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tailscale/util/dnsname/dnsname.go

127 lines
3.4 KiB
Go

// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package dnsname contains string functions for working with DNS names.
package dnsname
import "strings"
var separators = map[byte]bool{
' ': true,
'.': true,
'@': true,
'_': true,
}
func islower(c byte) bool {
return 'a' <= c && c <= 'z'
}
func isupper(c byte) bool {
return 'A' <= c && c <= 'Z'
}
func isalpha(c byte) bool {
return islower(c) || isupper(c)
}
func isalphanum(c byte) bool {
return isalpha(c) || ('0' <= c && c <= '9')
}
func isdnschar(c byte) bool {
return isalphanum(c) || c == '-'
}
func tolower(c byte) byte {
if isupper(c) {
return c + 'a' - 'A'
} else {
return c
}
}
// maxLabelLength is the maximal length of a label permitted by RFC 1035.
const maxLabelLength = 63
// SanitizeLabel takes a string intended to be a DNS name label
// and turns it into a valid name label according to RFC 1035.
func SanitizeLabel(label string) string {
var sb strings.Builder // TODO: don't allocate in common case where label is already fine
start, end := 0, len(label)
// This is technically stricter than necessary as some characters may be dropped,
// but labels have no business being anywhere near this long in any case.
if end > maxLabelLength {
end = maxLabelLength
}
// A label must start with a letter or number...
for ; start < end; start++ {
if isalphanum(label[start]) {
break
}
}
// ...and end with a letter or number.
for ; start < end; end-- {
// This is safe because (start < end) implies (end >= 1).
if isalphanum(label[end-1]) {
break
}
}
for i := start; i < end; i++ {
// Consume a separator only if we are not at a boundary:
// then we can turn it into a hyphen without breaking the rules.
boundary := (i == start) || (i == end-1)
if !boundary && separators[label[i]] {
sb.WriteByte('-')
} else if isdnschar(label[i]) {
sb.WriteByte(tolower(label[i]))
}
}
return sb.String()
}
// HasSuffix reports whether the provided name ends with the
// component(s) in suffix, ignoring any trailing or leading dots.
//
// If suffix is the empty string, HasSuffix always reports false.
func HasSuffix(name, suffix string) bool {
name = strings.TrimSuffix(name, ".")
suffix = strings.TrimSuffix(suffix, ".")
suffix = strings.TrimPrefix(suffix, ".")
nameBase := strings.TrimSuffix(name, suffix)
return len(nameBase) < len(name) && strings.HasSuffix(nameBase, ".")
}
// TrimSuffix trims any trailing dots from a name and removes the
// suffix ending if present. The name will never be returned with
// a trailing dot, even after trimming.
func TrimSuffix(name, suffix string) string {
if HasSuffix(name, suffix) {
name = strings.TrimSuffix(name, ".")
suffix = strings.Trim(suffix, ".")
name = strings.TrimSuffix(name, suffix)
}
return strings.TrimSuffix(name, ".")
}
// TrimCommonSuffixes returns hostname with some common suffixes removed.
func TrimCommonSuffixes(hostname string) string {
hostname = strings.TrimSuffix(hostname, ".local")
hostname = strings.TrimSuffix(hostname, ".localdomain")
hostname = strings.TrimSuffix(hostname, ".lan")
return hostname
}
// SanitizeHostname turns hostname into a valid name label according
// to RFC 1035.
func SanitizeHostname(hostname string) string {
hostname = TrimCommonSuffixes(hostname)
return SanitizeLabel(hostname)
}