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libnl/lib/doc.c
Thomas Graf 4fd5f7cb66 Documentation update
2008-12-03 19:58:41 +01:00

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/*
* lib/doc.c Documentation Purpose
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation version 2.1
* of the License.
*
* Copyright (c) 2003-2008 Thomas Graf <tgraf@suug.ch>
*/
/**
* @mainpage
*
* @section intro Introduction
*
* libnl is a set of libraries to deal with the netlink protocol and some
* of the high level protocols implemented on top of it. Its goal is to
* simplify netlink protocol usage and to create an abstraction layer using
* object based interfaces for various netlink based subsystems.The library
* was developed and tested on the 2.6.x kernel releases but it may work with
* older kernel series.
*
* @section toc Table of Contents
*
* - \subpage core_doc
* - \subpage route_doc
* - \subpage genl_doc
* - \subpage nf_doc
*
* @section remarks Remarks
*
* @subsection cache_alloc Allocation of Caches
*
* Almost all subsystem provide a function to allocate a new cache
* of some form. The function usually looks like this:
* @code
* struct nl_cache *<object name>_alloc_cache(struct nl_sock *sk);
* @endcode
*
* These functions allocate a new cache for the own object type,
* initializes it properly and updates it to represent the current
* state of their master, e.g. a link cache would include all
* links currently configured in the kernel.
*
* Some of the allocation functions may take additional arguments
* to further specify what will be part of the cache.
*
* All such functions return a newly allocated cache or NULL
* in case of an error.
*
* @subsection addr Setting of Addresses
* @code
* int <object name>_set_addr(struct nl_object *, struct nl_addr *)
* @endcode
*
* All attribute functions avaiable for assigning addresses to objects
* take a struct nl_addr argument. The provided address object is
* validated against the address family of the object if known already.
* The assignment fails if the address families mismatch. In case the
* address family has not been specified yet, the address family of
* the new address is elected to be the new requirement.
*
* The function will acquire a new reference on the address object
* before assignment, the caller is NOT responsible for this.
*
* All functions return 0 on success or a negative error code.
*
* @subsection flags Flags to Character StringTranslations
* All functions converting a set of flags to a character string follow
* the same principles, therefore, the following information applies
* to all functions convertings flags to a character string and vice versa.
*
* @subsubsection flags2str Flags to Character String
* @code
* char *<object name>_flags2str(int flags, char *buf, size_t len)
* @endcode
* @arg flags Flags.
* @arg buf Destination buffer.
* @arg len Buffer length.
*
* Converts the specified flags to a character string separated by
* commas and stores it in the specified destination buffer.
*
* @return The destination buffer
*
* @subsubsection str2flags Character String to Flags
* @code
* int <object name>_str2flags(const char *name)
* @endcode
* @arg name Name of flag.
*
* Converts the provided character string specifying a flag
* to the corresponding numeric value.
*
* @return Link flag or a negative value if none was found.
*
* @subsubsection type2str Type to Character String
* @code
* char *<object name>_<type>2str(int type, char *buf, size_t len)
* @endcode
* @arg type Type as numeric value
* @arg buf Destination buffer.
* @arg len Buffer length.
*
* Converts an identifier (type) to a character string and stores
* it in the specified destination buffer.
*
* @return The destination buffer or the type encoded in hexidecimal
* form if the identifier is unknown.
*
* @subsubsection str2type Character String to Type
* @code
* int <object name>_str2<type>(const char *name)
* @endcode
* @arg name Name of identifier (type).
*
* Converts the provided character string specifying a identifier
* to the corresponding numeric value.
*
* @return Identifier as numeric value or a negative value if none was found.
*
* @page core_doc Core Library (-lnl)
*
* @section core_intro Introduction
*
* The core library contains the fundamentals required to communicate over
* netlink sockets. It deals with connecting and unconnecting of sockets,
* sending and receiving of data, provides a customizeable receiving state
* machine, and provides a abstract data type framework which eases the
* implementation of object based netlink protocols where objects are added,
* removed, or modified with the help of netlink messages.
*
* @section core_toc Table of Contents
*
* - \ref proto_fund
* - \ref sk_doc
* - \ref rxtx_doc
* - \ref cb_doc
*
* @section proto_fund Netlink Protocol Fundamentals
*
* The netlink protocol is a socket based IPC mechanism used for communication
* between userspace processes and the kernel. The netlink protocol uses the
* \c AF_NETLINK address family and defines a protocol type for each subsystem
* protocol (e.g. NETLINK_ROUTE, NETLINK_NETFILTER, etc). Its addressing
* schema is based on a 32 bit port number, formerly referred to as PID, which
* uniquely identifies each peer.
