Cisco's implementation of the DTLS protocol unfortunately does not comply with the relevant standards. OpenSSL 0.9.8m or newer, and 1.0.0-beta2 or newer, contain a compatibility mode which allows interoperation with Cisco's servers. As long as you are using a current version of OpenSSL, you have nothing to worry about -- everything should work optimally. Without a suitable OpenSSL, the openconnect client will fall back to passing packets over the HTTPS connection. This will still work OK, but will suffer quite a lot if your connection has packet loss. For details of why that happens, see http://sites.inka.de/~W1011/devel/tcp-tcp.html If you insist on using ancient buggy versions of OpenSSL, these are the patches you require if you want DTLS to work: For versions of OpenSSL earlier than 0.9.8m, you'll need the Cisco compatibility support: http://cvs.openssl.org/chngview?cn=18037 For versions of OpenSSL earlier than 0.9.8j, a couple of other DTLS bug-fixes are also required: http://cvs.openssl.org/chngview?cn=17500 http://cvs.openssl.org/chngview?cn=17505