add the ability to control socks5 resolution

not all servers support remote resolution, and require one to send only ip addresses

this add a new SOCKS5 dialer creation function that lets one pass in a custom *net.Resolver.

if a resolver is set on the dialer, it will use it it convert non ip addresses into ip addresses.  For instance, can simply be used with net.DefaultResolver to do local resolution

internal/socks/client.go

@@ -23,6 +23,16 @@ func (d *Dialer) connect(ctx context.Context, c net.Conn, address string) (_ net
 	if err != nil {
 		return nil, err
 	}
+
+	if d.Resolver != nil {
+		if ip := net.ParseIP(host); ip == nil {
+			addresses, err := d.Resolver.LookupHost(ctx, host)
+			if err != nil {
+				return nil, err
+			}
+			host = addresses[0]
+		}
+	}
 	if deadline, ok := ctx.Deadline(); ok && !deadline.IsZero() {
 		c.SetDeadline(deadline)
 		defer c.SetDeadline(noDeadline)

internal/socks/socks.go

@@ -136,6 +136,8 @@ type Dialer struct {
 	// function. It must be non-nil when AuthMethods is not empty.
 	// It must return an error when the authentication is failed.
 	Authenticate func(context.Context, io.ReadWriter, AuthMethod) error
+
+	Resolver *net.Resolver
 }
 
 // DialContext connects to the provided address on the provided
@@ -266,8 +268,8 @@ func (d *Dialer) pathAddrs(address string) (proxy, dst net.Addr, err error) {
 
 // NewDialer returns a new Dialer that dials through the provided
 // proxy server's network and address.
-func NewDialer(network, address string) *Dialer {
-	return &Dialer{proxyNetwork: network, proxyAddress: address, cmd: CmdConnect}
+func NewDialer(network, address string, resolver *net.Resolver) *Dialer {
+	return &Dialer{proxyNetwork: network, proxyAddress: address, cmd: CmdConnect, Resolver: resolver}
 }
 
 const (

proxy/socks5.go

@@ -14,8 +14,9 @@ import (
 // SOCKS5 returns a Dialer that makes SOCKSv5 connections to the given
 // address with an optional username and password.
 // See RFC 1928 and RFC 1929.
-func SOCKS5(network, address string, auth *Auth, forward Dialer) (Dialer, error) {
-	d := socks.NewDialer(network, address)
+
+func socks5Internal(network, address string, auth *Auth, forward Dialer, resolver *net.Resolver) (Dialer, error) {
+	d := socks.NewDialer(network, address, resolver)
 	if forward != nil {
 		if f, ok := forward.(ContextDialer); ok {
 			d.ProxyDial = func(ctx context.Context, network string, address string) (net.Conn, error) {
@@ -40,3 +41,10 @@ func SOCKS5(network, address string, auth *Auth, forward Dialer) (Dialer, error)
 	}
 	return d, nil
 }
+func SOCKS5(network, address string, auth *Auth, forward Dialer) (Dialer, error) {
+	return socks5Internal(network, address, auth, forward, nil)
+}
+
+func SOCKS5WithResolver(network, address string, auth *Auth, forward Dialer, resolver *net.Resolver) (Dialer, error) {
+	return socks5Internal(network, address, auth, forward, resolver)
+}