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NAME

     netgraph — graph based kernel networking subsystem

DESCRIPTION

     The netgraph system provides a uniform and modular system for the implementation of kernel
     objects which perform various networking functions.  The objects, known as nodes, can be
     arranged into arbitrarily complicated graphs.  Nodes have hooks which are used to connect
     two nodes together, forming the edges in the graph.  Nodes communicate along the edges to
     process data, implement protocols, etc.

     The aim of netgraph is to supplement rather than replace the existing kernel networking
     infrastructure.  It provides:

     ·   A flexible way of combining protocol and link level drivers.
     ·   A modular way to implement new protocols.
     ·   A common framework for kernel entities to inter-communicate.
     ·   A reasonably fast, kernel-based implementation.

   Nodes and Types
     The most fundamental concept in netgraph is that of a node.  All nodes implement a number of
     predefined methods which allow them to interact with other nodes in a well defined manner.

     Each node has a type, which is a static property of the node determined at node creation
     time.  A node's type is described by a unique ASCII type name.  The type implies what the
     node does and how it may be connected to other nodes.

     In object-oriented language, types are classes, and nodes are instances of their respective
     class.  All node types are subclasses of the generic node type, and hence inherit certain
     common functionality and capabilities (e.g., the ability to have an ASCII name).

     Nodes may be assigned a globally unique ASCII name which can be used to refer to the node.
     The name must not contain the characters ‘.’ or ‘:’, and is limited to NG_NODESIZ characters
     (including the terminating NUL character).

     Each node instance has a unique ID number which is expressed as a 32-bit hexadecimal value.
     This value may be used to refer to a node when there is no ASCII name assigned to it.

   Hooks
     Nodes are connected to other nodes by connecting a pair of hooks, one from each node.  Data
     flows bidirectionally between nodes along connected pairs of hooks.  A node may have as many
     hooks as it needs, and may assign whatever meaning it wants to a hook.

     Hooks have these properties:

     ·   A hook has an ASCII name which is unique among all hooks on that node (other hooks on
         other nodes may have the same name).  The name must not contain the characters ‘.’ or
         ‘:’, and is limited to NG_HOOKSIZ characters (including the terminating NUL character).

     ·   A hook is always connected to another hook.  That is, hooks are created at the time they
         are connected, and breaking an edge by removing either hook destroys both hooks.

     ·   A hook can be set into a state where incoming packets are always queued by the input
         queueing system, rather than being delivered directly.  This can be used when the data
         is sent from an interrupt handler, and processing must be quick so as not to block other
         interrupts.

     ·   A hook may supply overriding receive data and receive message functions, which should be
         used for data and messages received through that hook in preference to the general node-
         wide methods.

     A node may decide to assign special meaning to some hooks.  For example, connecting to the
     hook named debug might trigger the node to start sending debugging information to that hook.

   Data Flow
     Two types of information flow between nodes: data messages and control messages.  Data
     messages are passed in mbuf chains along the edges in the graph, one edge at a time.  The
     first mbuf in a chain must have the M_PKTHDR flag set.  Each node decides how to handle data
     received through one of its hooks.

     Along with data, nodes can also receive control messages.  There are generic and type-
     specific control messages.  Control messages have a common header format, followed by type-
     specific data, and are binary structures for efficiency.  However, node types may also
     support conversion of the type-specific data between binary and ASCII formats, for debugging
     and human interface purposes (see the NGM_ASCII2BINARY and NGM_BINARY2ASCII generic control
     messages below).  Nodes are not required to support these conversions.

     There are three ways to address a control message.  If there is a sequence of edges
     connecting the two nodes, the message may be “source routed” by specifying the corresponding
     sequence of ASCII hook names as the destination address for the message (relative
     addressing).  If the destination is adjacent to the source, then the source node may simply
     specify (as a pointer in the code) the hook across which the message should be sent.
     Otherwise, the recipient node's global ASCII name (or equivalent ID-based name) is used as
     the destination address for the message (absolute addressing).  The two types of ASCII
     addressing may be combined, by specifying an absolute start node and a sequence of hooks.
     Only the ASCII addressing modes are available to control programs outside the kernel; use of
     direct pointers is limited to kernel modules.

     Messages often represent commands that are followed by a reply message in the reverse
     direction.  To facilitate this, the recipient of a control message is supplied with a
     “return address” that is suitable for addressing a reply.

     Each control message contains a 32-bit value, called a “typecookie”, indicating the type of
     the message, i.e. how to interpret it.  Typically each type defines a unique typecookie for
     the messages that it understands.  However, a node may choose to recognize and implement
     more than one type of messages.

