Using name-based virtual hosts with SSL adds another layer of complication. Without the SNI extension, it's not generally possible (though a subset of virtual host might work). With SNI, it's necessary to consider the configuration carefully to ensure security is maintained.
The Problem
The problem with using named virtual hosts over SSL is that named virtual hosts rely on knowing what hostname is being requested, and the request can't be read until the SSL connection is established. The ordinary behavior, then, is that the SSL connection is set up using the configuration in the default virtual host for the address where the connection was received.
While Apache can renegotiate the SSL connection later after seeing the hostname in the request (and does), that's too late to pick the right server certificate to use to match the request hostname during the initial handshake, resulting in browser warnings/errors about certificates having the wrong hostname in them.
And while it's possible to put multiple hostnames in a modern certificate and just use that one certificate in the default vhost, there are many hosting providers who are hosting far too many sites on a single address for that to be practical for them.
Prerequisites to use SNI
- Use OpenSSL 0.9.8f or later
- Build OpenSSL with the TLS Extensions option enabled (option enable-tlsext; OpenSSL 0.9.8k and later has this enabled by default).
- Apache must have been built with that OpenSSL (./configure --with-ssl=/path/to/your/openssl). In that case, mod_ssl will automatically detect the availability of the TLS extensions and support SNI.
- Apache must use that OpenSSL at run-time, which might require setting LD_LIBRARY_PATH or equivalent to point to that OpenSSL, maybe in bin/envvars. (You'll get unresolved symbol errors at Apache startup if Apache was built with SNI but isn't finding the right openssl libraries at run-time.)
How can you tell if your Apache build supports SNI? If you configure multiple name-based virtual hosts for an address where SSL is configured, and SNI isn't built into your Apache, then upon Apache startup a message like "You should not use name-based virtual hosts in conjunction with SSL!!" will occur in the error log. If SNI is built in, then the error log will show "[warn] Init: Name-based SSL virtual hosts only work for clients with TLS server name indication support (RFC 4366)".
The client browser must also support SNI. Here are some browsers that do:
- Mozilla Firefox 2.0 or later
- Opera 8.0 or later (with TLS 1.1 enabled)
- Internet Explorer 7.0 or later (on Vista, not XP)
- Google Chrome
- Safari 3.2.1 on Mac OS X 10.5.6
When Apache supports SNI and the client provided the hostname using SNI, the new environment variable SSL_TLS_SNI will be set to the hostname that the client provided.
Scenarios
SNI/Request hostname mismatch, or SNI provides hostname and request doesn't.
This is a browser bug. Apache will reject the request with a 400-type error.
Client doesn't support SNI.
If Apache has SNI support, and a request without the SNI hostname is received for a name-based virtual host over SSL, and SSLStrictSNIVHostCheck is on, it will be rejected (403) and this message logged:
[error] No hostname was provided via SNI for a name based virtual host
If SSLStrictSNIVHostCheck is off, then the request will be handled as if the server did not have SNI support; see above.
Example configuration
# Ensure that Apache listens on port 443
Listen 443
# Listen for virtual host requests on all IP addresses
NameVirtualHost *:443
# Go ahead and accept connections for these vhosts
# from non-SNI clients
SSLStrictSNIVHostCheck off
# Because this virtual host is defined first, it will
# be used as the default if the hostname is not received
# in the SSL handshake, e.g. if the browser doesn't support
# SNI.
DocumentRoot /www/example1
ServerName www.example.com
# Other directives here
DocumentRoot /www/example2
ServerName www.example2.org
# Other directives here
For more please refer:
- http://wiki.apache.org/httpd/NameBasedSSLVHostsWithSNI
- http://www.modssl.org/docs/2.8/ssl_intro.html
