From 7fe6ecf165bdb1525151bcbc8b6f5f92e05e0670 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Tue, 7 Sep 2021 15:45:28 -0700
Subject: [PATCH] ipn/ipnlocal: add MagicDNS records for IPv6-only nodes

Updates #2268

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 ipn/ipnlocal/dnsconfig_test.go | 11 ++++++++---
 ipn/ipnlocal/local.go          |  3 ++-
 net/tsaddr/tsaddr.go           | 21 +++++++++++++++++++++
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/ipn/ipnlocal/dnsconfig_test.go b/ipn/ipnlocal/dnsconfig_test.go
index 7245bf1c7..91166a082 100644
--- a/ipn/ipnlocal/dnsconfig_test.go
+++ b/ipn/ipnlocal/dnsconfig_test.go
@@ -68,15 +68,20 @@ func TestDNSConfigForNetmap(t *testing.T) {
 						Name:      "b.net",
 						Addresses: ipps("100.102.0.1", "100.102.0.2", "fe75::2"),
 					},
+					{
+						Name:      "v6-only.net",
+						Addresses: ipps("fe75::3"), // no IPv4, so we don't ignore IPv6
+					},
 				},
 			},
 			prefs: &ipn.Prefs{},
 			want: &dns.Config{
 				Routes: map[dnsname.FQDN][]dnstype.Resolver{},
 				Hosts: map[dnsname.FQDN][]netaddr.IP{
-					"b.net.":      ips("100.102.0.1", "100.102.0.2"),
-					"myname.net.": ips("100.101.101.101"),
-					"peera.net.":  ips("100.102.0.1", "100.102.0.2"),
+					"b.net.":       ips("100.102.0.1", "100.102.0.2"),
+					"myname.net.":  ips("100.101.101.101"),
+					"peera.net.":   ips("100.102.0.1", "100.102.0.2"),
+					"v6-only.net.": ips("fe75::3"),
 				},
 			},
 		},
diff --git a/ipn/ipnlocal/local.go b/ipn/ipnlocal/local.go
index 7171e3a7f..202aa4b17 100644
--- a/ipn/ipnlocal/local.go
+++ b/ipn/ipnlocal/local.go
@@ -1798,6 +1798,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 		if err != nil {
 			return // TODO: propagate error?
 		}
+		have4 := tsaddr.PrefixesContainsFunc(addrs, func(p netaddr.IPPrefix) bool { return p.IP().Is4() })
 		var ips []netaddr.IP
 		for _, addr := range addrs {
 			// Remove IPv6 addresses for now, as we don't
@@ -1807,7 +1808,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 			// https://github.com/tailscale/tailscale/issues/1152
 			// tracks adding the right capability reporting to
 			// enable AAAA in MagicDNS.
-			if addr.IP().Is6() {
+			if addr.IP().Is6() && have4 {
 				continue
 			}
 			ips = append(ips, addr.IP())
diff --git a/net/tsaddr/tsaddr.go b/net/tsaddr/tsaddr.go
index fc4ef26f9..46496dd54 100644
--- a/net/tsaddr/tsaddr.go
+++ b/net/tsaddr/tsaddr.go
@@ -164,3 +164,24 @@ func NewContainsIPFunc(addrs []netaddr.IPPrefix) func(ip netaddr.IP) bool {
 	}
 	return func(ip netaddr.IP) bool { return m[ip] }
 }
+
+// PrefixesContainsFunc reports whether f is true for any IPPrefix in
+// ipp.
+func PrefixesContainsFunc(ipp []netaddr.IPPrefix, f func(netaddr.IPPrefix) bool) bool {
+	for _, v := range ipp {
+		if f(v) {
+			return true
+		}
+	}
+	return false
+}
+
+// IPsContainsFunc reports whether f is true for any IP in ips.
+func IPsContainsFunc(ips []netaddr.IP, f func(netaddr.IP) bool) bool {
+	for _, v := range ips {
+		if f(v) {
+			return true
+		}
+	}
+	return false
+}