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// Package dnsutil contains higher-level methods useful with the dns
// package. While package dns implements the DNS protocols itself,
// these functions are related but not directly required for protocol
// processing. They are often useful in preparing input/output of the
// functions in package dns.
package dnsutil
import (
"strings"
"github.com/miekg/dns"
)
// AddOrigin adds origin to s if s is not already a FQDN.
// Note that the result may not be a FQDN. If origin does not end
// with a ".", the result won't either.
// This implements the zonefile convention (specified in RFC 1035,
// Section "5.1. Format") that "@" represents the
// apex (bare) domain. i.e. AddOrigin("@", "foo.com.") returns "foo.com.".
func AddOrigin(s, origin string) string {
// ("foo.", "origin.") -> "foo." (already a FQDN)
// ("foo", "origin.") -> "foo.origin."
// ("foo", "origin") -> "foo.origin"
// ("foo", ".") -> "foo." (Same as dns.Fqdn())
// ("foo.", ".") -> "foo." (Same as dns.Fqdn())
// ("@", "origin.") -> "origin." (@ represents the apex (bare) domain)
// ("", "origin.") -> "origin." (not obvious)
// ("foo", "") -> "foo" (not obvious)
if dns.IsFqdn(s) {
return s // s is already a FQDN, no need to mess with it.
}
if origin == "" {
return s // Nothing to append.
}
if s == "@" || s == "" {
return origin // Expand apex.
}
if origin == "." {
return dns.Fqdn(s)
}
return s + "." + origin // The simple case.
}
// TrimDomainName trims origin from s if s is a subdomain.
// This function will never return "", but returns "@" instead (@ represents the apex domain).
func TrimDomainName(s, origin string) string {
// An apex (bare) domain is always returned as "@".
// If the return value ends in a ".", the domain was not the suffix.
// origin can end in "." or not. Either way the results should be the same.
if s == "" {
return "@"
}
// Someone is using TrimDomainName(s, ".") to remove a dot if it exists.
if origin == "." {
return strings.TrimSuffix(s, origin)
}
original := s
s = dns.Fqdn(s)
origin = dns.Fqdn(origin)
if !dns.IsSubDomain(origin, s) {
return original
}
slabels := dns.Split(s)
olabels := dns.Split(origin)
m := dns.CompareDomainName(s, origin)
if len(olabels) == m {
if len(olabels) == len(slabels) {
return "@" // origin == s
}
if (s[0] == '.') && (len(slabels) == (len(olabels) + 1)) {
return "@" // TrimDomainName(".foo.", "foo.")
}
}
// Return the first (len-m) labels:
return s[:slabels[len(slabels)-m]-1]
}