
> - the handler gets compiled once the first time it is run, and the compiled
> code is re-used afterward
> - it can be smart, and only re-read the IP address list, and rebuild its
> internal table when the file changes
> - and in the meantime, it uses the table in memory
> So in that case you would not have to restart Apache, and any changes would
> take effect immediately.
> Also, something else :
> So far, you have been talking about blocking HTTP accesses at the Apache
> level. But maybe you want to block more than port 80 from those IP
> addresses, and maybe you should do this outside of Apache, before it even
> gets to Apache ?
> There are many solutions, but you are the one to decide which one you
> implement.

Thanks. You are right we should not even let these people get to Apache. We
have that process in place, but it often takes time to get that request
approved and processed by Network team. Meanwhile we want something that we
can block on ASAP. I am not sure how often this list will change. To begin
with this list is going to be empty. Only when we experience DOS then we
will update the IP.

We expect to get 1000s of requests per second. Since it's going to be highly
loaded server I started to think about something that would change
dynamically. You mentioned the code is compiled when apache restarts, which
means that if I keep list of IPs as an array inside the perl script is not
going to take affect until next restart. Only option I think then is to read
the list from flat file. I just have one basic question about mod_perl. Does
apache web server executes one process of perl per request ? Reason I am
asking is because you mentioned I could read the list from memory, and I am
not sure how would it read from memory when this script will be executed
every time it tries to process the request. Because if I try to read from
file then every request will try to open the file and read from it. It looks
like a stateless.

Thanks for detailed explanation. It does clear lot of things and also is
giving me different view points. Include directive was a great tip that I
wasn't aware of.

View raw message