     If a message is delivered to an address that implies that it arrived at that node through a
     particular hook (as opposed to having been directly addressed using its ID or global name)
     then that hook is identified to the receiving node.  This allows a message to be re-routed
     or passed on, should a node decide that this is required, in much the same way that data
     packets are passed around between nodes.  A set of standard messages for flow control and
     link management purposes are defined by the base system that are usually passed around in
     this manner.  Flow control message would usually travel in the opposite direction to the
     data to which they pertain.

   Netgraph is (Usually) Functional
     In order to minimize latency, most netgraph operations are functional.  That is, data and
     control messages are delivered by making function calls rather than by using queues and
     mailboxes.  For example, if node A wishes to send a data mbuf to neighboring node B, it
     calls the generic netgraph data delivery function.  This function in turn locates node B and
     calls B's “receive data” method.  There are exceptions to this.

     Each node has an input queue, and some operations can be considered to be writers in that
     they alter the state of the node.  Obviously, in an SMP world it would be bad if the state
     of a node were changed while another data packet were transiting the node.  For this
     purpose, the input queue implements a reader/writer semantic so that when there is a writer
     in the node, all other requests are queued, and while there are readers, a writer, and any
     following packets are queued.  In the case where there is no reason to queue the data, the
     input method is called directly, as mentioned above.

     A node may declare that all requests should be considered as writers, or that requests
     coming in over a particular hook should be considered to be a writer, or even that packets
     leaving or entering across a particular hook should always be queued, rather than delivered
     directly (often useful for interrupt routines who want to get back to the hardware quickly).
     By default, all control message packets are considered to be writers unless specifically
     declared to be a reader in their definition.  (See NGM_READONLY in <netgraph/ng_message.h>.)

     While this mode of operation results in good performance, it has a few implications for node
     developers:

     ·   Whenever a node delivers a data or control message, the node may need to allow for the
         possibility of receiving a returning message before the original delivery function call
         returns.

     ·   Netgraph provides internal synchronization between nodes.  Data always enters a “graph”
         at an edge node.  An edge node is a node that interfaces between netgraph and some other
         part of the system.  Examples of “edge nodes” include device drivers, the socket, ether,
         tty, and ksocket node type.  In these edge nodes, the calling thread directly executes
         code in the node, and from that code calls upon the netgraph framework to deliver data
         across some edge in the graph.  From an execution point of view, the calling thread will
         execute the netgraph framework methods, and if it can acquire a lock to do so, the input
         methods of the next node.  This continues until either the data is discarded or queued
         for some device or system entity, or the thread is unable to acquire a lock on the next
         node.  In that case, the data is queued for the node, and execution rewinds back to the
         original calling entity.  The queued data will be picked up and processed by either the
         current holder of the lock when they have completed their operations, or by a special
         netgraph thread that is activated when there are such items queued.

     ·   It is possible for an infinite loop to occur if the graph contains cycles.

     So far, these issues have not proven problematical in practice.

   Interaction with Other Parts of the Kernel
     A node may have a hidden interaction with other components of the kernel outside of the
     netgraph subsystem, such as device hardware, kernel protocol stacks, etc.  In fact, one of
     the benefits of netgraph is the ability to join disparate kernel networking entities
     together in a consistent communication framework.

     An example is the socket node type which is both a netgraph node and a socket(2) in the
     protocol family PF_NETGRAPH.  Socket nodes allow user processes to participate in netgraph.
     Other nodes communicate with socket nodes using the usual methods, and the node hides the
     fact that it is also passing information to and from a cooperating user process.

     Another example is a device driver that presents a node interface to the hardware.

   Node Methods
     Nodes are notified of the following actions via function calls to the following node
     methods, and may accept or reject that action (by returning the appropriate error code):

     Creation of a new node
         The constructor for the type is called.  If creation of a new node is allowed,
         constructor method may allocate any special resources it needs.  For nodes that
         correspond to hardware, this is typically done during the device attach routine.  Often
         a global ASCII name corresponding to the device name is assigned here as well.

     Creation of a new hook
         The hook is created and tentatively linked to the node, and the node is told about the
         name that will be used to describe this hook.  The node sets up any special data
         structures it needs, or may reject the connection, based on the name of the hook.

     Successful connection of two hooks
         After both ends have accepted their hooks, and the links have been made, the nodes get a
         chance to find out who their peer is across the link, and can then decide to reject the
         connection.  Tear-down is automatic.  This is also the time at which a node may decide
         whether to set a particular hook (or its peer) into the queueing mode.

