Limit_zone: Using other variables than $binary_remote_addr

Server NGINX
1

I want to restrict the number of connections people can have to our
download server. Limiting the concurrent connections by ip address is
not very useful, because if I only allow 1 connection per ip address
then a user cannot download multiple files at once. And if I allow n > 1
connections per IP address, then some download managers will create
multiple connections and other users complain about fairness.

Currently one can use the limit_zone module to restrict the number of
concurrent connections per ip address. An interesting use case would be
to use some token in the URL (for example a GET parameter or a part of
the requested path) instead of the ip address as the limit_zone
$variable mentioned in the wiki:
http://wiki.codemongers.com/NginxHttpLimitZoneModule

For example if I hand out the URL
http://example.org/file.zip?token={SOME_MD5_SUM} to a client I do not
want to allow more than one concurrent connection that uses the given
token. Is this currently possible in nginx? If yes, then which $variable
do I have to use in the limit_zone directive?

Of course, I would have to verify that the token is one that I have
given to the client and that it has not been constructed arbitrarily.
But this can be easily solved with a little PHP and the excellent
“X-Accel-Redirect” header that nginx supports.

Kind regards
Steffen W.

2

You can use any variable in limit_zone, e.g.,

limit_zone one $my_var 10m;
…
server {
…
set $my_var ;
limit_conn one 1;
}

3

Thanks! And just in case anybody is still unsure what to do in my case
(GET parameter “hash”) I was able to use

limit_zone downloads $arg_hash 10m;

Steffen

4

One further question: Can I somehow prevent that the requests denied by
limit_zone end up in my access log? Some download managers are quiet
aggressive and flood the log.

It would be okay if all requests denied with a 503 response code were
not logged. I think this should be possible by making the “access_log”
directive conditional (using an if clause). But I have not found a
suitable variable that I could use in the conditon.

Steffen