Initial commit.

Hardcodes query for google.com, but can fully interpret the resulting
answer.

dnsquery.c

@@ -0,0 +1,162 @@
+/* DNS query
+ *
+ * based off of https://datatracker.ietf.org/doc/html/rfc1035
+ *
+ * understanding compression required some study of strace(1) output
+ * from drill(1) and getent(1)
+ *
+ */
+
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+#define BYTE1(x) (x & 0xff)
+#define BYTE2(x) ((x>>8)&0xff)
+
+static void e(const char *str)
+{
+	fputs(str, stderr);
+	fputc('\n', stderr);
+	exit(1);
+}
+
+static void iter_name(const uint8_t *buf, size_t buf_len, char *name, size_t *name_pos, size_t *index, int recursion)
+{
+	if (recursion > 5) {
+		e("too much compression recursion!");
+	}
+
+	size_t l, ln = *name_pos, i = *index;
+	while ((l = buf[i++])) {
+		/* compression */
+		if (l & 0xc0) {
+			uint16_t offset = ((uint16_t)(l & ~0xc0) << 8) | buf[i++];
+			size_t j = offset;
+			iter_name(buf, buf_len, name, &ln, &j, recursion+1);
+			break;
+		} else {
+			memcpy(name+ln, buf+i, l);
+			i+=l;
+			ln+=l+1;
+			name[ln-1] = '.';
+		}
+	}
+	*name_pos = ln;
+	*index = i;
+}
+
+int main()
+{
+	uint8_t q[256], r[1024];
+
+	/* query id */
+	getentropy(q, 2);
+
+	/* QR(1) OPCODE(4) AA(1) TC(1) RD(1)
+	 * query | standard query */
+	q[2] = (0 << 7) | (0 << 3) | (0 << 2) | (0 << 1) | 1;
+	/* RA(1) Z(3) RCODE(4) */
+	q[3] = 0;
+
+	uint16_t qdcount = 1, ancount = 0, nscount = 0, arcount = 0;
+	q[4] = BYTE2(qdcount);
+	q[5] = BYTE1(qdcount);
+	q[6] = BYTE2(ancount);
+	q[7] = BYTE1(ancount);
+	q[8] = BYTE2(nscount);
+	q[9] = BYTE1(nscount);
+	q[10] = BYTE2(nscount);
+	q[11] = BYTE1(nscount);
+
+	size_t i = 12;
+
+	/* QNAME
+	 * length and then content */
+	q[i++] = 6;
+	memcpy(q+i, "google", 6);
+	i+=6;
+	q[i++] = 3;
+	memcpy(q+i, "com", 3);
+	i+=3;
+	/* terminator */
+	q[i++] = 0;
+
+	/* QTYPE
+	 * A record */
+	q[i++] = 0;
+	q[i++] = 1;
+	/* QCLASS 
+	 * IN class */
+	q[i++] = 0;
+	q[i++] = 1;
+
+	int s = socket(AF_INET, SOCK_DGRAM, 0);
+	struct sockaddr_in addr = {.sin_family=AF_INET, .sin_port=htons(53), .sin_addr=inet_addr("127.0.0.1")};
+	void *ap = &addr;
+	sendto(s, q, i, 0, ap, sizeof addr);
+
+	ssize_t ret = recv(s, r, sizeof r, 0);
+	if (ret < 0) {
+		perror("recv");
+		return 1;
+	}
+
+	if (memcmp(q, r, 2)) {
+		e("bad ID");
+	}
+
+	uint8_t rcode = r[3] & 0xf;
+	if (rcode) {
+		e("bad rcode");
+	}
+
+	ancount = ((uint16_t)r[6] << 8) | r[7];
+	if (ancount == 0) {
+		e("no answers");
+	}
+
+	/* begin parsing RRs */
+	i = 12;
+
+	for (size_t j=0; j < qdcount+ancount; j++) {
+		char name[64];
+		size_t ln = 0;
+		iter_name(r, ret, name, &ln, &i, 0);
+		name[ln-1] = '\0';
+		puts(name);
+
+		/* don't do anything else with query RRs */
+		if (j < qdcount) {
+			i+=4;
+			continue;
+		}
+
+		uint16_t type = ((uint16_t)r[i] << 8) | r[i+1];
+		i+=2;
+		uint16_t class = ((uint16_t)r[i] << 8) | r[i+1];
+		i+=2;
+		uint32_t ttl = ((uint32_t)r[i] << 24) | ((uint32_t)r[i+1] << 16) |
+			((uint32_t)r[i+2] << 8) | r[i+3];
+		i+=4;
+		printf("ttl: %u\n", (unsigned)ttl);
+
+		uint16_t rdlength = ((uint16_t)r[i] << 8) | r[i+1];
+		i+=2;
+
+		if (type == 1 && class == 1 && rdlength == 4) {
+			printf("IPv4: %d.%d.%d.%d\n", r[i], r[i+1], r[i+2], r[i+3]);
+		} else {
+			fputs("can't interpret answer\n", stderr);
+		}
+	}
+
+	return 0;
+}