The OpenConnect VPN Protocol Version 1.2

The OpenConnect protocol combines the TLS protocol , Datagram TLS protocol and HTTP protocols to provide an Internet-Layer VPN channel. The channel is designed to operate using UDP packets, and fallback on TCP if that's not possible. In brief the protocol initiates an HTTP over TLS connection on a known port, where client authentication is performed. After this step, the client initiates an HTTP CONNECT command to establish a VPN channel over TCP. A secondary VPN channel over UDP will be established using information provided by the server using HTTP headers. At that point the raw IP packets flow, over the VPN channels.

In the OpenConnect VPN protocol, the server is always authenticated using its certificate for the HTTP over TLS session. The server's identity in the certificate SHOULD be placed in the X.509 certificate's SubjectAlternativeName field, and be of type DNSName. This doesn't imply a particular certificate validation model. Clients also use an internet PKIX trust model, or trust on first use key validation model.

The OpenConnect VPN protocol allows for the following types of client authentication, or combinations of them. Password: a user can authenticate itself using a password. Certificate: a user can authenticate itself using a PKIX certificate it possesses. HTTP SPNEGO: a user can authenticate itself using a Kerberos ticket, or any other mechanism supported by SPNEGO (i.e., GSSAPI). It is important to note that during the password and HTTP SPNEGO authentication methods, any headers allowed by the HTTP protocol can be present. In fact, some legacy clients assume that the server will keep a state using cookies and send their username and password in different TLS and HTTP connections. This practice prevents the server from binding the TLS channel with the VPN session , and is discouraged. It is RECOMMENDED for clients to complete authentication in the same TLS session, and rely on TLS session resumption if reconnections to the server are needed.

The client and server establish a TLS connection over a known port, typically over 443, the port used for HTTPS. The client SHOULD negotiate TLS 1.2 or later.

A client initiates the session by start a TLS connection with the server. The initial TLS Client Hello will contain a number of extensions as mandated by the TLS protocol, but the following SHOULD be included. Server Name Indication : the client SHOULD provide the DNS name of the server in the TLS handshake. After the TLS session is established the client irrespective of the supported authentication methods, sends an HTTP POST request on "/" with a config-auth XML structure of type 'init'. The HTTP Content-Type to be used for these XML structures MUST be 'text/xml'. An example of its contents follow.

v5.01 ]]> The precise DTD declarations for the contents of XML messages are defined in this document and are listed in .

After the TLS session is established and the the config-auth XML structure of type 'init' is sent, the server requests the username and password using forms the client software prompts the user to fill in. The server's reply utilizes a config-auth XML structure of type 'auth-request'.

Please enter your username ]]> The client can be asked to provide the information in multiple, separate forms as the above message implies, and any number of passwords may be requested, e.g., when second factor authentication is available, a password and a second factor token may be requested. Alternatively, when the number of inputs are fixed the client may be provided a combined form as listed below.

Please enter your username ]]> The client software is expected to respond to the provided form(s) and send the responses to the server using an HTTP POST on the form action location as specified in the XML message (in the above examples it was "/auth"). The reply would then be of type 'auth-reply' as in the following example.

v5.01 test ]]> As mentioned above, the server may ask repeatedly for information until the user is authenticated. For example, the server could present a second form asking for the password after the username is provided, or ask for a second password if that is necessary, and may even use forms to prompt the user to change a password, provide additional information and so on. When multiple forms are provided the servers responds with an HTTP 200 OK status code and sends its new request. If client authentication fails, the server MUST respond with an HTTP 401 unauthorized status code. On successful authentication the server replies with a 200 HTTP code and use the 'complete' config-auth XML structure as follows.

0.1(1) SSL VPN Service ]]> Note, that including the username and password in XML messages will reveal the length of them to a passive eavesdropper. For that is is RECOMMENDED for clients to use an 'X-Pad' HTTP header, containing arbitrary printable data to make the message length a multiple of 64 bytes.

