diff options
Diffstat (limited to 'doc/doxygen/doc_protocol_overview.h')
| -rw-r--r-- | doc/doxygen/doc_protocol_overview.h | 195 |
1 files changed, 195 insertions, 0 deletions
diff --git a/doc/doxygen/doc_protocol_overview.h b/doc/doxygen/doc_protocol_overview.h new file mode 100644 index 0000000..0821222 --- /dev/null +++ b/doc/doxygen/doc_protocol_overview.h @@ -0,0 +1,195 @@ +/* + * OpenVPN -- An application to securely tunnel IP networks + * over a single TCP/UDP port, with support for SSL/TLS-based + * session authentication and key exchange, + * packet encryption, packet authentication, and + * packet compression. + * + * Copyright (C) 2010-2018 Fox Crypto B.V. <openvpn@fox-it.com> + * + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2 + * as published by the Free Software Foundation. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License along + * with this program; if not, write to the Free Software Foundation, Inc., + * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. + */ + +/** + * @file Network protocol overview documentation file. + */ + +/** + * @page network_protocol OpenVPN's network protocol + * + * Description of packet structure in OpenVPN's network protocol. + * + * This document describes the structure of packets exchanged between + * OpenVPN peers. It is based on the protocol description in the \c ssl.h + * file. + * + * @section network_protocol_external Outer structure of packets exchanged between OpenVPN peers + * + * VPN tunnel packets are transported between OpenVPN peers using the UDP + * or TCP protocols. Their structure is described below. + * + * @subsection network_protocol_external_structure External packet structure + * + * - packet length (16 bits, unsigned) [TCP-mode only]: always sent as + * plain text. Since TCP is a stream protocol, this packet length + * defines the packetization of the stream. + * - packet opcode and key_id (8 bits) [TLS-mode only]: + * - package message type (high 5 bits) + * - key_id (low 3 bits): the key_id refers to an already negotiated + * TLS session. OpenVPN seamlessly renegotiates the TLS session by + * using a new key_id for the new session. Overlap (controlled by + * user definable parameters) between old and new TLS sessions is + * allowed, providing a seamless transition during tunnel operation. + * - payload (n bytes) + * + * @subsection network_protocol_external_types Message types + * + * The type of a VPN tunnel packet is indicated by its opcode. The + * following describes the various opcodes available. + * + * - Control channel messages: + * - \ref P_CONTROL_HARD_RESET_CLIENT_V1 -- %Key method 1, initial %key + * from client, forget previous state. + * - \ref P_CONTROL_HARD_RESET_SERVER_V1 -- %Key method 1, initial %key + * from server, forget previous state. + * - \ref P_CONTROL_HARD_RESET_CLIENT_V2 -- %Key method 2, initial %key + * from client, forget previous state. + * - \ref P_CONTROL_HARD_RESET_SERVER_V2 -- %Key method 2, initial %key + * from server, forget previous state. + * - \ref P_CONTROL_SOFT_RESET_V1 -- New %key, with a graceful + * transition from old to new %key in the sense that a transition + * window exists where both the old or new key_id can be used. + * - \ref P_CONTROL_V1 -- Control channel packet (usually TLS + * ciphertext). + * - \ref P_ACK_V1 -- Acknowledgement for control channel packets + * received. + * - Data channel messages: + * - \ref P_DATA_V1 -- Data channel packet containing data channel + * ciphertext. + * - \ref P_DATA_V2 -- Data channel packet containing peer-id and data + * channel ciphertext. + * + * @subsection network_protocol_external_key_id Session IDs and Key IDs + * + * OpenVPN uses two different forms of packet identifiers: + * - The first form is 64 bits and is used for all control channel + * messages. This form is referred to as a \c session_id. + * - Data channel messages on the other hand use a shortened form of 3 + * bits for efficiency reasons since the vast majority of OpenVPN + * packets in an active tunnel will be data channel messages. This + * form is referred to as a \c key_id. + * + * The control and data channels use independent packet-id sequences, + * because the data channel is an unreliable channel while the control + * channel is a %reliable channel. Each use their own independent HMAC + * keys. + * + * @subsection network_protocol_external_reliable Control channel reliability layer + * + * Control