OpenVMS Source Code Demos
DNS_LOOKUP_DEMO
//===========================================================================================
// File : dns_lookup_demo.c
// Title : DNS Query Program on Linux (DNS Query Code in C with linux sockets)
// Author : Silver Moon (m00n.silv3r@gmail.com)
// Created : 29/4/2009
// References: http://www.binarytides.com/dns-query-code-in-c-with-linux-sockets/
// : http://www.binarytides.com/blog/dns-query-code-in-c-with-winsock/
// : https://github.com/elenaoat/proxy/blob/master/example.c
// platform : OpenVMS-8.4 Alpha with TCPIP Services v5.7
// compile : $cc dns_lookup_demo
// vms link : $link dns_lookup_demo (no linkage to stack libraries required)
// cli run : $run dns_lookup_demo
// Notes : 1) builds properly in 2014 on using HP C V7.3-010 on OpenVMS Alpha V8.4
// (tested with stack: "TCPIP Services for OpenVMS")
// : 2) this program employs two hardcoded dns address so will not work behind
// a firewall without modifications
// : 3) by switching this demo from T_A to T_MX we might be able to incorporate
// this in our outbound SMTP process)
// ver who when what
// --- ----------- --------------------------------------------------------------------------
// 100 Silver Moon 2009-04-29 original program
// 101 Neil Rieck 2014-10-13 minor cleanup and annotations for OpenVMS
//===========================================================================================
//
// Header Files
//
#include <stdio.h> // printf
#include <string.h> // strlen
#include <stdlib.h> // malloc
#include <sys/socket.h> // you know what this is for
#include <arpa/inet.h> // inet_addr , inet_ntoa , ntohs etc
#include <netinet/in.h> //
#include <unistd.h> // getpid
//
// List of DNS Servers registered on the system
//
char dns_servers[10][100];
int dns_server_count = 0;
//
// Types of DNS resource records
//
#define T_A 1 // Ipv4 address
#define T_NS 2 // Nameserver
#define T_CNAME 5 // canonical name
#define T_SOA 6 // start of authority zone
#define T_PTR 12 // domain name pointer
#define T_MX 15 // Mail server
//
// Function Prototypes
//
void ngethostbyname (unsigned char* , int);
void ChangetoDnsNameFormat (unsigned char*,unsigned char*);
unsigned char* ReadName (unsigned char*,unsigned char*,int*);
void get_dns_servers();
//
// DNS header structure
//
struct DNS_HEADER
{
unsigned short id; // identification number
unsigned char rd :1; // recursion desired
unsigned char tc :1; // truncated message
unsigned char aa :1; // authoritive answer
unsigned char opcode :4; // purpose of message
unsigned char qr :1; // query/response flag
unsigned char rcode :4; // response code
unsigned char cd :1; // checking disabled
unsigned char ad :1; // authenticated data
unsigned char z :1; // its z! reserved
unsigned char ra :1; // recursion available
unsigned short q_count; // number of question entries
unsigned short ans_count; // number of answer entries
unsigned short auth_count; // number of authority entries
unsigned short add_count; // number of resource entries
};
//Constant sized fields of query structure
struct QUESTION
{
unsigned short qtype;
unsigned short qclass;
};
//Constant sized fields of the resource record structure
#pragma pack(push, 1)
struct R_DATA
{
unsigned short type;
unsigned short _class;
unsigned int ttl;
unsigned short data_len;
};
#pragma pack(pop)
// Pointers to resource record contents
struct RES_RECORD
{
unsigned char *name;
struct R_DATA *resource;
unsigned char *rdata;
};
// Structure of a Query
typedef struct
{
unsigned char *name;
struct QUESTION *ques;
} QUERY;
//
// main (whew...)
