adns: multi server broadside

This changes the approach of Async DNS vs multiple DNS servers...

 - we will take all the platform tells us about
 - initially we will broadside the queries on all the servers
 - we'll measure the response performce over time, with decay
 - nonresponsive servers will get ignored and responsive ones
   will round-robin

Author: lws-team

READMEs/README.async-dns.md

@@ -38,6 +38,10 @@ Other features
  - Uses CNAME resolution inside the same response if present, otherwise
    recurses to resolve the CNAME (up to 3 deep)
  - ipv6 pieces only built if cmake `LWS_IPV6` enabled
+ - **Multi-server support**: automatically parses multiple nameservers from `/etc/resolv.conf`.
+ - **Adaptive server selection**: tracks DNS server response latencies using exponential weighted moving averages.
+ - **Parallel broadsiding**: initially sends queries to all configured servers simultaneously to determine the fastest responder.
+ - **Round-robin fallback**: utilizes the fastest server for subsequent queries or round-robins between equally performant servers.
 
 ## Api
 

include/libwebsockets/lws-async-dns.h

@@ -66,6 +66,7 @@ typedef struct lws * (*lws_async_dns_cb_t)(struct lws *wsi, const char *ads,
 
 struct lws_adns_q;
 struct lws_async_dns;
+struct lws_async_dns_server;
 
 /**
  * lws_async_dns_query() - perform a dns lookup using async dns
@@ -156,8 +157,12 @@ lws_adns_get_tid(struct lws_adns_q *q);
 LWS_VISIBLE LWS_EXTERN struct lws_async_dns *
 lws_adns_get_async_dns(struct lws_adns_q *q);
 
+LWS_VISIBLE LWS_EXTERN struct lws_async_dns_server *
+lws_adns_get_server(struct lws_adns_q *q);
+
 LWS_VISIBLE LWS_EXTERN void
-lws_adns_parse_udp(struct lws_async_dns *dns, const uint8_t *pkt, size_t len);
+lws_adns_parse_udp(struct lws_async_dns *dns, const uint8_t *pkt, size_t len,
+		   struct lws_async_dns_server *dsrv);
 
 /**
  * lws_plat_asyncdns_get_server() - Get system DNS server address

lib/core-net/private-lib-core-net.h

@@ -251,7 +251,9 @@ typedef struct lws_async_dns_server {
 	lws_dll2_t		list;
 	lws_sockaddr46 		sa46; /* nameserver */
 
-	lws_dll2_owner_t	waiting;
+	struct lws_adapt	*adapt; /* tracks ping latency */
+	lws_usec_t		last_queried_us;
+	lws_usec_t		last_responded_us;
 
 	int			refcount;
 
@@ -264,6 +266,7 @@ typedef struct lws_async_dns_server {
 typedef struct lws_async_dns {
 	lws_dll2_owner_t	nameservers; /* lws_async_dns_server_t */
 	lws_dll2_owner_t	cached;
+	lws_dll2_owner_t	waiting; /* queries waiting for a server */
 
 	struct lws_context	*cx;
 

lib/system/async-dns/async-dns-parse.c

@@ -134,7 +134,7 @@ lws_adns_parse_label(const uint8_t *pkt, int len, const uint8_t *ls, int budget,
 		 * it has no 00 terminator afterwards
 		 */
 
-		 return consumed;
+		return consumed;
 	}
 
 
@@ -607,7 +607,8 @@ lws_async_dns_store(const char *name, void *opaque, uint32_t ttl,
  */
 
 void
-lws_adns_parse_udp(lws_async_dns_t *dns, const uint8_t *pkt, size_t len)
+lws_adns_parse_udp(lws_async_dns_t *dns, const uint8_t *pkt, size_t len,
+		   lws_async_dns_server_t *dsrv)
 {
 	const char *nm, *nmcname;
 	lws_adns_cache_t *c;
@@ -665,6 +666,14 @@ lws_adns_parse_udp(lws_async_dns_t *dns, const uint8_t *pkt, size_t len)
 		return;
 	}
 
+	if (dsrv && q->broadsiding && q->issue_time) {
+		/* Record response time for the server that won the race! */
+		lws_adapt_report_val(dsrv->adapt, (uint64_t)(lws_now_usecs() - q->issue_time), lws_now_usecs());
+	} else if (q->dsrv && q->issue_time) {
+		/* Record response time for the pre-selected server */
+		lws_adapt_report_val(q->dsrv->adapt, (uint64_t)(lws_now_usecs() - q->issue_time), lws_now_usecs());
+	}
+
 	q->responded = (uint8_t)(q->responded | n);
 
 	/* did we get truncated? */
@@ -834,6 +843,7 @@ lws_adns_parse_udp(lws_async_dns_t *dns, const uint8_t *pkt, size_t len)
 	 * addrinfo results, if any, to all interested wsi, if any...
 	 */
 
+	lwsl_notice("%s: Calling lws_async_dns_complete for %s\n", __func__, q->firstcache ? q->firstcache->name : "NULL");
 	c->incomplete = 0;
 	lws_async_dns_complete(q, q->firstcache);