Question

I’m trying to recieve a DNS request (DNS content with DNS header) in C++ on Linux 2.6.32. The code works when I write a string over UDP with netcat. But if I do a “nslookup google.com”, it recieves only this:

root@EliteBook:/home/.../dns# xxd request.bin  
0000000: 8a2b 01

And if I use “tcpdump -x ‘udp port 53’ it shows me many bytes more.

My C++ code:

#include <string>
#include <iostream>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>

#define BUFLEN 1024*6
#define NPACK 10
#define PORT 53

using namespace std;

void diep(const string &s)
{
        perror(s.c_str());
        exit(1);
}

int main()
{
        struct sockaddr_in si_me;
        struct sockaddr si_other;
        int slen=sizeof(si_other);
        int s;                                          // UDP Socket
        char buf[BUFLEN];
        buf[0] = 'X';
        buf[1] = 'Y';
        buf[2] = 'Z';

        if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
                diep("Failed to create socket!");

        memset((char *) &si_me, 0, sizeof(si_me));
        si_me.sin_family = AF_INET;
        si_me.sin_port = htons(PORT);
        si_me.sin_addr.s_addr = htonl(INADDR_ANY);
        if (bind(s, (const sockaddr*) &si_me, sizeof(si_me))==-1)
                diep("bind");

        for (int i=0; i<NPACK; i++) {
                if (recvfrom(s, buf, BUFLEN, 0, &si_other, (socklen_t*) &slen)==-1)
                        diep("recvfrom()");

                struct sockaddr_in *si_other_in = (struct sockaddr_in *)&si_other;

                cout << "Pkg arrived: " << buf << " " << inet_ntoa(si_other_in->sin_addr) << ":" << ntohs(si_other_in->sin_port) << endl;

                FILE * pFile;
                pFile = fopen ("request.bin","wb");
                if (pFile!=NULL)
                {
                        fputs (buf, pFile);
                        fclose (pFile);
                }
        }
        close(s);

        return 0;
}

Compile this with

g++ -Wall main.cpp -o dns

So, what is my fault, did I forget something or must I use an other call to read binary data?

PS: Localhost is the first nameserver in /etc/resolv.conf

EDIT:

I looket at the return value from recvfrom, and it shows me:

size: **27**
Pkg arrived: �. 127.0.0.1:36686
size: **45**
Pkg arrived: l 127.0.0.1:38952

That should be OK. Why does my buffer doesn’t hold all bytes?

Answer 1

You are mostly ignoring the return value of recvfrom(). That return value tells you how many bytes you received.

From the man page:

Upon successful completion, recvfrom() returns the length of the message in bytes

Without that information, you cannot know that you didn’t receive all of the bytes.

So, what is my fault, did I forget something or must I use an other call to read binary data?

You are reading the binary data just fine, but you are writing it incorrectly. Your call to std::cout << buf and your call to fputs(buf, pFile) each operate in character strings, not on binary data. Specifically, they each write the bytes up to, but not including, the first zero-valued byte.

To fix your program, record the answer from recvfrom, skip the call to cout << buf, and replace your fputs() with an fwrite.

// UNTESTED
for (int i=0; i<NPACK; i++) {
    int bufsize;
    if ( (bufsize = recvfrom(s, buf, BUFLEN, 0, &si_other, (socklen_t*) &slen))==-1)
        diep("recvfrom()");

        struct sockaddr_in *si_other_in = (struct sockaddr_in *)&si_other;

        cout << "Pkg arrived: " << bufsize << " " << inet_ntoa(si_other_in->sin_addr) << ":" << ntohs(si_other_in->sin_port) << endl;

        FILE * pFile;
        pFile = fopen ("request.bin","wb");
        if (pFile!=NULL)
        {
            fwrite(buf, 1, bufsize, pFile);
            fclose (pFile);
        }
    }
}