net: LookupCNAME returns empty string if entry from /etc/hosts is used

[riton]

What version of Go are you using (go version)?

$ go version
go version go1.16 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/go/pkg/mod"
GONOPROXY="*.private-domain.fr"
GONOSUMDB="*.private-domain.fr"
GOOS="linux"
GOPATH="/home/user/go"
GOPRIVATE="*.private-domain.fr"
GOPROXY="direct"
GOROOT="/opt/go/go1.16"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/go/go1.16/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build937784123=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I'm trying to resolve a name that is present in my /etc/hosts using LookupCNAME

The simple program I'm using is:

package main                                               
                                                           
import (                                                   
        "fmt"                                              
        "net"                                              
        "os"                                               
)                                                          
                                                           
func main() {                                              
        name, err := net.LookupCNAME(os.Args[1])           
        fmt.Printf("name = %q, err = %v\n", name, err)     
}                                                          

The content of my /etc/hosts is:

140.82.121.4 github.com

(the behavior exists with other domains / adresses)

What did you expect to see?

Same behavior using cgo resolver and pure go resolver:

with cgo resolver`

$ GODEBUG="netdns=3" go run . github.com
go package net: dynamic selection of DNS resolver
go package net: hostLookupOrder() = cgo
name = "github.com.", err = <nil>

with pure go resolver`

$ GODEBUG="netdns=go+3" go run . github.com                 
go package net: GODEBUG setting forcing use of Go's resolver
go package net: hostLookupOrder() = files,dns               
go package net: hostLookupOrder(github.com) = files,dns     
name = "github.com.", err = <nil>                                                        

What did you see instead?

with cgo resolver`

$ GODEBUG="netdns=3" go run . github.com
go package net: dynamic selection of DNS resolver
go package net: hostLookupOrder() = cgo
name = "github.com.", err = <nil>

with pure go resolver`

$ GODEBUG="netdns=go+3" go run . github.com                 
go package net: GODEBUG setting forcing use of Go's resolver
go package net: hostLookupOrder() = files,dns               
go package net: hostLookupOrder(github.com) = files,dns     
name = "", err = <nil>                                                        

Pure GO returns an empty string while cgo resolves the name properly.

Extra informations

It seems that this behavior is from the dnsmessage.Name{} returned from https://github.com/golang/go/blob/go1.16/src/net/dnsclient_unix.go#L569 .

As soon as the /etc/hosts entry is removed, the behavior difference disappears.

Thanks in advance for helping me understand this.

[seankhliao]

I think this works as intended, an entry in /etc/hosts should be equivalent to an A or AAAA record and not CNAME

[dmitshur]

CC @bradfitz, @ianlancetaylor via owners.

[gopherbot]

Change https://golang.org/cl/382996 mentions this issue: net: add support for /etc/hosts aliases using go resolver

[neild]

The result of net.LookupCNAME with the cgo resolver is the contents of the ai_canonname ("canonical name for service location") field of the struct addrinfo returned by getaddrinfo. Many (most? all?) libc resolvers populate this field for results returned based on an entry in /etc/hosts.

With the go resolver, net.LookupCNAME issues an IN A or IN AAAA query and determines the canonical name from the response. The canonical name will be populated even if the name does not have a CNAME record; in this case, the name is just that of the original query.

I think that it makes sense to align the cgo and go resolvers and return a canonical name for a result based on an /etc/hosts entry

[ianlancetaylor]

Over in #50101 we decided to change net.LookupCNAME to first look up a CNAME record and then, if that failed, look up a A or AAAA record.

I'm not immediately sure how that affects this issue.