     Destruction of a hook
         The node is notified of a broken connection.  The node may consider some hooks to be
         critical to operation and others to be expendable: the disconnection of one hook may be
         an acceptable event while for another it may effect a total shutdown for the node.

     Preshutdown of a node
         This method is called before real shutdown, which is discussed below.  While in this
         method, the node is fully operational and can send a “goodbye” message to its peers, or
         it can exclude itself from the chain and reconnect its peers together, like the
         ng_tee(4) node type does.

     Shutdown of a node
         This method allows a node to clean up and to ensure that any actions that need to be
         performed at this time are taken.  The method is called by the generic (i.e.,
         superclass) node destructor which will get rid of the generic components of the node.
         Some nodes (usually associated with a piece of hardware) may be persistent in that a
         shutdown breaks all edges and resets the node, but does not remove it.  In this case,
         the shutdown method should not free its resources, but rather, clean up and then call
         the NG_NODE_REVIVE() macro to signal the generic code that the shutdown is aborted.  In
         the case where the shutdown is started by the node itself due to hardware removal or
         unloading (via ng_rmnode_self()), it should set the NGF_REALLY_DIE flag to signal to its
         own shutdown method that it is not to persist.

   Sending and Receiving Data
     Two other methods are also supported by all nodes:

     Receive data message
         A netgraph queueable request item, usually referred to as an item, is received by this
         function.  The item contains a pointer to an mbuf.

         The node is notified on which hook the item has arrived, and can use this information in
         its processing decision.  The receiving node must always NG_FREE_M() the mbuf chain on
         completion or error, or pass it on to another node (or kernel module) which will then be
         responsible for freeing it.  Similarly, the item must be freed if it is not to be passed
         on to another node, by using the NG_FREE_ITEM() macro.  If the item still holds
         references to mbufs at the time of freeing then they will also be appropriately freed.
         Therefore, if there is any chance that the mbuf will be changed or freed separately from
         the item, it is very important that it be retrieved using the NGI_GET_M() macro that
         also removes the reference within the item.  (Or multiple frees of the same object will
         occur.)

         If it is only required to examine the contents of the mbufs, then it is possible to use
         the NGI_M() macro to both read and rewrite mbuf pointer inside the item.

         If developer needs to pass any meta information along with the mbuf chain, he should use
         mbuf_tags(9) framework.  Note that old netgraph specific meta-data format is obsoleted
         now.

         The receiving node may decide to defer the data by queueing it in the netgraph NETISR
         system (see below).  It achieves this by setting the HK_QUEUE flag in the flags word of
         the hook on which that data will arrive.  The infrastructure will respect that bit and
         queue the data for delivery at a later time, rather than deliver it directly.  A node
         may decide to set the bit on the peer node, so that its own output packets are queued.

         The node may elect to nominate a different receive data function for data received on a
         particular hook, to simplify coding.  It uses the NG_HOOK_SET_RCVDATA(hook, fn) macro to
         do this.  The function receives the same arguments in every way other than it will
         receive all (and only) packets from that hook.

     Receive control message
         This method is called when a control message is addressed to the node.  As with the
         received data, an item is received, with a pointer to the control message.  The message
         can be examined using the NGI_MSG() macro, or completely extracted from the item using
         the NGI_GET_MSG() which also removes the reference within the item.  If the item still
         holds a reference to the message when it is freed (using the NG_FREE_ITEM() macro), then
         the message will also be freed appropriately.  If the reference has been removed, the
         node must free the message itself using the NG_FREE_MSG() macro.  A return address is
         always supplied, giving the address of the node that originated the message so a reply
         message can be sent anytime later.  The return address is retrieved from the item using
         the NGI_RETADDR() macro and is of type ng_ID_t.  All control messages and replies are
         allocated with the malloc(9) type M_NETGRAPH_MSG, however it is more convenient to use
         the NG_MKMESSAGE() and NG_MKRESPONSE() macros to allocate and fill out a message.
         Messages must be freed using the NG_FREE_MSG() macro.

         If the message was delivered via a specific hook, that hook will also be made known,
         which allows the use of such things as flow-control messages, and status change
         messages, where the node may want to forward the message out another hook to that on
         which it arrived.

         The node may elect to nominate a different receive message function for messages
         received on a particular hook, to simplify coding.  It uses the NG_HOOK_SET_RCVMSG(hook,
         fn) macro to do this.  The function receives the same arguments in every way other than
         it will receive all (and only) messages from that hook.

