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NETGRAPH(4)            FreeBSD Kernel Interfaces Manual            NETGRAPH(4)

     netgraph - graph based kernel networking subsystem

     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:

       +o   A flexible way of combining protocol and link level drivers
       +o   A modular way to implement new protocols
       +o   A common framework for kernel entities to inter-communicate
       +o   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 NUL byte).

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

     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:

       +o   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 a ``.'' or a ``:'' and is limited to NG_HOOKSIZ
           characters (including NUL byte).
       +o   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.
       +o   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 is used when the two joined nodes need to be
           decoupled, e.g. if they are running at different processor priority
           levels.  (spl)
       +o   A hook may supply over-riding 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 coming in on its

     Control messages are type-specific C structures sent from one node
     directly to some arbitrary other node.  Control messages have a common
     header format, followed by type-specific data, and are binary structures
     for efficiency.  However, node types also may support conversion of the
     type specific data between binary and ASCII for debugging and human
     interface purposes (see the NGM_ASCII2BINARY and NGM_BINARY2ASCII generic
     control messages below).  Nodes are not required to support these

     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 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, as use of direct pointers is limited of course to kernel

     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 message.

     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 rerouted 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 ng_message.h)

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

       +o   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.
       +o   Netgraph nodes and support routines generally run at splnet().
           However, some nodes may want to send data and control messages from
           a different priority level.  Netgraph supplies a mechanism which
           utilizes the NETISR system to move message and data delivery to
           splnet().  Nodes that run at other priorities (e.g. interfaces) can
           be directly linked to other nodes so that the combination runs at
           the other priority, however any interaction with nodes running at
           splnet MUST be achieved via the queueing functions, (which use the
           netisr() feature of the kernel).  Note that messages are always
           received at splnet().
       +o   It's possible for an infinite loop to occur if the graph contains

     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 node type socket which is both a netgraph node and a
     socket(2) BSD socket 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

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

   Node Methods
     Nodes are notified of the following actions via function calls to the
     following node methods (all at splnet()) 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, the constructor must call the generic node creation
          function (in object-oriented terms, the superclass constructor) and
          then 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 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 affect a total shutdown for the node.

     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 doesn't remove
          it. In this case the shutdown method should not free its resources,
          but rather, clean up and then clear the NG_INVALID flag 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 NG_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 the function.  The item contains a pointer to an mbuf
          and metadata about the packet.

          The node is notified on which hook the item 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 or metadata at
          the time of freeing then they will also be appropriately freed.
          Therefore, if there is any chance that the mbuf or metadata will be
          changed or freed separately from the item, it is very important that
          these fields be retrieved using the NGI_GET_M() and NGI_GET_META()
          macros that also remove 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 or the
          metadata, then it is possible to use the NGI_M() and NGI_META()
          macros to both read and rewrite these fields.

          In addition to the mbuf chain itself there may also be a pointer to
          a structure describing meta-data about the message (e.g. priority
          information). This pointer may be NULL if there is no additional
          information. The format for this information is described in
          sys/netgraph/netgraph.h.  The memory for meta-data must allocated
          via malloc() with type M_NETGRAPH_META.  As with the data itself, it
          is the receiver's responsibility to free() the meta-data. If the
          mbuf chain is freed the meta-data must be freed at the same time. If
          the meta-data is freed but the real data on is passed on, then a
          NULL pointer must be substituted. It is also the duty of the
          receiver to free the request item itself, or to use it to pass the
          message on further.

          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. This is used by device drivers running at
          different processor priorities to transfer packet delivery to the
          splnet() level at which the bulk of netgraph runs.

          The structure and use of meta-data is still experimental, but is
          presently used in frame-relay to indicate that management packets
          should be queued for transmission at a higher priority than data
          packets. This is required for conformance with Frame Relay

          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 malloc() type M_NETGRAPH_MSG, however
          it is more usual 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

          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 free'd 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.

     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 hex representation of the ID number within square brackets.
     Here are some examples of valid netgraph addresses:


     Consider 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 ]
                                                                  [    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 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 sys/netgraph/netgraph.h (for kernel structures
     only of interest to nodes) and sys/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:

            +o   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

            +o   node ID

                Of type ng_ID_t, This property can be retrieved using the
                macro NG_NODE_ID(node).

            +o   node name

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

                if (NG_NODE_NAME node [0]) ....

