net/dns/resolver: support magic resolution of via-<siteid>.<ip4> domains

Updates #3616

Signed-off-by: Tom DNetto <tom@tailscale.com>

net/dns/resolver/tsdns.go

@@ -17,6 +17,7 @@ import (
 	"os"
 	"runtime"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -633,6 +634,10 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netaddr.IP,
 			return tsaddr.TailscaleServiceIPv6(), dns.RCodeSuccess
 		}
 	}
+	// Special-case: 'via-<siteid>.<ipv4>' queries.
+	if ip, ok := r.parseViaDomain(domain, typ); ok {
+		return ip, dns.RCodeSuccess
+	}
 
 	r.mu.Lock()
 	hosts := r.hostToIP
@@ -708,6 +713,46 @@ 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.
+//
+// This exists as a convenient mapping into Tailscales 'Via Range'.
+func (r *Resolver) parseViaDomain(domain dnsname.FQDN, typ dns.Type) (netaddr.IP, bool) {
+	fqdn := string(domain.WithoutTrailingDot())
+	if typ != dns.TypeAAAA {
+		return netaddr.IP{}, false
+	}
+	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
+	}
+
+	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
+	}
+
+	prefix, err := strconv.ParseUint(siteID, 0, 32)
+	if err != nil {
+		return netaddr.IP{}, false // badly formed, dont respond
+	}
+
+	// MapVia will never error when given an ipv4 netaddr.IPPrefix.
+	out, _ := tsaddr.MapVia(uint32(prefix), netaddr.IPPrefixFrom(ip4, ip4.BitLen()))
+	return out.IP(), true
+}
+
 // resolveReverse returns the unique domain name that maps to the given address.
 func (r *Resolver) resolveLocalReverse(name dnsname.FQDN) (dnsname.FQDN, dns.RCode) {
 	var ip netaddr.IP

net/dns/resolver/tsdns_test.go

@@ -348,6 +348,9 @@ 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},
 	}
 
 	for _, tt := range tests {