dns.c

在UC-OS下开发的TCP-IP-PPP协议栈

/**
 * DNS Resolution Code
 * @author Craig Graham
 * @date 07/11/2001
 * @file dns.c
 */

#include <stdio.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>        /* for AF_INET */
#include <netinet/in.h>
#include <sys/time.h>
#include <string.h>

#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<<12)|DNSF_RECURSIVE;

	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]<<8)|packet[1];
	h->flags=(packet[2]<<8)|packet[3];
	h->questionCount=(packet[4]<<8)|packet[5];
	h->answerCount=(packet[6]<<8)|packet[7];
	h->authorityCount=(packet[8]<<8)|packet[9];
	h->additionalCount=(packet[10]<<8)|packet[11];
}

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)<<8)|input[1];
			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<0)
	{
#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<=0)
		{
			// 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]<<8)|p[1];
				qClass=(p[2]<<8)|p[3];
				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]<<8)|p[1];
				aClass=(p[2]<<8)|p[3];
				aTTL=(p[4]<<24)|(p[5]<<16)|(p[6]<<8)|p[7];
				aLength=(p[8]<<8)|p[9];
				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]<<8)|p[1];
				aClass=(p[2]<<8)|p[3];
				aTTL=(p[4]<<24)|(p[5]<<16)|(p[6]<<8)|p[7];
				aLength=(p[8]<<8)|p[9];
				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<DNS_CACHE_MAX; 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<0)
		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<DNS_CACHE_MAX; i++)
	{
		cache[i].name=NULL;
	}
}