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NAME
netgraph - graph based kernel networking subsystem
DESCRIPTION
The netgraph system provides a uniform and modular system for the imple‐
mentation 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 capabili‐
ties (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 cre‐
ated 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 inter‐
rupts.
A hook may supply overriding receive data and receive message func‐
tions 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 send‐
ing 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
hooks.
Along with data, nodes can also receive control messages. There are
generic and type-specific control messages. Control messages have a com‐
mon header format, followed by a type-specific data, and are binary
structures for efficiency. However, node types may also support conver‐
sion 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 global ASCII name (or
equivalent ID based name) is used as the destination address for the mes‐
sage (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 mod‐
ules.
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 address‐
ing a reply.
Each control message contains a 32 bit value called a typecookie indicat‐
ing 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 mes‐
sage 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 men‐
tioned 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 use‐
ful 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. Exam‐
ples 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 pro‐
cessed by either the current holder of the lock when they have com‐
pleted 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 proto‐
col 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 return‐
ing 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 corre‐
sponding 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 sig‐
nal 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 out‐
put 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 for‐
ward 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 compo‐
nent 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 enclos‐
ing 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 speci‐
fying the departing hook, with each node making the next routing deci‐
sion. 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 simi‐
lar 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 <net
graph/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 pro‐
vided to allow all the hooks to be tested for some condition.
NG_NODE_FOREACH_HOOK(node, fn, arg, rethook) where fn is a func‐
tion 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 decre‐
ment 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 subsys‐
tem. 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 con‐
structor 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 vari‐
able length data section which depends on the type cookie and the com‐
mand. 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 mes‐
sage.
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 generat‐
ing 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 lan‐
guage backslash escapes.
IP addresses have the obvious form.
Arrays are enclosed in square brackets, with the elements 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 ele‐
ment’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 neces‐
sary 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 possi‐
bly 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 addi‐
tion 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 mes‐
sage. 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 sec‐
tion 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 mod‐
ule 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.
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 send‐
ing 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 enter‐
ing 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 inter‐
face) 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 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.
KSOCKET
This intriguing node looks like a socket to the system but
diverts all data to and from the netgraph system for further pro‐
cessing. 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 recipi‐
ent 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.
There is a library for supporting user-mode programs that wish to inter‐
act 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_atm(4), ng_atmllc(4),
ng_atmpif(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 Commu‐
nications, 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〉.