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
The objects, known as nodes, can be arranged into arbitrari
ly 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
kernel networking infrastructure. It provides:
+o A flexible way of combining protocol and link level
+o A modular way to implement new protocols.
+o A common framework for kernel entities to inter-communi
+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
with other nodes in a well defined manner.
Each node has a type, which is a static property of the node
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 ter
minating NUL
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.
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 the characters `.' or `:', and is limited to
characters (including the terminating NUL character).
+o A hook is always connected to another hook. That is,
hooks are cre
ated 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
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.
+o A hook may supply overriding receive data and receive
message func
tions which should be used for data and messages re
ceived through
that hook in preference to the general node-wide meth
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
Along with data, nodes can also receive control messages.
There are
generic and type-specific control messages. Control mes
sages have a common header format, followed by a type-specific data, and are
structures for efficiency. However, node types may also
support conversion of the type specific data between binary and ASCII for
mats, for
debugging and human interface purposes (see the
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 address
ing). 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
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 address
ing may be
combined, by specifying an absolute start node and a se
quence 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 modules.
Messages often represent commands that are followed by a re
ply message in
the reverse direction. To facilitate this, the recipient of
a control
message is supplied with a ``return address'' that is suit
able 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
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
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. Obvious
ly, 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 di
rectly, 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
hook should always be queued, rather than delivered directly
(often useful for interrupt routines who want to get back to the hard
ware quickly).
By default, all control message packets are considered to be
unless specifically declared to be a reader in their defini
tion. (See
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 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 un
til 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.
+o It is possible for an infinite loop to occur if the
graph contains
So far, these issues have not proven problematical in prac
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
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 communi
cate 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 in
terface to the
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 re
sources 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
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
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 it
self from the
chain and reconnect its peers together, like the
ng_tee(4) node type
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 hard
ware removal or
unloading (via ng_rmnode_self()), it should set the
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 re
ceiving node must
always NG_FREE_M() the mbuf chain on completion or er
ror, or pass it
on to another node (or kernel module) which will then be
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


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 di
rectly. 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
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 ad
dressed to the
node. As with the received data, an item is received,
with a pointer
to the control message. The message can be examined us
ing 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 re
turn address
is always supplied, giving the address of the node that
the message so a reply message can be sent anytime lat
er. The return
address is retrieved from the item using the


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



If the message was delivered via a specific hook, that
hook will also
be made known, which allows the use of such things as
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 mes
sage 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.
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 ad
dress 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 se
quence of hooks.
A relative address includes only the sequence of hook names,
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 num
bers, by enclosing the hexadecimal representation of the ID number within
the square
brackets. Here are some examples of valid netgraph address

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>](dl
ci16)<-->(mux)[<un-named> ]
[ A ] [ B ](dlci20)<---+ [
C ]
[ type PPP ]
D ]
One could always send a control message to node C from any
where 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. Sim
``Frame1:uplink.dlci20'' could reliably be used to reach
node D, and node
A could refer to node B as ``.:uplink'', or simply ``up
link''. 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 origi
nating node.
This allows the option of hop-by-hop distribution of mes
sages 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
direction to outgoing data. For example a ``buffer nearly
full'' message
from ``Frame1:'' would be passed to node B which might de
cide 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 au
thors 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 struc
typedef struct ng_node *node_p;
The following properties are associated with a node, and
can be
accessed in the following manner:
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.
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


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] != ' ') ...
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


and retrieve this property, respectively.
Number of hooks
The NG_NODE_NUMHOOKS(node) macro is used to retrieve
this value.
The node may have a number of hooks. A traversal
method is provided to allow all the hooks to be tested for some
NG_NODE_FOREACH_HOOK(node, fn, arg, rethook) where
fn is a function that will be called for each hook with the form
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


and retrieve this property, respectively.
An associate node
The macro NG_HOOK_NODE(hook) finds the associated
A peer hook (hook_p)
The other hook in this connected pair. The


macro finds the peer.
The NG_HOOK_REF(hook) and NG_HOOK_UNREF(hook) macros
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