*
* The netlink protocol is based on messages each limited to the size of a
* memory page and consists of the netlink message header (struct nlmsghdr)
* plus the payload attached to it. The payload can consist of arbitary data
* but often contains a fixed sized family specifc header followed by a
* stream of \ref attr_doc. The use of attributes dramatically increases
* the flexibility of the protocol and allows for the protocol to be
* extended while maintaining backwards compatibility.
*
* The netlink message header (struct nlmsghdr):
* @code
* 0 1 2 3
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
* +-------------------------------------------------------------+
* | Length |
* +------------------------------+------------------------------+
* | Type | Flags |
* +------------------------------+------------------------------+
* | Sequence Number |
* +-------------------------------------------------------------+
* | Port (Address) |
* +-------------------------------------------------------------+
* @endcode
*
* Netlink differs between requests, notifications, and replies. Requests
* are messages which have the \c NLM_F_REQUEST flag set and are meant to
* request an action from the receiver. A request is typically sent from
* a userspace process to the kernel. Every request should be assigned a
* sequence number which should be incremented for each request sent on the
* sending side. Depending on the nature of the request, the receiver may
* reply to the request with regular netlink messages which should contain
* the same sequence number as the request it relates to. Notifications are
* of informal nature and don't expect a reply, therefore the sequence number
* is typically set to 0. It should be noted that unlike in protocols such as
* TCP there is no strict enforcment of the sequence number. The sole purpose
* of sequence numbers is to assist a sender in relating replies to the
* corresponding requests.
*
* @msc
* A,B;
* A=>B [label="GET (seq=1, NLM_F_REQUEST)"];
* A<=B [label="PUT (seq=1)"];
* ...;
* A<=B [label="NOTIFY (seq=0)"];
* @endmsc
*
* If the size of a reply exceeds the size of a memory page and thus exceeds
* the maximum message size, the reply can be split into a series of multipart
* messages. A multipart message has the \c flag NLM_F_MULTI set and the
* receiver is expected to continue parsing the reply until the special
* message type \c NLMSG_DONE is received.
*
* @msc
* A,B;
* A=>B [label="GET (seq=1, NLM_F_REQUEST)"];
* A<=B [label="PUT (seq=1, NLM_F_MULTI)"];
* ...;
* A<=B [label="PUT (seq=1, NLM_F_MULTI)"];
* A<=B [label="NLMSG_DONE (seq=1)"];
* @endmsc
*
* Errors can be reported using the standard message type \c NLMSG_ERROR which
* can carry an error code and the netlink mesage header of the request.
* Error messages should set their sequence number to the sequence number
* of the message which caused the error.
*
* @msc
* A,B;
* A=>B [label="GET (seq=1, NLM_F_REQUEST)"];
* A<=B [label="NLMSG_ERROR code=EINVAL (seq=1)"];
* @endmsc
*
* The \c NLMSG_ERROR message type is also used to send acknowledge messages.
* An acknowledge message can be requested by setting the \c NLM_F_ACK flag
* message except that the error code is set to 0.
*
* @msc
* A,B;
* A=>B [label="GET (seq=1, NLM_F_REQUEST | NLM_F_ACK)"];
* A<=B [label="ACK (seq=1)"];
* @endmsc
*
* @section sk_doc Dealing with Netlink Sockets
*
* In order to use the netlink protocol, a netlink socket is required. Each
* socket defines a completely independent context for sending and receiving
* of messages. The netlink socket and all its related attributes are
* represented by struct nl_sock.
*
* @code
* nl_socket_alloc() Allocate new socket structure.
* nl_socket_free(s) Free socket structure.
* @endcode
*
* @subsection local_port Local Port
* The local port number uniquely identifies the socket and is used to
* address it. A unique local port is generated automatically when the socket
* is allocated. It will consist of the Process ID (22 bits) and a random
* number (10 bits) to allow up to 1024 sockets per process.
*
* @code
* nl_socket_get_local_port(sk) Return the peer's port number.
* nl_socket_set_local_port(sk, port) Set the peer's port number.
