1 files changed, 38 insertions, 8 deletions

diff --git a/src/pkg/net/ipsock.go b/src/pkg/net/ipsock.go
index 80bc3eea5..e8bcac646 100644
--- a/src/pkg/net/ipsock.go
+++ b/src/pkg/net/ipsock.go
@@ -35,15 +35,28 @@ func kernelSupportsIPv6() bool {
 
 var preferIPv4 = !kernelSupportsIPv6()
 
-func firstSupportedAddr(addrs []string) (addr IP) {
+func firstSupportedAddr(filter func(IP) IP, addrs []string) IP {
 	for _, s := range addrs {
-		addr = ParseIP(s)
-		if !preferIPv4 || addr.To4() != nil {
-			break
+		if addr := filter(ParseIP(s)); addr != nil {
+			return addr
 		}
-		addr = nil
 	}
-	return addr
+	return nil
+}
+
+func anyaddr(x IP) IP  { return x }
+func ipv4only(x IP) IP { return x.To4() }
+
+func ipv6only(x IP) IP {
+	// Only return addresses that we can use
+	// with the kernel's IPv6 addressing modes.
+	// If preferIPv4 is set, it means the IPv6 stack
+	// cannot take IPv4 addresses directly (we prefer
+	// to use the IPv4 stack) so reject IPv4 addresses.
+	if x.To4() != nil && preferIPv4 {
+		return nil
+	}
+	return x
 }
 
 // TODO(rsc): if syscall.OS == "linux", we're supposd to read
@@ -131,7 +144,6 @@ func (e InvalidAddrError) String() string  { return string(e) }
 func (e InvalidAddrError) Timeout() bool   { return false }
 func (e InvalidAddrError) Temporary() bool { return false }
 
-
 func ipToSockaddr(family int, ip IP, port int) (syscall.Sockaddr, os.Error) {
 	switch family {
 	case syscall.AF_INET:
@@ -218,13 +230,31 @@ func hostPortToIP(net, hostport string) (ip IP, iport int, err os.Error) {
 		// Try as an IP address.
 		addr = ParseIP(host)
 		if addr == nil {
+			filter := anyaddr
+			if len(net) >= 4 && net[3] == '4' {
+				filter = ipv4only
+			} else if len(net) >= 4 && net[3] == '6' {
+				filter = ipv6only
+			}
 			// Not an IP address.  Try as a DNS name.
 			addrs, err1 := LookupHost(host)
 			if err1 != nil {
 				err = err1
 				goto Error
 			}
-			addr = firstSupportedAddr(addrs)
+			if filter == anyaddr {
+				// We'll take any IP address, but since the dialing code
+				// does not yet try multiple addresses, prefer to use
+				// an IPv4 address if possible.  This is especially relevant
+				// if localhost resolves to [ipv6-localhost, ipv4-localhost].
+				// Too much code assumes localhost == ipv4-localhost.
+				addr = firstSupportedAddr(ipv4only, addrs)
+				if addr == nil {
+					addr = firstSupportedAddr(anyaddr, addrs)
+				}
+			} else {
+				addr = firstSupportedAddr(filter, addrs)
+			}
 			if addr == nil {
 				// should not happen
 				err = &AddrError{"LookupHost returned invalid address", addrs[0]}