5 files changed, 56 insertions, 2 deletions
diff --git a/pkg/tcpip/iptables/BUILD b/pkg/tcpip/iptables/BUILD
index 2893c80cd..297eaccaf 100644
--- a/pkg/tcpip/iptables/BUILD
+++ b/pkg/tcpip/iptables/BUILD
@@ -14,5 +14,6 @@ go_library(
     deps = [
         "//pkg/log",
         "//pkg/tcpip",
+        "//pkg/tcpip/header",
     ],
 )
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..4f2a4d65e 100644
--- a/pkg/tcpip/iptables/types.go
+++ b/pkg/tcpip/iptables/types.go
@@ -151,6 +151,9 @@ 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 {
+	// IPHeaderFilter holds basic IP filtering fields common to every rule.
+	Filter IPHeaderFilter
+
 	// Matchers is the list of matchers for this rule.
 	Matchers []Matcher
 
@@ -158,6 +161,23 @@ type Rule struct {
 	Target Target
 }
 
+// TODO: This is gross.
+// TODO: Save this in SetEntries.
+// TODO: Utilize this when traversing tables.
+type IPHeaderFilter struct {
+	Source              [4]byte
+	Destination         [4]byte
+	SourceMask          [4]byte
+	DestinationMask     [4]byte
+	OutputInterface     string
+	InputInterface      string
+	OutputInterfaceMask string
+	InputInterfaceMask  string
+	Protocol            tcpip.TransportProtocolNumber
+	Flags               uint8
+	InverseFlags        uint8
+}
+
 // 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.
diff --git a/pkg/tcpip/packet_buffer.go b/pkg/tcpip/packet_buffer.go
index ab24372e7..7a036b93c 100644
--- a/pkg/tcpip/packet_buffer.go
+++ b/pkg/tcpip/packet_buffer.go
@@ -13,7 +13,9 @@
 
 package tcpip
 
-import "gvisor.dev/gvisor/pkg/tcpip/buffer"
+import (
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+)
 
 // A PacketBuffer contains all the data of a network packet.
 //
@@ -65,3 +67,24 @@ func (pk PacketBuffer) Clone() PacketBuffer {
 	pk.Data = pk.Data.Clone(nil)
 	return pk
 }
+
+//// TransportProtocol returns the transport protocol of pk.
+////
+//// Precondition: pk.NetworkHeader is set.
+//func (pk PacketBuffer) TransportProtocolIPv4() uint16 {
+//	if pk.NetworkHeader == nil {
+//		panic("This should only be called when pk.NetworkHeader is set.")
+//	}
+//	return header.IPv4(pk.NetworkHeader).TransportProtocol()
+//}
+
+// func (pk Packet) findNetHeader() header.IPv4 {
+// 	// Inbound:
+// 	// Data holds everything, but may have had some headers shaved off.
+// 	// Figure out whether it's set or still somewhere in data and return
+// 	// appropriately.
+
+// 	// Outbound:
+// 	// NetworkHeader will be set if we've added one. Otherwise there's no
+// 	// header.
+// }