各位高手，帮忙分析一下这个程序

[lionqin]

void
__arp_handler(pktbuf_t *pkt)
{
arp_header_t *arp = pkt->arp_hdr;
int hw_type, protocol;

/*
* Only handle ethernet hardware and IP protocol.
*/
protocol = ntohs(arp->protocol);
hw_type = ntohs(arp->hw_type);
if ((hw_type == ARP_HW_ETHER) && (protocol == ETH_TYPE_IP)) {
/*
* Handle requests for our ethernet address.
*/
if (!memcmp(arp->target_ip, __local_ip_addr, 4)) {
if (ntohs(arp->opcode) == ARP_REQUEST) {
/* format response. */
arp->opcode = htons(ARP_REPLY);
memcpy(arp->target_ip, arp->sender_ip,
sizeof(ip_addr_t));
memcpy(arp->target_enet, arp->sender_enet,
sizeof(enet_addr_t));
memcpy(arp->sender_ip, __local_ip_addr,
sizeof(ip_addr_t));
memcpy(arp->sender_enet, __local_enet_addr,
sizeof(enet_addr_t));
pkt->pkt_bytes = sizeof(arp_header_t);
__enet_send(pkt, &arp->target_enet, ETH_TYPE_ARP);

} else if (ntohs(arp->opcode) == ARP_REPLY && arp_req.waiting) {
if (!memcmp(arp_req.ip, arp->sender_ip, sizeof(ip_addr_t))) {
memcpy(arp_req.eth, arp->sender_enet, sizeof(enet_addr_t));
arp_req.waiting = 0;
}
}
}
}
__pktbuf_free(pkt);
}


/*
* Find the ethernet address of the machine with the given
* ip address.
* Return 0 and fills in 'eth_addr' if successful,
* -1 if unsuccessful.
*/
int
__arp_request(ip_addr_t *ip_addr, enet_addr_t *eth_addr, int allow_self)
{
pktbuf_t *pkt;
arp_header_t *arp;
unsigned long retry_start;
enet_addr_t bcast_addr;
int retry;

if (!allow_self) {
// Special case request for self
if (!memcmp(ip_addr, __local_ip_addr, 4)) {
memcpy(eth_addr, __local_enet_addr, sizeof(enet_addr_t));
return 0;
}
}

/* just fail if can't get a buffer */
if ((pkt = __pktbuf_alloc(ARP_PKT_SIZE)) == NULL)
return -1;

arp = pkt->arp_hdr;
arp->opcode = htons(ARP_REQUEST);
arp->hw_type = htons(ARP_HW_ETHER);
arp->protocol = htons(0x800);
arp->hw_len = sizeof(enet_addr_t);
arp->proto_len = sizeof(ip_addr_t);

memcpy(arp->sender_ip, __local_ip_addr, sizeof(ip_addr_t));
memcpy(arp->sender_enet, __local_enet_addr, sizeof(enet_addr_t));
memcpy(arp->target_ip, ip_addr, sizeof(ip_addr_t));

bcast_addr[0] = 255;
bcast_addr[1] = 255;
bcast_addr[2] = 255;
bcast_addr[3] = 255;
bcast_addr[4] = 255;
bcast_addr[5] = 255;

arp_req.eth = (char *)eth_addr;
arp_req.ip = (char *)ip_addr;
arp_req.waiting = 1;

retry = 8;
while (retry-- > 0) {

/* send the packet */
pkt->pkt_bytes = sizeof(arp_header_t);
__enet_send(pkt, &bcast_addr, ETH_TYPE_ARP);

retry_start = MS_TICKS();
while ((MS_TICKS_DELAY() - retry_start) < 250) {
__enet_poll();
if (!arp_req.waiting) {
__pktbuf_free(pkt);
return 0;
}
}
}
__pktbuf_free(pkt);
return -1;
}

#define NUM_ARP 16
static ip_route_t routes[NUM_ARP];

int
__arp_lookup(ip_addr_t *host, ip_route_t *rt)
{
int i;
static int next_arp = 0;

for (i = 0; i < NUM_ARP; i++) {
if (memcmp(host, &routes[i].ip_addr, sizeof(*host)) == 0) {
// This is a known host
memcpy(rt, &routes[i], sizeof(*rt));
return 0;
}
}
memcpy(&rt->ip_addr, host, sizeof(*host));
if (((*host)[0] == 0xFF) && ((*host)[1] == 0xFF) && ((*host)[2] == 0xFF)) {
memset(&rt->enet_addr, 0xFF, sizeof(&rt->enet_addr));
return 0;
#ifdef CYGSEM_REDBOOT_NETWORKING_USE_GATEWAY
} else if (!__ip_addr_local(host)) {
// non-local IP address -- look up Gateway's Ethernet address
host = &__local_ip_gate;
#endif
}
if (__arp_request(host, &rt->enet_addr, 0) < 0) {
return -1;
} else {
memcpy(&routes[next_arp], rt, sizeof(*rt));
if (++next_arp == NUM_ARP) next_arp = 0;
return 0;
}
}

[Joran]

好像是LINUX中TCP/IP关于arp协议处理的一段代码，建议察看相关的书籍。如《LINUX2。4源代码情景分析》

[lichenzhou333]

也可以查看《TCP/IP协议详解卷2〉。