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NG_BRIDGE(4)		 BSD Kernel Interfaces Manual		  NG_BRIDGE(4)

     ng_bridge -- Ethernet bridging netgraph node type

     #include <sys/types.h>
     #include <netgraph/ng_bridge.h>

     The bridge	node type performs Ethernet bridging over one or more links.
     Each link (represented by a connected hook) is used to transmit and re-
     ceive raw Ethernet	frames.	 As packets are	received, the node learns
     which link	each host resides on.  Packets unicast to a known host are di-
     rected out	the appropriate	link only, and other links are spared the
     traffic.  This behavior is	in contrast to a hub, which always forwards
     every received packet to every other link.

     The bridge	node incorporates a simple loop	detection algorithm.  A	loop
     is	when two ports are connected to	the same physical medium.  Loops are
     important to avoid	because	of packet storms, which	severely degrade per-
     formance.	A packet storm results when the	same packet is sent and	re-
     ceived over and over again.  If a host is detected	on link	A, and is then
     detected on link B	within a certain time period after first being de-
     tected on link A, then link B is considered to be a looped	back link.
     The time period is	called the minimum stable time.

     A looped back link	will be	temporarily muted, i.e., all traffic received
     on	that link is ignored.

     Processing	of IP packets via the ipfirewall(4) mechanism on a per-link
     basis is not yet implemented.

     This node type supports an	unlimited number of hooks.  Each connected
     hook represents a bridged link.  The hooks	are named link0, link1,	etc.
     Typically these hooks are connected to the	lower hooks of one or more
     ng_ether(4) nodes.	 To connect the	host machine to	a bridged network,
     simply connect the	upper hook of an ng_ether(4) node to the bridge	node.

     This node type supports the generic control messages, plus	the following:

     NGM_BRIDGE_SET_CONFIG (setconfig)
	  Set the node configuration.  This command takes a struct
	  ng_bridge_config as an argument:

	  /* Node configuration	structure */
	  struct ng_bridge_config {
	    u_char	debugLevel;	      /* debug level */
	    uint32_t	loopTimeout;	      /* link loopback mute time */
	    uint32_t	maxStaleness;	      /* max host age before nuking */
	    uint32_t	minStableAge;	      /* min time for a	stable host */

	  The debugLevel field sets the	debug level on the node.  At level of
	  2 or greater,	detected loops are logged.  The	default	level is 1.

	  The loopTimeout determines how long (in seconds) a looped link is
	  muted.  The default is 60 seconds.  The maxStaleness parameter de-
	  termines how long a period of	inactivity before a host's entry is
	  forgotten.  The default is 15	minutes.  The minStableAge determines
	  how quickly a	host must jump from one	link to	another	before we de-
	  clare	a loopback condition.  The default is one second.

     NGM_BRIDGE_GET_CONFIG (getconfig)
	  Returns the current configuration as a struct	ng_bridge_config.

     NGM_BRIDGE_RESET (reset)
	  Causes the node to forget all	hosts and unmute all links.  The node
	  configuration	is not changed.

     NGM_BRIDGE_GET_STATS (getstats)
	  This command takes a four byte link number as	an argument and	re-
	  turns	a struct ng_bridge_link_stats containing statistics for	the
	  corresponding	link, which must be currently connected:

	  /* Statistics	structure (one for each	link) */
	  struct ng_bridge_link_stats {
	    uint64_t   recvOctets;     /* total	octets rec'd on	link */
	    uint64_t   recvPackets;    /* total	pkts rec'd on link */
	    uint64_t   recvMulticasts; /* multicast pkts rec'd on link */
	    uint64_t   recvBroadcasts; /* broadcast pkts rec'd on link */
	    uint64_t   recvUnknown;    /* pkts rec'd with unknown dest addr */
	    uint64_t   recvRunts;      /* pkts rec'd less than 14 bytes	*/
	    uint64_t   recvInvalid;    /* pkts rec'd with bogus	source addr */
	    uint64_t   xmitOctets;     /* total	octets xmit'd on link */
	    uint64_t   xmitPackets;    /* total	pkts xmit'd on link */
	    uint64_t   xmitMulticasts; /* multicast pkts xmit'd	on link	*/
	    uint64_t   xmitBroadcasts; /* broadcast pkts xmit'd	on link	*/
	    uint64_t   loopDrops;      /* pkts dropped due to loopback */
	    uint64_t   loopDetects;    /* number of loop detections */
	    uint64_t   memoryFailures; /* times	couldn't get mem or mbuf */

     NGM_BRIDGE_CLR_STATS (clrstats)
	  This command takes a four byte link number as	an argument and	clears
	  the statistics for that link.

     NGM_BRIDGE_GETCLR_STATS (getclrstats)
	  Same as NGM_BRIDGE_GET_STATS,	but also atomically clears the statis-
	  tics as well.

     NGM_BRIDGE_GET_TABLE (gettable)
	  Returns the current host mapping table used to direct	packets, in a
	  struct ng_bridge_host_ary.

     NGM_BRIDGE_SET_PERSISTENT (setpersistent)
	  This command sets the	persistent flag	on the node, and takes no ar-

     This node shuts down upon receipt of a NGM_SHUTDOWN control message, or
     when all hooks have been disconnected.  Setting the persistent flag via a
     NGM_BRIDGE_SET_PERSISTENT control message disables	automatic node shut-
     down when the last	hook gets disconnected.

	       Example script showing how to set up a bridging network

     if_bridge(4), netgraph(4),	ng_ether(4), ng_hub(4),	ng_one2many(4),

     The ng_bridge node	type was implemented in	FreeBSD	4.2.

     Archie Cobbs <>

BSD				October	2, 2019				   BSD


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