Casper Wandahl Schmidt wrote:
Den 02-11-2011 10:50, André Warnier skrev:
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
<ch...@christopherschultz.net> wrote:
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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
http://tomcat.apache.org/tomcat-7.0-doc/config/valve.html#Remote_Address_Filter)
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
(http://tomcat.apache.org/tomcat-7.0-doc/config/valve.html#Remote_Host_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 "*.customer1.com" or "*.customer2.org") ?
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 google.com, and on that page is a link to your website,
clicking it does not mean that the request "comes from google.com".
It comes from your browser.
I guess that depends how you read the word "site" (I believe you read it
as like a building site, a psysical location - or well semi-psysical due
to the fact that LAN's can be spread to multiple psysical locations -
whereas I read it as website and then simply ignore the fact that he
does not have the full understanding of who and how requests are made) :)
Right. I was thinking of the following type of scenario, which we encounter
frequently :
A large organisation has a large internal LAN, spread over multiple physical locations;
all accesses to the WWW from workstations within that LAN, go over one of 3 HTTP
firewalls/proxies. So to an external webserver, all accesses from workstations in that LAN
appear as coming from only 3 distinct IP addresses (the external IP addresses of the
proxies). So it makes it easy to filter using the RemoteAddress filter.
(Not preventing hot-linking, but allowing only requests "coming from one site", with a
certain meaning of "site").
One more thing, if Ricardo is still listening : no matter which filtering you want to do
in the end, and in addition to the filters mentioned above, you may want to have a look at
the UrlRewriteFilter, at http://www.tuckey.org.
It resembles, and is inspired by, the Apache httpd mod_rewrite module, and it provides /a
lot/ of possibilities in terms of filtering requests.
It works as a servlet filter, not as a Valve. So it will not filter requests as soon as
they come in Tomcat, but it will filter them at the level of one web application.
Depending on your needs, this may be an advantage or an inconvenient.
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