net/dns: ensure multiple hosts with the same IP address are combined into a single HostEntry

This ensures that each line has a unique IP address.

Fixes #11939

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/dns/manager.go

@@ -17,6 +17,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	xmaps "golang.org/x/exp/maps"
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/net/dns/resolver"
@@ -122,6 +123,7 @@ func (m *Manager) Set(cfg Config) error {
 // The returned list is sorted by the first hostname in each entry.
 func compileHostEntries(cfg Config) (hosts []*HostEntry) {
 	didLabel := make(map[string]bool, len(cfg.Hosts))
+	hostsMap := make(map[netip.Addr]*HostEntry, len(cfg.Hosts))
 	for _, sd := range cfg.SearchDomains {
 		for h, ips := range cfg.Hosts {
 			if !sd.Contains(h) || h.NumLabels() != (sd.NumLabels()+1) {
@@ -136,15 +138,23 @@ func compileHostEntries(cfg Config) (hosts []*HostEntry) {
 				if cfg.OnlyIPv6 && ip.Is4() {
 					continue
 				}
-				hosts = append(hosts, &HostEntry{
-					Addr:  ip,
-					Hosts: ipHosts,
-				})
+				if e := hostsMap[ip]; e != nil {
+					e.Hosts = append(e.Hosts, ipHosts...)
+				} else {
+					hostsMap[ip] = &HostEntry{
+						Addr:  ip,
+						Hosts: ipHosts,
+					}
+				}
 				// Only add IPv4 or IPv6 per host, like we do in the resolver.
 				break
 			}
 		}
 	}
+	if len(hostsMap) == 0 {
+		return nil
+	}
+	hosts = xmaps.Values(hostsMap)
 	slices.SortFunc(hosts, func(a, b *HostEntry) int {
 		if len(a.Hosts) == 0 && len(b.Hosts) == 0 {
 			return 0

net/dns/manager_test.go

@@ -87,8 +87,7 @@ func TestCompileHostEntries(t *testing.T) {
 				{Addr: netip.MustParseAddr("1.1.1.1"), Hosts: []string{"a.foo.ts.net.", "a"}},
 				{Addr: netip.MustParseAddr("1.1.1.2"), Hosts: []string{"b.foo.ts.net.", "b"}},
 				{Addr: netip.MustParseAddr("1.1.1.3"), Hosts: []string{"c.foo.ts.net.", "c"}},
-				{Addr: netip.MustParseAddr("1.1.1.4"), Hosts: []string{"d.foo.beta.tailscale.net."}},
-				{Addr: netip.MustParseAddr("1.1.1.4"), Hosts: []string{"d.foo.ts.net.", "d"}},
+				{Addr: netip.MustParseAddr("1.1.1.4"), Hosts: []string{"d.foo.ts.net.", "d", "d.foo.beta.tailscale.net."}},
 				{Addr: netip.MustParseAddr("1.1.1.5"), Hosts: []string{"e.foo.beta.tailscale.net.", "e"}},
 			},
 		},
@@ -103,8 +102,7 @@ func TestCompileHostEntries(t *testing.T) {
 				SearchDomains: []dnsname.FQDN{"foo.ts.net.", "foo.beta.tailscale.net."},
 			},
 			want: []*HostEntry{
-				{Addr: netip.MustParseAddr("1.1.1.5"), Hosts: []string{"e.foo.beta.tailscale.net."}},
-				{Addr: netip.MustParseAddr("1.1.1.5"), Hosts: []string{"e.foo.ts.net.", "e"}},
+				{Addr: netip.MustParseAddr("1.1.1.5"), Hosts: []string{"e.foo.ts.net.", "e", "e.foo.beta.tailscale.net."}},
 			},
 		},
 		{
@@ -120,8 +118,7 @@ func TestCompileHostEntries(t *testing.T) {
 				SearchDomains: []dnsname.FQDN{"foo.ts.net.", "foo.beta.tailscale.net."},
 			},
 			want: []*HostEntry{
-				{Addr: netip.MustParseAddr("1.1.1.4"), Hosts: []string{"d.foo.beta.tailscale.net."}},
-				{Addr: netip.MustParseAddr("1.1.1.4"), Hosts: []string{"d.foo.ts.net.", "d"}},
+				{Addr: netip.MustParseAddr("1.1.1.4"), Hosts: []string{"d.foo.ts.net.", "d", "d.foo.beta.tailscale.net."}},
 			},
 		},
 		{
@@ -139,8 +136,7 @@ func TestCompileHostEntries(t *testing.T) {
 			want: []*HostEntry{
 				{Addr: netip.MustParseAddr("1.1.1.2"), Hosts: []string{"h1.foo.beta.tailscale.net."}},
 				{Addr: netip.MustParseAddr("1.1.1.3"), Hosts: []string{"h1.foo.ts.net.", "h1"}},
-				{Addr: netip.MustParseAddr("1.1.1.1"), Hosts: []string{"h2.foo.beta.tailscale.net."}},
-				{Addr: netip.MustParseAddr("1.1.1.1"), Hosts: []string{"h2.foo.ts.net.", "h2"}},
+				{Addr: netip.MustParseAddr("1.1.1.1"), Hosts: []string{"h2.foo.ts.net.", "h2", "h2.foo.beta.tailscale.net."}},
 			},
 		},
 	}