tailcfg: dns name mangling

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

tailcfg/name.go

@@ -0,0 +1,108 @@
+// Copyright (c) 2020 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 tailcfg
+
+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
+
+// SanitizeNameLabel takes a string intended to be a DNS name label
+// and turns it into a valid name label according to RFC 1035.
+func SanitizeNameLabel(label string) string {
+	var sb strings.Builder
+	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...
+	for ; start < end; start++ {
+		if isalpha(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 hypen 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()
+}
+
+// SanitizeName takes a string intended to be a DNS name
+// and turns it into a valid name according to RFC 1035.
+//
+// All dots in the string are preserved, defining its division into labels,
+// unless the string represents an email address,
+// in which case the local part of the address is treated as a single label.
+func SanitizeName(name string) string {
+	// The local part may be a quoted string containing @, so we split on the last @.
+	if idx := strings.LastIndexByte(name, '@'); idx != -1 {
+		localPart := SanitizeNameLabel(name[:idx])
+		domain := SanitizeName(name[idx+1:])
+		return localPart + "." + domain
+	}
+
+	labels := strings.Split(name, ".")
+	for i, label := range labels {
+		labels[i] = SanitizeNameLabel(label)
+	}
+
+	return strings.Join(labels, ".")
+}

tailcfg/name_test.go

@@ -0,0 +1,63 @@
+// Copyright (c) 2020 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 tailcfg
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSanitizeNameLabel(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{"empty", "", ""},
+		{"space", " ", ""},
+		{"upper", "OBERON", "oberon"},
+		{"mixed", "Avery's iPhone 4(SE)", "averys-iphone-4se"},
+		{"dotted", "mon.ipn.dev", "mon-ipn-dev"},
+		{"email", "admin@example.com", "admin-example-com"},
+		{"boudary", ".bound.ary.", "bound-ary"},
+		{"bad_trailing", "a-", "a"},
+		{"bad_leading", "-a", "a"},
+		{"bad_both", "-a-", "a"},
+		{
+			"overlong",
+			strings.Repeat("test.", 20),
+			"test-test-test-test-test-test-test-test-test-test-test-test-tes",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := SanitizeNameLabel(tt.in)
+			if got != tt.want {
+				t.Errorf("want %s; got %s", tt.want, got)
+			}
+		})
+	}
+}
+
+func TestSanitizeName(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{"single_label", "OBERON", "oberon"},
+		{"dotted", "MON.IPN.DEV", "mon.ipn.dev"},
+		{"email", "first.last@example.com", "first-last.example.com"},
+		{"weird", "\"first..last(c+d)?\"@email.com", "first--lastcd.email.com"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := SanitizeName(tt.in)
+			if got != tt.want {
+				t.Errorf("want %s; got %s", tt.want, got)
+			}
+		})
+	}
+}