Understanding Linux Network Internals

Handling Ingress Traffic

We saw in Chapter 10 how ingress traffic is processed by netif_receive_skb. In particular, we saw how the function calls handle_bridge (defined in net/core/dev.c) before passing each ingress frame to the upper-layer protocol handler.

When the kernel does not have support for bridging, handle_bridge is defined as a NULL pointer and netif_receive_skb hands ingress frames to other protocol handlers. When the kernel does support bridging, and a frame is received on a bridge port, handle_bridge processes the frame with br_handle_frame_hook. The latter pointer is initialized to br_handle_bridge when the bridging module is initialized.

#if defined(CONFIG_BRIDGE) || defined (CONFIG_BRIDGE_MODULE)
...
static _ _inline_ _ int handle_bridge(struct sk_buff **pskb,
                    struct packet_type **pt_prev, int *ret)
{
    struct net_bridge_port *port;

    if ((*pskb)->pkt_type == PACKET_LOOPBACK ||
        (port = rcu_dereference((*pskb)->dev->br_port)) == NULL)
        return 0;

    if (*pt_prev) {
        *ret = deliver_skb(*pskb, *pt_prev);
        *pt_prev = NULL;
    }

    return br_handle_frame_hook(port, pskb);
}
#else
#define handle_bridge(skb, pt_prev, ret)    (0)
#endif

In the following subsections, we will see how handle_bridge processes ingress frames, distinguishing between data frames and STP BPDUs (Figure 16-11, circle a). For data frames, the function also distinguishes between unicast frames and multicast or broadcast frames (Figure 16-11, circle b).

Data Frames Versus BPDUs

On a Linux system with support for bridging, not ...