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README

      1 Netlink Protocol Library
      2 
      3 This library is a clean room re-implementation of libnl 2.0 and
      4 re-licensed under Apache 2.0. It was developed primarily to support
      5 wpa_supplicant. However, with additional development can be extended
      6 to support other netlink applications.
      7 
      8 Netlink Protocol Format (RFC3549)
      9 
     10 +-----------------+-+-------------------+-+
     11 |Netlink Message  |P| Generic Netlink   |P|
     12 |    Header       |A|  Message Header   |A|
     13 |(struct nlmsghdr)|D|(struct genlmsghdr)|D|
     14 +-----------------+-+-------------------+-+-------------+
     15 |len:4|type:2|flags:2|seq:4 pid:4|cmd:1|ver:1|reserved:2|
     16 +--------------------------------+----------------------+
     17 +-----------------+-+-----------------+-+-----------------+-+-----------------+-+---+
     18 |Netlink Attribute|P|Netlink Attribute|P|Netlink Attribute|P|Netlink Attribute|P|...|
     19 |    #0 Header    |A|   #0 Payload    |A|   #1 Header     |A|   #1 Payload    |A|   |
     20 | (struct nlattr) |D|     (void)      |D| (struct nlattr) |D|     (void)      |D|   |
     21 +-----------------+-+-----------------+-+-----------------+-+-----------------+-+---+
     22 |len:2(==4+payload)|type:2|payload|pad|
     23 +-------------------------+-------+---+
     24 
     25 NETLINK OVERVIEW
     26 
     27 * Each netlink message consists of a bitstream with a netlink header.
     28 * After this header a second header *can* be used specific to the netlink
     29   family in use. This library was tested using the generic netlink
     30   protocol defined by struct genlmsghdr to support nl80211.
     31 * After the header(s) netlink attributes can be appended to the message
     32   which hold can hold basic types such as unsigned integers and strings.
     33 * Attributes can also be nested. This is accomplished by calling "nla_nest_start"
     34   which creates an empty attribute with nest attributes as its payload. Then to
     35   close the nest, "nla_nest_end" is called.
     36 * All data structures in this implementation are byte-aligned (Currently 4 bytes).
     37 * Acknowledgements (ACKs) are sent as NLMSG_ERROR netlink message types (0x2) and
     38   have an error value of 0.
     39 
     40 KNOWN ISSUES
     41 
     42   GENERAL
     43   * Not tested for thread safety
     44 
     45   Android.mk
     46   * No dynamic library because of netlink cache not implemented and
     47     not tested for thread safety
     48 
     49   attr.c
     50   * nla_parse - does not use nla_policy argument
     51 
     52   cache.c
     53   * netlink cache not implemented and only supports one netlink family id
     54     which is stored in the nl_cache pointer instead of an actual cache
     55 
     56   netlink.c
     57   * nl_recvmsgs - does not support nl_cb_overwrite_recv()
     58   * nl_recv - sets/unsets asynchronous socket flag
     59 
     60 SOURCE FILES
     61 
     62 * Android.mk - Android makefile
     63 * README - This file
     64 * attr.c - Netlink attributes
     65 * cache.c - Netlink cache
     66 * genl/family.c - Generic netlink family id
     67 * genl/genl.c - Generic netlink
     68 * handlers.c - Netlink callbacks
     69 * msg.c - Netlink messages construction
     70 * netlink.c - Netlink socket communication
     71 * object.c - libnl object wrapper
     72 * socket.c - Netlink kernel socket utils
     73 
     74 IMPORTANT HEADER FILES - NOTE: These are based on the the origin GPL libnl headers
     75 
     76 * netlink-types.h - Contains many important structs for libnl
     77   to represent netlink objects
     78 * netlink/netlink-kernel.h - Netlink kernel headers and field constants.
     79 * netlink/msg.h - macros for iterating over netlink messages
     80 * netlink/attr.h - netlink attribute constants, iteration macros and setters
     81 
     82 REFERENCES
     83 
     84 * nl80211.h
     85 * netlink_types.h
     86 * $LINUX_KERNEL/net/wireless/nl80211.c
     87 * http://www.infradead.org/~tgr/libnl/doc-3.0/index.html
     88 * http://www.netfilter.org/projects/libmnl/doxygen/index.html
     89