linux下检测ip冲突

原理其实很简单，那就是广播一个arp包，然后recv，如果没有数据(这里要设置延时)，那么说明这个ip是可用的，否则就检测这个数据是否为回复我们发出的arp的应答包.如果是则证明ip已被使用，否则继续等待.

这里可以看下busybox的dhcp中的检测程序。
networking/udhcp/arpping.c

/* vi: set sw=4 ts=4: */
/*
 * arpping.c
 *
 * Mostly stolen from: dhcpcd - DHCP client daemon
 * by Yoichi Hariguchi <[e-mail]>
 */

#include <netinet/if_ether.h>
#include <net/if_arp.h>

#include "common.h"
#include "dhcpd.h"

//这里是arp包的格式,其中的数据格式都是宏了，比如uint_8_t为无符char.
struct arpMsg {
	/* Ethernet header */
	uint8_t  h_dest[6];     /* 00 destination ether addr */
	uint8_t  h_source[6];   /* 06 source ether addr */
	uint16_t h_proto;       /* 0c packet type ID field */

	/* ARP packet */
	uint16_t htype;         /* 0e hardware type (must be ARPHRD_ETHER) */
	uint16_t ptype;         /* 10 protocol type (must be ETH_P_IP) */
	uint8_t  hlen;          /* 12 hardware address length (must be 6) */
	uint8_t  plen;          /* 13 protocol address length (must be 4) */
	uint16_t operation;     /* 14 ARP opcode */
	uint8_t  sHaddr[6];     /* 16 sender's hardware address */
	uint8_t  sInaddr[4];    /* 1c sender's IP address */
	uint8_t  tHaddr[6];     /* 20 target's hardware address */
	uint8_t  tInaddr[4];    /* 26 target's IP address */
	uint8_t  pad[18];       /* 2a pad for min. ethernet payload (60 bytes) */
} PACKED;

enum {
	ARP_MSG_SIZE = 0x2a
};


/* Returns 1 if no reply received */

//主程序，如果返回1说明此ip可用
int arpping(uint32_t test_ip, uint32_t from_ip, uint8_t *from_mac, const char *interface)
{

	int timeout_ms;
//这里使用poll来检测句柄。
	struct pollfd pfd[1];
#define s (pfd[0].fd)           /* socket */
	int rv = 1;             /* "no reply received" yet */
	struct sockaddr addr;   /* for interface name */
	struct arpMsg arp;

//建立scoket.由于我们是要直接访问访问链路层并自己组arp包.因此我们使用PF_PACKET协议簇.socket类型为SOCK_PACKET.

	s = socket(PF_PACKET, SOCK_PACKET, htons(ETH_P_ARP));
	if (s == -1) {
		bb_perror_msg(bb_msg_can_not_create_raw_socket);
		return -1;
	}

	if (setsockopt_broadcast(s) == -1) {
		bb_perror_msg("cannot enable bcast on raw socket");
		goto ret;
	}
//进行组包，由于是要广播，因此目的mac地址为全0.
	/* send arp request */
	memset(&arp, 0, sizeof(arp));
	memset(arp.h_dest, 0xff, 6);                    /* MAC DA */
	memcpy(arp.h_source, from_mac, 6);              /* MAC SA */
	arp.h_proto = htons(ETH_P_ARP);                 /* protocol type (Ethernet) */
	arp.htype = htons(ARPHRD_ETHER);                /* hardware type */
	arp.ptype = htons(ETH_P_IP);                    /* protocol type (ARP message) */
	arp.hlen = 6;                                   /* hardware address length */
	arp.plen = 4;                                   /* protocol address length */
	arp.operation = htons(ARPOP_REQUEST);           /* ARP op code */
	memcpy(arp.sHaddr, from_mac, 6);                /* source hardware address */
	memcpy(arp.sInaddr, &from_ip, sizeof(from_ip)); /* source IP address */
	/* tHaddr is zero-fiiled */                     /* target hardware address */
	memcpy(arp.tInaddr, &test_ip, sizeof(test_ip)); /* target IP address */

	memset(&addr, 0, sizeof(addr));
	safe_strncpy(addr.sa_data, interface, sizeof(addr.sa_data));
//广播arp包.
	if (sendto(s, &arp, sizeof(arp), 0, &addr, sizeof(addr)) < 0) {
		// TODO: error message? caller didn't expect us to fail,
		// just returning 1 "no reply received" misleads it.
		goto ret;
	}

	/* wait for arp reply, and check it */
//等待时间，超时则认为此ip地址可用
	timeout_ms = 2000;
	do {
		int r;
		unsigned prevTime = monotonic_us();

		pfd[0].events = POLLIN;
//这边他是害怕poll被信号打断，因此加了层循环，其实这边我们还可以使用ppoll的，就可以了。
		r = safe_poll(pfd, 1, timeout_ms);
		if (r < 0)
			break;
		if (r) {
//读取返回数据.
			r = read(s, &arp, sizeof(arp));
			if (r < 0)
				break;
//检测是否为应打包，发送ip是否为我们所请求的ip,这里是为了防止其他的数据包干扰我们检测。
			if (r >= ARP_MSG_SIZE
			 && arp.operation == htons(ARPOP_REPLY)
			 /* don't check it: Linux doesn't return proper tHaddr (fixed in 2.6.24?) */
			 /* && memcmp(arp.tHaddr, from_mac, 6) == 0 */
			 && *((uint32_t *) arp.sInaddr) == test_ip
			) {
//说明ip地址已被使用
				rv = 0;
				break;
			}
		}
		timeout_ms -= ((unsigned)monotonic_us() - prevTime) / 1000;
	} while (timeout_ms > 0);

 ret:
	close(s);
	DEBUG("%srp reply received for this address", rv ? "No a" : "A");
	return rv;
}

评论

1 楼 xiangjie88 2014-03-25  

能否给出setsockopt_broadcast的头文件或函数