From 74a7d76c9777820fcd7bd6002481eb959f58e247 Mon Sep 17 00:00:00 2001
From: Kevin Krakauer <krakauer@google.com>
Date: Wed, 3 Jun 2020 19:57:39 -0700
Subject: iptables: loopback traffic skips prerouting chain

Loopback traffic is not affected by rules in the PREROUTING chain.

This change is also necessary for istio's envoy to talk to other
components in the same pod.
---
 pkg/tcpip/network/ipv4/ipv4.go | 49 +++++++++++++++++-------------------------
 pkg/tcpip/stack/nic.go         |  3 ++-
 2 files changed, 22 insertions(+), 30 deletions(-)

(limited to 'pkg')

diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 9cd7592f4..959f7e007 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -258,38 +258,24 @@ func (e *endpoint) WritePacket(r *stack.Route, gso *stack.GSO, params stack.Netw
 		return nil
 	}
 
+	// If the packet is manipulated as per NAT Ouput rules, handle packet
+	// based on destination address and do not send the packet to link layer.
+	// TODO(gvisor.dev/issue/170): We should do this for every packet, rather than
+	// only NATted packets, but removing this check short circuits broadcasts
+	// before they are sent out to other hosts.
 	if pkt.NatDone {
-		// If the packet is manipulated as per NAT Ouput rules, handle packet
-		// based on destination address and do not send the packet to link layer.
 		netHeader := header.IPv4(pkt.NetworkHeader)
 		ep, err := e.stack.FindNetworkEndpoint(header.IPv4ProtocolNumber, netHeader.DestinationAddress())
 		if err == nil {
-			src := netHeader.SourceAddress()
-			dst := netHeader.DestinationAddress()
-			route := r.ReverseRoute(src, dst)
-
-			views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
-			views[0] = pkt.Header.View()
-			views = append(views, pkt.Data.Views()...)
-			ep.HandlePacket(&route, &stack.PacketBuffer{
-				Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views),
-			})
+			route := r.ReverseRoute(netHeader.SourceAddress(), netHeader.DestinationAddress())
+			handleLoopback(&route, pkt, ep)
 			return nil
 		}
 	}
 
 	if r.Loop&stack.PacketLoop != 0 {
-		// The inbound path expects the network header to still be in
-		// the PacketBuffer's Data field.
-		views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
-		views[0] = pkt.Header.View()
-		views = append(views, pkt.Data.Views()...)
 		loopedR := r.MakeLoopedRoute()
-
-		e.HandlePacket(&loopedR, &stack.PacketBuffer{
-			Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views),
-		})
-
+		handleLoopback(&loopedR, pkt, e)
 		loopedR.Release()
 	}
 	if r.Loop&stack.PacketOut == 0 {
@@ -305,6 +291,17 @@ func (e *endpoint) WritePacket(r *stack.Route, gso *stack.GSO, params stack.Netw
 	return nil
 }
 
+func handleLoopback(route *stack.Route, pkt *stack.PacketBuffer, ep stack.NetworkEndpoint) {
+	// The inbound path expects the network header to still be in
+	// the PacketBuffer's Data field.
+	views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
+	views[0] = pkt.Header.View()
+	views = append(views, pkt.Data.Views()...)
+	ep.HandlePacket(route, &stack.PacketBuffer{
+		Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views),
+	})
+}
+
 // WritePackets implements stack.NetworkEndpoint.WritePackets.
 func (e *endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.PacketBufferList, params stack.NetworkHeaderParams) (int, *tcpip.Error) {
 	if r.Loop&stack.PacketLoop != 0 {
@@ -347,13 +344,7 @@ func (e *endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.Packe
 				src := netHeader.SourceAddress()
 				dst := netHeader.DestinationAddress()
 				route := r.ReverseRoute(src, dst)
-
-				views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
-				views[0] = pkt.Header.View()
-				views = append(views, pkt.Data.Views()...)
-				ep.HandlePacket(&route, &stack.PacketBuffer{
-					Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views),
-				})
+				handleLoopback(&route, pkt, ep)
 				n++
 				continue
 			}
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index ec8e3cb85..6664aea06 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -1229,7 +1229,8 @@ func (n *NIC) DeliverNetworkPacket(remote, local tcpip.LinkAddress, protocol tcp
 	}
 
 	// TODO(gvisor.dev/issue/170): Not supporting iptables for IPv6 yet.
-	if protocol == header.IPv4ProtocolNumber {
+	// Loopback traffic skips the prerouting chain.
+	if protocol == header.IPv4ProtocolNumber && !n.isLoopback() {
 		// iptables filtering.
 		ipt := n.stack.IPTables()
 		address := n.primaryAddress(protocol)
-- 
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