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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

     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.

     A similar graph might be used to represent multi-link PPP running over an
     ISDN line:

     [ type BRI ](B1)<--->(link1)[ type MPP  ]
     [  "ISDN1" ](B2)<--->(link2)[ (no name) ]
     [          ](D) <-+
                       |
      +----------------+
      |
      +->(switch)[ type Q.921 ](term1)<---->(datalink)[ type Q.931 ]
                 [ (no name)  ]                       [ (no name)  ]

   Netgraph Structures
     Structures are defined in #include <netgraph/netgraph.h>
     (for kernel structures only of interest to nodes) and #include
     <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 nul) */

     struct ng_mesg {
       struct ng_msghdr {
         u_char      version;        /* Must equal NG_VERSION */
         u_char      spare;          /* Pad to 2 bytes */
         u_short     arglen;         /* Length of cmd/resp data */
         u_long      flags;          /* Message status flags */
         u_long      token;          /* Reply should have the same token */
         u_long      typecookie;     /* Node type understanding this message */
         u_long      cmd;            /* Command identifier */
         u_char      cmdstr[NG_CMDSTRSIZ]; /* Cmd string (for debug) */
       } header;
       char  data[0];                /* Start of cmd/resp data */
     };

     #define NG_ABI_VERSION  5               /* Netgraph kernel ABI version */
     #define NG_VERSION      4               /* Netgraph message version */
     #define NGF_ORIG        0x0000          /* Command */
     #define NGF_RESP        0x0001          /* 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
             insures 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 #include <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 #include
     <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/mpd 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/mpd 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

             Definitions for use solely within the kernel by netgraph nodes.

             Definitions needed by any file that needs to deal with netgraph
             messages.

             Definitions needed to use netgraph socket type nodes.

             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_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_ksocket(4), ng_l2cap(4), ng_l2tp(4), ng_lmi(4), ng_mppc(4),
     ng_netflow(4), ng_one2many(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〉.