When a user is authenticated using a certificate, during the initial TLS protocol handshake the server will require a client certificate to be presented. Because under TLS 1.2 the client certificate is sent in the clear during the handshake, the certificate SHOULD NOT contain other identifying information other than a username, or a pseudonymus identifier. It is RECOMMENDED to place the user identifier in the DN field of the certificate, using the UID object identifier (0.9.2342.19200300.100.1.1) . After the TLS session is established and the the config-auth XML structure of type 'init' is sent, the server responds according to certificate validation status. If the certificate sent by the client was successfully validated, the server should reply using the HTTP response code 200, and the contents of the reply should be a config-auth XML structure of type 'complete', as follows.

0.1(1) SSL VPN Service ]]> In that case the client should proceed to the establishment of the primary CSTP channel as in .

The HTTP SPNEGO protocol enables among others authentication using Kerberos tickets. The HTTP SPNEGO method is available using the Generic Security Service API . A client which supports the HTTP SPNEGO protocol, indicates it using the following header on in its initial request to the server with the config-auth 'init' XML structure.

After that the server would report a "401 Unauthorized" status code and authentication would proceed as specified in the HTTP SPNEGO protocol. The server may utilize the following header, to indicate that alternative authentication methods are available (e.g., with plain password), if authentication fails.

If client authentication fails, the server MUST respond with an HTTP 401 unauthorized status code. In that case, a client which received the previous header should retry authenticating to the server without advertising HTTP SPNEGO, meaning the "X-Support-HTTP-Auth: true" header will not be included. Otherwise, on successful authentication the server should reply with a 200 HTTP code and use the 'complete' config-auth XML structure as in . Once the client is successfully validated, the server should reply using the HTTP response code 200, and the contents of the reply should be a config-auth XML structure of type 'complete', as with the certificate authentication.

By the receipt of a 'complete' config-auth XML structure, the client issues an HTTP CONNECT request to initiate the VPN tunnel. An example CONNECT request is shown below.

As each client supports different capabilities, the following HTTP headers are used during the CONNECT request to advertise them. X-CSTP-Address-Type: A comma separated list of the requested address types. IPv4: when the client only supports IPv4 addresses. IPv6: when the client only supports IPv6 addresses. IPv4,IPv6: when the client supports both types of IP addresses. X-CSTP-Base-MTU: The MTU of the link as estimated by the client. X-CSTP-Accept-Encoding: A comma separated list of accepted compression algorithms for the CSTP channel (optional). Compression on encrypted streams introduces additional risk, see for more information. User-Agent: A string identifying the client software. By convention OpenConnect clients identify as "Open AnyConnect VPN Agent". This string is informative to the server and its operator.

After a successful receipt of an HTTP CONNECT request, the server responds and provides the client with configuration parameters. The available tunnel configuration options are listed below. X-CSTP-Address: The IPv4 address of the client, if IPv4 has been requested. X-CSTP-Netmask: An IPv4 netmask to be pushed to the client, if IPv4 has been requested. This should contain the mask on the P-t-P link and is RECOMMENDED the server address to be the first in defined network. X-CSTP-Address-IP6: The IPv6 address of the client in CIDR notation, if IPv6 has been requested. The prefix length is RECOMMENDED to be set to 127-bits according to . X-CSTP-DNS: The IP address of a DNS server that can be used for that session. X-CSTP-Default-Domain: The DNS default search domains. Typically a subset of X-CSTP-Split-DNS. If multiple, the domains are space separated. X-CSTP-Split-DNS: A DNS domain the provided DNS servers respond for. Multiple such headers may be present for different domains. X-CSTP-Split-Include: The network address of a route which is provided by this server. Multiple such headers may be present. X-CSTP-Split-Exclude: The network address of a route that is not provided by this server. Multiple such headers may be present. X-CSTP-Base-MTU: The MTU of the link as estimated by this server. X-CSTP-DynDNS: Set to "true" if the server is operating with a dynamic DNS address. X-CSTP-Content-Encoding: if present is it set to one of the values presented by the client in 'X-CSTP-Accept-Encoding' header. It contains the compression algorithm used in the CSTP channel. X-DTLS-Content-Encoding: if present is it set to one of the values presented by the client in 'X-DTLS-Accept-Encoding' header. It will be the compression algorithm used in the DTLS channel. The received options are the client's tunnel networking configuration. If no "X-CSTP-Split-Include" headers are present, the client is expected to assign its default route through the VPN. After the server's response to the CONNECT request, the VPN tunnel is established. This tunnel consists of two channels, the CSTP channel and the (optional) DTLS channel that are described in the next sections.

