Create WEAVE-EXPOSE iptables chain

Docker 1.13 has changed a default policy of FORWARD chain to DROP
(https://github.com/moby/moby/pull/28257) which makes containers
inaccessible from a remote host when the bridge is exposed.

The change breaks e.g. the AWSVPC mode. To overcome this we install
an explicit rule for accepting forwarded ingress traffic to an
exposed subnet which is appended to the WEAVE-EXPOSE chain.

The chain is a target of the rule "-t filter -A FORWARD -o weave".

net/bridge.go

@@ -455,6 +455,15 @@ func configureIPTables(config *BridgeConfig) error {
 			return err
 		}
 	}
+
+	if !config.NPC {
+		// Create a chain for allowing ingress traffic when the bridge is exposed
+		_ = ipt.NewChain("filter", "WEAVE-EXPOSE")
+		if err = ipt.AppendUnique("filter", "FORWARD", "-o", config.WeaveBridgeName, "-j", "WEAVE-EXPOSE"); err != nil {
+			return err
+		}
+	}
+
 	// Forward from weave to the rest of the world
 	if err = ipt.AppendUnique("filter", "FORWARD", "-i", config.WeaveBridgeName, "!", "-o", config.WeaveBridgeName, "-j", "ACCEPT"); err != nil {
 		return err

net/expose.go

@@ -19,16 +19,31 @@ import (
 // * "removeDefaultRoute" - whether to remove a default route installed by the kernel (used only in the AWSVPC mode).
 // * "npc" - whether is Weave NPC running.
 func Expose(bridgeName string, ipAddr *net.IPNet, removeDefaultRoute, npc bool) error {
+	ipt, err := iptables.New()
+	if err != nil {
+		return errors.Wrap(err, "iptables.New")
+	}
+	cidr := ipAddr.String()
+
 	if err := addBridgeIPAddr(bridgeName, ipAddr, removeDefaultRoute); err != nil {
 		return errors.Wrap(err, "addBridgeIPAddr")
 	}
 
-	if err := exposeNAT(ipAddr); err != nil {
+	if err := exposeNAT(ipt, cidr); err != nil {
 		return errors.Wrap(err, "exposeNAT")
 	}
 
 	if !npc {
-		// TODO comment why not in npc mode && add filter rules and docs
+		// Docker 1.13 has changed a default policy of FORWARD chain to DROP
+		// (https://github.com/moby/moby/pull/28257) which makes containers
+		// inaccessible from a remote host when the bridge is exposed.
+		//
+		// The change breaks e.g. the AWSVPC mode. To overcome this we install
+		// an explicit rule for accepting forwarded ingress traffic to an
+		// exposed subnet.
+		if err := ipt.AppendUnique("filter", "WEAVE-EXPOSE", "-d", cidr, "-j", "ACCEPT"); err != nil {
+			return errors.Wrap(err, "ipt.AppendUnique")
+		}
 	}
 
 	return nil
@@ -71,13 +86,7 @@ func addBridgeIPAddr(bridgeName string, addr *net.IPNet, removeDefaultRoute bool
 	return nil
 }
 
-func exposeNAT(ipnet *net.IPNet) error {
-	ipt, err := iptables.New()
-	if err != nil {
-		return err
-	}
-	cidr := ipnet.String()
-
+func exposeNAT(ipt *iptables.IPTables, cidr string) error {
 	if err := addNatRule(ipt, "-s", cidr, "-d", "224.0.0.0/4", "-j", "RETURN"); err != nil {
 		return err
 	}

weave

@@ -508,6 +508,11 @@ destroy_bridge() {
     run_iptables -t filter -D FORWARD -o $BRIDGE -m state --state NEW -j NFLOG --nflog-group 86 2>/dev/null || true
     run_iptables -t filter -D FORWARD -o $BRIDGE -j DROP 2>/dev/null || true
     run_iptables -X WEAVE-NPC >/dev/null 2>&1 || true
+
+    run_iptables -F WEAVE-EXPOSE >/dev/null 2>&1 || true
+    run_iptables -t filter -D FORWARD -o $BRIDGE -j WEAVE-EXPOSE 2>/dev/null || true
+    run_iptables -X WEAVE-EXPOSE >/dev/null 2>&1 || true
+
     run_iptables -t nat -F WEAVE >/dev/null 2>&1 || true
     run_iptables -t nat -D POSTROUTING -j WEAVE >/dev/null 2>&1 || true
     run_iptables -t nat -D POSTROUTING -o $BRIDGE -j ACCEPT >/dev/null 2>&1 || true
@@ -1494,6 +1499,7 @@ case "$COMMAND" in
                 ip addr del dev $BRIDGE $CIDR
                 delete_iptables_rule nat WEAVE -d $CIDR ! -s $CIDR -j MASQUERADE
                 delete_iptables_rule nat WEAVE -s $CIDR ! -d $CIDR -j MASQUERADE
+                delete_iptables_rule filter WEAVE-EXPOSE -d $CIDR -j ACCEPT
                 when_weave_running delete_dns weave:expose $CIDR
             fi
         done