traceroute， 也就是 trace route，跟踪路由。这个程序最早是Van Jacobson实现的。源码在网上可以找到，不过我还没有去找。是IP路由过程中对数据包TTL(Time to Live，存活时间)的处理。当路由器收到一个IP包时，会修改IP包的TTL（及由此造成的头部检验和checksum变化）。每收到一个包，检查这个 的TTL是否是0或1。如果是，表明这个包还没有到达目的地，而且剩余时间不多了，肯定是到不了目的地了。这样路由器就简单地丢弃这个包，并给源主机发送 ICMP通知，说这个包已经超时了。ICMP的通知信息里包含当前路由器发送时所用的IP。
这样就可以通过构造数据包，来间接检查到 达一个主机时经过了哪些路由。一开始发送一个TTL为1的包，这样到达第一个路由器的时候就已经超时了，第一个路由器就发通知说包超时，这样就可以记录下 所经过的第一个路由器的IP。然后TTL加1，安全通过第一个路由器，而第二个路由器的的处理与第一个相同，丢包，发通知说包超时了，这样记录下第二个路 由器IP，由此可以一直进行下去，直到这个数据包到达目标主机，由此打印出所有经过的路由器。
#include<stdio.h> #include<sys/time.h> #include<errno.h> #include<signal.h> #include<time.h> #include<stdlib.h> #include<unistd.h> #include<netdb.h> #include<string.h> #include<strings.h> #include<sys/socket.h> #include<arpa/inet.h> #include<netinet/in_systm.h> #include<netinet/ip.h> #include<netinet/ip_icmp.h> #include<netinet/udp.h> #include<netinet/in_systm.h> #define BUFSIZE 1500 struct rec{ u_short rec_seq; u_short rec_ttl; struct timeval rec_tv; }; char recvbuf[BUFSIZE]; char sendbuf[BUFSIZE]; int datalen; char *host; u_short sport,dport; int nsent; pid_t pid; int probe,nprobes; int sendfd,recvfd; int ttl,max_ttl; int verbose; struct proto{ const char*(*icmpcode)(int); int (*recv)(int,struct timeval*); struct sockaddr *sasend; //dest addr, the destination struct sockaddr *sarecv; //recv addr, store who send the message struct sockaddr *salast; struct sockaddr *sabind; //bind the source port socklen_t salen; int icmpproto; int ttllevel; int ttloptname; }*pr; int gotalarm; const char *icmpcode_v4(int code){ static char errbuf[100]; switch(code){ case 0:return("network unreachable"); case 1:return("host unreachable"); case 2:return("protocol unreachable"); case 3:return("port unreachable"); case 4:return("fragmentation required but DF bit set"); case 5:return("source route failed"); case 6:return("destination network unknown"); case 7:return("destination host unknown"); case 8:return("source host isolated(obsolete)"); case 9:return("destination network administartively prohibited"); case 10:return("destination host administartively prohibited"); case 11:return("network unreachable for TOS"); case 12:return("host unreachable for TOS"); case 13:return("communication error"); case 14:return("host recedenc violation"); case 15:return("precedence cutoff in effect"); default:sprintf(errbuf,"unknown code %d",code); } return errbuf; } void sig_alrm(int signo){ gotalarm=1; return; } void tv_sub(struct timeval *out,struct timeval *in){ if((out->tv_usec-=in->tv_usec)<0){ --out->tv_sec; out->tv_sec+=1000000; } out->tv_sec-=in->tv_sec; } void traceloop(void){ int seq,code,done; double rtt; struct rec *rec; struct timeval tvrecv; if((recvfd=socket(pr->sasend->sa_family,SOCK_RAW,pr->icmpproto))<0){ printf("recvfd:socket failed "); return; } setuid(getuid()); if((sendfd=socket(pr->sasend->sa_family,SOCK_DGRAM,0))<0){ printf("sendfd:socket failed "); return; } pr->sabind->sa_family=pr->sasend->sa_family; sport=(getpid()&0xffff) | 0x8000; ((struct sockaddr_in*)pr->sabind)->sin_port=htons(sport); if(bind(sendfd,pr->sabind,pr->salen)<0){ printf("bind error "); return; } sig_alrm(SIGALRM); seq=0; done=0; for(ttl=1;ttl<=max_ttl&&done==0;ttl++){ setsockopt(sendfd,pr->ttllevel,pr->ttloptname,&ttl,sizeof(int));//modify ttl bzero(pr->salast,pr->salen); printf("%2d ",ttl); fflush(stdout); for(probe=0;probe<nprobes;probe++){ /* * *these sendbuf is just * *used to exam if the received data is sended by our program * */ rec=(struct rec*)sendbuf; rec->rec_seq=++seq; rec->rec_ttl=ttl; gettimeofday(&rec->rec_tv,NULL); ((struct sockaddr_in*)pr->sasend)->sin_port=htons(dport+seq); if(sendto(sendfd,sendbuf,datalen,0,pr->sasend,pr->salen)<0){//send to dest with ttl added perror("bad sendto"); continue; } //if time_out print * else print info if((code=(*pr->recv)(seq,&tvrecv))==-3){ printf(" *"); }else{ char str[NI_MAXHOST]; if(memcmp(pr->sarecv,pr->salast,pr->salen)!