Why can't I use SSL with name-based/non-IP-based virtual hosts?
The reason is very technical. Actually it's some sort of a chicken and egg problem: The SSL protocol layer stays below the HTTP protocol layer and encapsulates HTTP. When an SSL connection (HTTPS) is established Apache/mod_ssl has to negotiate the SSL protocol parameters with the client. For this mod_ssl has to consult the configuration of the virtual server (for instance it has to look for the cipher suite, the server certificate, etc.). But in order to dispatch to the correct virtual server Apache has to know the Host HTTP header field. For this the HTTP request header has to be read. This cannot be done before the SSL handshake is finished. But the information is already needed at the SSL handshake phase. Bingo!
Why is it not possible to use Name-Based Virtual Hosting to identify different SSL virtual hosts?
Name-Based Virtual Hosting is a very popular method of identifying different virtual hosts. It allows you to use the same IP address and the same port number for many different sites. When people move on to SSL, it seems natural to assume that the same method can be used to have lots of different SSL virtual hosts on the same server.
It comes as rather a shock to learn that it is impossible.
The reason is that the SSL protocol is a separate layer which encapsulates the HTTP protocol. So the problem is that the SSL session is a separate transaction that takes place before the HTTP session even starts. Therefore all the server receives is an SSL request on IP address X and port Y (usually 443). Since the SSL request does not contain any Host: field, the server has no way to decide which SSL virtual host to use. Usually, it will just use the first one it finds that matches the port and IP address.
You can, of course, use Name-Based Virtual Hosting to identify many non-SSL virtual hosts (all on port 80, for example) and then you can have no more than 1 SSL virtual host (on port 443). But if you do this, you must make sure to put the non-SSL port number on the NameVirtualHost directive, e.g.