diff --git a/src/pkg/net/lookup.go b/src/pkg/net/lookup.go
index 28d439d643..0cd1993129 100644
--- a/src/pkg/net/lookup.go
+++ b/src/pkg/net/lookup.go
@@ -8,6 +8,19 @@ import (
 	"time"
 )
 
+// protocols contains minimal mappings between internet protocol
+// names and numbers for platforms that don't have a complete list of
+// protocol numbers.
+//
+// See http://www.iana.org/assignments/protocol-numbers
+var protocols = map[string]int{
+	"icmp": 1, "ICMP": 1,
+	"igmp": 2, "IGMP": 2,
+	"tcp": 6, "TCP": 6,
+	"udp": 17, "UDP": 17,
+	"ipv6-icmp": 58, "IPV6-ICMP": 58, "IPv6-ICMP": 58,
+}
+
 var lookupGroup singleflight
 
 // lookupHostMerge wraps lookupHost, but makes sure that for any given
diff --git a/src/pkg/net/lookup_unix.go b/src/pkg/net/lookup_unix.go
index fa98eed5f2..a39e726192 100644
--- a/src/pkg/net/lookup_unix.go
+++ b/src/pkg/net/lookup_unix.go
@@ -11,15 +11,11 @@ import (
 	"sync"
 )
 
-var (
-	protocols         map[string]int
-	onceReadProtocols sync.Once
-)
+var onceReadProtocols sync.Once
 
 // readProtocols loads contents of /etc/protocols into protocols map
 // for quick access.
 func readProtocols() {
-	protocols = make(map[string]int)
 	if file, err := open("/etc/protocols"); err == nil {
 		for line, ok := file.readLine(); ok; line, ok = file.readLine() {
 			// tcp    6   TCP    # transmission control protocol
@@ -31,9 +27,13 @@ func readProtocols() {
 				continue
 			}
 			if proto, _, ok := dtoi(f[1], 0); ok {
-				protocols[f[0]] = proto
+				if _, ok := protocols[f[0]]; !ok {
+					protocols[f[0]] = proto
+				}
 				for _, alias := range f[2:] {
-					protocols[alias] = proto
+					if _, ok := protocols[alias]; !ok {
+						protocols[alias] = proto
+					}
 				}
 			}
 		}
diff --git a/src/pkg/net/lookup_windows.go b/src/pkg/net/lookup_windows.go
index 6d20b7976a..dcbb6d74f1 100644
--- a/src/pkg/net/lookup_windows.go
+++ b/src/pkg/net/lookup_windows.go
@@ -42,6 +42,11 @@ func lookupProtocol(name string) (proto int, err error) {
 		ch <- result{proto: proto, err: err}
 	}()
 	r := <-ch
+	if r.err != nil {
+		if proto, ok := protocols[name]; ok {
+			return protol, nil
+		}
+	}
 	return r.proto, r.err
 }