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 [Toc] [Back]
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.
Hooks [Toc] [Back]
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 [Toc] [Back]
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.
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 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 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 modules.
Messages often represent commands that are followed by a reply message in
the reverse direction. To facilitate this, the recipient of a control
message is supplied with a ``return address'' that is suitable for
addressing a reply.
Each control message contains a 32 bit value called a typecookie indicating
the type of the message, i.e., how to interpret it. Typically each
type defines a unique typecookie for the messages that it understands.
However, a node may choose to recognize and implement more than one type
of 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 [Toc] [Back]
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
cycles.
So far, these issues have not proven problematical in practice.
Interaction With Other Parts of the Kernel [Toc] [Back]
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
process.
Another example is a device driver that presents a node interface to the
hardware.
Node Methods [Toc] [Back]
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 [Toc] [Back]
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 standards.
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 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 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.
Addressing [Toc] [Back]
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:
.:
[3f]:
foo:
.:hook1
foo:hook1.hook2
[d80]:hook1
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 ]
+>(mux)[<un-named>]
[ D ]
One could always send a control message to node C from anywhere by using
the name Frame1:uplink.dlci16. In this case, node C would also be notified
that the message reached it via its hook ``mux''. Similarly,
Frame1:uplink.dlci20 could reliably be used to reach node D, and node A
could refer to node B as .:uplink, or simply uplink. Conversely, B can
refer to A as data. The address mux.data could be used by both nodes C
and D to address a message to node A.
Note that this is only for control messages. In each of these cases,
where a relative addressing mode is used, the recipient is notified of
the hook on which the message arrived, as well as the originating node.
This allows the option of hop-by-hop distribution of messages and state
information. Data messages are only routed one hop at a time, by specifying
the departing hook, with each node making the next routing decision.
So when B receives a frame on hook ``data'' it decodes the frame
relay header to determine the DLCI, and then forwards the unwrapped frame
to either C or D.
In a similar way, flow control messages may be routed in the reverse
direction to outgoing data. For example a "buffer nearly full" message
from Frame1: would be passed to node B which might decide to send similar
messages to both nodes C and D. The nodes would use Direct hook pointer
addressing to route the messages. The message may have travelled from
Frame1: to B as a synchronous reply, saving time and cycles.
A similar graph might be used to represent multi-link PPP running over an
ISDN line:
[ type BRI ](B1)<--->(link1)[ type MPP ]
[ "ISDN1" ](B2)<--->(link2)[ (no name) ]
[ ](D) <-+
|
+----------------+
|
+->(switch)[ type Q.921 ](term1)<---->(datalink)[ type Q.931 ]
[ (no name) ] [ (no name) ]
Netgraph Structures [Toc] [Back]
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 completed.
+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 [Toc] [Back]
Control messages have the following structure:
#define NG_CMDSTRSIZ 32 /* Max command string (including nul) */
struct ng_mesg {
struct ng_msghdr {
u_char version; /* Must equal NG_VERSION */
u_char spare; /* Pad to 2 bytes */
u_short arglen; /* Length of cmd/resp data */
u_long flags; /* Message status flags */
u_long token; /* Reply should have the same token */
u_long typecookie; /* Node type understanding this message */
u_long cmd; /* Command identifier */
u_char cmdstr[NG_CMDSTRSIZ]; /* Cmd string (for debug) */
} header;
char data[0]; /* Start of cmd/resp data */
};
#define NG_ABI_VERSION 5 /* Netgraph kernel ABI version */
#define NG_VERSION 4 /* Netgraph message version */
#define NGF_ORIG 0x0000 /* Command */
#define NGF_RESP 0x0001 /* Response */
Control messages have the fixed header shown above, followed by a variable
length data section which depends on the type cookie and the command.
Each field is explained below:
version
Indicates the version of the netgraph message protocol itself. The
current version is NG_VERSION.
arglen
This is the length of any extra arguments, which begin at data.
flags
Indicates whether this is a command or a response control message.
token
The token is a means by which a sender can match a reply message to
the corresponding command message; the reply always has the same
token.
typecookie
The corresponding node type's unique 32-bit value. If a node
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 automatic.
command
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 [Toc] [Back]
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 predefined
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 type.
Generic Control Messages [Toc] [Back]
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.
NGM_CONNECT
Connect to another node, using the supplied hook names on either
end.
NGM_MKPEER
Construct a node of the given type and then connect to it using the
supplied hook names.
NGM_SHUTDOWN
The target node should disconnect from all its neighbours and shut
down. Persistent nodes such as those representing physical hardware
might not disappear from the node namespace, but only reset themselves.
The node must disconnect all of its hooks. This may result
in neighbors shutting themselves down, and possibly a cascading
shutdown of the entire connected graph.
NGM_NAME
Assign a name to a node. Nodes can exist without having a name, and
this is the default for nodes created using the NGM_MKPEER method.
Such nodes can only be addressed relatively or by their ID number.
NGM_RMHOOK
Ask the node to break a hook connection to one of its neighbours.
Both nodes will have their ``disconnect'' method invoked. Either
node may elect to totally shut down as a result.
NGM_NODEINFO
Asks the target node to describe itself. The four returned fields
are the node name (if named), the node type, the node ID and the
number of hooks attached. The ID is an internal number unique to
that node.
NGM_LISTHOOKS
This returns the information given by NGM_NODEINFO, but in addition
includes an array of fields describing each link, and the description
for the node at the far end of that link.
NGM_LISTNAMES
This returns an array of node descriptions (as for NGM_NODEINFO)
where each entry of the array describes a named node. All named
nodes will be described.
