net: add Resolver.StrictErrors

When LookupIP is performing multiple subqueries, this option causes a
timeout/servfail affecting a single query to abort the whole operation,
instead of returning a partial (IPv4/IPv6-only) result.

Similarly, operations that walk the DNS search list will also abort when
encountering one of these errors.

Fixes #17448

Change-Id: Ice22e4aceb555c5a80d19bd1fde8b8fe87ac9517
Reviewed-on: https://go-review.googlesource.com/32572Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient_unix.go

@@ -200,6 +200,11 @@ func tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype uint16)
 				if nerr, ok := err.(Error); ok && nerr.Timeout() {
 					lastErr.(*DNSError).IsTimeout = true
 				}
+				// Set IsTemporary for socket-level errors. Note that this flag
+				// may also be used to indicate a SERVFAIL response.
+				if _, ok := err.(*OpError); ok {
+					lastErr.(*DNSError).IsTemporary = true
+				}
 				continue
 			}
 			// libresolv continues to the next server when it receives
@@ -314,7 +319,7 @@ func (conf *resolverConfig) releaseSema() {
 	<-conf.ch
 }
 
-func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
+func (r *Resolver) lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
 	if !isDomainName(name) {
 		// We used to use "invalid domain name" as the error,
 		// but that is a detail of the specific lookup mechanism.
@@ -332,6 +337,11 @@ func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs [
 		if err == nil {
 			break
 		}
+		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.StrictErrors {
+			// If we hit a temporary error with StrictErrors enabled,
+			// stop immediately instead of trying more names.
+			break
+		}
 	}
 	if err, ok := err.(*DNSError); ok {
 		// Show original name passed to lookup, not suffixed one.
@@ -432,11 +442,11 @@ func (o hostLookupOrder) String() string {
 // Normally we let cgo use the C library resolver instead of
 // depending on our lookup code, so that Go and C get the same
 // answers.
-func goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
-	return goLookupHostOrder(ctx, name, hostLookupFilesDNS)
+func (r *Resolver) goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
+	return r.goLookupHostOrder(ctx, name, hostLookupFilesDNS)
 }
 
-func goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
+func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		// Use entries from /etc/hosts if they match.
 		addrs = lookupStaticHost(name)
@@ -444,7 +454,7 @@ func goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder)
 			return
 		}
 	}
-	ips, _, err := goLookupIPCNAMEOrder(ctx, name, order)
+	ips, _, err := r.goLookupIPCNAMEOrder(ctx, name, order)
 	if err != nil {
 		return
 	}
@@ -470,13 +480,13 @@ func goLookupIPFiles(name string) (addrs []IPAddr) {
 
 // goLookupIP is the native Go implementation of LookupIP.
 // The libc versions are in cgo_*.go.
-func goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
+func (r *Resolver) goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
 	order := systemConf().hostLookupOrder(host)
-	addrs, _, err = goLookupIPCNAMEOrder(ctx, host, order)
+	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
 	return
 }
 
-func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname string, err error) {
+func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname string, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		addrs = goLookupIPFiles(name)
 		if len(addrs) > 0 || order == hostLookupFiles {
@@ -506,11 +516,16 @@ func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrde
 				lane <- racer{cname, rrs, err}
 			}(qtype)
 		}
+		hitStrictError := false
 		for range qtypes {
 			racer := <-lane
 			if racer.error != nil {
-				// Prefer error for original name.
-				if lastErr == nil || fqdn == name+"." {
+				if nerr, ok := racer.error.(Error); ok && nerr.Temporary() && r.StrictErrors {
+					// This error will abort the nameList loop.
+					hitStrictError = true
+					lastErr = racer.error
+				} else if lastErr == nil || fqdn == name+"." {
+					// Prefer error for original name.
 					lastErr = racer.error
 				}
 				continue
@@ -520,6 +535,13 @@ func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrde
 				cname = racer.cname
 			}
 		}
+		if hitStrictError {
+			// If either family hit an error with StrictErrors enabled,
+			// discard all addresses. This ensures that network flakiness
+			// cannot turn a dualstack hostname IPv4/IPv6-only.
+			addrs = nil
+			break
+		}
 		if len(addrs) > 0 {
 			break
 		}
@@ -543,9 +565,9 @@ func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrde
 }
 
 // goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
-func goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
+func (r *Resolver) goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
 	order := systemConf().hostLookupOrder(host)
-	_, cname, err = goLookupIPCNAMEOrder(ctx, host, order)
+	_, cname, err = r.goLookupIPCNAMEOrder(ctx, host, order)
 	return
 }
 
