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;}

 

转载于  http://simohayha.iteye.com/blog/267955