net/dns/resolver: add support for <IPv4>.via-<site-id>

Currently we only support "via-<site-id>.<IPv4>", however that does not
work with Google Chrome which parses `http://via-1.10.0.0.1` as a search
string and not as a URL. This commit introduces "<IPv4>.via-<site-id>"
(`http://10.0.0.1.via-1`) which is parsed correctly by Chrome.

Updates #3616

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/dns/resolver/tsdns.go

@@ -261,7 +261,6 @@ func (r *Resolver) Close() {
 // bound on per-query resource usage.
 const dnsQueryTimeout = 10 * time.Second
 
-
 func (r *Resolver) Query(ctx context.Context, bs []byte, from netaddr.IPPort) ([]byte, error) {
 	metricDNSQueryLocal.Add(1)
 	select {
@@ -617,11 +616,16 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netaddr.IP,
 	}
 }
 
-// parseViaDomain synthesizes an IP address for quad-A DNS requests of
-// the form 'via-<X>.<IPv4-address>', where X is a decimal, or hex-encoded
-// number with a '0x' prefix.
+// parseViaDomain synthesizes an IP address for quad-A DNS requests of the form
+// `<IPv4-address>.via-<X>` and the deprecated form `via-<X>.<IPv4-address>`,
+// where X is a decimal, or hex-encoded number with a '0x' prefix.
 //
 // This exists as a convenient mapping into Tailscales 'Via Range'.
+//
+// TODO(maisem/bradfitz/tom): `<IPv4-address>.via-<X>` was introduced
+// (2022-06-02) to work around an issue in Chrome where it would treat
+// "http://via-1.1.2.3.4" as a search string instead of a URL. We should rip out
+// the old format in early 2023.
 func (r *Resolver) parseViaDomain(domain dnsname.FQDN, typ dns.Type) (netaddr.IP, bool) {
 	fqdn := string(domain.WithoutTrailingDot())
 	if typ != dns.TypeAAAA {
@@ -630,18 +634,29 @@ func (r *Resolver) parseViaDomain(domain dnsname.FQDN, typ dns.Type) (netaddr.IP
 	if len(fqdn) < len("via-X.0.0.0.0") {
 		return netaddr.IP{}, false // too short to be valid
 	}
-	if !strings.HasPrefix(fqdn, "via-") {
-		return netaddr.IP{}, false
-	}
 
-	firstDot := strings.Index(fqdn, ".")
-	if firstDot < 0 {
-		return netaddr.IP{}, false // missing dot delimiters
+	var siteID string
+	var ip4Str string
+	if strings.HasPrefix(fqdn, "via-") {
+		firstDot := strings.Index(fqdn, ".")
+		if firstDot < 0 {
+			return netaddr.IP{}, false // missing dot delimiters
+		}
+		siteID = fqdn[len("via-"):firstDot]
+		ip4Str = fqdn[firstDot+1:]
+	} else {
+		lastDot := strings.LastIndex(fqdn, ".")
+		if lastDot < 0 {
+			return netaddr.IP{}, false // missing dot delimiters
+		}
+		suffix := fqdn[lastDot+1:]
+		if !strings.HasPrefix(suffix, "via-") {
+			return netaddr.IP{}, false
+		}
+		siteID = suffix[len("via-"):]
+		ip4Str = fqdn[:lastDot]
 	}
 
-	siteID := fqdn[len("via-"):firstDot]
-	ip4Str := fqdn[firstDot+1:]
-
 	ip4, err := netaddr.ParseIP(ip4Str)
 	if err != nil {
 		return netaddr.IP{}, false // badly formed, dont respond

net/dns/resolver/tsdns_test.go

@@ -343,9 +343,12 @@ func TestResolveLocal(t *testing.T) {
 		{"ns-nxdomain", "test3.ipn.dev.", dns.TypeNS, netaddr.IP{}, dns.RCodeNameError},
 		{"onion-domain", "footest.onion.", dns.TypeA, netaddr.IP{}, dns.RCodeNameError},
 		{"magicdns", dnsSymbolicFQDN, dns.TypeA, netaddr.MustParseIP("100.100.100.100"), dns.RCodeSuccess},
-		{"via_hex", dnsname.FQDN("via-0xff.1.2.3.4."), dns.TypeAAAA, netaddr.MustParseIP("fd7a:115c:a1e0:b1a:0:ff:102:304"), dns.RCodeSuccess},
-		{"via_dec", dnsname.FQDN("via-1.10.0.0.1."), dns.TypeAAAA, netaddr.MustParseIP("fd7a:115c:a1e0:b1a:0:1:a00:1"), dns.RCodeSuccess},
-		{"via_invalid", dnsname.FQDN("via-."), dns.TypeA, netaddr.IP{}, dns.RCodeRefused},
+		{"via_hex", dnsname.FQDN("via-0xff.1.2.3.4."), dns.TypeAAAA, netaddr.MustParseIP("fd7a:115c:a1e0:b1a:0:ff:1.2.3.4"), dns.RCodeSuccess},
+		{"via_dec", dnsname.FQDN("via-1.10.0.0.1."), dns.TypeAAAA, netaddr.MustParseIP("fd7a:115c:a1e0:b1a:0:1:10.0.0.1"), dns.RCodeSuccess},
+		{"x_via_hex", dnsname.FQDN("4.3.2.1.via-0xff."), dns.TypeAAAA, netaddr.MustParseIP("fd7a:115c:a1e0:b1a:0:ff:4.3.2.1"), dns.RCodeSuccess},
+		{"x_via_dec", dnsname.FQDN("1.0.0.10.via-1."), dns.TypeAAAA, netaddr.MustParseIP("fd7a:115c:a1e0:b1a:0:1:1.0.0.10"), dns.RCodeSuccess},
+		{"via_invalid", dnsname.FQDN("via-."), dns.TypeAAAA, netaddr.IP{}, dns.RCodeRefused},
+		{"via_invalid_2", dnsname.FQDN("2.3.4.5.via-."), dns.TypeAAAA, netaddr.IP{}, dns.RCodeRefused},
 	}
 
 	for _, tt := range tests {