@@ -554,7 +576,7 @@ func goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
 // only if cgoLookupPTR is the stub in cgo_stub.go).
 // Normally we let cgo use the C library resolver instead of depending
 // on our lookup code, so that Go and C get the same answers.
-func goLookupPTR(ctx context.Context, addr string) ([]string, error) {
+func (r *Resolver) goLookupPTR(ctx context.Context, addr string) ([]string, error) {
 	names := lookupStaticAddr(addr)
 	if len(names) > 0 {
 		return names, nil
@@ -563,7 +585,7 @@ func goLookupPTR(ctx context.Context, addr string) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	_, rrs, err := lookup(ctx, arpa, dnsTypePTR)
+	_, rrs, err := r.lookup(ctx, arpa, dnsTypePTR)
 	if err != nil {
 		return nil, err
 	}

src/net/lookup.go

@@ -97,6 +97,16 @@ type Resolver struct {
 	// GODEBUG=netdns=go, but scoped to just this resolver.
 	PreferGo bool
 
+	// StrictErrors controls the behavior of temporary errors
+	// (including timeout, socket errors, and SERVFAIL) when using
+	// Go's built-in resolver. For a query composed of multiple
+	// sub-queries (such as an A+AAAA address lookup, or walking the
+	// DNS search list), this option causes such errors to abort the
+	// whole query instead of returning a partial result. This is
+	// not enabled by default because it may affect compatibility
+	// with resolvers that process AAAA queries incorrectly.
+	StrictErrors bool
+
 	// TODO(bradfitz): optional interface impl override hook
 	// TODO(bradfitz): Timeout time.Duration?
 }

src/net/lookup_unix.go

@@ -57,12 +57,12 @@ func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string,
 		// cgo not available (or netgo); fall back to Go's DNS resolver
 		order = hostLookupFilesDNS
 	}
-	return goLookupHostOrder(ctx, host, order)
+	return r.goLookupHostOrder(ctx, host, order)
 }
 
 func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
 	if r.PreferGo {
-		return goLookupIP(ctx, host)
+		return r.goLookupIP(ctx, host)
 	}
 	order := systemConf().hostLookupOrder(host)
 	if order == hostLookupCgo {
@@ -72,7 +72,7 @@ func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, e
 		// cgo not available (or netgo); fall back to Go's DNS resolver
 		order = hostLookupFilesDNS
 	}
-	addrs, _, err = goLookupIPCNAMEOrder(ctx, host, order)
+	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
 	return
 }
 
@@ -98,17 +98,17 @@ func (r *Resolver) lookupCNAME(ctx context.Context, name string) (string, error)
 			return cname, err
 		}
 	}
-	return goLookupCNAME(ctx, name)
+	return r.goLookupCNAME(ctx, name)
 }
 
-func (*Resolver) lookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
+func (r *Resolver) lookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
 	var target string
 	if service == "" && proto == "" {
 		target = name
 	} else {
 		target = "_" + service + "._" + proto + "." + name
 	}
-	cname, rrs, err := lookup(ctx, target, dnsTypeSRV)
+	cname, rrs, err := r.lookup(ctx, target, dnsTypeSRV)
 	if err != nil {
 		return "", nil, err
 	}
@@ -121,8 +121,8 @@ func (*Resolver) lookupSRV(ctx context.Context, service, proto, name string) (st
 	return cname, srvs, nil
 }
 
-func (*Resolver) lookupMX(ctx context.Context, name string) ([]*MX, error) {
-	_, rrs, err := lookup(ctx, name, dnsTypeMX)
+func (r *Resolver) lookupMX(ctx context.Context, name string) ([]*MX, error) {
+	_, rrs, err := r.lookup(ctx, name, dnsTypeMX)
 	if err != nil {
 		return nil, err
 	}
@@ -135,8 +135,8 @@ func (*Resolver) lookupMX(ctx context.Context, name string) ([]*MX, error) {
 	return mxs, nil
 }
 
-func (*Resolver) lookupNS(ctx context.Context, name string) ([]*NS, error) {
-	_, rrs, err := lookup(ctx, name, dnsTypeNS)
+func (r *Resolver) lookupNS(ctx context.Context, name string) ([]*NS, error) {
+	_, rrs, err := r.lookup(ctx, name, dnsTypeNS)
 	if err != nil {
 		return nil, err
 	}
@@ -148,7 +148,7 @@ func (*Resolver) lookupNS(ctx context.Context, name string) ([]*NS, error) {
 }
 
 func (r *Resolver) lookupTXT(ctx context.Context, name string) ([]string, error) {
-	_, rrs, err := lookup(ctx, name, dnsTypeTXT)
+	_, rrs, err := r.lookup(ctx, name, dnsTypeTXT)
 	if err != nil {
 		return nil, err
 	}
@@ -165,5 +165,5 @@ func (r *Resolver) lookupAddr(ctx context.Context, addr string) ([]string, error
 			return ptrs, err
 		}
 	}
-	return goLookupPTR(ctx, addr)
+	return r.goLookupPTR(ctx, addr)
 }

src/net/netgo_unix_test.go

@@ -22,7 +22,7 @@ func TestGoLookupIP(t *testing.T) {
 	if err != nil {
 		t.Error(err)
 	}
-	if _, err := goLookupIP(ctx, host); err != nil {
+	if _, err := DefaultResolver.goLookupIP(ctx, host); err != nil {
 		t.Error(err)
 	}
 }