The previous HTTP message is the last HTTP message sent by the server. After that message, the established TCP connection forms a channel that is transports IP packets between the client and the server. We refer to it as the CSTP channel in the rest of this document. The encoding of the transferred packets is described further in .

The secondary DTLS based channel over UDP is established optionally by clients and servers that wish to avoid the drawbacks of tunneling TCP over TCP. This channel -referred to as the DTLS channel- is established if the client advertises support for it during the HTTP CONNECT request (see ). This is done by the client including the following headers in the request. X-DTLS-CipherSuite: Must contain the keyword PSK-NEGOTIATE. X-DTLS-Accept-Encoding: A comma separated list of accepted compression algorithms for the DTLS channel (optional). The same risks as with the primary CSTP channel apply for compression. An example HTTP CONNECT request that advertises support for the DTLS channel is shown below.

The server's response to the HTTP CONNECT request, includes the following headers, if the server wishes to establish the DTLS channel. X-DTLS-App-ID: A hex encoded value to be used as a DTLS application-specific identifier by the client. It serves as an identifier for the server to associate the incoming DTLS session with the TLS session. The identifier (before encoding) can be from 16 to 32 bytes. X-DTLS-Port: The port number to which the client should send UDP packets for DTLS. X-DTLS-CipherSuite: It must contain the value "PSK-NEGOTIATE" without any quotes. X-DTLS-Rekey-Time: The time (in seconds) after which the DTLS session should rekey, see . Only considered if applicable to the negotiated DTLS protocol. X-DTLS-Rekey-Method: The method used in DTLS rekey, see . Only considered if applicable to the negotiated DTLS protocol.

After the DTLS channel is negotiated over the CSTP channel, it is established by the client initiating a DTLS session. The client initiates a UDP connection to the IP address if the server and port as specified by the X-DTLS-Port value. The new UDP connection uses the DTLS 1.2 protocol (or any later version) with the PSK key exchange method. The pre-shared key material for this channel are generated by both the server and the client independently and is not exchanged. The pre-shared key is a 256-bit value generated with an exporter from the TLS session of the CSTP channel. The key material exporter uses the label "EXPORTER-openconnect-psk" without the quotes, and without any context value. In its DTLS Client Hello message the client must copy the value received in the 'X-DTLS-App-ID' header after hex decoding it, to the session ID field of the DTLS Client Hello. That identifier is not used for session resumption, and is used by the server when it receives the first UDP message to associate the new DTLS protocol connection with the corresponding CSTP channel.

An overview of the established tunnel and channels is shown in .

| | | | | TLS handshake Finished | | | <----------------------------------- | | | | | HTTP POST config-auth init | ,--------------------!. | -----------------------------------> |This is an HTTP over|_\ | | |TLS session. | | | `----------------------' | config-auth auth-request | | | <----------------------------------- | | | | | HTTP POST config-auth auth-reply | | | -----------------------------------> | | | | | config-auth complete | | | <----------------------------------- | | | | | HTTP CONNECT | | | -----------------------------------> | | | | | | | | =================================== | ====================== CSTP VPN session is established ======================= | =================================== | | | | | | ,-------------------------!. | TLS record packet with CSTP payload| |These packets show |_\ | -----------------------------------> |that IP traffic can start | | | |prior to the DTLS channel | | | |establishment. | | | `---------------------------' | TLS record packet with CSTP payload| | | <----------------------------------- | | | | | DTLS handshake Client Hello | | - - - - - - - - - - - - - - - - - - - - - - - - - - - > | | | | DTLS handshake Finished | | <- - - - - - - - - - - - - - - - - - - - - - - - - - - - | | | | | | | =================================== | ====================== DTLS VPN channel is established ======================= | =================================== | | | | | DTLS record packet with payload | | - - - - - - - - - - - - - - - - - - - - - - - - - - - > | | | | DTLS record packet with payload | | <- - - - - - - - - - - - - - - - - - - - - - - - - - - - Client ,--+---. ,----+-----. ,-. |Server| |ServerDTLS| `-' `------' `----------' /|\ | / \ ]]>

