discovery+server: use network-specific functions for fallback SRV lookup

In this commit, we fix a bug where a fallback SRV lookup would leak
information if `lnd` was set to route connections over Tor. We solve
this by using the network-specific functions rather than the standard
ones found in the `net` package.

discovery/bootstrapper.go

@@ -12,6 +12,7 @@ import (
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/autopilot"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/tor"
 	"github.com/miekg/dns"
 	"github.com/roasbeef/btcd/btcec"
 	"github.com/roasbeef/btcutil/bech32"
@@ -246,9 +247,8 @@ type DNSSeedBootstrapper struct {
 	// in the tuple is a special A record that we'll query in order to
 	// receive the IP address of the current authoritative DNS server for
 	// the network seed.
-	dnsSeeds   [][2]string
-	lookupHost func(string) ([]string, error)
-	lookupSRV  func(string, string, string) (string, []*net.SRV, error)
+	dnsSeeds [][2]string
+	net      tor.Net
 }
 
 // A compile time assertion to ensure that DNSSeedBootstrapper meets the
@@ -262,14 +262,8 @@ var _ NetworkPeerBootstrapper = (*ChannelGraphBootstrapper)(nil)
 // used as a fallback for manual TCP resolution in the case of an error
 // receiving the UDP response. The second host should return a single A record
 // with the IP address of the authoritative name server.
-func NewDNSSeedBootstrapper(seeds [][2]string, lookupHost func(string) ([]string, error),
-	lookupSRV func(string, string, string) (string, []*net.SRV, error)) (
-	NetworkPeerBootstrapper, error) {
-	return &DNSSeedBootstrapper{
-		dnsSeeds:   seeds,
-		lookupHost: lookupHost,
-		lookupSRV:  lookupSRV,
-	}, nil
+func NewDNSSeedBootstrapper(seeds [][2]string, net tor.Net) NetworkPeerBootstrapper {
+	return &DNSSeedBootstrapper{dnsSeeds: seeds, net: net}
 }
 
 // fallBackSRVLookup attempts to manually query for SRV records we need to
@@ -280,12 +274,14 @@ func NewDNSSeedBootstrapper(seeds [][2]string, lookupHost func(string) ([]string
 // the records we return are currently too large for a class of resolvers,
 // causing them to be filtered out. The targetEndPoint is the original end
 // point that was meant to be hit.
-func fallBackSRVLookup(soaShim string, targetEndPoint string) ([]*net.SRV, error) {
+func (d *DNSSeedBootstrapper) fallBackSRVLookup(soaShim string,
+	targetEndPoint string) ([]*net.SRV, error) {
+
 	log.Tracef("Attempting to query fallback DNS seed")
 
 	// First, we'll lookup the IP address of the server that will act as
 	// our shim.
-	addrs, err := net.LookupHost(soaShim)
+	addrs, err := d.net.LookupHost(soaShim)
 	if err != nil {
 		return nil, err
 	}
@@ -293,7 +289,7 @@ func fallBackSRVLookup(soaShim string, targetEndPoint string) ([]*net.SRV, error
 	// Once we have the IP address, we'll establish a TCP connection using
 	// port 53.
 	dnsServer := net.JoinHostPort(addrs[0], "53")
-	conn, err := net.Dial("tcp", dnsServer)
+	conn, err := d.net.Dial("tcp", dnsServer)
 	if err != nil {
 		return nil, err
 	}
@@ -356,10 +352,12 @@ search:
 			// keys of nodes. We use the lndLookupSRV function for
 			// this task.
 			primarySeed := dnsSeedTuple[0]
-			_, addrs, err := d.lookupSRV("nodes", "tcp", primarySeed)
+			_, addrs, err := d.net.LookupSRV("nodes", "tcp", primarySeed)
 			if err != nil {
-				log.Tracef("Unable to lookup SRV records via " +
-					"primary seed, falling back to secondary")
+				log.Tracef("Unable to lookup SRV records via "+
+					"primary seed: %v", err)
+
+				log.Trace("Falling back to secondary")
 
 				// If the host of the secondary seed is blank,
 				// then we'll bail here as we can't proceed.
@@ -371,7 +369,7 @@ search:
 				// the primary seed, we'll fallback to the
 				// secondary seed before concluding failure.
 				soaShim := dnsSeedTuple[1]
-				addrs, err = fallBackSRVLookup(
+				addrs, err = d.fallBackSRVLookup(
 					soaShim, primarySeed,
 				)
 				if err != nil {
@@ -397,7 +395,7 @@ search:
 				// key. We use the lndLookup function for this
 				// task.
 				bechNodeHost := nodeSrv.Target
-				addrs, err := d.lookupHost(bechNodeHost)
+				addrs, err := d.net.LookupHost(bechNodeHost)
 				if err != nil {
 					return nil, err
 				}

server.go

@@ -720,15 +720,9 @@ func initNetworkBootstrappers(s *server) ([]discovery.NetworkPeerBootstrapper, e
 			srvrLog.Infof("Creating DNS peer bootstrapper with "+
 				"seeds: %v", dnsSeeds)
 
-			dnsBootStrapper, err := discovery.NewDNSSeedBootstrapper(
-				dnsSeeds,
-				cfg.net.LookupHost,
-				cfg.net.LookupSRV,
+			dnsBootStrapper := discovery.NewDNSSeedBootstrapper(
+				dnsSeeds, cfg.net,
 			)
-			if err != nil {
-				return nil, err
-			}
-
 			bootStrappers = append(bootStrappers, dnsBootStrapper)
 		}
 	}