=0){ if(getnameinfo(pr->sarecv,pr->salen,str,sizeof(str),NULL,0,0)==0){ printf(" %s (%s)",str,inet_ntoa(((struct sockaddr_in*)pr->sarecv)->sin_addr)); }else{ printf(" %s",inet_ntoa(((struct sockaddr_in*)pr->sarecv)->sin_addr)); } memcpy(pr->salast,pr->sarecv,pr->salen); } tv_sub(&tvrecv,&rec->rec_tv); rtt=tvrecv.tv_sec*1000.0+tvrecv.tv_usec/1000; printf(" %.3f ms",rtt); if(code==-1){ //reach the dest done++; }else if(code>0){ printf(" (ICMP %s)",(*pr->icmpcode)(code)); } } fflush(stdout); } printf(" "); } } int recv_v4(int seq,struct timeval *tv){ int hlen1,hlen2,icmplen,ret; socklen_t len; ssize_t n; struct ip *ip,*hip; struct icmp *icmp; struct udphdr *udp; gotalarm=0; for(;;){ if(gotalarm){ return -3; } len=pr->salen; alarm(3); n=recvfrom(recvfd,recvbuf,sizeof(recvbuf),0,pr->sarecv,&len);//data len if(n<0){ if(errno==EINTR){ continue; }else{ printf("recvfrom error "); return 0; } }else{ //if recvfrom ok , close the alarm alarm(0); } //read data ip=(struct ip*)recvbuf; hlen1=ip->ip_hl<<2;//ip len icmp=(struct icmp*)(recvbuf+hlen1); if((icmplen=n-hlen1)<8){ continue; } if(icmp->icmp_type==ICMP_TIMXCEED&& icmp->icmp_code==ICMP_TIMXCEED_INTRANS){ if(icmplen<8+sizeof(struct ip)){ continue; } //get icmp data hip=(struct ip*)(recvbuf+hlen1+8); hlen2=hip->ip_hl<<2; if(icmplen<8+hlen2+4){ continue; } udp=(struct udphdr *)(recvbuf+hlen1+8+hlen2); if(hip->ip_p==IPPROTO_UDP&& udp->source==htons(sport)&& udp->dest==htons(dport+seq)){ ret=-2; break; } }else if(icmp->icmp_type==ICMP_UNREACH){ if(icmplen<8+sizeof(struct ip)) continue; hip=(struct ip*)(recvbuf+hlen1+8); hlen2=hip->ip_hl<<2; if(icmplen<8+hlen2+4) continue; udp=(struct udphdr*)(recvbuf+hlen1+8+hlen2); if(hip->ip_p==IPPROTO_UDP&& udp->source==htons(sport)&& udp->dest==htons(dport+seq)){ if(icmp->icmp_code==ICMP_UNREACH_PORT) ret=-1; //reach the destination else ret=icmp->icmp_code; break; } } } gettimeofday(tv,NULL); return ret; } struct proto proto_v4={icmpcode_v4,recv_v4,NULL,NULL,NULL,NULL,0,IPPROTO_ICMP,IPPROTO_IP,IP_TTL}; int datalen=sizeof(struct rec); int max_ttl=30; int nprobes=3; u_short dport=32768+666;//hope the port of dest is not used struct addrinfo *host_serv(const char *host,const char *serv,int family,int socktype){ int n; struct addrinfo hints,*res; bzero(&hints,sizeof(hints)); hints.ai_flags=AI_CANONNAME; hints.ai_family=family; hints.ai_socktype=socktype; if((n=getaddrinfo(host,serv,&hints,&res))!=0){ return NULL; } return (res); } int main(int argc,char *argv[]){ int c; struct addrinfo *ai; struct sigaction s_action; char h[20]={0}; while((c=getopt(argc,argv,"m:v"))!=-1){ switch(c){ case 'm': if((max_ttl=atoi(optarg))<0){ printf("invalid input "); } break; case 'v': verbose++; break; case '?': printf("unrecognized "); return -1; } } if(optind!=argc-1){ printf("error input "); return -1; } host=argv[optind]; pid=getpid(); bzero(&s_action,sizeof(s_action)); s_action.sa_handler=sig_alrm; s_action.sa_flags=SA_INTERRUPT; sigaction(SIGALRM,&s_action,NULL); // signal(SIGALRM,sig_alrm); ai=host_serv(host,NULL,0,0); inet_ntop(AF_INET,&((struct sockaddr_in*)(ai->ai_addr))->sin_addr,h,sizeof(h)); printf("traceroute to %s (%s): %d hops max, %d data bytes ", ai->ai_canonname?ai->ai_canonname:h,h,max_ttl,datalen); if(ai->ai_family==AF_INET){ pr=&proto_v4; }else{ printf("UNKNOW address family "); return -1; } pr->sasend=ai->ai_addr; pr->sarecv=(struct sockaddr*)calloc(1,ai->ai_addrlen); pr->salast=(struct sockaddr*)calloc(1,ai->ai_addrlen); pr->sabind=(struct sockaddr*)calloc(1,ai->ai_addrlen); pr->salen=ai->ai_addrlen; traceloop(); exit(0); }