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Demonstration that OS X has working resolver(3) functions in its standard library.
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| #include <stdio.h> | |
| #include <stdlib.h> | |
| #include <string.h> | |
| // Uncomment this and link with -lresolv | |
| //#include <resolv.h> | |
| #include <arpa/nameser_compat.h> | |
| #include <arpa/inet.h> | |
| // We want minimum headers for now, so define NS_QFIXEDSZ if needed | |
| #ifndef NS_QFIXEDSZ | |
| #define NS_QFIXEDSZ 4 /* #/bytes of fixed data in query */ | |
| #endif | |
| #define A_RECORD 1 | |
| int | |
| main(int argc, char* argv[]) { | |
| if (argc != 2) { | |
| fprintf(stderr, "Specify a domain to resolve.\n"); | |
| return EXIT_FAILURE; | |
| } | |
| if (res_init() != 0) { | |
| fprintf(stderr, "Unable to initialize the resolver.\n"); | |
| return EXIT_FAILURE; | |
| } | |
| char answer[1024] = ""; | |
| int answer_length = 0; | |
| answer_length = res_search(argv[1], 1, A_RECORD, answer, sizeof(answer)); | |
| if (answer_length < 0) { | |
| fprintf(stderr, "Unable to resolve %s\n", argv[1]); | |
| return EXIT_FAILURE; | |
| } | |
| // Get a DNS Header and figure out how many answer we have | |
| HEADER* hdr = (HEADER*) answer; | |
| hdr->ancount = ntohs(hdr->ancount); | |
| hdr->qdcount = ntohs(hdr->qdcount); | |
| char* record = answer + sizeof(HEADER); | |
| char* end = answer + answer_length; | |
| int i = 0, skip = 0; | |
| char name[1024] = ""; | |
| // First, iterate over the question records and skip them | |
| for (i = 0; (i < hdr->qdcount); i++) { | |
| skip = dn_expand(answer, end, record, name, sizeof(name)); | |
| char* proposed_record = record + skip + QFIXEDSZ; | |
| if (proposed_record >= end) { | |
| fprintf(stderr, "Something went wrong while parsing the DNS response.\n"); | |
| return EXIT_FAILURE; | |
| } | |
| record = proposed_record; | |
| } | |
| for (i = 0; (i < hdr->ancount); i++) { | |
| skip = dn_expand(answer, end, record, name, sizeof(name)); | |
| char* proposed_ip = record + skip + 10; | |
| if (proposed_ip >= end) { | |
| fprintf(stderr, "Something went wrong while finding the IP in the DNS response.\n"); | |
| return EXIT_FAILURE; | |
| } | |
| char* values = record + skip; | |
| // Array indices as per RFC 1035 | |
| int type = ntohs(*(short*)(values + 0)); | |
| int class = ntohs(*(short*)(values + 2)); | |
| int len = ntohs(*(short*)(values + 8)); | |
| record = proposed_ip + len; | |
| if (type != A_RECORD) { | |
| continue; | |
| } | |
| int ip = ntohl(*(int*) proposed_ip); | |
| printf("Response: %d.%d.%d.%d\n", | |
| (ip >> 24) & 0xFF, | |
| (ip >> 16) & 0xFF, | |
| (ip >> 8) & 0xFF, | |
| (ip >> 0) & 0xFF); | |
| } | |
| return EXIT_SUCCESS; | |
| } |
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