     Much use has been made of reference counts, so that nodes being freed of all references are
     automatically freed, and this behaviour has been tested and debugged to present a consistent
     and trustworthy framework for the “type module” writer to use.

   Addressing
     The netgraph framework provides an unambiguous and simple to use method of specifically
     addressing any single node in the graph.  The naming of a node is independent of its type,
     in that another node, or external component need not know anything about the node's type in
     order to address it so as to send it a generic message type.  Node and hook names should be
     chosen so as to make addresses meaningful.

     Addresses are either absolute or relative.  An absolute address begins with a node name or
     ID, followed by a colon, followed by a sequence of hook names separated by periods.  This
     addresses the node reached by starting at the named node and following the specified
     sequence of hooks.  A relative address includes only the sequence of hook names, implicitly
     starting hook traversal at the local node.

     There are a couple of special possibilities for the node name.  The name ‘.’ (referred to as
     ‘.:’) always refers to the local node.  Also, nodes that have no global name may be
     addressed by their ID numbers, by enclosing the hexadecimal representation of the ID number
     within the square brackets.  Here are some examples of valid netgraph addresses:

           .:
           [3f]:
           foo:
           .:hook1
           foo:hook1.hook2
           [d80]:hook1

     The following set of nodes might be created for a site with a single physical frame relay
     line having two active logical DLCI channels, with RFC 1490 frames on DLCI 16 and PPP frames
     over DLCI 20:

     [type SYNC ]                  [type FRAME]                 [type RFC1490]
     [ "Frame1" ](uplink)<-->(data)[<un-named>](dlci16)<-->(mux)[<un-named>  ]
     [    A     ]                  [    B     ](dlci20)<---+    [     C      ]
                                                           |
                                                           |      [ type PPP ]
                                                           +>(mux)[<un-named>]
                                                                  [    D     ]

     One could always send a control message to node C from anywhere by using the name
     “Frame1:uplink.dlci16”.  In this case, node C would also be notified that the message
     reached it via its hook mux.  Similarly, “Frame1:uplink.dlci20” could reliably be used to
     reach node D, and node A could refer to node B as “.:uplink”, or simply “uplink”.
     Conversely, B can refer to A as “data”.  The address “mux.data” could be used by both nodes
     C and D to address a message to node A.

     Note that this is only for control messages.  In each of these cases, where a relative
     addressing mode is used, the recipient is notified of the hook on which the message arrived,
     as well as the originating node.  This allows the option of hop-by-hop distribution of
     messages and state information.  Data messages are only routed one hop at a time, by
     specifying the departing hook, with each node making the next routing decision.  So when B
     receives a frame on hook data, it decodes the frame relay header to determine the DLCI, and
     then forwards the unwrapped frame to either C or D.

     In a similar way, flow control messages may be routed in the reverse direction to outgoing
     data.  For example a “buffer nearly full” message from “Frame1:” would be passed to node B
     which might decide to send similar messages to both nodes C and D.  The nodes would use
     direct hook pointer addressing to route the messages.  The message may have travelled from
     “Frame1:” to B as a synchronous reply, saving time and cycles.

   Netgraph Structures
     Structures are defined in <netgraph/netgraph.h> (for kernel structures only of interest to
     nodes) and <netgraph/ng_message.h> (for message definitions also of interest to user
     programs).

     The two basic object types that are of interest to node authors are nodes and hooks.  These
     two objects have the following properties that are also of interest to the node writers.

     struct ng_node
         Node authors should always use the following typedef to declare their pointers, and
         should never actually declare the structure.

         typedef struct ng_node *node_p;

         The following properties are associated with a node, and can be accessed in the
         following manner:

         Validity
             A driver or interrupt routine may want to check whether the node is still valid.  It
             is assumed that the caller holds a reference on the node so it will not have been
             freed, however it may have been disabled or otherwise shut down.  Using the
             NG_NODE_IS_VALID(node) macro will return this state.  Eventually it should be almost
             impossible for code to run in an invalid node but at this time that work has not
             been completed.

         Node ID (ng_ID_t)
             This property can be retrieved using the macro NG_NODE_ID(node).

         Node name
             Optional globally unique name, NUL terminated string.  If there is a value in here,
             it is the name of the node.

                   if (NG_NODE_NAME(node)[0] != '\0') ...

                   if (strcmp(NG_NODE_NAME(node), "fred") == 0) ...

         A node dependent opaque cookie
             Anything of the pointer type can be placed here.  The macros
             NG_NODE_SET_PRIVATE(node, value) and NG_NODE_PRIVATE(node) set and retrieve this
             property, respectively.

