iptables: filter by IP address (and range)

Enables commands such as:
$ iptables -A INPUT -d 127.0.0.1 -j ACCEPT
$ iptables -t nat -A PREROUTING ! -d 127.0.0.1 -j REDIRECT

Also adds a bunch of REDIRECT+destination tests.

2 files changed, 37 insertions, 3 deletions

diff --git a/pkg/tcpip/iptables/iptables.go b/pkg/tcpip/iptables/iptables.go
index dbaccbb36..262b6448d 100644
--- a/pkg/tcpip/iptables/iptables.go
+++ b/pkg/tcpip/iptables/iptables.go
@@ -240,9 +240,8 @@ func (it *IPTables) checkChain(hook Hook, pkt tcpip.PacketBuffer, table Table, r
 func (it *IPTables) checkRule(hook Hook, pkt tcpip.PacketBuffer, table Table, ruleIdx int) (RuleVerdict, int) {
 	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() {
+	// Check whether the packet matches the IP header filter.
+	if !filterMatch(rule.Filter, header.IPv4(pkt.NetworkHeader)) {
 		// Continue on to the next rule.
 		return RuleJump, ruleIdx + 1
 	}
@@ -263,3 +262,26 @@ func (it *IPTables) checkRule(hook Hook, pkt tcpip.PacketBuffer, table Table, ru
 	// All the matchers matched, so run the target.
 	return rule.Target.Action(pkt)
 }
+
+func filterMatch(filter IPHeaderFilter, hdr header.IPv4) bool {
+	// TODO(gvisor.dev/issue/170): Support other fields of the filter.
+	// Check the transport protocol.
+	if filter.Protocol != 0 && filter.Protocol != hdr.TransportProtocol() {
+		return false
+	}
+
+	// Check the destination IP.
+	dest := hdr.DestinationAddress()
+	matches := true
+	for i := range filter.Dst {
+		if dest[i]&filter.DstMask[i] != filter.Dst[i] {
+			matches = false
+			break
+		}
+	}
+	if matches == filter.DstInvert {
+		return false
+	}
+
+	return true
+}
diff --git a/pkg/tcpip/iptables/types.go b/pkg/tcpip/iptables/types.go
index 7d032fd23..e7fcf6bff 100644
--- a/pkg/tcpip/iptables/types.go
+++ b/pkg/tcpip/iptables/types.go
@@ -144,6 +144,18 @@ type Rule struct {
 type IPHeaderFilter struct {
 	// Protocol matches the transport protocol.
 	Protocol tcpip.TransportProtocolNumber
+
+	// Dst matches the destination IP address.
+	Dst tcpip.Address
+
+	// DstMask masks bits of the destination IP address when comparing with
+	// Dst.
+	DstMask tcpip.Address
+
+	// DstInvert inverts the meaning of the destination IP check, i.e. when
+	// true the filter will match packets that fail the destination
+	// comparison.
+	DstInvert bool
 }
 
 // A Matcher is the interface for matching packets.