//
int main( int argc , char *argv[])
{
unsigned char hostname[100];
//Get the DNS servers from the resolv.conf file
get_dns_servers();
//Get the hostname from the terminal
printf("Enter Hostname to Lookup : ");
scanf("%s" , hostname);
//Now get the ip of this hostname , A record
ngethostbyname(hostname , T_A);
return 0;
}
//
// Perform a DNS query by sending a packet
//
void ngethostbyname(unsigned char *host , int query_type)
{
unsigned char buf[65536],*qname,*reader;
// int i , j , stop , s;
unsigned int i;
int j , stop , s;
struct sockaddr_in a;
struct RES_RECORD answers[20],auth[20],addit[20]; //the replies from the DNS server
struct sockaddr_in dest;
struct DNS_HEADER *dns = NULL;
struct QUESTION *qinfo = NULL;
printf("Resolving %s" , host);
s = socket(AF_INET , SOCK_DGRAM , IPPROTO_UDP); //UDP packet for DNS queries
dest.sin_family = AF_INET;
dest.sin_port = htons(53);
dest.sin_addr.s_addr = inet_addr(dns_servers[0]); //dns servers
// Set the DNS structure to standard queries
dns = (struct DNS_HEADER *)&buf;
dns->id = (unsigned short) htons(getpid());
dns->qr = 0; //This is a query
dns->opcode = 0; //This is a standard query
dns->aa = 0; //Not Authoritative
dns->tc = 0; //This message is not truncated
dns->rd = 1; //Recursion Desired
dns->ra = 0; //Recursion not available! hey we dont have it (lol)
dns->z = 0;
dns->ad = 0;
dns->cd = 0;
dns->rcode = 0;
dns->q_count = htons(1); //we have only 1 question
dns->ans_count = 0;
dns->auth_count = 0;
dns->add_count = 0;
// point to the query portion
qname =(unsigned char*)&buf[sizeof(struct DNS_HEADER)];
ChangetoDnsNameFormat(qname , host);
qinfo =(struct QUESTION*)&buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname) + 1)]; //fill it
qinfo->qtype = htons( query_type ); //type of the query , A , MX , CNAME , NS etc
qinfo->qclass = htons(1); //its internet (lol)
printf("\nSending Packet...");
if( sendto(s,(char*)buf,sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION),0,(struct sockaddr*)&dest,sizeof(dest)) < 0)
{
perror("sendto failed");
}
printf("Done");
// Receive the answer
i = sizeof dest;
printf("\nReceiving answer...");
// from the Linux man pages
// ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);
if (
// recvfrom( s, (char*)buf, 65536, 0, (struct sockaddr*)&dest, (socklen_t*)&i) < 0)
recvfrom( s, (char*)buf, 65536, 0, (struct sockaddr*)&dest, &i) < 0)
{
perror("recvfrom failed");
}
printf("Done");
dns = (struct DNS_HEADER*) buf;
// move ahead of the dns header and the query field
reader = &buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION)];
printf("\nThe response contains : ");
printf("\n %d Questions.",ntohs(dns->q_count));
printf("\n %d Answers.",ntohs(dns->ans_count));
printf("\n %d Authoritative Servers.",ntohs(dns->auth_count));
printf("\n %d Additional records.\n\n",ntohs(dns->add_count));
// Start reading answers
stop=0;
for(i=0;i<ntohs(dns->ans_count);i++)
{
answers[i].name=ReadName(reader,buf,&stop);
reader = reader + stop;
answers[i].resource = (struct R_DATA*)(reader);
reader = reader + sizeof(struct R_DATA);
if(ntohs(answers[i].resource->type) == 1) //if its an ipv4 address
{
answers[i].rdata = (unsigned char*)malloc(ntohs(answers[i].resource->data_len));
for(j=0 ; j<ntohs(answers[i].resource->data_len) ; j++)
{
answers[i].rdata[j]=reader[j];
}
answers[i].rdata[ntohs(answers[i].resource->data_len)] = '\0';
reader = reader + ntohs(answers[i].resource->data_len);
}
else
{
answers[i].rdata = ReadName(reader,buf,&stop);
reader = reader + stop;
}
}
// read authorities
for(i=0;i<ntohs(dns->auth_count);i++)
{
auth[i].name=ReadName(reader,buf,&stop);
reader+=stop;
auth[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);
