mateusz@test:~$ cat /etc/nsswitch.conf
(..)
hosts:          files dns
(..)
mateusz@test:~$ cat /etc/hosts
127.0.0.1       localhost
::1             localhost ip6-localhost ip6-loopback
ff02::1         ip6-allnodes
ff02::2         ip6-allrouters

127.0.0.1       test.test test
127.0.0.2       test2 test2.test
127.0.0.3       test3.test
127.0.0.4       test4 test5

mateusz@test:~$ cat main.go
package main

import (
        "fmt"
        "net"
        "os"
)

func main() {
        a, err := net.LookupCNAME(os.Args[1])
        if err != nil {
                fmt.Printf("ERROR: %v\n", err)
                os.Exit(1)
        }
        fmt.Println(a)

}

Before:

    mateusz@test:/tmp$ DOMAIN=google.com && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    google.com.
    google.com.
    mateusz@test:/tmp$ DOMAIN=www.iana.org && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    ianawww.vip.icann.org.
    ianawww.vip.icann.org.
    mateusz@test:/tmp$ DOMAIN=test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test.test.
    ERROR: lookup test: DNS response contained records which contain invalid names
    mateusz@test:/tmp$ DOMAIN=test.test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test.test.
    ERROR: lookup test.test: DNS response contained records which contain invalid names
    mateusz@test:/tmp$ DOMAIN=test2 && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test2.
    ERROR: lookup test2: DNS response contained records which contain invalid names
    mateusz@test:/tmp$ DOMAIN=test2.test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test2.
    ERROR: lookup test2.test: DNS response contained records which contain invalid names
    mateusz@test:/tmp$ DOMAIN=test3.test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test3.test.
    ERROR: lookup test3.test: DNS response contained records which contain invalid names
    mateusz@test:/tmp$ DOMAIN=test4 && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test4.
    ERROR: lookup test4: DNS response contained records which contain invalid names
    mateusz@test:/tmp$ DOMAIN=test5 && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test4.
    ERROR: lookup test5: DNS response contained records which contain invalid names

After:

    mateusz@test:~/t/test$ DOMAIN=google.com && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    google.com.
    google.com.
    mateusz@test:~/t/test$ DOMAIN=www.iana.org && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    ianawww.vip.icann.org.
    ianawww.vip.icann.org.
    mateusz@test:~/t/test$ DOMAIN=test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test.test.
    test.test.
    mateusz@test:~/t/test$ DOMAIN=test.test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test.test.
    test.test.
    mateusz@test:~/t/test$ DOMAIN=test2 && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test2.
    test2
    mateusz@test:~/t/test$ DOMAIN=test2.test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test2.
    test2
    mateusz@test:~/t/test$ DOMAIN=test3.test && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test3.test.
    test3.test.
    mateusz@test:~/t/test$ DOMAIN=test4 && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test4.
    test4
    mateusz@test:~/t/test$ DOMAIN=test5 && GODEBUG=netdns=cgo ./main "$DOMAIN" ; GODEBUG=netdns=go ./main "$DOMAIN"
    test4.
    test4