/** * DNS Resolution Code * @author Craig Graham * @date 07/11/2001 * @file dns.c */ #include #include #include #include /* for AF_INET */ #include #include #include #include "dns.h" #define DNS_MAX_RETRY 5 #define DNS_CACHE_MAX 10 #define DNS_DEBUG 1 /** DNS lookup cache (used to avoid doing lookups for every connection) */ static DNS_CACHE cache[DNS_CACHE_MAX]; /** Index of next DNS cache slot to use */ static int cachePtr=0; /** Query ID */ static unsigned short queryId=0x1234; /** * Build up a DNS query packet header */ void dns_makeHeader(unsigned char packet[],DNS_OPCODE opcode) { unsigned short flags; flags=(opcode packet[0]=(queryId>>8)&0xff; packet[1]=(queryId)&0xff; packet[2]=(flags>>8)&0xff; packet[3]=flags&0xff; packet[4]=0; // questions count MSB packet[5]=1; // questions count LSB packet[6]=0; // answers count MSB packet[7]=0; // answers count LSB packet[8]=0; // authority count MSB packet[9]=0; // authority count LSB packet[10]=0; // additional count MSB packet[11]=0; // additional count LSB queryId++; } /** * Decode a DNS response packet header */ static void dns_decodeHeader(unsigned char packet[], DNS_HEADER *h) { h->id=(packet[0] h->flags=(packet[2] h->questionCount=(packet[4] h->answerCount=(packet[6] h->authorityCount=(packet[8] h->additionalCount=(packet[10] } static char *encodeDotted(const char *name,char *output) { int l; const char *startSegment; while(*name) { l=0; for(startSegment=name; (*name)&&(*name!='.'); name++) { l++; output[l]=*name; } *output++=l; output+=l; if(*name) name++; } *output++='\0'; return output; } static char *decodeDotted(char *packet, char *input, char *output) { int segLen; unsigned short offset; char *rtn=NULL; while(1) { while((*input&(0xC0))==0xC0) { offset=((input[0]&~0xc0) if(rtn==NULL) rtn=&input[2]; input=&packet[offset]; } segLen=*input++; while(segLen--) *output++=*input++; if(!*input) break; *output++='.'; } *output='\0'; return (rtn)?rtn:++input; } static int dns_sendQuery(DNS_CONTEXT *c) { unsigned char packet[1024],*p; unsigned long l,pl; struct sockaddr_in ns; int ret; pl=l=strlen(c->name); dns_makeHeader(packet,DNS_QUERY); p=encodeDotted(c->name,&packet[12]); *p++=(DNS_T_A>>8); *p++=DNS_T_A&0xff; //type = host address query *p++=0; *p++=1; //class 1 - internet address pl=((unsigned long)p)-(unsigned long)packet; c->s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); if(c->s { #if DNS_DEBUG printf("dns_sendQuery: cann't create socket\n"); #endif return -1; } memset(&ns, 0 , sizeof(ns)) ; ns.sin_family = AF_INET ; /* Using TCP/IP protocol */ ns.sin_port = htons(53) ; /* converts to network short */ memcpy(&ns.sin_addr, c->ns, 4) ; if (connect (c->s, (struct sockaddr *)&ns, sizeof(ns)) != 0) { #if DNS_DEBUG printf("dns_sendQuery: cann't connect to socket\n"); #endif return -2; } ret=write(c->s,packet,pl); if(ret!=pl) { #if DNS_DEBUG printf("dns_sendQuery: write failled\n"); #endif close(c->s); return -3; } return 0; } #if DNS_DEBUG char recursionIndicator[100]; int recursionIndicatorI=0; void incR(void) { recursionIndicator[recursionIndicatorI++]='|'; recursionIndicator[recursionIndicatorI++]=' '; recursionIndicator[recursionIndicatorI]='\0'; } void decR(void) { recursionIndicatorI-=2; recursionIndicator[recursionIndicatorI]='\0'; } #else #define incR() #define decR() #endif static int dns_getResponse(DNS_CONTEXT *c) { unsigned char packet[1024]; unsigned char name[1024],*p,*aS; DNS_HEADER h; int ret,cnt,gotAddress=0; unsigned short qClass,aClass,aLength; DNS_T_TYPE qType,aType; unsigned long aTTL; unsigned char aAddress[4]; struct timeval timeout; fd_set readFds; int retryCount=0; timeout.tv_sec=2; timeout.tv_usec=0; while(!gotAddress) { __retry: FD_ZERO(&readFds); FD_SET(c->s,&readFds); ret=select(c->s+1,&readFds,NULL,NULL, &timeout); if(ret { // timeout or error, retry... #if DNS_DEBUG if(ret) printf("%serror, retry\n",recursionIndicator); else printf("%stimeout, retry\n",recursionIndicator); #endif close(c->s); retryCount++; if(retryCount==DNS_MAX_RETRY) return -4; ret=dns_sendQuery(c); if(ret) return ret; continue; } ret=read(c->s,packet,1024); close(c->s); if(ret>=12) { dns_decodeHeader(packet,&h); #if DNS_DEBUG printf("%sgot response from ns %d.%d.%d.