DNS-Resolver in C

A real DNS resolver that speaks the DNS wire protocol over UDP. It hand-builds query packets, sends them to Google's 8.8.8.8 nameserver on port 53, and parses the binary response — decoding A, AAAA, MX, TXT, NS, and CNAME resource records. No system resolver libraries are used; every byte is constructed manually.

Part of the Corg-Labs collection of single-file C programs.

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Features

• Wire-format query construction using #pragma pack(1) structs
• Handles DNS name compression pointers in responses
• Decodes A (IPv4), AAAA (IPv6), MX, TXT, CNAME, and NS record types
• 5-second receive timeout via SO_RCVTIMEO
• Displays query type, answer count, TTL, and resolved value for each RR

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Tutorial

1. DNS Wire Format: The Header

Every DNS message begins with a fixed 12-byte header packed to match the wire layout exactly:

#pragma pack(push, 1)
typedef struct {
    uint16_t id;       /* random transaction ID         */
    uint16_t flags;    /* QR, opcode, AA, TC, RD, RA…  */
    uint16_t qdcount;  /* number of questions           */
    uint16_t ancount;  /* number of answers             */
    uint16_t nscount;
    uint16_t arcount;
} dns_header_t;
#pragma pack(pop)

The query sets flags = 0x0100 (RD=1, recursion desired). All multi-byte fields use htons() for big-endian network byte order.

2. Encoding a Domain Name

DNS names are length-prefixed labels, not plain strings:

"www.google.com"  →  \x03 w w w \x06 g o o g l e \x03 c o m \x00

encode_name() scans for . delimiters, writes each label length followed by the label bytes, and appends a zero terminator.

3. UDP Socket Setup

int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

struct timeval tv = { .tv_sec = 5 };
setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));

struct sockaddr_in srv = {
    .sin_family = AF_INET,
    .sin_port   = htons(53),
};
inet_pton(AF_INET, "8.8.8.8", &srv.sin_addr);

sendto(sock, query, qlen, 0, (struct sockaddr *)&srv, sizeof(srv));
ssize_t rlen = recvfrom(sock, response, sizeof(response), 0, NULL, NULL);

4. DNS Name Compression Pointers

A pointer is identified by the top two bits being set (0xC0):

if ((len & 0xC0) == 0xC0) {
    int target = ((len & 0x3F) << 8) | msg[pos + 1];
    pos = target;   /* jump to referenced offset */
    continue;
}

decode_name() follows pointer chains transparently until it reaches a zero-length label.

5. Decoding Record Types

if (type == QTYPE_A && rdlen == 4) {
    inet_ntop(AF_INET, msg + pos, ip, sizeof(ip));
} else if (type == QTYPE_AAAA && rdlen == 16) {
    inet_ntop(AF_INET6, msg + pos, ip, sizeof(ip));
} else if (type == QTYPE_MX) {
    uint16_t pref = ntohs(*(uint16_t *)(msg + pos));
    decode_name(msg, msglen, pos + 2, mx, sizeof(mx));
} else if (type == QTYPE_TXT) {
    uint8_t tlen = msg[tpos++];
    fwrite(msg + tpos, 1, tlen, stdout);
}

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Build

gcc dns.c -o dns

Run

./dns google.com
./dns -AAAA ipv6.google.com
./dns -MX gmail.com
./dns -TXT google.com

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Concepts Practiced

• DNS wire protocol (RFC 1035)
• UDP socket programming (SOCK_DGRAM)
• Network byte order (htons/ntohs/ntohl)
• Binary packet construction and parsing
• DNS name compression pointer resolution

Dependencies

Standard C library + POSIX sockets (arpa/inet.h, netinet/in.h, sys/socket.h). No external libraries.