* @endcode
*
* @subsection peer_port Peer Port
* A peer port can be assigned to the socket which will result in all unicast
* messages sent over the socket to be addresses to the corresponding peer. If
* no peer is specified, the kernel will try to automatically bind the socket
* to a kernel side socket of the same netlink protocol family. It is common
* practice not to bind the socket to a peer port as typically only one kernel
* side socket exists per netlink protocol family.
*
* @code
* nl_socket_get_peer_port(sk) Return the local port number.
* nl_socket_set_peer_port(sk, port) Set the local port number.
* @endcode
*
* @subsection sock_fd File Descriptor
* The file descriptor of the socket(2).
*
* @code
* nl_socket_get_fd(sk) Return file descriptor.
* nl_socket_set_buffer_size(sk, rx, tx) Set buffer size of socket.
* nl_socket_set_nonblocking(sk) Set socket to non-blocking state.
* @endcode
*
* @subsection group_sub Group Subscriptions
* Each socket can subscribe to multicast groups of the netlink protocol
* family it is bound to. The socket will then receive a copy of each
* message sent to any of the groups. Multicast groups are commonly used
* to implement event notifications. Prior to kernel 2.6.14 the group
* subscription was performed using a bitmask which limited the number of
* groups per protocol family to 32. This outdated interface can still be
* accessed via the function nl_join_groups even though it is not recommended
* for new code. Starting with 2.6.14 a new method was introduced which
* supports subscribing to an almost unlimited number of multicast groups.
*
* @code
* nl_socket_add_membership(sk, group) Become a member of a multicast group.
* nl_socket_drop_membership(sk, group) Drop multicast group membership.
* nl_join_groups(sk, groupmask) Join a multicast group (obsolete).
* @endcode
*
* @subsection seq_num Sequence Numbers
* The socket keeps track of the sequence numbers used. The library will
* automatically verify the sequence number of messages received unless
* the check was disabled using the function nl_socket_disable_seq_check().
* When a message is sent using nl_send_auto_complete(), the sequence number
* is automatically filled in, and replies will be verified correctly.
*
* @code
* nl_socket_disable_seq_check(sk) Disable checking of sequece numbers.
* nl_socket_use_seq(sk) Use sequence number and bump to next.
* @endcode
*
* @subsection sock_cb Callback Configuration
* Every socket is associated a callback configuration which enables the
* applications to hook into various internal functions and control the
* receiving and sendings semantics. For more information, see section
* \ref cb_doc.
*
* @code
* nl_socket_alloc_cb(cb) Allocate socket based on callback set.
* nl_socket_get_cb(sk) Return callback configuration.
* nl_socket_set_cb(sk, cb) Replace callback configuration.
* nl_socket_modify_cb(sk, ...) Modify a specific callback function.
* @endcode
*
* @subsection sk_other Other Functions
* @code
* nl_socket_enable_auto_ack(sock) Enable automatic request of ACK.
* nl_socket_disable_auto_ack(sock) Disable automatic request of ACK.
* nl_socket_enable_msg_peek(sock) Enable message peeking.
* nl_socket_disable_msg_peek(sock) Disable message peeking.
* nl_socket_set_passcred(sk, state) Enable/disable credential passing.
* nl_socket_recv_pktinfo(sk, state) Enable/disable packet information.