fn) macros can be used to set override methods that
will be used
in preference to the generic receive data and re
ceive message
functions. To unset these, use the macros to set
them to NULL.
They will only be used for data and messages re
ceived 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 remem
ber 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 (in
cluding 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 de
bug) */
} 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 proto
col itself.
The current version is NG_VERSION.
arglen This is the length of any extra arguments, which be
gin at data.
flags Indicates whether this is a command or a response
control mes
token The token is a means by which a sender can match a
reply message
to the corresponding command message; the reply al
ways has the
same token.
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 out
put of ``date
-u +%s'').
There is a predefined typecookie NGM_GENERIC_COOKIE
for the
generic node type, and a corresponding set of gener
ic messages
which all nodes understand. The handling of these
messages is
cmd The identifier for the message command. This is
type specific,
and is defined in the same header file as the type
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. How
ever, 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 ex
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 predefined datatypes in ASCII with this simple syntax:
+o Integer types are represented by base 8, 10, or 16 num
+o Strings are enclosed in double quotes and respect the
normal C lan
guage backslash escapes.
+o IP addresses have the obvious form.
+o Arrays are enclosed in square brackets, with the ele
ments listed con
secutively 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
+o Array elements and structure fields may be specified in
any order.
Each node type may define its own arbitrary types by provid
ing 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 it
self. These are
defined in #include <netgraph/ng_message.h> along with the basic layout of messages and other similar
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 repre
senting physical
hardware might not disappear from the node names
pace, 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
Assign a name to a node. Nodes can exist without
having a name,
and this is the default for nodes created using the
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 in
voked. Either
node may elect to totally shut down as a result.
Asks the target node to describe itself. The four
fields are the node name (if named), the node type,
the node ID
and the number of hooks attached. The ID is an in
ternal 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
This returns an array of node descriptions (as for
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
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
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 con
tained 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
messages that it itself understands, so the target
node of the
NGM_BINARY2ASCII is often the same node that would
receive that message.
The opposite of NGM_BINARY2ASCII. The entire con
trol message to
be converted, in ASCII form, is contained in the ar
guments section of the NGM_ASCII2BINARY and need only have the
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 de
fined. 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 utilis
ing flow control, and will be in the likely path of these messages. The
action of a node that does not understand these messages
should be to
pass them onto the next node. Hopefully some helper func
tions will
assist in this eventually. These messages are also defined
in #include
and have a separate cookie NG_FLOW_COOKIE to help identify
them. They
will not be covered in depth here.


The base netgraph code may either be statically compiled in
to 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 se
lected node
types in the kernel compilation, for example:

options NETGRAPH
Once the netgraph subsystem is loaded, individual node types
may be
loaded at any time as KLD modules via kldload(8). Moreover,
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
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 document
ed 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_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 con
trol messages
are passed using the sendto(2) and recvfrom(2) sys
tem 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
ECHO Responds only to generic messages and always echoes
data back
through the hook from which it arrived. Returns any
messages as their own response. Useful for testing.
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.
Encapsulates/de-encapsulates frames encoded accord
ing 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 man
agement interface) operations and packets. Automatically probes
and detects
which of several LMI standards is in use at the ex
TTY This node is also a line discipline. It simply con
verts 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 asyn
chronous frames
according to RFC 1662. This is used in conjunction
with the TTY
node type for supporting PPP links over asynchronous
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.
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 inter
faces are named
``ng0'', ``ng1'', etc.
This node implements a simple round-robin multiplex
er. It can be
used for example to make several LAN ports act to
gether 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 sup
ports PPTP VPNs
using the PPTP node.
PPPOE A server and client side implementation of PPPoE.
Used in con
junction with either ppp(8) or the net/mpd port.
BRIDGE This node, together with the Ethernet nodes, allows
a very flexi
ble bridging system to be implemented.
This intriguing node looks like a socket to the sys
tem 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
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. Messages sent using one of NG_SEND_MSG_*() family
macros are freed
by the recipient. As in the case above, the addresses
with the message are freed by whatever allocated them
so the recipient should copy them if it wants to keep that informa
3. Both control messages and data are delivered and queued
with a
netgraph item. The item must be freed using


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
Definitions needed to use netgraph type nodes, in
cluding the type
cookie definition.
The netgraph subsystem loadable KLD module.
Loadable KLD module for node type type.
Skeleton netgraph node. Use this as a starting
point for new
node types.


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 debug
ging 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_atm(4), ng_atm
ng_atmpif(4), ng_bluetooth(4), ng_bpf(4), ng_bridge(4),
ng_btsocket(4), ng_cisco(4), ng_device(4), ng_echo(4),
ng_etf(4), ng_ether(4), ng_fec(4), ng_frame_relay(4),
ng_gif_demux(4), ng_h4(4), ng_hci(4), ng_hole(4), ng_hub(4),
ng_ip_input(4), ng_ksocket(4), ng_l2cap(4), ng_l2tp(4),
ng_mppc(4), ng_netflow(4), ng_one2many(4), ng_ppp(4), ng_pp
ng_pptpgre(4), ng_rfc1490(4), ng_socket(4), ng_split(4),
ng_sscfu(4), ng_sscop(4), ng_tee(4), ng_tty(4), ng_ubt(4),
ng_uni(4), ng_vjc(4), ng_vlan(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
BSD July 1, 2004
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