What steps will reproduce the problem?
If possible, include a link to a program on play.golang.org.

1. Set up the first two CNAMES in dns, one that points to a valid A record, and one that
points to a missing hostname.

$ host -t CNAME defined.0xkarl.com
defined.0xkarl.com is an alias for www.0xkarl.com.

$ host -t CNAME undefined.0xkarl.com
undefined.0xkarl.com is an alias for stillundefined.0xkarl.com.

$ host www.0xkarl.com
www.0xkarl.com has address 69.64.236.169
$ host stillundefined.0xkarl.com
Host stillundefined.0xkarl.com not found: 3(NXDOMAIN)

2. Compile and run http://play.golang.org/p/BLtnb_hXh9

What is the expected output?
CNAME for defined.0xkarl.com is: www.0xkarl.com.
CNAME for undefined.0xkarl.com is: stillundefined.0xkarl.com.

What do you see instead?

CNAME for defined.0xkarl.com is:www.0xkarl.com.
Error looking up undefined.0xkarl.com: lookup undefined.0xkarl.com: no such host

Which compiler are you using (5g, 6g, 8g, gccgo)?

6g

Which operating system are you using?

Linux 3.8.2 

Which version are you using?  (run 'go version')

go version devel +2bd4d868a758 Wed Mar 13 02:40:56 2013 +0800 linux/amd64

Please provide any additional information below.

I do expect LookupHost or LookupIP to return "no such host" in this situation,
and they do, since they resolve all the way to the end, but by calling LookupCNAME I'm
trying to get the CNAME itself, which is in DNS, so I don't want it to resolve all the
way to the end and fail, I just want the first CNAME.

If I modify lookup_unix.go to always call goLookupCNAME instead of cgoLookupCNAME, I get
the output I'd expect:

diff -r 2bd4d868a758 src/pkg/net/lookup_unix.go
--- a/src/pkg/net/lookup_unix.go        Wed Mar 13 02:40:56 2013 +0800
+++ b/src/pkg/net/lookup_unix.go        Tue Mar 12 15:43:05 2013 -0700
@@ -77,10 +77,10 @@
 }
 
 func lookupCNAME(name string) (cname string, err error) {
-       cname, err, ok := cgoLookupCNAME(name)
-       if !ok {
-               cname, err = goLookupCNAME(name)
-       }
+       //      cname, err, ok := cgoLookupCNAME(name)
+       //      if !ok {
+       cname, err = goLookupCNAME(name)
+       //      }
        return
 }

CNAME for defined.0xkarl.com is:www.0xkarl.com.
CNAME for undefined.0xkarl.com is:stillundefined.0xkarl.com.