Looking up a CNAME in Go
3 jan 2021 | pattern | golang | development
Let's learn today how to get the details of a DNS CNAME record using Go. Go provides us with the net package to do exactly this. The function we need is called LookupCNAME.
package main
import (
"errors"
"log"
"net"
"strings"
)
func lookupCNAME(host string) (string, error) {
if host == "" {
return "", errors.New("Empty host name")
}
cname, err := net.LookupCNAME(host)
if err != nil {
return "", errors.New("Domain name doesn't exist")
}
cname = strings.TrimSuffix(cname, ".")
host = strings.TrimSuffix(host, ".")
if cname == "" || cname == host {
return "", errors.New("Domain name is not a CNAME")
}
return cname, nil
}
func main() {
result, err := lookupCNAME("api.yellowduck.be")
if err != nil {
log.Fatal(err)
}
log.Println(result)
}
The LookupCNAME function is described as follows:
func LookupCNAME(host string) (cname string, err error)
LookupCNAMEreturns the canonical name for the givenhost. Callers that do not care about the canonical name can callLookupHostorLookupIPdirectly; both take care of resolving the canonical name as part of the lookup.A canonical name is the final name after following zero or more
CNAMErecords.LookupCNAMEdoes not return an error if host does not contain DNS "CNAME" records, as long as host resolves to address records.