auth[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}
// read additional
for(i=0;i<ntohs(dns->add_count);i++)
{
addit[i].name=ReadName(reader,buf,&stop);
reader+=stop;
addit[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);
if(ntohs(addit[i].resource->type)==1)
{
addit[i].rdata = (unsigned char*)malloc(ntohs(addit[i].resource->data_len));
for(j=0;j<ntohs(addit[i].resource->data_len);j++)
addit[i].rdata[j]=reader[j];
addit[i].rdata[ntohs(addit[i].resource->data_len)]='\0';
reader+=ntohs(addit[i].resource->data_len);
}
else
{
addit[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}
}
// print answers
printf("\nAnswer Records : %d \n" , ntohs(dns->ans_count) );
for(i=0 ; i < ntohs(dns->ans_count) ; i++)
{
printf("Name : %s ",answers[i].name);
if( ntohs(answers[i].resource->type) == T_A) //IPv4 address
{
long *p;
p=(long*)answers[i].rdata;
a.sin_addr.s_addr=(*p); //working without ntohl
printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
}
if(ntohs(answers[i].resource->type)==5)
{
//Canonical name for an alias
printf("has alias name : %s",answers[i].rdata);
}
printf("\n");
}
// print authorities
printf("\nAuthoritive Records : %d \n" , ntohs(dns->auth_count) );
for( i=0 ; i < ntohs(dns->auth_count) ; i++)
{
printf("Name : %s ",auth[i].name);
if(ntohs(auth[i].resource->type)==2)
{
printf("has nameserver : %s",auth[i].rdata);
}
printf("\n");
}
// print additional resource records
printf("\nAdditional Records : %d \n" , ntohs(dns->add_count) );
for(i=0; i < ntohs(dns->add_count) ; i++)
{
printf("Name : %s ",addit[i].name);
if(ntohs(addit[i].resource->type)==1)
{
long *p;
p=(long*)addit[i].rdata;
a.sin_addr.s_addr=(*p);
printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
}
printf("\n");
}
return;
}
//
// like the name says
//
u_char* ReadName(unsigned char* reader,unsigned char* buffer,int* count)
{
unsigned char *name;
unsigned int p=0,jumped=0,offset;
int i , j;
*count = 1;
name = (unsigned char*)malloc(256);
name[0]='\0';
// read the names in 3www6google3com format
while(*reader!=0)
{
if(*reader>=192)
{
offset = (*reader)*256 + *(reader+1) - 49152; //49152 = 11000000 00000000 ;)
reader = buffer + offset - 1;
jumped = 1; //we have jumped to another location so counting wont go up!
}
else
{
name[p++]=*reader;
}
reader = reader+1;
if(jumped==0)
{
*count = *count + 1; //if we havent jumped to another location then we can count up
}
}
name[p]='\0'; //string complete
if(jumped==1)
{
*count = *count + 1; //number of steps we actually moved forward in the packet
}
//now convert 3www6google3com0 to www.google.com
for(i=0;i<(int)strlen((const char*)name);i++)
{
p=name[i];
for(j=0;j<(int)p;j++)
{
name[i]=name[i+1];
i=i+1;
}
name[i]='.';
}
name[i-1]='\0'; //remove the last dot
return name;
}
//
// Get the DNS servers from /etc/resolv.conf file on Linux
//
void get_dns_servers()
{
FILE *fp;
char line[200] , *p;
if((fp = fopen("/etc/resolv.conf" , "r")) == NULL)
{
printf("Failed opening /etc/resolv.conf file \n");
}
while(fgets(line , 200 , fp))
{
if(line[0] == '#')
{
continue;
}
if(strncmp(line , "nameserver" , 10) == 0)
{
p = strtok(line , " ");
p = strtok(NULL , " ");
//p now is the dns ip :)
//????
}
}
strcpy(dns_servers[0] , "208.67.222.222"); //
strcpy(dns_servers[1] , "208.67.220.220"); //
}
//
// convert "www.google.com" to "3www6google3com0"
// +---+------+---+--- these uchar values indicate stub length
//
void ChangetoDnsNameFormat(unsigned char* dns,unsigned char* host)
{
int lock = 0 , i;
strcat((char*)host,".");
for(i = 0 ; i < strlen((char*)host) ; i++)
{
if(host[i]=='.')
{
*dns++ = i-lock;
for(;lock<i;lock++)
{
*dns++=host[lock];
}
lock++; //or lock=i+1;
}
}
*dns++='\0';
}