Skip to content

Instantly share code, notes, and snippets.

@scaryghost
Last active February 23, 2020 15:27
Show Gist options
  • Save scaryghost/6565447 to your computer and use it in GitHub Desktop.
Save scaryghost/6565447 to your computer and use it in GitHub Desktop.
How to do a SRV or TXT record lookup in C/C++ on Linux.
/**
* compile: g++ -std=c++0x -o dnssd dnssd.cpp -lresolv
* Your g++ compiler must have lambda support
*
* Code based on examples from:
* http://www.cs.columbia.edu/~zwb/project/dotslash/dots_apache/dnsc.c
* http://people.samba.org/bzr/jerry/slag/unix/query-srv.c
*/
#include <arpa/nameser.h>
#include <functional>
#include <iostream>
#include <netinet/in.h>
#include <resolv.h>
#include <string.h>
#include <map>
using namespace std;
int main(int argc, char **argv) {
if (argc < 3) {
cerr << "Usage: " << argv[0] << " [SRV|TXT] [service]" << endl;
return 1;
}
res_init();
int response;
unsigned char query_buffer[1024];
{
ns_type type;
if (!strcmp(argv[1], "SRV")) {
type= ns_t_srv;
} else if (!strcmp(argv[1], "TXT")) {
type= ns_t_txt;
} else {
cerr << "Invalid argument: '" << argv[1] << "'. Must be [SRV|TXT]" << endl;
return 1;
}
response= res_query(argv[2], C_IN, type, query_buffer, sizeof(query_buffer));
if (response < 0) {
cerr << "Error looking up service: '" << argv[2] << "'" << endl;
return 2;
}
}
ns_msg nsMsg;
ns_initparse(query_buffer, response, &nsMsg);
map<ns_type, function<void (const ns_rr &rr)>> callbacks;
callbacks[ns_t_srv]= [&nsMsg](const ns_rr &rr) -> void {
cout << ns_rr_ttl(rr) << endl;
cout << ntohs(*(unsigned short*)ns_rr_rdata(rr)) << endl;
cout << ntohs(*((unsigned short*)ns_rr_rdata(rr) + 1)) << endl;
cout << ntohs(*((unsigned short*)ns_rr_rdata(rr) + 2)) << endl;
char name[1024];
dn_expand(ns_msg_base(nsMsg), ns_msg_end(nsMsg),
ns_rr_rdata(rr) + 6, name, sizeof(name));
cout << name << endl;
};
callbacks[ns_t_txt]= [&nsMsg](const ns_rr &rr) -> void {
cout << ns_rr_rdata(rr) + 1 << endl;
};
for(int x= 0; x < ns_msg_count(nsMsg, ns_s_an); x++) {
ns_rr rr;
ns_parserr(&nsMsg, ns_s_an, x, &rr);
ns_type type= ns_rr_type(rr);
if (callbacks.count(type)) {
callbacks[type](rr);
}
}
return 0;
}
Copy link

ghost commented Aug 31, 2016

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment