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NG_BRIDGE(4)           FreeBSD 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
     receive raw Ethernet frames.  As packets are received, the node learns
     which link each host resides on.  Packets unicast to a known host are
     directed 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
     performance.  A packet storm results when the same packet is sent and
     received 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
     detected 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 up to NG_BRIDGE_MAX_LINKS 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      ipfw[NG_BRIDGE_MAX_LINKS]; /* enable ipfw */
            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 ipfw array enables ipfirewall(4) processing of IP packets
          received on the corresponding links.  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
          determines 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
          declare 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
          returns 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
          statistics 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

     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
     shutdown 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 <>

FreeBSD 11.0-PRERELEASE           May 5, 2010          FreeBSD 11.0-PRERELEASE


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