NGM_LISTNODES
This is the same as NGM_LISTNAMES except that all nodes are listed
regardless of whether they have a name or not.
NGM_LISTTYPES
This returns a list of all currently installed netgraph types.
NGM_TEXT_STATUS
The node may return a text formatted status message. The status
information is determined entirely by the node type. It is the only
"generic" message that requires any support within the node itself
and as such the node may elect to not support this message. The text
response must be less than NG_TEXTRESPONSE bytes in length
(presently 1024). This can be used to return general status information
in human readable form.
NGM_BINARY2ASCII
This message converts a binary control message to its ASCII form.
The entire control message to be converted is contained within the
arguments field of the NGM_BINARY2ASCII message itself. If successful,
the reply will contain the same control message in ASCII form.
A node will typically only know how to translate messages that it
itself understands, so the target node of the NGM_BINARY2ASCII is
often the same node that would actually receive that message.
NGM_ASCII2BINARY
The opposite of NGM_BINARY2ASCII. The entire control message to be
converted, in ASCII form, is contained in the arguments section of
the NGM_ASCII2BINARY and need only have the flags, cmdstr, and
arglen header fields filled in, plus the NUL-terminated string version
of the arguments in the arguments field. If successful, the
reply contains the binary version of the control message.
Flow Control Messages [Toc] [Back]
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.
Metadata [Toc] [Back]
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,
include
options NETGRAPH
in your kernel configuration file. You may also include selected node
types in the kernel compilation, for example:
options NETGRAPH
options NETGRAPH_SOCKET
options NETGRAPH_ECHO
Once the netgraph subsystem is loaded, individual node types may be
loaded at any time as KLD modules via kldload(8). Moreover, netgraph
knows how to automatically do this; when a request to create a new node
of unknown type type is made, netgraph will attempt to load the KLD module
ng_type.ko.
Types can also be installed at boot time, as certain device drivers may
want to export each instance of the device as a netgraph node.
In general, new types can be installed at any time from within the kernel
by calling ng_newtype(), supplying a pointer to the type's struct ng_type
structure.
The NETGRAPH_INIT() macro automates this process by using a linker set.
Several node types currently exist. Each is fully documented in its own
man page:
SOCKET
The socket type implements two new sockets in the new protocol
domain PF_NETGRAPH. The new sockets protocols are NG_DATA and
NG_CONTROL, both of type SOCK_DGRAM. Typically one of each is associated
with a socket node. When both sockets have closed, the node
will shut down. The NG_DATA socket is used for sending and receiving
data, while the NG_CONTROL socket is used for sending and
receiving control messages. Data and control messages are passed
using the sendto(2) and recvfrom(2) calls, using a struct
sockaddr_ng socket address.
HOLE
Responds only to generic messages and is a ``black hole'' for data,
Useful for testing. Always accepts new hooks.
ECHO
Responds only to generic messages and always echoes data back
through the hook from which it arrived. Returns any non generic messages
as their own response. Useful for testing. Always accepts new
hooks.
TEE This node is useful for ``snooping''. It has 4 hooks: left, right,
left2right, and right2left. Data entering from the right is passed
to the left and duplicated on right2left, and data entering from the
left is passed to the right and duplicated on left2right. Data
entering from left2right is sent to the right and data from
right2left to left.
RFC1490 MUX
Encapsulates/de-encapsulates frames encoded according to RFC 1490.
Has a hook for the encapsulated packets (``downstream'') and one
hook for each protocol (i.e., IP, PPP, etc.).
FRAME RELAY MUX
Encapsulates/de-encapsulates Frame Relay frames. Has a hook for the
encapsulated packets (``downstream'') and one hook for each DLCI.
FRAME RELAY LMI
Automatically handles frame relay ``LMI'' (link management interface)
operations and packets. Automatically probes and detects
which of several LMI standards is in use at the exchange.
TTY This node is also a line discipline. It simply converts between mbuf
frames and sequential serial data, allowing a tty to appear as a
netgraph node. It has a programmable ``hotkey'' character.
ASYNC
This node encapsulates and de-encapsulates asynchronous frames
according to RFC 1662. This is used in conjunction with the TTY node
type for supporting PPP links over asynchronous serial lines.
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 Mpd port can use these modules to make a very low latency high
capacity ppp system. It also supports PPTP vpns using the PPTP node.
PPPOE
A server and client side implementation of PPPoE. Used in conjunction
with either ppp(8) or the mpd port.
BRIDGE
This node, together with the ethernet nodes allows a very flexible
bridging system to be implemented.
KSOCKET
This intriguing node looks like a socket to the system but diverts
all data to and from the netgraph system for further processing.
This allows such things as UDP tunnels to be almost trivially implemented
from the command line.
Refer to the section at the end of this man page for more nodes types.
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 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.
/sys/netgraph/netgraph.h
Definitions for use solely within the kernel by netgraph nodes.
/sys/netgraph/ng_message.h
Definitions needed by any file that needs to deal with netgraph
messages.
/sys/netgraph/ng_socket.h
Definitions needed to use netgraph socket type nodes.
/sys/netgraph/ng_{type}.h
Definitions needed to use netgraph {type} nodes, including the
type cookie definition.
/boot/kernel/netgraph.ko
Netgraph subsystem loadable KLD module.
/boot/kernel/ng_{type}.ko
Loadable KLD module for node type {type}.
/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 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 <[email protected]>, with contributions by Archie Cobbs
<[email protected]>.
FreeBSD 5.2.1 January 19, 1999 FreeBSD 5.2.1 [ Back ] |
