wgengine/magicsock: prefer link-local addresses to private ones

Since link-local addresses are definitionally more likely to be a direct
(lower-latency, more reliable) connection than a non-link-local private
address, give those a bit of a boost when selecting endpoints.

Updates #8097

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I93fdeb07de55ba39ba5fcee0834b579ca05c2a4e

wgengine/magicsock/endpoint.go

@@ -1665,14 +1665,20 @@ func betterAddr(a, b addrQuality) bool {
 	// pay for the bandwidth in a cloud environment.
 	//
 	// Additionally, prefer any loopback address strongly over non-loopback
-	// addresses.
+	// addresses, and prefer link-local unicast addresses over other types
+	// of private IP addresses since it's definitionally more likely that
+	// they'll be on the same network segment than a general private IP.
 	if a.Addr().IsLoopback() {
 		aPoints += 50
+	} else if a.Addr().IsLinkLocalUnicast() {
+		aPoints += 30
 	} else if a.Addr().IsPrivate() {
 		aPoints += 20
 	}
 	if b.Addr().IsLoopback() {
 		bPoints += 50
+	} else if b.Addr().IsLinkLocalUnicast() {
+		bPoints += 30
 	} else if b.Addr().IsPrivate() {
 		bPoints += 20
 	}

wgengine/magicsock/magicsock_test.go

@@ -1747,6 +1747,19 @@ func TestBetterAddr(t *testing.T) {
 			b:    al("192.168.0.1:555", 100*ms),
 			want: false,
 		},
+
+		// Link-local unicast addresses are preferred over other
+		// private IPs, but not as much as localhost addresses.
+		{
+			a:    al("[fe80::ce8:474a:a27e:113b]:555", 101*ms),
+			b:    al("[fd89:1a8a:8888:9999:aaaa:bbbb:cccc:dddd]:555", 100*ms),
+			want: true,
+		},
+		{
+			a:    al("[fe80::ce8:474a:a27e:113b]:555", 101*ms),
+			b:    al("[::1]:555", 100*ms),
+			want: false,
+		},
 	}
 	for i, tt := range tests {
 		got := betterAddr(tt.a, tt.b)