net: disable RFC 6724 Rule 9 for IPv4 addresses

Rule 9 arguably doesn't make sense for IPv4 addresses, and so far it
has only caused problems (#13283, #18518). Disable it until we hear
from users that actually want/need it.

Fixes #18518.

Change-Id: I7b0dd75d03819cab8e0cd4c29f0c1dc8d2e9c179
Reviewed-on: https://go-review.googlesource.com/34914
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 5 insertions, 131 deletions

diff --git a/src/net/addrselect.go b/src/net/addrselect.go
index 0b9d160fd4..1ab9fc5326 100644
--- a/src/net/addrselect.go
+++ b/src/net/addrselect.go
@@ -188,33 +188,17 @@ func (s *byRFC6724) Less(i, j int) bool {
 
 	// Rule 9: Use longest matching prefix.
 	// When DA and DB belong to the same address family (both are IPv6 or
-	// both are IPv4): If CommonPrefixLen(Source(DA), DA) >
+	// both are IPv4 [but see below]): If CommonPrefixLen(Source(DA), DA) >
 	// CommonPrefixLen(Source(DB), DB), then prefer DA.  Similarly, if
 	// CommonPrefixLen(Source(DA), DA) < CommonPrefixLen(Source(DB), DB),
 	// then prefer DB.
-	da4 := DA.To4() != nil
-	db4 := DB.To4() != nil
-	if da4 == db4 {
+	//
+	// However, applying this rule to IPv4 addresses causes
+	// problems (see issues 13283 and 18518), so limit to IPv6.
+	if DA.To4() == nil && DB.To4() == nil {
 		commonA := commonPrefixLen(SourceDA, DA)
 		commonB := commonPrefixLen(SourceDB, DB)
 
-		// CommonPrefixLen doesn't really make sense for IPv4, and even
-		// causes problems for common load balancing practices
-		// (e.g., https://golang.org/issue/13283).  Glibc instead only
-		// uses CommonPrefixLen for IPv4 when the source and destination
-		// addresses are on the same subnet, but that requires extra
-		// work to find the netmask for our source addresses. As a
-		// simpler heuristic, we limit its use to when the source and
-		// destination belong to the same special purpose block.
-		if da4 {
-			if !sameIPv4SpecialPurposeBlock(SourceDA, DA) {
-				commonA = 0
-			}
-			if !sameIPv4SpecialPurposeBlock(SourceDB, DB) {
-				commonB = 0
-			}
-		}
-
 		if commonA > commonB {
 			return preferDA
 		}
@@ -404,28 +388,3 @@ func commonPrefixLen(a, b IP) (cpl int) {
 	}
 	return
 }
-
-// sameIPv4SpecialPurposeBlock reports whether a and b belong to the same
-// address block reserved by the IANA IPv4 Special-Purpose Address Registry:
-// http://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml
-func sameIPv4SpecialPurposeBlock(a, b IP) bool {
-	a, b = a.To4(), b.To4()
-	if a == nil || b == nil || a[0] != b[0] {
-		return false
-	}
-	// IANA defines more special-purpose blocks, but these are the only
-	// ones likely to be relevant to typical Go systems.
-	switch a[0] {
-	case 10: // 10.0.0.0/8: Private-Use
-		return true
-	case 127: // 127.0.0.0/8: Loopback
-		return true
-	case 169: // 169.254.0.0/16: Link Local
-		return a[1] == 254 && b[1] == 254
-	case 172: // 172.16.0.0/12: Private-Use
-		return a[1]&0xf0 == 16 && b[1]&0xf0 == 16
-	case 192: // 192.168.0.0/16: Private-Use
-		return a[1] == 168 && b[1] == 168
-	}
-	return false
-}
diff --git a/src/net/addrselect_test.go b/src/net/addrselect_test.go
index 80aa4eb195..d6e0e63c3b 100644
--- a/src/net/addrselect_test.go
+++ b/src/net/addrselect_test.go
@@ -117,27 +117,6 @@ func TestSortByRFC6724(t *testing.T) {
 			},
 			reverse: false,
 		},
-
-		// Prefer longer common prefixes, but only for IPv4 address
-		// pairs in the same special-purpose block.
-		{
-			in: []IPAddr{
-				{IP: ParseIP("1.2.3.4")},
-				{IP: ParseIP("10.55.0.1")},
-				{IP: ParseIP("10.66.0.1")},
-			},
-			srcs: []IP{
-				ParseIP("1.2.3.5"),
-				ParseIP("10.66.1.2"),
-				ParseIP("10.66.1.2"),
-			},
-			want: []IPAddr{
-				{IP: ParseIP("10.66.0.1")},
-				{IP: ParseIP("10.55.0.1")},
-				{IP: ParseIP("1.2.3.4")},
-			},
-			reverse: true,
-		},
 	}
 	for i, tt := range tests {
 		inCopy := make([]IPAddr, len(tt.in))
@@ -268,67 +247,3 @@ func TestRFC6724CommonPrefixLength(t *testing.T) {
 	}
 
 }
-
-func mustParseCIDRs(t *testing.T, blocks ...string) []*IPNet {
-	res := make([]*IPNet, len(blocks))
-	for i, block := range blocks {
-		var err error
-		_, res[i], err = ParseCIDR(block)
-		if err != nil {
-			t.Fatalf("ParseCIDR(%s) failed: %v", block, err)
-		}
-	}
-	return res
-}
-
-func TestSameIPv4SpecialPurposeBlock(t *testing.T) {
-	blocks := mustParseCIDRs(t,
-		"10.0.0.0/8",
-		"127.0.0.0/8",
-		"169.254.0.0/16",
-		"172.16.0.0/12",
-		"192.168.0.0/16",
-	)
-
-	addrs := []struct {
-		ip    IP
-		block int // index or -1
-	}{
-		{IP{1, 2, 3, 4}, -1},
-		{IP{2, 3, 4, 5}, -1},
-		{IP{10, 2, 3, 4}, 0},
-		{IP{10, 6, 7, 8}, 0},
-		{IP{127, 0, 0, 1}, 1},
-		{IP{127, 255, 255, 255}, 1},
-		{IP{169, 254, 77, 99}, 2},
-		{IP{169, 254, 44, 22}, 2},
-		{IP{169, 255, 0, 1}, -1},
-		{IP{172, 15, 5, 6}, -1},
-		{IP{172, 16, 32, 41}, 3},
-		{IP{172, 31, 128, 9}, 3},
-		{IP{172, 32, 88, 100}, -1},
-		{IP{192, 168, 1, 1}, 4},
-		{IP{192, 168, 128, 42}, 4},
-		{IP{192, 169, 1, 1}, -1},
-	}
-
-	for i, addr := range addrs {
-		for j, block := range blocks {
-			got := block.Contains(addr.ip)
-			want := addr.block == j
-			if got != want {
-				t.Errorf("%d/%d. %s.Contains(%s): got %v, want %v", i, j, block, addr.ip, got, want)
-			}
-		}
-	}
-
-	for i, addr1 := range addrs {
-		for j, addr2 := range addrs {
-			got := sameIPv4SpecialPurposeBlock(addr1.ip, addr2.ip)
-			want := addr1.block >= 0 && addr1.block == addr2.block
-			if got != want {
-				t.Errorf("%d/%d. sameIPv4SpecialPurposeBlock(%s, %s): got %v, want %v", i, j, addr1.ip, addr2.ip, got, want)
-			}
-		}
-	}
-}