Use modern cryptographic practices when setting up SSL and TLS services on your server
To make sure that your iOS and OS X clients can connect to your server, use Diffie-Hellman keys with a group size of 2048 bits or greater.
To ensure security and privacy for your users, and interoperability with Apple products, server administrators should use a group size of 2048 bits or greater when using Diffie-Hellman key exchange.
Services that might use these types of connections include:
Enterprise Wi-Fi (802.1X)
Secure email connections
Secure web connections (HTTPS)
Secure Internet printing (IPP over TLS/SSL)
iOS 8.4 and OS X v10.10.4
If your users can't establish a secure connection after updating, check the configuration of your server. Refer to these links for more information: