net: LookupAddr changed result form in Go 1.5 #12189

rsc · 2015-08-18T23:22:27Z

Reported via email: LookupAddr("8.8.8.8") returned "google-public-dns-a.google.com." (with trailing dot) in all Go releases to date. In Go 1.5 candidate it returns "google-public-dns.a.google.com" (no trailing dot).

This was an intentional change, in CL 3420, but I believe it was a mistake. The argument made there was that the result was inconsistent depending on whether the C library resolver or the pure Go DNS resolver was being used. But Go 1.4 and earlier never used the C library resolver for this query. So the inconsistency indicated was only introduced after Go 1.4 and does not justify deviating from the Go 1.4 behavior.

I am still investigating and will post more details when I'm done, but right now it looks like we should put the dot back for consistency with all previous Go releases.

rsc · 2015-08-19T00:41:48Z

I checked all the Lookup functions. Here is a program to do that (comment out the NS code in Go 1):

package main

import (
    "bytes"
    "fmt"
    "net"
)

func main() {
    names, err := net.LookupAddr("8.8.8.8")
    fmt.Printf("LookupAddr(8.8.8.8) = %v, %v\n", names, err)

    cname, err := net.LookupCNAME("research.swtch.com")
    fmt.Printf("LookupCNAME(research.swtch.com) = %v, %v\n", cname, err)

    cname, err = net.LookupCNAME("www.mit.edu")
    fmt.Printf("LookupCNAME(www.mit.edu) = %v, %v\n", cname, err)

    mx, err := net.LookupMX("google.com")
    fmt.Printf("LookupMX(google.com) = %#v, %v\n", strmx(mx), err)

    ns, err := net.LookupNS("google.com")
    fmt.Printf("LookupNS(google.com) = %#v, %v\n", strns(ns), err)

    cname, addrs, err := net.LookupSRV("xmpp-server", "tcp", "google.com")
    fmt.Printf("LookupSRV(xmpp-server, tcp, google.com) = %v, %v, %v\n", cname, strsrv(addrs), err)

    txt, err := net.LookupTXT("google.com")
    fmt.Printf("LookupTXT(google.com) = %v, %v\n", txt, err)
}

func strmx(mxs []*net.MX) string {
    var buf bytes.Buffer
    sep := ""
    fmt.Fprintf(&buf, "[")
    for _, mx := range mxs {
        fmt.Fprintf(&buf, "%s%s:%d", sep, mx.Host, mx.Pref)
        sep = " "
    }
    fmt.Fprintf(&buf, "]")
    return buf.String()
}

func strns(nss []*net.NS) string {
    var buf bytes.Buffer
    sep := ""
    fmt.Fprintf(&buf, "[")
    for _, ns := range nss {
        fmt.Fprintf(&buf, "%s%s", sep, ns.Host)
        sep = " "
    }
    fmt.Fprintf(&buf, "]")
    return buf.String()
}

func strsrv(srvs []*net.SRV) string {
    var buf bytes.Buffer
    sep := ""
    fmt.Fprintf(&buf, "[")
    for _, srv := range srvs {
        fmt.Fprintf(&buf, "%s%s:%d:%d:%d", sep, srv.Target, srv.Port, srv.Priority, srv.Weight)
        sep = " "
    }
    fmt.Fprintf(&buf, "]")
    return buf.String()
}

Here are some Go versions running on Linux:

$ go1 version
go version go1
$ go1.1 version
go version go1.1.2 linux/amd64
$ go1.2 version
go version go1.2.2 linux/amd64
$ go1.3 version
go version go1.3.3 linux/amd64
$ go1.4 version
go version go1.4.2 linux/amd64
$ go version
go version devel +ee0658d Tue Aug 18 12:39:14 2015 -0400 linux/amd64
$

Compiled with plain make.bash (so cgo is enabled):

$ go1 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.1 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns3.google.com. ns2.google.com. ns4.google.com. ns1.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.2 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns2.google.com. ns3.google.com. ns4.google.com. ns1.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.3 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns3.google.com. ns2.google.com. ns4.google.com. ns1.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt4.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.4 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns1.google.com. ns3.google.com. ns4.google.com. ns2.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns4.google.com. ns1.google.com. ns3.google.com. ns2.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

Compiled with CGO_ENABLED=0 make.bash:

$ go1 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.1 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns3.google.com. ns2.google.com. ns4.google.com. ns1.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.2 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns3.google.com. ns2.google.com. ns4.google.com. ns1.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.3 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns1.google.com. ns3.google.com. ns2.google.com. ns4.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go1.4 run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com.], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns3.google.com. ns1.google.com. ns4.google.com. ns2.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

$ go run lookup.go
LookupAddr(8.8.8.8) = [google-public-dns-a.google.com], <nil>
LookupCNAME(research.swtch.com) = ghs.l.google.com., <nil>
LookupCNAME(www.mit.edu) = e9566.dscb.akamaiedge.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns1.google.com. ns3.google.com. ns2.google.com. ns4.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = _xmpp-server._tcp.google.com., [xmpp-server.l.google.com.:5269:5:0 alt3.xmpp-server.l.google.com.:5269:20:0 alt1.xmpp-server.l.google.com.:5269:20:0 alt4.xmpp-server.l.google.com.:5269:20:0 alt2.xmpp-server.l.google.com.:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

It may be different on non-Linux systems, but it seems like literally every release of Go to date has had the trailing dot, while the current 1.5 tree has dropped it.

