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