net/netcheck: flesh out some logging in error paths

Updates tailscale/corp#20654

Change-Id: Ie190f956b864985668f79b5b986438bbe07ce905

net/netcheck/netcheck.go

@@ -340,6 +340,18 @@ const (
 	probeHTTPS                   // HTTPS
 )
 
+func (p probeProto) String() string {
+	switch p {
+	case probeIPv4:
+		return "v4"
+	case probeIPv6:
+		return "v4"
+	case probeHTTPS:
+		return "https"
+	}
+	return "?"
+}
+
 type probe struct {
 	// delay is when the probe is started, relative to the time
 	// that GetReport is called. One probe in each probePlan
@@ -1229,7 +1241,7 @@ func (c *Client) measureICMPLatency(ctx context.Context, reg *tailcfg.DERPRegion
 	// TODO(andrew-d): this is a bit ugly
 	nodeAddr := c.nodeAddr(ctx, node, probeIPv4)
 	if !nodeAddr.IsValid() {
-		return 0, false, fmt.Errorf("no address for node %v", node.Name)
+		return 0, false, fmt.Errorf("no address for node %v (v4-for-icmp)", node.Name)
 	}
 	addr := &net.IPAddr{
 		IP:   net.IP(nodeAddr.Addr().AsSlice()),
@@ -1481,7 +1493,7 @@ func (rs *reportState) runProbe(ctx context.Context, dm *tailcfg.DERPMap, probe
 
 	addr := c.nodeAddr(ctx, node, probe.proto)
 	if !addr.IsValid() {
-		c.logf("netcheck.runProbe: named node %q has no address", probe.node)
+		c.logf("netcheck.runProbe: named node %q has no %v address", probe.node, probe.proto)
 		return
 	}
 
@@ -1607,12 +1619,15 @@ func (c *Client) nodeAddr(ctx context.Context, n *tailcfg.DERPNode, proto probeP
 	c.mu.Unlock()
 
 	probeIsV4 := proto == probeIPv4
-	addrs, _ := lookupIPAddr(ctx, n.HostName)
+	addrs, err := lookupIPAddr(ctx, n.HostName)
 	for _, a := range addrs {
 		if (a.Is4() && probeIsV4) || (a.Is6() && !probeIsV4) {
 			return netip.AddrPortFrom(a, uint16(port))
 		}
 	}
+	if err != nil {
+		c.logf("netcheck: DNS lookup error for %q (node %q region %v): %v", n.HostName, n.Name, n.RegionID, err)
+	}
 	return
 }
 

net/netcheck/netcheck_test.go

@@ -624,18 +624,6 @@ func (p probe) String() string {
 	return fmt.Sprintf("%s-%s%s%s", p.node, p.proto, delay, wait)
 }
 
-func (p probeProto) String() string {
-	switch p {
-	case probeIPv4:
-		return "v4"
-	case probeIPv6:
-		return "v4"
-	case probeHTTPS:
-		return "https"
-	}
-	return "?"
-}
-
 func TestLogConciseReport(t *testing.T) {
 	dm := &tailcfg.DERPMap{
 		Regions: map[int]*tailcfg.DERPRegion{