* @endcode
*
* @section rxtx_doc Sending and Receiving of Data
*
* @subsection recv_semantisc Receiving Semantics
* @code
* nl_recvmsgs_default(set)
* | cb = nl_socket_get_cb(sk)
* v
* nl_recvmsgs(sk, cb)
* | [Application provides nl_recvmsgs() replacement]
* |- - - - - - - - - - - - - - - v
* | cb->cb_recvmsgs_ow()
* |
* | [Application provides nl_recv() replacement]
* +-------------->|- - - - - - - - - - - - - - - v
* | nl_recv() cb->cb_recv_ow()
* | +----------->|<- - - - - - - - - - - - - - -+
* | | v
* | | Parse Message
* | | |- - - - - - - - - - - - - - - v
* | | | NL_CB_MSG_IN()
* | | |<- - - - - - - - - - - - - - -+
* | | |
* | | |- - - - - - - - - - - - - - - v
* | | Sequence Check NL_CB_SEQ_CHECK()
* | | |<- - - - - - - - - - - - - - -+
* | | |
* | | |- - - - - - - - - - - - - - - v [ NLM_F_ACK is set ]
* | | | NL_CB_SEND_ACK()
* | | |<- - - - - - - - - - - - - - -+
* | | |
* | | +-----+------+--------------+----------------+--------------+
* | | v v v v v
* | | Valid Message ACK NO-OP Message End of Multipart Error
* | | | | | | |
* | | v v v v v
* | |NL_CB_VALID() NL_CB_ACK() NL_CB_SKIPPED() NL_CB_FINISH() cb->cb_err()
* | | | | | | |
* | | +------------+--------------+----------------+ v
* | | | (FAILURE)
* | | | [Callback returned NL_SKIP]
* | | [More messages to be parsed] |<-----------
* | +----------------------------------|
* | |
* | [is Multipart message] |
* +-------------------------------------| [Callback returned NL_STOP]
* |<-----------
* v
* (SUCCESS)
*
* At any time:
* Message Format Error
* |- - - - - - - - - - - - v
* v NL_CB_INVALID()
* (FAILURE)
*
* Message Overrun (Kernel Lost Data)
* |- - - - - - - - - - - - v
* v NL_CB_OVERRUN()
* (FAILURE)
*
* Callback returned negative error code
* (FAILURE)
* @endcode
*
* @subsection send_semantics Sending Semantisc
*
* @code
* nl_send_auto_complete(sk, msg)
* | [Automatically completes netlink message header]
* | [(local port, sequence number) ]
* |
* | [Application provies nl_send() replacement]
* |- - - - - - - - - - - - - - - - - - - - v
* v cb->cb_send_ow()
* nl_send(sk, msg)
* | [If available, add peer port and credentials]
* v
* nl_sendmsg(sk, msg, msghdr)
* |- - - - - - - - - - - - - - - - - - - - v
* | NL_CB_MSG_OUT()
* |<- - - - - - - - - - - - - - - - - - - -+
* v
* sendmsg()
* @endcode
*
* @section cb_doc Callback Configurations
* Callbacks and overwriting capabilities are provided to control various
* semantics of the library. All callback functions are packed together in
* struct nl_cb which is attached to a netlink socket or passed on to
* the respective functions directly.
*
* @subsection cb_ret_doc Callback Return Values
* Callback functions can control the flow of the calling layer by returning
* appropriate error codes:
* @code
* Action ID | Description
* -----------------+-------------------------------------------------------
* NL_OK | Proceed with whatever comes next.
* NL_SKIP | Skip message currently being processed and continue
* | with next message.
* NL_STOP | Stop parsing and discard all remaining messages in
* | this set of messages.
* @endcode
*
* All callbacks are optional and a default action is performed if no
* application specific implementation is provided:
*
* @code
* Callback ID | Default Return Value
* ------------------+----------------------
* NL_CB_VALID | NL_OK
* NL_CB_FINISH | NL_STOP
* NL_CB_OVERRUN | NL_STOP
* NL_CB_SKIPPED | NL_SKIP
* NL_CB_ACK | NL_STOP
* NL_CB_MSG_IN | NL_OK
* NL_CB_MSG_OUT | NL_OK
* NL_CB_INVALID | NL_STOP
* NL_CB_SEQ_CHECK | NL_OK
* NL_CB_SEND_ACK | NL_OK
* |
* Error Callback | NL_STOP
* @endcode
*
* In order to simplify typical usages of the library, different sets of
* default callback implementations exist:
* @code
* NL_CB_DEFAULT: No additional actions
* NL_CB_VERBOSE: Automatically print warning and error messages to a file
* descriptor as appropriate. This is useful for CLI based
* applications.
* NL_CB_DEBUG: Print informal debugging information for each message
* received. This will result in every message beint sent or
* received to be printed to the screen in a decoded,
* human-readable format.
* @endcode
*
* @par 1) Setting up a callback set
* @code
* // Allocate a callback set and initialize it to the verbose default set
* struct nl_cb *cb = nl_cb_alloc(NL_CB_VERBOSE);
*
* // Modify the set to call my_func() for all valid messages
* nl_cb_set(cb, NL_CB_VALID, NL_CB_CUSTOM, my_func, NULL);
*
* // Set the error message handler to the verbose default implementation
* // and direct it to print all errors to the given file descriptor.
* FILE *file = fopen(...);
* nl_cb_err(cb, NL_CB_VERBOSE, NULL, file);
* @endcode
*
* @page route_doc Routing Family
*
* @page genl_doc Generic Netlink Family
*
* @page nf_doc Netfilter Subsystem
*/
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