Datalink manual

Datalink File Filter

Datalink File Filter (DLFF) component authorizes access to files linked with READ ACCESS DB link control option.

For such datalinks, SQL functions DLURLCOMPLETE() and DLURLPATH() will produce addresses with embeded read access token. This token is in turn used by the SQL function datalink.dl_authorize() to authorize access to the file. This function is then used by the various front-ends.

Firstly, let us make some datalinks:

mydb=# create table t ( link datalink(52) );
NOTICE:  DATALINK DDL:TRIGGER on t
CREATE TABLE

mydb=# insert into t values (dlvalue('/var/www/datalink/test1.txt'));
NOTICE:  DATALINK LINK:/var/www/datalink/test1.txt
INSERT 0 1

Apache Module

Provided is Apache 2 mod_perl module datalink.conf, which is installed into /etc/apache2/sites-available.

Enable it with:

% a2ensite datalink

This can be used with links provided by DLURLCOMPLETE() SQL function.

Be sure to also set dirurl in datalink.directory to enable file-to-URL mapping to get HTTP instead of FILE URLs.

mydb=# update datalink.directory set dirurl='http://localhost/datalink/' where dirpath='/var/www/datalink/';

You will probably want to eventually set dirurl to a fully qualified URL for your server.

mydb=# select dlurlcomplete(link) from t;
                                dlurlcomplete                                 
------------------------------------------------------------------------------
http://localhost/datalink/738a74c2-4126-4350-947b-e9f0c0735411;test1.txt
(1 row)

dlcat

This is similar to cat UNIX shell command, but it works with filenames with embedded read tokens. It will print the contents of the file(s) to standard output. It is a SUID/SGID command (with www-data:www-data), which accesses the files and database pretending to be a web server.

This can be used with filenames provided by DLURLPATH() SQL function.

my_db=# select dlurlpath(link) from t;
                            dlurlpath                             
------------------------------------------------------------------
/var/www/datalink/738a74c2-4126-4350-947b-e9f0c0735411;test1.txt
(1 row)

$ dlcat '/var/www/datalink/738a74c2-4126-4350-947b-e9f0c0735411;test1.txt'

SQL file functions

Functions datalink.read_text and datalink.read_lines provide support for read access tokens.

my_db=# select datalink.read_text('/var/www/datalink/738a74c2-4126-4350-947b-e9f0c0735411;test1.txt');

It will probably be easier still to use them with datalinks directly:

my_db=# select datalink.read_text(link) from t;

Direct file system access (libc)

File filter is currently not implemented for direct file system access (using libc).

Perhaps FUSE filesystem might make sense here.

Datalink manual