NG_BRIDGE(4) MidnightBSD 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:


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 */
u_int32_t loopTimeout; /* link loopback mute time */
u_int32_t maxStaleness; /* max host age before nuking */
u_int32_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.


Returns the current configuration as a struct ng_bridge_config.


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


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 {
u_int64_t recvOctets; /* total octets rec’d on link */
u_int64_t recvPackets; /* total pkts rec’d on link */
u_int64_t recvMulticasts; /* multicast pkts rec’d on link */
u_int64_t recvBroadcasts; /* broadcast pkts rec’d on link */
u_int64_t recvUnknown; /* pkts rec’d with unknown dest addr */
u_int64_t recvRunts; /* pkts rec’d less than 14 bytes */
u_int64_t recvInvalid; /* pkts rec’d with bogus source addr */
u_int64_t xmitOctets; /* total octets xmit’d on link */
u_int64_t xmitPackets; /* total pkts xmit’d on link */
u_int64_t xmitMulticasts; /* multicast pkts xmit’d on link */
u_int64_t xmitBroadcasts; /* broadcast pkts xmit’d on link */
u_int64_t loopDrops; /* pkts dropped due to loopback */
u_int64_t loopDetects; /* number of loop detections */
u_int64_t memoryFailures; /* times couldn’t get mem or mbuf */


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


Same as NGM_BRIDGE_GET_STATS, but also atomically clears the statistics as well.


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


This node shuts down upon receipt of a NGM_SHUTDOWN control message, or when all hooks have been disconnected.


Example script showing how to set up a bridging network


bridge(4), netgraph(4), ng_ether(4), ng_hub(4), ng_one2many(4), ngctl(8)


The ng_bridge node type was implemented in FreeBSD 4.2.


Archie Cobbs 〈〉

MidnightBSD 0.3 August 31, 2000 MidnightBSD 0.3