Merge pull request #52 from dreamscached/srv-record

Implement SRV record lookups

net.go

@@ -1,6 +1,7 @@
 package minequery
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"strings"
@@ -58,6 +59,30 @@ func (p *Pinger) openUDPConnWithLocalAddr(host string, remotePort int, localAddr
 	return conn, nil
 }
 
+// resolveSRV performs SRV lookup of a Minecraft server hostname.
+//
+// In case there are no records found, an empty string, a zero port and nil error are returned.
+//
+// In case when there is more than one record, the hostname and port of the first record with
+// the least weight is returned.
+func (p *Pinger) resolveSRV(host string) (string, uint16, error) {
+	_, records, err := net.LookupSRV("minecraft", "tcp", host)
+	if err != nil {
+		var dnsError *net.DNSError
+		if errors.As(err, &dnsError) && dnsError.IsNotFound {
+			return "", 0, nil
+		}
+
+		return "", 0, err
+	}
+
+	if len(records) == 0 {
+		return "", 0, nil
+	}
+	target := records[0]
+	return target.Target, target.Port, nil
+}
+
 func shouldWrapIPv6(host string) bool {
 	return len(host) >= 2 && !(host[0] == '[' && host[1] == ']') && strings.Count(host, ":") >= 2
 }

ping_14.go

@@ -39,6 +39,14 @@ func Ping14(host string, port int) (*Status14, error) {
 // Ping14 pings 1.4 to 1.6 (exclusively) Minecraft servers (Notchian servers of more late versions also respond to
 // this ping packet.)
 func (p *Pinger) Ping14(host string, port int) (*Status14, error) {
+	status, err := p.pingGeneric(p.ping14, host, port)
+	if err != nil {
+		return nil, err
+	}
+	return status.(*Status14), nil
+}
+
+func (p *Pinger) ping14(host string, port int) (interface{}, error) {
 	conn, err := p.openTCPConn(host, port)
 	if err != nil {
 		return nil, err

ping_16.go

@@ -446,6 +446,14 @@ func Ping16(host string, port int) (*Status16, error) {
 // Ping16 pings 1.6 to 1.7 (exclusively) Minecraft servers (Notchian servers of more late versions also respond
 // to this ping packet.)
 func (p *Pinger) Ping16(host string, port int) (*Status16, error) {
+	status, err := p.pingGeneric(p.ping16, host, port)
+	if err != nil {
+		return nil, err
+	}
+	return status.(*Status16), nil
+}
+
+func (p *Pinger) ping16(host string, port int) (interface{}, error) {
 	conn, err := p.openTCPConn(host, port)
 	if err != nil {
 		return nil, err

ping_17.go

@@ -1745,6 +1745,14 @@ func Ping17(host string, port int) (*Status17, error) {
 
 // Ping17 pings 1.7+ Minecraft servers.
 func (p *Pinger) Ping17(host string, port int) (*Status17, error) {
+	status, err := p.pingGeneric(p.ping17, host, port)
+	if err != nil {
+		return nil, err
+	}
+	return status.(*Status17), nil
+}
+
+func (p *Pinger) ping17(host string, port int) (interface{}, error) {
 	conn, err := p.openTCPConn(host, port)
 	if err != nil {
 		return nil, err

ping_beta18.go

@@ -41,6 +41,14 @@ func PingBeta18(host string, port int) (*StatusBeta18, error) {
 // PingBeta18 pings Beta 1.8 to Release 1.4 (exclusively) Minecraft servers (Notchian servers of more late versions
 // also respond to this ping packet.)
 func (p *Pinger) PingBeta18(host string, port int) (*StatusBeta18, error) {
+	status, err := p.pingGeneric(p.pingBeta18, host, port)
+	if err != nil {
+		return nil, err
+	}
+	return status.(*StatusBeta18), nil
+}
+
+func (p *Pinger) pingBeta18(host string, port int) (interface{}, error) {
 	conn, err := p.openTCPConn(host, port)
 	if err != nil {
 		return nil, err

ping_generic.go

@@ -0,0 +1,50 @@
+package minequery
+
+// defaultMinecraftPort is a default port Minecraft server runs on and which
+// will be used when server port is left as zero value.
+const defaultMinecraftPort = 25565
+
+// pingGeneric accepts version-specific ping function and host/port pair. Then it performs
+// (if necessary, see PreferSRVRecord) SRV lookup, and attempts to use the SRV record hostname and port
+// (first returned record is used if more than one is returned, see net.LookupSRV documentation)
+// to ping, if lookup fails or ping fails, the provided hostname/port pair is used directly.
+func (p *Pinger) pingGeneric(pingFn func(string, int) (interface{}, error), host string, port int) (interface{}, error) {
+	// Use default Minecraft port if port is 0
+	if port == 0 {
+		port = defaultMinecraftPort
+	}
+
+	if p.PreferSRVRecord {
+		// When SRV record is preferred, try resolving it
+		srvHost, srvPort, err := p.resolveSRV(host)
+		if err != nil {
+			if p.UseStrict {
+				// If UseStrict, SRV lookup error is fatal
+				return nil, err
+			}
+
+			// If not UseStrict, continue pinging on the desired host/port
+
+		} else {
+			// If SRV lookup is successful, check if there are any records
+			if srvHost != "" {
+				status, err := pingFn(srvHost, int(srvPort))
+				if err != nil {
+					// If pinging on the SRV record failed and UseStrict is set,
+					// this is fatal enough to raise an error
+					if p.UseStrict {
+						return nil, err
+					}
+
+				} else {
+					// Success, SRV record ping passed
+					return status, nil
+				}
+			}
+
+		}
+	}
+
+	// Otherwise just ping normally
+	return pingFn(host, port)
+}

pinger.go

@@ -39,6 +39,15 @@ func WithUseStrict(useStrict bool) PingerOption {
 	return func(p *Pinger) { p.UseStrict = useStrict }
 }
 
+// WithPreferSRVRecord sets Pinger PreferSRVRecord to the provided value.
+//
+//goland:noinspection GoUnusedExportedFunction
+func WithPreferSRVRecord(preferSRV bool) PingerOption {
+	return func(p *Pinger) {
+		p.PreferSRVRecord = preferSRV
+	}
+}
+
 // WithProtocolVersion16 sets Pinger ProtocolVersion16 value.
 //
 //goland:noinspection GoUnusedExportedFunction
@@ -121,6 +130,10 @@ type Pinger struct {
 	// that are by default silently ignored should be actually returned as errors.
 	UseStrict bool
 
+	// PreferSRVRecord is a configuration value that defines if Pinger will prefer SRV records, which is the
+	// default behavior of Minecraft clients.
+	PreferSRVRecord bool
+
 	// UnmarshalFunc is the function used to unmarshal JSON (used by Ping17 for responses from 1.7+ servers).
 	// By default, it uses json.Unmarshal function.
 	UnmarshalFunc UnmarshalFunc
@@ -151,6 +164,7 @@ func newDefaultPinger() *Pinger {
 	WithDialer(&net.Dialer{})(p)
 	WithQueryCacheExpiry(30*time.Second, 5*time.Minute)(p)
 	WithTimeout(15 * time.Second)(p)
+	WithPreferSRVRecord(true)(p)
 	WithUnmarshaller(json.Unmarshal)(p)
 	WithImageEncoding(base64.StdEncoding)(p)
 	WithImageDecoder(png.Decode)(p)