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

            +o   A node dependent opaque cookie

                You may place anything of type pointer here.  Use the macros
                NG_NODE_SET_PRIVATE(node, value) and NG_NODE_PRIVATE(node) to
                set and retrieve this property.

            +o   number of hooks

                Use NG_NODE_NUMHOOKS(node) to retrieve this value.

            +o   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:

            +o   A node dependent opaque cookie.

                You may place anything of type pointer here.  Use the macros
                NG_HOOK_SET_PRIVATE(hook, value) and NG_HOOK_PRIVATE(hook) to
                set and retrieve this property.

            +o   An associate node.

                You may use the macro NG_HOOK_NODE(hook) to find the
                associated node.

            +o   A peer hook

                The other hook in this connected pair. Of type hook_p. You can
                use NG_HOOK_PEER(hook) to find the peer.

            +o   references

                NG_HOOK_REF(hook) and NG_HOOK_UNREF(hook) 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.

            +o   Over-ride receive functions.

                The NG_HOOK_SET_RCVDATA(hook, fn) and NG_HOOK_SET_RCVMSG(hook,
                fn) macros can be used to set over-ride 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
          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:

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

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

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

          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

          The corresponding node type's unique 32-bit value.  If a node
          doesn't 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

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

          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:

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

     o    Integer types are represented by base 8, 10, or 16 numbers.
     o    Strings are enclosed in double quotes and respect the normal C
          language backslash escapes.
     o    IP addresses have the obvious form.
     o    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.
     o    Structures are enclosed in curly braces, and each field is specified
          in the form ``fieldname=value''.
     o    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.
     o    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 documentation for that node

   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 ng_message.h along with the basic layout of messages and other
     similar information.

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

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

          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.

          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.

          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.

          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.

          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.

          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.

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

          This returns a list of all currently installed netgraph types.

          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.

          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.

          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 doesn't 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
     sys/netgraph/ng_message.h and have a separate cookie NG_FLOW_COOKIE to
     help identify them.  They will not be covered in depth here.

     Data moving through the netgraph system can be accompanied by meta-data
     that describes some aspect of that data.  The form of the meta-data is a
     fixed header, which contains enough information for most uses, and can
     optionally be supplemented by trailing option structures, which contain a
     cookie (see the section on control messages), an identifier, a length and
     optional data. If a node does not recognize the cookie associated with an
     option, it should ignore that option.

     Meta data might include such things as priority, discard eligibility, or
     special processing requirements.  It might also mark a packet for debug
     status, etc.  The use of meta-data is still experimental.

     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,

           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

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

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

          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) calls, using a struct
          sockaddr_ng socket address.

          Responds only to generic messages and is a ``black hole'' for data,
          Useful for testing. Always accepts new hooks.

          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.).

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

          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.

          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.

          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.

          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 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.

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

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

          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.

     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

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

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

     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     Any meta-data information traveling with the data has the same
           restriction.  It might be freed by any node the data passes
           through, and a NULL passed onwards, but the caller will never free
           it.  Two macros NG_FREE_META(meta) and NG_FREE_M(m) should be used
           if possible to free data and meta data (see netgraph.h).

     3     Messages sent using ng_send_message() 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.

     4     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.

            Definitions for use solely within the kernel by netgraph nodes.
            Definitions needed by any file that needs to deal with netgraph
            Definitions needed to use netgraph socket type nodes.
            Definitions needed to use netgraph {type} nodes, including the
            type cookie definition.
            Netgraph subsystem loadable KLD module.
            Loadable KLD module for node type {type}.
            Skeleton netgraph node.  Use this as a starting point for new node

     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.

     socket(2), netgraph(3), ng_async(4), ng_bpf(4), ng_bridge(4),
     ng_cisco(4), ng_echo(4), ng_ether(4), ng_frame_relay(4), ng_hole(4),
     ng_iface(4), ng_ksocket(4), ng_lmi(4), ng_mppc(4), ng_ppp(4),
     ng_pppoe(4), ng_pptpgre(4), ng_rfc1490(4), ng_socket(4), ng_tee(4),
     ng_tty(4), ng_UI(4), ng_vjc(4), ngctl(8), nghook(8)

     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.

     Julian Elischer <>, with contributions by Archie Cobbs

FreeBSD 11.0-PRERELEASE        January 19, 1999        FreeBSD 11.0-PRERELEASE


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