         Number of hooks
             The NG_NODE_NUMHOOKS(node) macro is used to retrieve this value.

         Hooks
             The node may have a number of hooks.  A traversal method is provided to allow all
             the hooks to be tested for some condition.  NG_NODE_FOREACH_HOOK(node, fn, arg,
             rethook) where fn is a function that will be called for each hook with the form
             fn(hook, arg) and returning 0 to terminate the search.  If the search is terminated,
             then rethook will be set to the hook at which the search was terminated.

     struct ng_hook
         Node authors should always use the following typedef to declare their hook pointers.

         typedef struct ng_hook *hook_p;

         The following properties are associated with a hook, and can be accessed in the
         following manner:

         A hook dependent opaque cookie
             Anything of the pointer type can be placed here.  The macros
             NG_HOOK_SET_PRIVATE(hook, value) and NG_HOOK_PRIVATE(hook) set and retrieve this
             property, respectively.

         An associate node
             The macro NG_HOOK_NODE(hook) finds the associated node.

         A peer hook (hook_p)
             The other hook in this connected pair.  The NG_HOOK_PEER(hook) macro finds the peer.

         References
             The NG_HOOK_REF(hook) and NG_HOOK_UNREF(hook) macros increment and decrement the
             hook reference count accordingly.  After decrement you should always assume the hook
             has been freed unless you have another reference still valid.

         Override receive functions
             The NG_HOOK_SET_RCVDATA(hook, fn) and NG_HOOK_SET_RCVMSG(hook, fn) macros can be
             used to set override methods that will be used in preference to the generic receive
             data and receive message functions.  To unset these, use the macros to set them to
             NULL.  They will only be used for data and messages received on the hook on which
             they are set.

         The maintenance of the names, reference counts, and linked list of hooks for each node
         is handled automatically by the netgraph subsystem.  Typically a node's private info
         contains a back-pointer to the node or hook structure, which counts as a new reference
         that must be included in the reference count for the node.  When the node constructor is
         called, there is already a reference for this calculated in, so that when the node is
         destroyed, it should remember to do a NG_NODE_UNREF() on the node.

         From a hook you can obtain the corresponding node, and from a node, it is possible to
         traverse all the active hooks.

         A current example of how to define a node can always be seen in
         src/sys/netgraph/ng_sample.c and should be used as a starting point for new node
         writers.

   Netgraph Message Structure
     Control messages have the following structure:

     #define NG_CMDSTRSIZ    32      /* Max command string (including null) */

     struct ng_mesg {
       struct ng_msghdr {
         u_char      version;        /* Must equal NG_VERSION */
         u_char      spare;          /* Pad to 4 bytes */
         uint16_t    spare2;
         uint32_t    arglen;         /* Length of cmd/resp data */
         uint32_t    cmd;            /* Command identifier */
         uint32_t    flags;          /* Message status flags */
         uint32_t    token;          /* Reply should have the same token */
         uint32_t    typecookie;     /* Node type understanding this message */
         u_char      cmdstr[NG_CMDSTRSIZ];  /* cmd string +   */
       } header;
       char  data[];                 /* placeholder for actual data */
     };

     #define NG_ABI_VERSION  12              /* Netgraph kernel ABI version */
     #define NG_VERSION      8               /* Netgraph message version */
     #define NGF_ORIG        0x00000000      /* The msg is the original request */
     #define NGF_RESP        0x00000001      /* The message is a response */

     Control messages have the fixed header shown above, followed by a variable length data
     section which depends on the type cookie and the command.  Each field is explained below:

     version
             Indicates the version of the netgraph message protocol itself.  The current version
             is NG_VERSION.

     arglen  This is the length of any extra arguments, which begin at data.

     flags   Indicates whether this is a command or a response control message.

     token   The token is a means by which a sender can match a reply message to the
             corresponding command message; the reply always has the same token.

     typecookie
             The corresponding node type's unique 32-bit value.  If a node does not recognize the
             type cookie it must reject the message by returning EINVAL.

             Each type should have an include file that defines the commands, argument format,
             and cookie for its own messages.  The typecookie ensures that the same header file
             was included by both sender and receiver; when an incompatible change in the header
             file is made, the typecookie must be changed.  The de-facto method for generating
             unique type cookies is to take the seconds from the Epoch at the time the header
             file is written (i.e., the output of “date -u +%s”).