%d : \n",recursionIndicator,c->ns[0],c->ns[1],c->ns[2],c->ns[3]); printf("%s questionCount=%d\n",recursionIndicator,h.questionCount); printf("%s answerCount=%d\n",recursionIndicator,h.answerCount); printf("%s authorityCount=%d\n",recursionIndicator,h.authorityCount); #endif p=&packet[sizeof(DNS_HEADER)]; // process query section of packet for(cnt=h.questionCount; cnt; cnt--) { p=decodeDotted(packet,p, name); qType=(p[0] qClass=(p[2] p+=4; #if DNS_DEBUG printf("%squestion = \"%s\" type=%x class=%x\n",recursionIndicator,name,qType,qClass); #endif } // process answer section of packet aS=p; for(cnt=h.answerCount; cnt; cnt--) { p=decodeDotted(packet, p, name); aType=(p[0] aClass=(p[2] aTTL=(p[4] aLength=(p[8] if(aType==DNS_T_A) { // we've got an address, so we can return here.... c->addr[0]=p[10]; c->addr[1]=p[11]; c->addr[2]=p[12]; c->addr[3]=p[13]; #if DNS_DEBUG printf("%sanswer: \"%s\" type=%x class=%x TTL=%lx address=%d.%d.%d.%d\n",recursionIndicator, name,aType,aClass,aTTL,c->addr[0],c->addr[1],c->addr[2],c->addr[3]); #endif return 0; } #if DNS_DEBUG printf("%sanswer: \"%s\" type=%x class=%x TTL=%lx\n",recursionIndicator,name,aType,aClass,aTTL); #endif p+=10+aLength; } // process authority section of packet for(cnt=h.authorityCount; cnt; cnt--) { p=decodeDotted(packet, p, name); aType=(p[0] aClass=(p[2] aTTL=(p[4] aLength=(p[8] p+=10; #if DNS_DEBUG printf("%sauthority: \"%s\" type=%x class=%x TTL=%lx\n",recursionIndicator,name,aType,aClass,aTTL); #endif if(aType==DNS_T_NS) { // Try to resolve authoritive nameserver for this address.... struct hostent *authoritiveServer; p=decodeDotted(packet, p, name); #if DNS_DEBUG printf("%s nameServer=\"%s\"\n",recursionIndicator,name); #endif if(strcmp(name,c->name)) { authoritiveServer=gethostbyname(name); if(authoritiveServer) { // we found the authoritive server, re-target our original request at the authoritive nameserver.. memcpy(c->ns,authoritiveServer->h_addr_list[0],4); #if DNS_DEBUG printf("%s!!!! trying authoritive server %s\n",recursionIndicator,name); #endif incR(); ret=dns_sendQuery(c); if(!ret) { ret=dns_getResponse(c); decR(); if(!ret) return 0; #if DNS_DEBUG printf("%s!!!! server %s didn't respond :(\n",recursionIndicator,name); #endif }else{ decR(); #if DNS_DEBUG printf("%s!!!! couldn't contact server %s :(\n",recursionIndicator,name); #endif } } } } } } ret=dns_sendQuery(c); if(ret) return ret; } return 0; } char nameserver[4]={195,184,228,6}; void setNameServer(const char *ns1, const char *ns2) { UI32 a=ntohl(inet_addr(ns1)); memcpy(nameserver,&a,4); } struct hostent *addCacheEntry(DNS_CONTEXT *c) { struct hostent *rtn; // got a response, cache it and return the cache entry... printf("dns lookup, got address for %s, caching in slot %d\n",c->name,cachePtr); if(cache[cachePtr].name) free(cache[cachePtr].name); memcpy(&cache[cachePtr].in, c->addr, sizeof(cache[cachePtr].in)); cache[cachePtr].name=strdup(c->name); cache[cachePtr].h.h_name = cache[cachePtr].name; cache[cachePtr].h.h_addrtype = AF_INET; cache[cachePtr].h.h_length = sizeof(cache[cachePtr].in); cache[cachePtr].h.h_addr_list = (char **) cache[cachePtr].addr_list; cache[cachePtr].addr_list[0]=&cache[cachePtr].in; cache[cachePtr].addr_list[1]=NULL; rtn=&cache[cachePtr].h; if(++cachePtr==DNS_CACHE_MAX) cachePtr=0; return rtn; } struct hostent *getHostFromCache(const char *name) { int i; // do dns cache lookup... for(i=0; i { if((cache[i].name)&&(!strcmp(name,cache[i].name))) { printf("getHostFromCache(%s): got cached address in slot %d\n",name,i); return &cache[i].h; } } return NULL; } struct hostent *gethostbyaddr(const char *addr, int len, int type) { DNS_CONTEXT c; if(inet_aton(addr,(struct in_addr*)&c.addr)) { c.name=addr; return addCacheEntry(&c); } return NULL; } struct hostent *gethostbyname(const char *name) { DNS_CONTEXT c; int ret; struct hostent *h; #if DNS_DEBUG printf("gethostbyname(%s)\n",name); #endif // try for cached address from a previous lookup... h=getHostFromCache(name); if(h) return h; // not a totally valid thing to do here, but we check for numeric address's // here anyway...(makes higher level code simpler) h=gethostbyaddr(name,4,1); if(h) return h; // ok, it's not a numeric address and it's not in the DNS cache, // so we'll have to do a proper DNS lookup... c.name=name; c.ns[0]=nameserver[0]; c.ns[1]=nameserver[1]; c.ns[2]=nameserver[2]; c.ns[3]=nameserver[3]; ret=dns_sendQuery(&c); if(ret return NULL; ret=dns_getResponse(&c); if(ret==0) { return addCacheEntry(&c); } return NULL; } void dns_init(void) { int i; #if DNS_DEBUG recursionIndicator[0]='\0'; recursionIndicatorI=0; #endif printf("dns_init()\n"); for(i=0; i { cache[i].name=NULL; } }
