fix(dns): retry configurable resolvers

Treat resolver transport failures as inconclusive checks so they do not open outage alerts.

checks/cdns/local_query.go

@@ -4,7 +4,6 @@ import (
 	"errors"
 	"fmt"
 	"log"
-	"strings"
 
 	"github.com/miekg/dns"
 )
@@ -14,7 +13,6 @@ func localQuery(mychan chan DNSreply, qname string, qtype uint16) {
 		fmt.Printf("DEBUG: start of DNS request \"%s\" / %d\n", qname, qtype)
 	}
 	var result DNSreply
-	var trials uint
 	result.qname = qname
 	result.qtype = qtype
 	result.r = nil
@@ -24,41 +22,29 @@ func localQuery(mychan chan DNSreply, qname string, qtype uint16) {
 	localm.RecursionDesired = true
 	localm.Question = make([]dns.Question, 1)
 	localm.SetEdns0(bufsize, false) // Even if no EDNS requested, see #9 May be we should retry without it if timeout?
-	localc := new(dns.Client)
-	localc.ReadTimeout = timeout
 	localm.Question[0] = dns.Question{Name: qname, Qtype: qtype, Qclass: dns.ClassINET}
-Tests:
-	for trials = 0; trials < uint(maxTrials); trials++ {
-		for serverIndex := range conf.Servers {
-			server := conf.Servers[serverIndex]
-			result.nameserver = server
-			// Brackets around the server address are necessary for IPv6 name servers
-			// Brackets required for IPv6; do not use net.JoinHostPort (see check-soa commit 3e4edb1)
-			r, rtt, err := localc.Exchange(localm, "["+server+"]:"+conf.Port)
-			if r == nil {
-				result.r = nil
-				result.err = err
-				log.Println(err.Error())
-				if strings.Contains(err.Error(), "timeout") {
-					// Try another resolver
-					continue
-				}
-				// We give in
-				break Tests
+	for _, server := range conf.Servers {
+		result.nameserver = server
+		localc := &dns.Client{ReadTimeout: timeout}
+		// Brackets are required for IPv6; do not use net.JoinHostPort (see check-soa commit 3e4edb1).
+		r, rtt, err := localc.Exchange(localm, "["+server+"]:"+conf.Port)
+		if err != nil || r == nil {
+			result.r = nil
+			result.err = err
+			if err != nil {
+				log.Println(err)
 			}
-			result.rtt = rtt
-			if r.Rcode == dns.RcodeSuccess {
-				// TODO: NODATA (NOERROR/ANSWER=0) are silently ignored (e.g. name exists but no IP address)
-				// TODO: for rcodes like SERVFAIL, trying another resolver could make sense
-				result.r = r
-				result.err = nil
-				break Tests
-			}
-			// All the other codes are errors
-			result.r = r
-			result.err = errors.New(dns.RcodeToString[r.Rcode])
-			break Tests
+			continue
 		}
+		result.rtt = rtt
+		if r.Rcode == dns.RcodeSuccess {
+			result.r = r
+			result.err = nil
+			break
+		}
+		result.r = r
+		result.err = errors.New(dns.RcodeToString[r.Rcode])
+		break
 	}
 	if debug {
 		fmt.Printf("DEBUG: end of DNS request \"%s\" / %d\n", qname, qtype)