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_cli.c

@@ -0,0 +1,63 @@
+/*
+ * Command line interface of the 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 "dns.h"
+
+#pragma codeseg BANK2
+#pragma constseg BANK2
+
+extern __xdata uint8_t cmd_buffer[CMD_BUF_SIZE];
+extern __xdata uint8_t cmd_words_len;
+extern __xdata uint8_t cmd_words_b[15];
+extern __xdata uint8_t ip[4];
+uint8_t cmd_compare(uint8_t start, __code const uint8_t * cmd);
+uint8_t parse_ip(uint8_t idx);
+
+
+void dns_parse(void) __banked __reentrant
+{
+	__xdata uint8_t *s;
+	uint8_t n;
+
+	if (cmd_words_len < 2) {
+		dns_show();
+		return;
+	}
+	if (cmd_compare(1, "server") && cmd_words_len >= 3 && parse_ip(cmd_words_b[2])) {
+		memcpy(dns_state.server[0], ip, 4);
+		memset(dns_state.server[1], 0, 4);
+		if (cmd_words_len >= 4) {
+			if (!parse_ip(cmd_words_b[3]))
+				goto err;
+			memcpy(dns_state.server[1], ip, 4);
+		}
+		return;
+	}
+	if (cmd_compare(1, "lookup") && cmd_words_len >= 3) {
+		if (dns_state.status == DNS_PENDING) {
+			print_string("DNS: a lookup is already running\n");
+			return;
+		}
+		s = &cmd_buffer[cmd_words_b[2]];
+		for (n = 0; s[n] && s[n] != ' '; n++) {
+			if (n == DNS_NAME_LEN - 1)
+				goto err;
+			dns_state.name[n] = s[n];
+		}
+		dns_state.name[n] = 0;
+		dns_state.verbose = 1;
+		dns_lookup();
+		if (dns_state.status == DNS_DONE) {
+			print_ip(dns_state.addr);
+			write_char('\n');
+		}
+		return;
+	}
+err:
+	print_string("Error: dns [server <ip> [<ip>]|lookup <name>]\n");
+}