DAAP Authentication
Beginning with iTunes 4.2, Apple introduced authentication to DAAP sharing, meaning that the only clients that could connect to iTunes servers were other instances of iTunes. This was further modified in iTunes 4.5 to use a custom hashing algorithm, rather than the standard MD5 function used previously. Both authentication methods were successfully reverse engineered within months of release.
With iTunes 7.0, a new 'Client-DAAP-Validation' header hash is needed when connecting to an iTunes 7.0 server. This does not affect third-party DAAP servers, but all current DAAP clients (including official iTunes before iTunes 7.0) will fail to connect to an iTunes 7.0 server, receiving a '403 Forbidden' HTTP error. The iTunes 7.0 authentication traffic analysis seem to indicate that a certificate exchange is performed to calculate the hash sent in the 'Client-DAAP-Validation' header.
As of August 2012, the iTunes 7.0+ DAAP authentication still hasn't been reverse engineered, so no third-party application can stream from iTunes 7.x, 8.x, 9.x or 10.x servers.
Read more about this topic: Digital Audio Access Protocol