=encoding utf-8

=head1 NAME

ngx_http_secure_link_module - Module ngx_http_secure_link_module




=head1



The C<ngx_http_secure_link_module> module (0.7.18)
is used to check authenticity of requested links,
protect resources from unauthorized access,
and limit link lifetime.





The authenticity of a requested link is verified by comparing the
checksum value passed in a request with the value computed
for the request.
If a link has a limited lifetime and the time has expired,
the link is considered outdated.
The status of these checks is made available in the
C<$secure_link> variable.





The module provides two alternative operation modes.
The first mode is enabled by the L</secure_link_secret>
directive and is used to check authenticity of requested links
as well as protect resources from unauthorized access.
The second mode (0.8.50) is enabled by the
L</secure_link> and L</secure_link_md5>
directives and is also used to limit lifetime of links.





This module is not built by default, it should be enabled with the
C<--with-http_secure_link_module>
configuration parameter.




=head1 Directives

=head2 secure_link


B<syntax:> secure_link I<I<C<expression>>>



B<context:> I<http>


B<context:> I<server>


B<context:> I<location>





Defines a string with variables from which the
checksum value and lifetime of a link will be extracted.





Variables used in an I<C<expression>> are usually associated
with a request; see example below.





The checksum value extracted from the string is compared with
the MD5 hash value of the expression defined by the
L</secure_link_md5> directive.
If the checksums are different, the C<$secure_link> variable
is set to an empty string.
If the checksums are the same, the link lifetime is checked.
If the link has a limited lifetime and the time has expired,
the C<$secure_link> variable is set to “C<0>”.
Otherwise, it is set to “C<1>”.
The MD5 hash value passed in a request is encoded in
L<base64url|http://tools.ietf.org/html/rfc4648#section-5>.





If a link has a limited lifetime, the expiration time
is set in seconds since Epoch (Thu, 01 Jan 1970 00:00:00 GMT).
The value is specified in the expression after the MD5 hash,
and is separated by a comma.
The expiration time passed in a request is available through
the C<$secure_link_expires> variable for a use in
the L</secure_link_md5> directive.
If the expiration time is not specified, a link has the unlimited
lifetime.







=head2 secure_link_md5


B<syntax:> secure_link_md5 I<I<C<expression>>>



B<context:> I<http>


B<context:> I<server>


B<context:> I<location>





Defines an expression for which the MD5 hash value will
be computed and compared with the value passed in a request.





The expression should contain the secured part of a link (resource)
and a secret ingredient.
If the link has a limited lifetime,
the expression should also contain C<$secure_link_expires>.





To prevent unauthorized access, the expression may contain some
information about the client, such as its address and browser version.





Example:

    
    location /s/ {
        secure_link $arg_md5,$arg_expires;
        secure_link_md5 "$secure_link_expires$uri$remote_addr secret";
    
        if ($secure_link = "") {
            return 403;
        }
    
        if ($secure_link = "0") {
            return 410;
        }
    
        ...
    }


The
“C<E<sol>sE<sol>link?md5=_e4Nc3iduzkWRm01TBBNYwE<amp>expires=2147483647>”
link
restricts access to “C<E<sol>sE<sol>link>” for the client with the
IP address 127.0.0.1.
The link also has the limited lifetime until January 19, 2038 (GMT).





On UNIX, the I<C<md5>> request argument value can be obtained as:

    
    echo -n '2147483647/s/link127.0.0.1 secret' | \
        openssl md5 -binary | openssl base64 | tr +/ -_ | tr -d =









=head2 secure_link_secret


B<syntax:> secure_link_secret I<I<C<word>>>



B<context:> I<location>





Defines a secret I<C<word>> used to check authenticity
of requested links.





The full URI of a requested link looks as follows:

    
    /<value>prefix</value>/<value>hash</value>/<value>link</value>


where I<C<hash>> is a hexadecimal representation of the
MD5 hash computed for the concatenation of the link and secret word,
and I<C<prefix>> is an arbitrary string without slashes.





If the requested link passes the authenticity check,
the C<$secure_link> variable is set to the link
extracted from the request URI.
Otherwise, the C<$secure_link> variable
is set to an empty string.





Example:

    
    location /p/ {
        secure_link_secret secret;
    
        if ($secure_link = "") {
            return 403;
        }
    
        rewrite ^ /secure/$secure_link;
    }
    
    location /secure/ {
        internal;
    }


A request of “C<E<sol>pE<sol>5e814704a28d9bc1914ff19fa0c4a00aE<sol>link>”
will be internally redirected to
“C<E<sol>secureE<sol>link>”.





On UNIX, the hash value for this example can be obtained as:

    
    echo -n 'linksecret' | openssl md5 -hex









=head1 Embedded Variables




=over



=item C<$secure_link>




The status of a link check.
The specific value depends on the selected operation mode.



=item C<$secure_link_expires>





The lifetime of a link passed in a request;
intended to be used only in the
L</secure_link_md5> directive.




=back