dns: resolve host names

A small resolver for IPv4 address records, so that clients such as NTP can
be given a host name: "dns server <ip> [<ip>]", "dns lookup <name>" and
"dns". Without a configured server it uses the one DHCP handed out. A query
goes to the servers in turn every two seconds and gives up after six tries;
answers with compressed names and CNAME records ahead of the address are
understood. A name that already is a dotted address is taken without a
query.

The resolver sits in BANK3, the command line in BANK2 next to the helpers it
uses. The System page shows and sets the servers.

dns.c

@@ -0,0 +1,226 @@
+/*
+ * DNS resolver for the RTL837x platform
+ * This code is in the Public Domain
+ */
+
+#include <stdint.h>
+#include "rtl837x_common.h"
+#include "cmd_parser.h"
+#include "uip/uip.h"
+#include "dhcp.h"
+#include "dns.h"
+
+#pragma codeseg BANK3
+#pragma constseg BANK3
+
+#define DNS_PORT	53
+#define DNS_TRIES	6
+#define DNS_TIMEOUT	(2 * SYS_TICK_HZ)
+
+extern __xdata struct dhcp_state dhcp_state;
+extern volatile __xdata uint32_t ticks;
+
+__xdata struct dns_state dns_state;
+__xdata uip_ipaddr_t dns_ip;
+uint8_t * __xdata dns_cand[3];
+
+
+static uint8_t dns_used(__xdata uint8_t *a) __reentrant
+{
+	return a[0] | a[1] | a[2] | a[3];
+}
+
+
+static uint8_t dns_name_is_ip(void) __reentrant
+{
+	__xdata char *s = dns_state.name;
+	uint8_t i, n;
+	uint16_t v;
+
+	for (i = 0; i < 4; i++) {
+		v = 0;
+		n = 0;
+		while (*s >= '0' && *s <= '9') {
+			v = v * 10 + (*s++ - '0');
+			if (++n > 3 || v > 255)
+				return 0;
+		}
+		if (!n)
+			return 0;
+		dns_state.addr[i] = v;
+		if (i < 3 && *s++ != '.')
+			return 0;
+	}
+	return !*s;
+}
+
+
+static void dns_next(void) __reentrant
+{
+	uint8_t k = 0;
+
+	if (dns_state.conn) {
+		uip_udp_remove(dns_state.conn);
+		dns_state.conn = 0;
+	}
+	if (dns_used(dns_state.server[0]))
+		dns_cand[k++] = dns_state.server[0];
+	if (dns_used(dns_state.server[1]))
+		dns_cand[k++] = dns_state.server[1];
+	if (dns_used(dhcp_state.dns))
+		dns_cand[k++] = dhcp_state.dns;
+	if (!k || dns_state.tries >= DNS_TRIES) {
+		dns_state.status = DNS_FAILED;
+		if (dns_state.verbose)
+			print_string(k ? "DNS: no answer\n" : "DNS: no server\n");
+		return;
+	}
+	k = dns_state.tries % k;
+	uip_ipaddr(dns_ip, dns_cand[k][0], dns_cand[k][1], dns_cand[k][2], dns_cand[k][3]);
+	dns_state.conn = uip_udp_new(&dns_ip, HTONS(DNS_PORT));
+	if (!dns_state.conn) {
+		dns_state.status = DNS_FAILED;
+		print_string("DNS: no free UDP connection\n");
+		return;
+	}
+	dns_state.send = 1;
+}
+
+
+static void dns_query(void) __reentrant
+{
+	__xdata uint8_t *p = (__xdata uint8_t *)uip_appdata;
+	__xdata char *n = dns_state.name;
+	__xdata uint8_t *label;
