3 files changed, 23 insertions, 2 deletions
diff --git a/pkg/tcpip/iptables/iptables.go b/pkg/tcpip/iptables/iptables.go
index 605a71679..fc06b5b87 100644
--- a/pkg/tcpip/iptables/iptables.go
+++ b/pkg/tcpip/iptables/iptables.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 )
 
 // Table names.
@@ -184,8 +185,16 @@ func (it *IPTables) checkTable(hook Hook, pkt tcpip.PacketBuffer, tablename stri
 	panic(fmt.Sprintf("Traversed past the entire list of iptables rules in table %q.", tablename))
 }
 
+// Precondition: pk.NetworkHeader is set.
 func (it *IPTables) checkRule(hook Hook, pkt tcpip.PacketBuffer, table Table, ruleIdx int) Verdict {
 	rule := table.Rules[ruleIdx]
+
+	// First check whether the packet matches the IP header filter.
+	// TODO(gvisor.dev/issue/170): Support other fields of the filter.
+	if rule.Filter.Protocol != 0 && rule.Filter.Protocol != header.IPv4(pkt.NetworkHeader).TransportProtocol() {
+		return Continue
+	}
+
 	// Go through each rule matcher. If they all match, run
 	// the rule target.
 	for _, matcher := range rule.Matchers {
diff --git a/pkg/tcpip/iptables/types.go b/pkg/tcpip/iptables/types.go
index 9f6906100..a8b972f1b 100644
--- a/pkg/tcpip/iptables/types.go
+++ b/pkg/tcpip/iptables/types.go
@@ -14,7 +14,9 @@
 
 package iptables
 
-import "gvisor.dev/gvisor/pkg/tcpip"
+import (
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
 
 // A Hook specifies one of the hooks built into the network stack.
 //
@@ -151,6 +153,9 @@ func (table *Table) SetMetadata(metadata interface{}) {
 // packets this rule applies to. If there are no matchers in the rule, it
 // applies to any packet.
 type Rule struct {
+	// Filter holds basic IP filtering fields common to every rule.
+	Filter IPHeaderFilter
+
 	// Matchers is the list of matchers for this rule.
 	Matchers []Matcher
 
@@ -158,6 +163,12 @@ type Rule struct {
 	Target Target
 }
 
+// IPHeaderFilter holds basic IP filtering data common to every rule.
+type IPHeaderFilter struct {
+	// Protocol matches the transport protocol.
+	Protocol tcpip.TransportProtocolNumber
+}
+
 // A Matcher is the interface for matching packets.
 type Matcher interface {
 	// Match returns whether the packet matches and whether the packet
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 0a1453b31..85512f9b2 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -353,7 +353,8 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt tcpip.PacketBuffer) {
 	}
 	pkt.NetworkHeader = headerView[:h.HeaderLength()]
 
-	// iptables filtering.
+	// iptables filtering. All packets that reach here are intended for
+	// this machine and will not be forwarded.
 	ipt := e.stack.IPTables()
 	if ok := ipt.Check(iptables.Input, pkt); !ok {
 		// iptables is telling us to drop the packet.