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From André Warnier>
Subject Re: Filter by HTTP_REFERER
Date Wed, 02 Nov 2011 09:50:35 GMT
André Warnier wrote:
> Casper Wandahl Schmidt wrote:
>> Den 02-11-2011 10:17, André Warnier skrev:
>>> Pid * wrote:
>>>> On 31 Oct 2011, at 18:25, Christopher Schultz
>>>> <> wrote:
>>>>> Hash: SHA1
>>>>> Richardo,
>>>>> On 10/31/2011 12:33 PM, Ricardo Bayley wrote:
>>>>>> You are right. What I intend to do is prevent hot linking.
>>>>> We get what you are trying to do: you'll just have to write your own
>>>>> code to do it. Tomcat ships with a Filter called RequestFilter that
>>>>> you can subclass if you need that kind of flexibility. If you don't
>>>>> need such flexibility, just write it yourself: it's pretty much a
>>>>> one-liner.
>>>> It'll still be fragile and open to exploitation. An AJAX call can set
>>>> any request headers it likes. You be better off using authentication
>>>> if you want anything more than a casual defence.
>>>> p
>>>>>> My webapp, is working as a REST webservice.
>>>>>> So I would like to have tomcat reply only when requests come from
>>>>>> specific sites.
>>>>> You mean when the requests are referred from specific sites, right?
>>>>> - -chris
>>> I think that a bit of clarification is in order now.
>>> 1) When receiving a request, Tomcat "knows" from which client IP this 
>>> request is coming.
>>> This is because there is a TCP connection nbetween the client and 
>>> Tomcat, and the TCP/IP stack on the Tomcat machine "knows" the IP 
>>> address and TCP port from which the remote client is making this 
>>> connection.
>>> Tomcat can allow/block requests originating from specific IP 
>>> addresses, using the RemoteAddress filter (See 

>>> or the Remote IP Valve (also mentioned there).
>>> This is fairly efficient, because Tomcat already has the information 
>>> needed to make the decision.
>>> 2) Because Tomcat has the client's remote IP address, it can also 
>>> make a "reverse DNS lookup", to find out which domain name 
>>> corresponds to this IP address, and then allow/deny the request based 
>>> on the remote host's domain name.  This is done via the RemoteHost 
>>> filter 
>>> (

>>> It is less efficient than option (1), because a DNS lookup has to 
>>> take place.
>>> Furthermore, this DNS lookup is not guaranteed to succeed, because 
>>> not all IP addresses are mapped in reverse to a hostname.
>>> I also wonder what happens exactly when the remote host's IP address 
>>> corresponds to multiple DNS names, as can happen.
>>> 3) the HTTP "Referer" header in a HTTP request is a different animal.
>>> First, as pointed out by Pid, it can be easily faked by a client.
>>> (So anything based on this should certainly /not/ be considered as 
>>> any kind of security-enhancing feature).
>>> Second, it works as follows :
>>> Imagine a browser B, some website W, and the Tomcat web server T.
>>> The browser B gets a web page from site W. This is now the browser's 
>>> "current page", and its origin is (to the browser) something like : 
>>> "http://website-W/some-document.html".
>>> In that page, there is a URL link to the Tomcat website on T, for 
>>> example something like :
>>> <a href="http://website-T/some-REST-url">click here</a>.
>>> When the user clicks on that link, the browser will send a HTTP 
>>> request to Tomcat on server T.  And the browser, in this request, 
>>> will add a "Referer:" header containing 
>>> "http://website-W/some-document.html".
>>> That is because the request originates from the current page, which 
>>> had been obtained from "http://website-W/some-document.html".  That 
>>> is what "referrer" means.
>>> As you can see, this is different from cases (1) and (2), because (1) 
>>> and (2) refer to the client's own IP address or name, while (3) 
>>> refers to another server altogether.
>>> Now the point is to know exactly what Ricardo wants, as per this 
>>> phrase :
>>> >>> So I would like to have tomcat reply only when requests come from
>>> >>> specific sites.
>>> Does that mean :
>>> - Tomcat should reject requests coming from workstations that are not 
>>> themselves within a particular range of IP addresses ? (for example, 
>>> only from some customer LANs L1, L2, L3,..) of which the IP addresses 
>>> are known in advance ?
>>> OR
>>> - Tomcat should reject requests coming from clients whose own IP 
>>> addresses cannot be resolved to hostnames that are members of some 
>>> DNS domain (like "*" or "*") ?
>>> OR
>>> - Tomcat should reject requests (from any client), unless these 
>>> requests come from a link which itself appears only on some websites ?
>>> (and the possibility of a malicious client "faking" such a "Referer" 
>>> can be ignored).
>> According to OP he want's to avoid hot-linking which would be this 
>> last case or have I misunderstood something?
> Well, "hot-linking" is not a term he himself mentioned, it is a term 
> someone else mentioned, after interpreting the above phrase that he 
> wrote.  It is not so clear to me what he really wants.
Sorry, I stand corrected. He did use "hot-linking".  But then, the next phrase kind of 
contradicts that, because "request comes from a specific site" means something else (to me

at least).  When you get a page from, and on that page is a link to your 
website, clicking it does not mean that the request "comes from".  It comes 
from your browser.

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