fix resolver fallback

go.mod

@@ -16,7 +16,7 @@ require (
 	github.com/gorilla/websocket v1.4.2
 	github.com/klauspost/compress v1.13.6
 	github.com/lucas-clemente/quic-go v0.26.0
-	github.com/miekg/dns v1.1.43
+	github.com/miekg/dns v1.1.47
 	github.com/milosgajdos/tenus v0.0.3
 	github.com/ryanuber/go-glob v1.0.0
 	github.com/shadowsocks/go-shadowsocks2 v0.1.5

go.sum

@@ -141,8 +141,8 @@ github.com/marten-seemann/qtls-go1-18 v0.1.1 h1:qp7p7XXUFL7fpBvSS1sWD+uSqPvzNQK4
 github.com/marten-seemann/qtls-go1-18 v0.1.1/go.mod h1:mJttiymBAByA49mhlNZZGrH5u1uXYZJ+RW28Py7f4m4=
 github.com/matttproud/golang_protobuf_extensions v1.0.1/go.mod h1:D8He9yQNgCq6Z5Ld7szi9bcBfOoFv/3dc6xSMkL2PC0=
 github.com/microcosm-cc/bluemonday v1.0.1/go.mod h1:hsXNsILzKxV+sX77C5b8FSuKF00vh2OMYv+xgHpAMF4=
-github.com/miekg/dns v1.1.43 h1:JKfpVSCB84vrAmHzyrsxB5NAr5kLoMXZArPSw7Qlgyg=
-github.com/miekg/dns v1.1.43/go.mod h1:+evo5L0630/F6ca/Z9+GAqzhjGyn8/c+TBaOyfEl0V4=
+github.com/miekg/dns v1.1.47 h1:J9bWiXbqMbnZPcY8Qi2E3EWIBsIm6MZzzJB9VRg5gL8=
+github.com/miekg/dns v1.1.47/go.mod h1:e3IlAVfNqAllflbibAZEWOXOQ+Ynzk/dDozDxY7XnME=
 github.com/milosgajdos/tenus v0.0.3 h1:jmaJzwaY1DUyYVD0lM4U+uvP2kkEg1VahDqRFxIkVBE=
 github.com/milosgajdos/tenus v0.0.3/go.mod h1:eIjx29vNeDOYWJuCnaHY2r4fq5egetV26ry3on7p8qY=
 github.com/mmcloughlin/avo v0.0.0-20200803215136-443f81d77104/go.mod h1:wqKykBG2QzQDJEzvRkcS8x6MiSJkF52hXZsXcjaB3ls=
@@ -297,6 +297,7 @@ golang.org/x/net v0.0.0-20201021035429-f5854403a974/go.mod h1:sp8m0HH+o8qH0wwXwY
 golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
 golang.org/x/net v0.0.0-20210405180319-a5a99cb37ef4/go.mod h1:p54w0d4576C0XHj96bSt6lcn1PtDYWL6XObtHCRCNQM=
 golang.org/x/net v0.0.0-20210428140749-89ef3d95e781/go.mod h1:OJAsFXCWl8Ukc7SiCT/9KSuxbyM7479/AVlXFRxuMCk=
+golang.org/x/net v0.0.0-20210726213435-c6fcb2dbf985/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20211015210444-4f30a5c0130f/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20211112202133-69e39bad7dc2/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20220325170049-de3da57026de h1:pZB1TWnKi+o4bENlbzAgLrEbY4RMYmUIRobMcSmfeYc=
@@ -334,7 +335,6 @@ golang.org/x/sys v0.0.0-20200808120158-1030fc2bf1d9/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20200930185726-fdedc70b468f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210112080510-489259a85091/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210303074136-134d130e1a04/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210330210617-4fbd30eecc44/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210510120138-977fb7262007/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
@@ -371,6 +371,7 @@ golang.org/x/tools v0.0.0-20200425043458-8463f397d07c/go.mod h1:EkVYQZoAsY45+roY
 golang.org/x/tools v0.0.0-20200808161706-5bf02b21f123/go.mod h1:njjCfa9FT2d7l9Bc6FUM5FLjQPp3cFF28FI3qnDFljA=
 golang.org/x/tools v0.0.0-20201224043029-2b0845dc783e/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.1.1/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
+golang.org/x/tools v0.1.6-0.20210726203631-07bc1bf47fb2/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
 golang.org/x/tools v0.1.10 h1:QjFRCZxdOhBJ/UNgnBZLbNV13DlbnK0quyivTnXJM20=
 golang.org/x/tools v0.1.10/go.mod h1:Uh6Zz+xoGYZom868N8YTex3t7RhtHDBrE8Gzo9bV56E=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

resolver.go

@@ -263,9 +263,7 @@ func (r *resolver) copyServers() []NameServer {
 	defer r.mux.RUnlock()
 
 	servers := make([]NameServer, len(r.servers))
-	for i := range r.servers {
-		servers[i] = r.servers[i]
-	}
+	copy(servers, r.servers)
 
 	return servers
 }
@@ -312,17 +310,28 @@ func (r *resolver) resolve(ctx context.Context, ex Exchanger, host string) (ips
 	r.mux.RUnlock()
 
 	if prefer == "ipv6" { // prefer ipv6
-		mq := &dns.Msg{}
-		mq.SetQuestion(dns.Fqdn(host), dns.TypeAAAA)
-		ips, err = r.resolveIPs(ctx, ex, mq)
-		if err != nil || len(ips) > 0 {
+		if ips, err = r.resolve6(ctx, ex, host); len(ips) > 0 {
 			return
 		}
+		return r.resolve4(ctx, ex, host)
 	}
 
-	mq := &dns.Msg{}
+	if ips, err = r.resolve4(ctx, ex, host); len(ips) > 0 {
+		return
+	}
+	return r.resolve6(ctx, ex, host)
+}
+
+func (r *resolver) resolve4(ctx context.Context, ex Exchanger, host string) (ips []net.IP, err error) {
+	mq := dns.Msg{}
 	mq.SetQuestion(dns.Fqdn(host), dns.TypeA)
-	return r.resolveIPs(ctx, ex, mq)
+	return r.resolveIPs(ctx, ex, &mq)
+}
+
+func (r *resolver) resolve6(ctx context.Context, ex Exchanger, host string) (ips []net.IP, err error) {
+	mq := dns.Msg{}
+	mq.SetQuestion(dns.Fqdn(host), dns.TypeAAAA)
+	return r.resolveIPs(ctx, ex, &mq)
 }
 
 func (r *resolver) resolveIPs(ctx context.Context, ex Exchanger, mq *dns.Msg) (ips []net.IP, err error) {
@@ -710,6 +719,7 @@ func (ex *dnsExchanger) Exchange(ctx context.Context, query []byte) ([]byte, err
 
 	conn := &dns.Conn{
 		Conn:    c,
+		UDPSize: 1024,
 	}
 	if _, err = conn.Write(query); err != nil {
 		return nil, err