             There is a predefined typecookie NGM_GENERIC_COOKIE for the generic node type, and a
             corresponding set of generic messages which all nodes understand.  The handling of
             these messages is automatic.

     cmd     The identifier for the message command.  This is type specific, and is defined in
             the same header file as the typecookie.

     cmdstr  Room for a short human readable version of command (for debugging purposes only).

     Some modules may choose to implement messages from more than one of the header files and
     thus recognize more than one type cookie.

   Control Message ASCII Form
     Control messages are in binary format for efficiency.  However, for debugging and human
     interface purposes, and if the node type supports it, control messages may be converted to
     and from an equivalent ASCII form.  The ASCII form is similar to the binary form, with two
     exceptions:

     1.   The cmdstr header field must contain the ASCII name of the command, corresponding to
          the cmd header field.

     2.   The arguments field contains a NUL-terminated ASCII string version of the message
          arguments.

     In general, the arguments field of a control message can be any arbitrary C data type.
     Netgraph includes parsing routines to support some pre-defined datatypes in ASCII with this
     simple syntax:

     ·   Integer types are represented by base 8, 10, or 16 numbers.

     ·   Strings are enclosed in double quotes and respect the normal C language backslash
         escapes.

     ·   IP addresses have the obvious form.

     ·   Arrays are enclosed in square brackets, with the elements listed consecutively starting
         at index zero.  An element may have an optional index and equals sign (‘=’) preceding
         it.  Whenever an element does not have an explicit index, the index is implicitly the
         previous element's index plus one.

     ·   Structures are enclosed in curly braces, and each field is specified in the form
         fieldname=value.

     ·   Any array element or structure field whose value is equal to its “default value” may be
         omitted.  For integer types, the default value is usually zero; for string types, the
         empty string.

     ·   Array elements and structure fields may be specified in any order.

     Each node type may define its own arbitrary types by providing the necessary routines to
     parse and unparse.  ASCII forms defined for a specific node type are documented in the
     corresponding man page.

   Generic Control Messages
     There are a number of standard predefined messages that will work for any node, as they are
     supported directly by the framework itself.  These are defined in <netgraph/ng_message.h>
     along with the basic layout of messages and other similar information.

     NGM_CONNECT
             Connect to another node, using the supplied hook names on either end.

     NGM_MKPEER
             Construct a node of the given type and then connect to it using the supplied hook
             names.

     NGM_SHUTDOWN
             The target node should disconnect from all its neighbours and shut down.  Persistent
             nodes such as those representing physical hardware might not disappear from the node
             namespace, but only reset themselves.  The node must disconnect all of its hooks.
             This may result in neighbors shutting themselves down, and possibly a cascading
             shutdown of the entire connected graph.

     NGM_NAME
             Assign a name to a node.  Nodes can exist without having a name, and this is the
             default for nodes created using the NGM_MKPEER method.  Such nodes can only be
             addressed relatively or by their ID number.

     NGM_RMHOOK
             Ask the node to break a hook connection to one of its neighbours.  Both nodes will
             have their “disconnect” method invoked.  Either node may elect to totally shut down
             as a result.

     NGM_NODEINFO
             Asks the target node to describe itself.  The four returned fields are the node name
             (if named), the node type, the node ID and the number of hooks attached.  The ID is
             an internal number unique to that node.

     NGM_LISTHOOKS
             This returns the information given by NGM_NODEINFO, but in addition includes an
             array of fields describing each link, and the description for the node at the far
             end of that link.

     NGM_LISTNAMES
             This returns an array of node descriptions (as for NGM_NODEINFO) where each entry of
             the array describes a named node.  All named nodes will be described.

     NGM_LISTNODES
             This is the same as NGM_LISTNAMES except that all nodes are listed regardless of
             whether they have a name or not.

     NGM_LISTTYPES
             This returns a list of all currently installed netgraph types.

     NGM_TEXT_STATUS
             The node may return a text formatted status message.  The status information is
             determined entirely by the node type.  It is the only “generic” message that
             requires any support within the node itself and as such the node may elect to not
             support this message.  The text response must be less than NG_TEXTRESPONSE bytes in
             length (presently 1024).  This can be used to return general status information in
             human readable form.

     NGM_BINARY2ASCII
             This message converts a binary control message to its ASCII form.  The entire
             control message to be converted is contained within the arguments field of the
             NGM_BINARY2ASCII message itself.  If successful, the reply will contain the same
             control message in ASCII form.  A node will typically only know how to translate
             messages that it itself understands, so the target node of the NGM_BINARY2ASCII is
             often the same node that would actually receive that message.