The format of the packets sent over the primary channel consists of an 8-bytes header followed by data. The whole packet in encapsulated in a TLS record (see ). The bytes of the header indicate the type of data that follow, and their contents are explained in . byte value 0fixed to 0x53 (S) 1fixed to 0x54 (T) 2fixed to 0x46 (F) 3fixed to 0x01 4-5The length of the packet that follows this header in big endian order 6The type of the payload that follows (see for available types) 7fixed to 0x00 The available payload types are listed in . Value Description 0x00DATA: the TLS record packet contains an IPv4 or IPv6 packet 0x03DPD-REQ: used for dead peer detection. Once sent the peer should reply with a DPD-RESP packet, that has the same contents as the original request. 0x04DPD-RESP: used as a response to a previously received DPD-REQ. 0x05DISCONNECT: sent by the client (or server) to terminate the session. This is followed by one byte indicating the disconnect reason. When the reason is '0xb0' the session should be invalidated after the request. 0x07KEEPALIVE: sent by any peer. No data is associated with this request. 0x08COMPRESSED DATA: a Data packet which is compressed prior to encryption. 0x09TERMINATE: sent by the server to indicate that the server is shutting down. No data is associated with this request.

The format of the packets sent over the DTLS channel consists of an 1-byte header followed by data. The header byte indicates the type of data that follow as in . The header and the data are encapsulated in a DTLS record packet (see ).

During the exchange of session parameters (), the server advertises the methods available for session rekey using the "X-CSTP-Rekey-Method" and "X-DTLS-Rekey-Method" HTTP headers. The available options for both the server and client are listed below. none: no rekey; the session will go on until 2^48 DTLS records have been exchanged, or 2^64 TLS records. ssl: a TLS or DTLS rekey will be performed periodically. Under TLS/DTLS 1.2 this is performed using a rehandshake, and in later versions using a rekey. new-tunnel: the session will tear down and the client will reconnect periodically. When the value is other than "none" the rekey period is determined by the "X-CSTP-Rekey-Time" and "X-DTLS-Rekey-Time" headers. These headers contain the time in seconds after which a session should rekey. It should be noted that when the "ssl" rekey option is used under TLS1.2, care must be taken by both the client and the server to ensure that either safe renegotiation is used (), or that the identity of the peer remains the same.

In OpenConnect there are two packet types that can be used for keep-alive or dead peer detection, as shown in . These are the DPD-REQ and KeepAlive packets. The timings of the transmission of these packets are set by the server, and they for the DPD are advisory to a client. However, any peer receiving these packets MUST response with the appropriate packet. For DPD-REQ packets, the response MUST be DPD-RESP, and for KeepAlive packets the response must be another KeepAlive packet. The main difference between these two types of packets, is that the DPD packets similarly to are sent when there is no traffic or when the other party requests them, and allow for arbitrary data to be attached, making them suitable for Path MTU detection. The server advertises the suggested periods during the tunnel establishment (). The available headers are listed below. X-CSTP-DPD: applicable to CSTP channel; contains a relative time in seconds. X-CSTP-Keepalive: applicable to CSTP channel; contains a relative time in seconds. X-DTLS-DPD: applicable to DTLS channel; contains a relative time in seconds. X-DTLS-Keepalive: applicable to DTLS channel; contains a relative time in seconds.