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.

Makefile

@@ -69,6 +69,8 @@ SRCS = \
 	rtlplayground.c \
 	boot.c \
 	sfp.c \
+	dns.c \
+	dns_cli.c \
 	syslog.c \
 	udp_apps.c
 

cmd_parser.c

@@ -19,6 +19,7 @@
 #include "sfp.h"
 #include "dhcp.h"
 #include "syslog.h"
+#include "dns.h"
 #include "uip/uip.h"
 #include "version.h"
 
@@ -1802,6 +1803,8 @@ void cmd_parser(void) __banked
 			parse_mtu();
 		} else if (cmd_compare(0, "syslog")) {
 			parse_syslog();
+		} else if (cmd_compare(0, "dns")) {
+			dns_parse();
 		} else if (cmd_compare(0, "ip")) {
 			if (cmd_compare(1, "dhcp")) {
 				dhcp_start();

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();
+	}
+}

dns.h

@@ -0,0 +1,36 @@
+#ifndef _DNS_H_
+#define _DNS_H_
+
+#include <stdint.h>
+
+#define DNS_NAME_LEN	48
+
+#define DNS_IDLE	0
+#define DNS_PENDING	1
+#define DNS_DONE	2
+#define DNS_FAILED	3
+
+struct uip_udp_conn;
+
+struct dns_state {
+	uint8_t server[2][4];	/* configured servers, 0.0.0.0 = unused */
+	char name[DNS_NAME_LEN];	/* name of the current or last lookup */
+	uint8_t addr[4];	/* address found by the last lookup */
+	uint8_t status;		/* DNS_* */
+	uint8_t verbose;	/* print the result of the lookup */
+	uint8_t tries;		/* queries sent for the current lookup */
+	uint8_t send;		/* a query is due on the connection */
+	uint16_t id;
+	uint32_t sent;		/* ticks when the last query went out */
+	struct uip_udp_conn *conn;
+};
+
+extern __xdata struct dns_state dns_state;
+
+void dns_init(void) __banked;
+void dns_lookup(void) __banked;
+void dns_show(void) __banked;
+void dns_callback(uint16_t lport) __banked __reentrant;
+void dns_parse(void) __banked __reentrant;
+
+#endif

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");
+}

doc/dns.md

@@ -0,0 +1,19 @@
+# DNS
+
+The switch can resolve host names to IPv4 addresses, for the NTP client and
+for anything else that is given a name instead of an address.
+
+```
+dns server <ip> [<ip>]   # one or two servers, 0.0.0.0 removes them
+dns lookup <name>        # resolve a name and print the address
+dns                      # show the servers
+```
+
+Without a configured server the resolver uses the one handed out by DHCP.
+Only address records are looked up; a query goes to the servers in turn,
+every two seconds, and gives up after six tries. A name that already is a
+dotted address is taken as it is, without a query.
+
+The DNS servers are set in the Network card of the System page as well.
+Settings made there or on the console are kept only once the configuration is
+saved.

