Years after that, I was contacted by David Woodhouse, proposing changes to GnuTLS in order to support an early draft version of Datagram TLS (DTLS). Since we already had support for the final version of DTLS (i.e, 1.0), I couldn't understand the request. As it seems David was working on openconnect, a client for the CISCO AnyConnect SSL VPN protocol.
That intrigued me, as it was the first SSL VPN solution I had heard of that used Datagram TLS to transfer data. My interest increased as I learned more about openconnect, so much that I even ended-up writing the server counterpart of openconnect. Because the details of the protocol are still confined to David's head and mine, I'll attempt in this post to describe them on a higher level than source code.
Key exchange & Authentication
The protocol is very simple in nature and is HTTP-based. Initially the client connects to the server over TLS (note that TLS runs only over TCP, something that I take as granted on the rest of this text). On that TLS session the server is authenticated using its certificate, and the client may optionally be authenticated using a certificate as well. After the TLS key exchange is complete, the client obtains an authentication page by issuing an HTTP "POST /" request containing the following.
<config-auth client="vpn" type="init">
<version who="vpn">v5.01</version>
<device-id>linux-64</device-id>
<group-access>https://example.com</group-access>
</config-auth>
If the client did not present a certificate, or if additional information is required (e.g., an one-time password), the following takes place. The server replies with a request for the client's username that looks like:
<auth id="main"> <message>Please enter your username</message> <form action="/auth" method="post"> </form>
</auth>Which effectively contains the message to be printed to the user, as well the URL (/auth) where the string obtained by the user should be sent. The client subsequently replies with a POST containing his username.
<config-auth client="vpn" type="auth-reply">
<version who="vpn">v5.01</version>
<device-id>linux-64</device-id>
<auth><username>test</username></auth>
</config-auth>
Once the username is received by the server, a similar conversation continues for the number of passwords that are required by the user. The message format remains essencially the same so we skip this part.
VPN tunnel establishment
When authenticated, the client issues an HTTP CONNECT request. That effectively terminates the HTTP session and initiates a VPN tunnel over the existing TLS session. In short the client issues:CONNECT /CSCOSSLC/tunnel HTTP/1.1
User-Agent: Open AnyConnect VPN Agent v5.01 X-CSTP-Version: 1 X-CSTP-MTU: 1280 X-CSTP-Address-Type: IPv6,IPv4 X-DTLS-Master-Secret: DAA8F66082E7661AE593 [truncated] X-DTLS-CipherSuite: AES256-SHA:AES128-SHA:DES-CBC3-SHA
and the server replies with something that looks like the following.
HTTP/1.1 200 CONNECTED X-CSTP-Version: 1 X-CSTP-DPD: 440 X-CSTP-Address: 192.168.1.191 X-CSTP-Netmask: 255.255.255.0 X-CSTP-DNS: 192.168.1.190 X-CSTP-Split-Include: 192.168.1.0/255.255.255.0 X-CSTP-Keepalive: 32400 X-CSTP-Rekey-Time: 115200 X-CSTP-Rekey-Method: new-tunnel X-DTLS-Session-ID: 767a9ad8 [truncated] X-DTLS-DPD: 440 X-DTLS-Port: 443 X-DTLS-Rekey-Time: 115200 X-DTLS-Keepalive: 32400 X-DTLS-CipherSuite: AES128-SHA X-DTLS-MTU: 1214 X-CSTP-MTU: 1214
This completes the HTTP authentication phase of the protocol. At this point a VPN tunnel is established over TLS, and the client obtains the IP address present in the "X-CSTP-Address" header, and adds the "X-CSTP-Split-Include" routes to its routing table. The IP packets read from the local TUN device are sent via the tunnel to the peer with an 8-byte prefix, that allows distinguishing IP data from various VPN packets such as keep-alive or dead-peer-detection.
It is, however, well known that TCP over TCP is far from being optimal, and for this reason the server provides the option to the client for an additional tunnel over UDP and DTLS.
VPN tunnel over UDP
To initiate the Datagram TLS over UDP session the client sends the "X-DTLS-Master-Secret" and "X-DTLS-CipherSuite" headers at its CONNECT request (see above). The former contains the key to be used as the pre-master key, in TLS terminology, and the latter contains a list of ciphersuites as read by OpenSSL. The server replies on these requests by accepting a ciphersuite and presenting it in its "X-DTLS-CipherSuite" header, adding the headers such as "X-DTLS-Port" and "X-DTLS-Session-ID" and others less relevant for this description.At the receipt of that information the client initiates a Datagram TLS session (using a draft version of DTLS 1.0) on the port indicated by the server. A good question at this point is how is the server associating the new DTLS session request with this particular client. The hint here is that the client copies the value in the "X-DTLS-Session-ID" header to its Session ID field of the DTLS Client Hello. That session is in effect handled as a session resumption that uses the "X-DTLS-Master-Secret" as pre-master secret, the previous session ID and the ciphersuite negotiated in the "X-DTLS-CipherSuite" header (presumably that hackish approach exists because this solution pre-dates TLS with preshared keys).
That completes the DTLS negotiation over UDP, and the establishment of the second VPN tunnel. From this point this tunnel is being used as primary, and if for some reason it goes down the traffic is redirected to the backup tunnel over TLS. A difference of the DTLS tunnel with the TLS one, is that the VPN packet header is a single byte instead of 8. The smaller header is an important save since DTLS packets are restricted by the link MTU which is further reduced by the headers of IP, UDP, and DTLS.
Rekeying
DTLS allows the transfer up to 248 packets (or 220 petabytes - assuming an average of 800 bytes per packet) in a single session. To allow for even larger transfers and to refresh the keys used the protocol enforces re-keying by time as indicated by the "X-DTLS-Rekey-Time". At the moment openconnect implements that by tearing up both the TLS and DTLS tunnels and reconnecting to the server.
Reconnections
Reconnections in this protocol, e.g., because of the client switching networks and changing IP, are handled on the HTTP level. That is the client accepts a cookie by the server and uses it on any subsequent connection.
Data transfer
In the VPN tunnel establishment we show the negotiation of the ciphersuite used for the DTLS tunnel and actual data transfer. Due to the protocol's restriction to pre-DTLS 1.0 the available ciphersuite options are the following three:
- AES256-CBC-SHA
- AES128-CBC-SHA
- 3DES-CBC-SHA
