A teacher asked for a directory on https://wwwsec.cs.uu.nl/ to be limited to a specific client certificate. Configuring this exact limitation turned out to be quite some searching as I couldn't find it in the modssl documentation (the site being down a lot of the time this week didn't help either). The correct way to write that you want to limit to a certain subject DN turns out to be: %{SSL_CLIENT_S_DN} eq "/C=NL/ST=Utrecht/L=Utrecht/O=Universiteit Utrecht/... "