html/app.js

@@ -83,7 +83,7 @@ sy_ip_err:"Invalid IP / netmask / gateway",sy_host_err:"Hostname: 1-23 printable
 sy_net_q:"Apply network settings?",sy_net_d:"The management IP changes to {ip}: this page will need to be reopened there.",
 sy_ip_changed:"IP changed, reconnect at http://{ip}/",sy_dhcp_q:"Switch to DHCP?",
 sy_dhcp_d:"The switch requests an address via DHCP. You must find its new IP to reconnect.",
-sy_sysip_err:"Invalid syslog server IP",sy_sysport_err:"Syslog port must be 1-65535",
+sy_sysip_err:"Invalid syslog server IP",sy_dns1:"DNS server",sy_dns2:"Second DNS",sy_dns_err:"Invalid DNS server IP",sy_dns_dhcp:"from DHCP: {ip}",sy_sysport_err:"Syslog port must be 1-65535",
 sy_pw_len:"Password: 1-20 characters",sy_pw_space:"Password cannot contain spaces",sy_pw_match:"Passwords do not match",
 sy_pw_q:"Change admin password?",sy_pw_d:"Takes effect immediately for new logins. Save to flash to persist.",
 sy_reboot_q:"Reboot the switch?",sy_reboot_d:"There are UNSAVED changes and they will be lost. Save to flash first if you want to keep them.",
@@ -658,6 +658,7 @@ var CONF_CMDS=[
   /^gw\s+(\d{1,3}\.){3}\d{1,3}$/,/^netmask\s+(\d{1,3}\.){3}\d{1,3}$/,
   /^syslog\s+(on|off)$/,/^syslog\s+ip\s+(\d{1,3}\.){3}\d{1,3}$/,/^syslog\s+port\s+\d{1,5}$/,
   /^session\s+\d{1,5}$/,
+  /^dns\s+server\s+(\d{1,3}\.){3}\d{1,3}(\s+(\d{1,3}\.){3}\d{1,3})?$/,
   /^passwd\s+\S+$/,/^hostname\s+\S{1,23}$/,
   /^vlan\s+\d{1,4}\s+d$/,/^vlan\s+\d{1,4}\s+mgmt$/,
   /^vlan\s+\d{1,4}(\s+[a-zA-Z]\w*)?(\s+\d{1,2}t?)+$/,
@@ -1744,6 +1745,16 @@ function sysLoad(){
     if(S.info.session_timeout)$("sy-sesstmo").value=S.info.session_timeout;
   }).catch(function(){});
   cfgReload();
+  dnsLoad();
+}
+function dnsLoad(){
+  return api("/cmd",{method:"POST",body:"dns"}).then(function(r){
+    var m=r.body.match(/DNS servers ([\d.]+) ([\d.]+), from DHCP ([\d.]+)/);
+    if(!m)return;
+    function v(x){return x==="0.0.0.0"?"":x}
+    $("sy-dns1").value=v(m[1]);$("sy-dns2").value=v(m[2]);
+    $("sy-dnsdhcp").textContent=m[3]==="0.0.0.0"?"":t("sy_dns_dhcp",{ip:m[3]});
+  }).catch(function(){});
 }
 function cfgParseKnown(txt){
   var igmp=false,syslog=false;
@@ -1760,13 +1771,15 @@ function cfgParseKnown(txt){
 $("sy-apply").addEventListener("click",function(){
   var ip=$("sy-ip").value.trim(),mask=$("sy-mask").value.trim(),gw=$("sy-gw").value.trim();
   if(!okIp(ip)||!okIp(mask)||!okIp(gw)){toast(t("sy_ip_err"),"err");return;}
+  var d1=$("sy-dns1").value.trim(),d2=$("sy-dns2").value.trim();
+  if((d1&&!okIp(d1))||(d2&&!okIp(d2))){toast(t("sy_dns_err"),"err");return;}
   var cmds=[];
   var hn=$("sy-host").value.trim();
   if(hn&&hn!==S.info.hostname){
     if(!/^[\x21-\x7e]{1,23}$/.test(hn)||/["\\]/.test(hn)){toast(t("sy_host_err"),"err");return;}
     cmds.push("hostname "+hn);
   }
-  cmds.push("ip "+ip+"\nnetmask "+mask+"\ngw "+gw);
+  cmds.push("ip "+ip+"\nnetmask "+mask+"\ngw "+gw+"\ndns server "+(d1||"0.0.0.0")+(d2?" "+d2:""));
   var changingIp=ip!==S.info.ip_address;
   confirmModal(t("sy_net_q"),changingIp?t("sy_net_d",{ip:ip}):"",function(){
     postCmds(cmds).then(function(){
@@ -1862,7 +1875,7 @@ tabHooks.system={enter:sysLoad};
 
 var CONF_OVERWRITE=[
   /^ip\b/,/^gw\b/,/^netmask\b/,/^hostname\b/,
-  /^syslog\s+ip\b/,/^syslog\s+port\b/,/^passwd\b/,/^session\b/,
+  /^syslog\s+ip\b/,/^syslog\s+port\b/,/^dns\s+server\b/,/^passwd\b/,/^session\b/,
   /^vlan\s+\d{1,4}\s+mgmt$/,/^vlan\s+\d{1,4}(?!\s+mgmt\b)/,
   /^pvid\s+\d{1,2}\b/,
   /^port\s+\d{1,2}(?!\s+name\b)/,/^port\s+\d{1,2}\s+name\b/,

html/index.html

@@ -360,6 +360,9 @@ button.danger{background:var(--bad);color:#fff;border-color:transparent}
           <label class="row"><span class="lb" data-i18n="i_ip">IP address</span><input class="in" id="sy-ip" size="15"></label>
           <label class="row"><span class="lb" data-i18n="i_mask">Netmask</span><input class="in" id="sy-mask" size="15"></label>
           <label class="row"><span class="lb" data-i18n="i_gw">Gateway</span><input class="in" id="sy-gw" size="15"></label>
+          <label class="row"><span class="lb" data-i18n="sy_dns1">DNS server</span><input class="in" id="sy-dns1" size="15"></label>
+          <label class="row"><span class="lb" data-i18n="sy_dns2">Second DNS</span><input class="in" id="sy-dns2" size="15"></label>
+          <p class="small mut" id="sy-dnsdhcp"></p>
           <div style="display:flex;gap:10px;margin-top:12px">
             <button class="ctl pri" id="sy-apply" data-i18n="c_apply">Apply</button>
             <button class="ctl" id="sy-dhcp" data-i18n="sy_dhcp" data-i18n-t="sy_dhcp_t">Use DHCP</button>

rtlplayground.c

@@ -25,6 +25,7 @@
 #include "machine.h"
 #include "phy.h"
 #include "syslog.h"
+#include "dns.h"
 #include "httpd/page_impl.h"
 #include "boot.h"
 #include "sfp.h"
@@ -1756,6 +1757,7 @@ void main(void)
 	check_and_flash_update_image();
 
 	syslog_init();
+	dns_init();
 
 #ifdef DEBUG
 	// This register seems to work on the RTL8373 only if also the SDS

udp_apps.c

@@ -6,4 +6,5 @@ void udp_callbacks(void)
 {
 	dhcp_callback(uip_udp_conn->lport); 	// let the application decide if this is for it or not
 	syslog_callback(uip_udp_conn->lport);	// let the application decide if this is for it or not
+	dns_callback(uip_udp_conn->lport);
 }

udp_apps.h

@@ -3,6 +3,7 @@
 
 #include "dhcp.h"
 #include "syslog.h"
+#include "dns.h"
 
 void udp_callbacks(void);