+	uint16_t len = 12;
+
+	dns_state.id++;
+	p[0] = dns_state.id >> 8;
+	p[1] = dns_state.id;
+	p[2] = 0x01;
+	p[3] = 0x00;
+	p[4] = 0;
+	p[5] = 1;
+	p[6] = p[7] = p[8] = p[9] = p[10] = p[11] = 0;
+	while (*n) {
+		label = p + len++;
+		*label = 0;
+		while (*n && *n != '.') {
+			p[len++] = *n++;
+			(*label)++;
+		}
+		if (*n)
+			n++;
+	}
+	p[len++] = 0;
+	p[len++] = 0;
+	p[len++] = 1;
+	p[len++] = 0;
+	p[len++] = 1;
+	uip_udp_send(len);
+}
+
+
+static uint16_t dns_skip_name(__xdata uint8_t *p, uint16_t i, uint16_t len) __reentrant
+{
+	while (i < len) {
+		if (!p[i])
+			return i + 1;
+		if ((p[i] & 0xc0) == 0xc0)
+			return i + 2;
+		i += p[i] + 1;
+	}
+	return len;
+}
+
+
+static void dns_answer(void) __reentrant
+{
+	__xdata uint8_t *p = (__xdata uint8_t *)uip_appdata;
+	uint16_t len = uip_datalen();
+	uint16_t i, an;
+
+	if (dns_state.status != DNS_PENDING || len < 12 || !(p[2] & 0x80)
+	    || p[0] != (uint8_t)(dns_state.id >> 8) || p[1] != (uint8_t)dns_state.id)
+		return;
+	an = ((uint16_t)p[6] << 8) | p[7];
+	i = dns_skip_name(p, 12, len) + 4;
+	while (!(p[3] & 0x0f) && an-- && i + 10 <= len) {
+		i = dns_skip_name(p, i, len);
+		if (i + 10 > len)
+			break;
+		if (!p[i] && p[i + 1] == 1 && !p[i + 8] && p[i + 9] == 4 && i + 14 <= len) {
+			memcpy(dns_state.addr, p + i + 10, 4);
+			dns_state.status = DNS_DONE;
+			uip_udp_remove(dns_state.conn);
+			dns_state.conn = 0;
+			if (dns_state.verbose) {
+				print_string("DNS: ");
+				print_string_x(dns_state.name);
+				print_string(" is ");
+				print_ip(dns_state.addr);
+				write_char('\n');
+			}
+			return;
+		}
+		i += 10 + (((uint16_t)p[i + 8] << 8) | p[i + 9]);
+	}
+	dns_state.tries = DNS_TRIES;
+	dns_next();
+}
+
+
+void dns_init(void) __banked
+{
+	memset(dns_state.server, 0, sizeof(dns_state.server));
+	dns_state.name[0] = 0;
+	dns_state.status = DNS_IDLE;
+	dns_state.conn = 0;
+}
+
+
+void dns_lookup(void) __banked
+{
+	if (dns_name_is_ip()) {
+		dns_state.status = DNS_DONE;
+		return;
+	}
+	dns_state.tries = 0;
+	dns_state.status = DNS_PENDING;
+	dns_next();
+}
+
+
+void dns_show(void) __banked
+{
+	print_string("DNS servers ");
+	print_ip(dns_state.server[0]);
+	write_char(' ');
+	print_ip(dns_state.server[1]);
+	print_string(", from DHCP ");
+	print_ip(dhcp_state.dns);
+	write_char('\n');
+}
+
+
+void dns_callback(uint16_t lport) __banked __reentrant
+{
+	if (!dns_state.conn || lport != dns_state.conn->lport)
+		return;
+	if (uip_newdata()) {
+		dns_answer();
+		return;
+	}
+	if (dns_state.status != DNS_PENDING)
+		return;
+	if (dns_state.send) {
+		dns_state.send = 0;
+		dns_state.sent = ticks;
+		dns_query();
+		return;
+	}
+	if (ticks - dns_state.sent >= DNS_TIMEOUT) {
+		dns_state.tries++;
+		dns_next();
+	}
+}