linux下写程序检测ip冲突(ARP)

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

C代码



/* vi: set sw=4 ts=4: */

/*

* arpping.c

*

* Mostly stolen from: dhcpcd - DHCP client daemon

* by Yoichi Hariguchi <yoichi@fore.com>

*/

#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;

}

Ref: http://simohayha.javaeye.com/blog/267955