SSL: the "ssl_verify_client" directive parameter "optional_no_ca". This parameter allows to don't require certificate to be signed by a trusted CA, e.g. if CA certificate isn't known in advance, like in WebID protocol. Note that it doesn't add any security unless the certificate is actually checked to be trusted by some external means (e.g. by a backend). Patch by Mike Kazantsev, Eric O'Connor.