Furthermore, the dropping of the trailing dot in LookupAddr appears to be inconsistent with all the other functions, even in Go 1.5. (LookupTXT doesn't count, since it returns uninterpreted text, but I wanted to run all the Lookups.)

For both these reasons I think we should put the final dot back in.

I will prepare a CL.

rsc · 2015-08-19T01:51:22Z

This is even worse than I thought. There is no code that takes the dot out from the Go lookups. The problem is that by default, and even when you set GODEBUG=netdns=go, the code tries the new C library wrappers before trying pure Go.

So not only did the result change, but now each blocked LookupAddr takes a full thread instead of a goroutine. That behavior definitely has to be rolled back.

tpng · 2015-08-19T02:16:42Z

Same test ran on go version go1.4.2 windows/386, Windows 7 64-bit.

LookupAddr(8.8.8.8) = [google-public-dns-a.google.com], <nil>
LookupCNAME(research.swtch.com) = ghs.google.com., <nil>
LookupCNAME(www.mit.edu) = www.mit.edu.edgekey.net., <nil>
LookupMX(google.com) = "[aspmx.l.google.com.:10 alt1.aspmx.l.google.com.:20 alt2.aspmx.l.google.com.:30 alt3.aspmx.l.google.com.:40 alt4.aspmx.l.google.com.:50]", <nil>
LookupNS(google.com) = "[ns2.google.com. ns3.google.com. ns4.google.com. ns1.google.com.]", <nil>
LookupSRV(xmpp-server, tcp, google.com) = google.com, [xmpp-server.l.google.com:5269:5:0 alt4.xmpp-server.l.google.com:5269:20:0 alt1.xmpp-server.l.google.com:5269:20:0 alt2.xmpp-server.l.google.com:5269:20:0 alt3.xmpp-server.l.google.com:5269:20:0], <nil>
LookupTXT(google.com) = [v=spf1 include:_spf.google.com ~all], <nil>

gopherbot · 2015-08-19T03:00:20Z

CL https://golang.org/cl/13697 mentions this issue.

gopherbot · 2015-11-27T04:01:39Z

CL https://golang.org/cl/17216 mentions this issue.

… of local source The builtin name resolver using various resolution techniques is a bit complicated and we sometimes fotget to take care of all the go and cgo code paths and exchanging information to local and remote sources. This change makes LookupAddr return absolute domain names even in the case of local source. Updates #12189. Fixes #12240. Change-Id: Icdd3375bcddc7f5d4d3b24f134d93815073736fc Reviewed-on: https://go-review.googlesource.com/17216 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

gopherbot · 2015-12-04T09:01:28Z

CL https://golang.org/cl/17411 mentions this issue.

This change makes existing Lookup API test cases conform to the new return value form that all the Lookup APIs except LookupTXT must return a single or multiple absolute domain names. Updates #12189. Fixes #12193. Change-Id: I03ca09be5bff80e818fbcdc26039daa33d5440a8 Reviewed-on: https://go-review.googlesource.com/17411 Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

rsc self-assigned this Aug 18, 2015

rsc added this to the Go1.5 milestone Aug 18, 2015

rsc mentioned this issue Aug 19, 2015

net: LookupAddr force use of cgo no matter what #12190

Closed

rsc mentioned this issue Aug 19, 2015

net: windows and plan 9 return unrooted DNS names #12193

Closed

rsc closed this as completed in 18d27b2 Aug 19, 2015

mikioh mentioned this issue Aug 20, 2015

net: LookupAddr returns non-absolute FQDNs when GODEBUG=netdns=go #12240

Closed

golang locked and limited conversation to collaborators Dec 14, 2016

gopherbot added the FrozenDueToAge label Dec 14, 2016

rsc removed their assignment Jun 23, 2022

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net: LookupAddr changed result form in Go 1.5 #12189

net: LookupAddr changed result form in Go 1.5 #12189

rsc commented Aug 18, 2015

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gopherbot commented Dec 4, 2015

net: LookupAddr changed result form in Go 1.5 #12189

net: LookupAddr changed result form in Go 1.5 #12189

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