     NGM_ASCII2BINARY
             The opposite of NGM_BINARY2ASCII.  The entire control message to be converted, in
             ASCII form, is contained in the arguments section of the NGM_ASCII2BINARY and need
             only have the flags, cmdstr, and arglen header fields filled in, plus the
             NUL-terminated string version of the arguments in the arguments field.  If
             successful, the reply contains the binary version of the control message.

   Flow Control Messages
     In addition to the control messages that affect nodes with respect to the graph, there are
     also a number of flow control messages defined.  At present these are not handled
     automatically by the system, so nodes need to handle them if they are going to be used in a
     graph utilising flow control, and will be in the likely path of these messages.  The default
     action of a node that does not understand these messages should be to pass them onto the
     next node.  Hopefully some helper functions will assist in this eventually.  These messages
     are also defined in <netgraph/ng_message.h> and have a separate cookie NG_FLOW_COOKIE to
     help identify them.  They will not be covered in depth here.

INITIALIZATION

     The base netgraph code may either be statically compiled into the kernel or else loaded
     dynamically as a KLD via kldload(8).  In the former case, include

           options NETGRAPH

     in your kernel configuration file.  You may also include selected node types in the kernel
     compilation, for example:

           options NETGRAPH
           options NETGRAPH_SOCKET
           options NETGRAPH_ECHO

     Once the netgraph subsystem is loaded, individual node types may be loaded at any time as
     KLD modules via kldload(8).  Moreover, netgraph knows how to automatically do this; when a
     request to create a new node of unknown type type is made, netgraph will attempt to load the
     KLD module ng_type.ko.

     Types can also be installed at boot time, as certain device drivers may want to export each
     instance of the device as a netgraph node.

     In general, new types can be installed at any time from within the kernel by calling
     ng_newtype(), supplying a pointer to the type's struct ng_type structure.

     The NETGRAPH_INIT() macro automates this process by using a linker set.

EXISTING NODE TYPES

     Several node types currently exist.  Each is fully documented in its own man page:

     SOCKET  The socket type implements two new sockets in the new protocol domain PF_NETGRAPH.
             The new sockets protocols are NG_DATA and NG_CONTROL, both of type SOCK_DGRAM.
             Typically one of each is associated with a socket node.  When both sockets have
             closed, the node will shut down.  The NG_DATA socket is used for sending and
             receiving data, while the NG_CONTROL socket is used for sending and receiving
             control messages.  Data and control messages are passed using the sendto(2) and
             recvfrom(2) system calls, using a struct sockaddr_ng socket address.

     HOLE    Responds only to generic messages and is a “black hole” for data.  Useful for
             testing.  Always accepts new hooks.

     ECHO    Responds only to generic messages and always echoes data back through the hook from
             which it arrived.  Returns any non-generic messages as their own response.  Useful
             for testing.  Always accepts new hooks.

     TEE     This node is useful for “snooping”.  It has 4 hooks: left, right, left2right, and
             right2left.  Data entering from the right is passed to the left and duplicated on
             right2left, and data entering from the left is passed to the right and duplicated on
             left2right.  Data entering from left2right is sent to the right and data from
             right2left to left.

     RFC1490 MUX
             Encapsulates/de-encapsulates frames encoded according to RFC 1490.  Has a hook for
             the encapsulated packets (downstream) and one hook for each protocol (i.e., IP, PPP,
             etc.).

     FRAME RELAY MUX
             Encapsulates/de-encapsulates Frame Relay frames.  Has a hook for the encapsulated
             packets (downstream) and one hook for each DLCI.

     FRAME RELAY LMI
             Automatically handles frame relay “LMI” (link management interface) operations and
             packets.  Automatically probes and detects which of several LMI standards is in use
             at the exchange.

     TTY     This node is also a line discipline.  It simply converts between mbuf frames and
             sequential serial data, allowing a TTY to appear as a netgraph node.  It has a
             programmable “hotkey” character.

     ASYNC   This node encapsulates and de-encapsulates asynchronous frames according to RFC
             1662.  This is used in conjunction with the TTY node type for supporting PPP links
             over asynchronous serial lines.

     ETHERNET
             This node is attached to every Ethernet interface in the system.  It allows
             capturing raw Ethernet frames from the network, as well as sending frames out of the
             interface.

     INTERFACE
             This node is also a system networking interface.  It has hooks representing each
             protocol family (IP, AppleTalk, IPX, etc.) and appears in the output of ifconfig(8).
             The interfaces are named “ng0”, “ng1”, etc.

