diff --git a/src/net/hook.go b/src/net/hook.go
new file mode 100644
index 0000000000..14dc19f68f
--- /dev/null
+++ b/src/net/hook.go
@@ -0,0 +1,7 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+var testHookLookupIP = func(fn func(string) ([]IPAddr, error), host string) ([]IPAddr, error) { return fn(host) }
diff --git a/src/net/lookup.go b/src/net/lookup.go
index 65abc81309..be4b0c2df6 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -46,7 +46,7 @@ var lookupGroup singleflight
 // is always owned by the caller.
 func lookupIPMerge(host string) (addrs []IPAddr, err error) {
 	addrsi, err, shared := lookupGroup.Do(host, func() (interface{}, error) {
-		return lookupIP(host)
+		return testHookLookupIP(lookupIP, host)
 	})
 	return lookupIPReturn(addrsi, err, shared)
 }
@@ -84,7 +84,7 @@ func lookupIPDeadline(host string, deadline time.Time) (addrs []IPAddr, err erro
 	defer t.Stop()
 
 	ch := lookupGroup.DoChan(host, func() (interface{}, error) {
-		return lookupIP(host)
+		return testHookLookupIP(lookupIP, host)
 	})
 
 	select {