Why do I get an error about mod_auth_pgsql when I start my site?

On CentOS 4 or Red Hat Enterprise Linux 4, you may get an error at startup that suggests an Apache problem.

The error looks like this: Starting httpd: Syntax error on line 6 of /etc/httpd/conf.d/auth_pgsql.conf: Cannot load /etc/httpd/modules/mod_auth_pgsql.so into server: libpq.so.3: cannot open shared object file: No such file or directory [FAILED]

This can result from installing Red Hat or CentOS with the wrong options. You can fix it by creating this symlink:

• ln -s /usr/lib/libpq.so.4 /usr/lib/libpq.so.3