     ONE2MANY
             This node implements a simple round-robin multiplexer.  It can be used for example
             to make several LAN ports act together to get a higher speed link between two
             machines.

     Various PPP related nodes
             There is a full multilink PPP implementation that runs in netgraph.  The net/mpd5
             port can use these modules to make a very low latency high capacity PPP system.  It
             also supports PPTP VPNs using the PPTP node.

     PPPOE   A server and client side implementation of PPPoE.  Used in conjunction with either
             ppp(8) or the net/mpd5 port.

     BRIDGE  This node, together with the Ethernet nodes, allows a very flexible bridging system
             to be implemented.

     KSOCKET
             This intriguing node looks like a socket to the system but diverts all data to and
             from the netgraph system for further processing.  This allows such things as UDP
             tunnels to be almost trivially implemented from the command line.

     Refer to the section at the end of this man page for more nodes types.

NOTES

     Whether a named node exists can be checked by trying to send a control message to it (e.g.,
     NGM_NODEINFO).  If it does not exist, ENOENT will be returned.

     All data messages are mbuf chains with the M_PKTHDR flag set.

     Nodes are responsible for freeing what they allocate.  There are three exceptions:

     1.   Mbufs sent across a data link are never to be freed by the sender.  In the case of
          error, they should be considered freed.

     2.   Messages sent using one of NG_SEND_MSG_*() family macros are freed by the recipient.
          As in the case above, the addresses associated with the message are freed by whatever
          allocated them so the recipient should copy them if it wants to keep that information.

     3.   Both control messages and data are delivered and queued with a netgraph item.  The item
          must be freed using NG_FREE_ITEM(item) or passed on to another node.

FILES

     <netgraph/netgraph.h>
             Definitions for use solely within the kernel by netgraph nodes.

     <netgraph/ng_message.h>
             Definitions needed by any file that needs to deal with netgraph messages.

     <netgraph/ng_socket.h>
             Definitions needed to use netgraph socket type nodes.

     <netgraph/ng_>⟨type⟩.h
             Definitions needed to use netgraph type nodes, including the type cookie definition.

     /boot/kernel/netgraph.ko
             The netgraph subsystem loadable KLD module.

     /boot/kernel/ng_⟨type⟩.ko
             Loadable KLD module for node type type.

     src/sys/netgraph/ng_sample.c
             Skeleton netgraph node.  Use this as a starting point for new node types.

USER MODE SUPPORT

     There is a library for supporting user-mode programs that wish to interact with the netgraph
     system.  See netgraph(3) for details.

     Two user-mode support programs, ngctl(8) and nghook(8), are available to assist manual
     configuration and debugging.

     There are a few useful techniques for debugging new node types.  First, implementing new
     node types in user-mode first makes debugging easier.  The tee node type is also useful for
     debugging, especially in conjunction with ngctl(8) and nghook(8).

     Also look in /usr/share/examples/netgraph for solutions to several common networking
     problems, solved using netgraph.

SEE ALSO

     socket(2), netgraph(3), ng_async(4), ng_atm(4), ng_atmllc(4), ng_bluetooth(4), ng_bpf(4),
     ng_bridge(4), ng_bt3c(4), ng_btsocket(4), ng_car(4), ng_cisco(4), ng_device(4), ng_echo(4),
     ng_eiface(4), ng_etf(4), ng_ether(4), ng_fec(4), ng_frame_relay(4), ng_gif(4),
     ng_gif_demux(4), ng_h4(4), ng_hci(4), ng_hole(4), ng_hub(4), ng_iface(4), ng_ip_input(4),
     ng_ipfw(4), ng_ksocket(4), ng_l2cap(4), ng_l2tp(4), ng_lmi(4), ng_mppc(4), ng_nat(4),
     ng_netflow(4), ng_one2many(4), ng_patch(4), ng_ppp(4), ng_pppoe(4), ng_pptpgre(4),
     ng_rfc1490(4), ng_socket(4), ng_split(4), ng_sppp(4), ng_sscfu(4), ng_sscop(4), ng_tee(4),
     ng_tty(4), ng_ubt(4), ng_UI(4), ng_uni(4), ng_vjc(4), ng_vlan(4), ngctl(8), nghook(8)

HISTORY

     The netgraph system was designed and first implemented at Whistle Communications, Inc. in a
     version of FreeBSD 2.2 customized for the Whistle InterJet.  It first made its debut in the
     main tree in FreeBSD 3.4.

AUTHORS

     Julian Elischer <julian@FreeBSD.org>, with contributions by Archie Cobbs
     <archie@FreeBSD.org>.