channel messages (\c P_CONTROL_* and \c P_ACK_* message types) + * are TLS ciphertext packets which have been encapsulated inside of a + * reliability layer. The reliability layer is implemented as a + * straightforward acknowledge and retransmit model. + * + * Acknowledgments of received messages can be encoded in either the + * dedicated \c P_ACK_* record or they can be prepended to a \c + * P_CONTROL_* message. + * + * See the \link reliable Reliability Layer\endlink module for a detailed + * description. + * + * @section network_protocol_control Structure of control channel messages + * + * @subsection network_protocol_control_ciphertext Structure of ciphertext control channel messages + * + * Control channel packets in ciphertext form consist of the following + * parts: + * + * - local \c session_id (random 64 bit value to identify TLS session). + * - HMAC signature of entire encapsulation header for HMAC firewall + * [only if \c --tls-auth is specified] (usually 16 or 20 bytes). + * - packet-id for replay protection (4 or 8 bytes, includes sequence + * number and optional \c time_t timestamp). + * - acknowledgment packet-id array length (1 byte). + * - acknowledgment packet-id array (if length > 0). + * - acknowledgment remote session-id (if length > 0). + * - packet-id of this message (4 bytes). + * - TLS payload ciphertext (n bytes) (only for \c P_CONTROL_V1). + * + * Note that when \c --tls-auth is used, all message types are protected + * with an HMAC signature, even the initial packets of the TLS handshake. + * This makes it easy for OpenVPN to throw away bogus packets quickly, + * without wasting resources on attempting a TLS handshake which will + * ultimately fail. + * + * @subsection network_protocol_control_key_methods Control channel key methods + * + * Once the TLS session has been initialized and authenticated, the TLS + * channel is used to exchange random %key material for bidirectional + * cipher and HMAC keys which will be used to secure data channel packets. + * OpenVPN currently implements two %key methods. %Key method 1 directly + * derives keys using random bits obtained from the \c rand_bytes() function. + * %Key method 2 mixes random %key material from both sides of the connection + * using the TLS PRF mixing function. %Key method 2 is the preferred method and + * is the default for OpenVPN 2.0+. + * + * The @ref key_generation "Data channel key generation" related page + * describes the %key methods in more detail. + * + * @subsection network_protocol_control_plaintext Structure of plaintext control channel messages + * + * - %Key method 1 (support removed in OpenVPN 2.5): + * - Cipher %key length in bytes (1 byte). + * - Cipher %key (n bytes). + * - HMAC %key length in bytes (1 byte). + * - HMAC %key (n bytes). + * - %Options string (n bytes, null terminated, client/server %options + * string should match). + * - %Key method 2: + * - Literal 0 (4 bytes). + * - %Key method (1 byte). + * - \c key_source structure (\c key_source.pre_master only defined + * for client -> server). + * - %Options string length, including null (2 bytes). + * - %Options string (n bytes, null terminated, client/server %options + * string must match). + * - [The username/password data below is optional, record can end at + * this point.] + * - Username string length, including null (2 bytes). + * - Username string (n bytes, null terminated). + * - Password string length, including null (2 bytes). + * - Password string (n bytes, null terminated). + * + * @section network_protocol_data Structure of data channel messages + * + * The P_DATA_* payload represents encapsulated tunnel packets which tend to be + * either IP packets or Ethernet frames. This is essentially the "payload" of + * the VPN. Data channel packets consist of a data channel header, and a + * payload. There are two possible formats: + * + * @par P_DATA_V1 + * P_DATA_V1 packets have a 1-byte header, carrying the \ref P_DATA_V1 \c opcode + * and \c key_id, followed by the payload:\n + * <tt> [ 5-bit opcode | 3-bit key_id ] [ payload ] </tt> + * + * @par P_DATA_V2 + * P_DATA_V2 packets have the same 1-byte opcode/key_id, but carrying the \ref + * P_DATA_V2 opcode, followed by a 3-byte peer-id, which uniquely identifies + * the peer:\n + * <tt> [ 5-bit opcode | 3-bit key_id ] [ 24-bit peer-id ] [ payload ] </tt> + * + * See @ref data_crypto for details on the data channel payload format. + * + */ |