From: Thomas Graf <tgraf@redhat.com>
Date: Tue, 31 Mar 2009 20:52:35 +0200
Subject: [net] iptables NAT port randomisation
Message-id: 20090331185235.GA19481@plip.localdomain
O-Subject: [RHEL5.4 PATCH] net: iptables NAT port randomisation (bz459943)
Bugzilla: 459943
RH-Acked-by: David Miller <davem@redhat.com>
RH-Acked-by: Neil Horman <nhorman@redhat.com>

Hey all -

This patch adds port randomisation to the netfilter NAT code. The
feature was indirectly requested by the security team with the
option to ignore the request if found not worthwhile. I considered
it a good idea for 5.4 even though not having randomised NAT ports
is not a security issue at all.

Fixes BZ459943.

diff --git a/drivers/char/random.c b/drivers/char/random.c
index ced24b4..f9578f3 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1584,6 +1584,7 @@ u32 secure_ipv4_port_ephemeral(__u32 saddr, __u32 daddr, __u16 dport)
 
 	return half_md4_transform(hash, keyptr->secret);
 }
+EXPORT_SYMBOL_GPL(secure_ipv4_port_ephemeral);
 
 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
 u32 secure_ipv6_port_ephemeral(const __u32 *saddr, const __u32 *daddr, __u16 dport)
diff --git a/include/linux/netfilter_ipv4/ip_nat.h b/include/linux/netfilter_ipv4/ip_nat.h
index e9f5ed1..7812698 100644
--- a/include/linux/netfilter_ipv4/ip_nat.h
+++ b/include/linux/netfilter_ipv4/ip_nat.h
@@ -16,6 +16,7 @@ enum ip_nat_manip_type
 
 #define IP_NAT_RANGE_MAP_IPS 1
 #define IP_NAT_RANGE_PROTO_SPECIFIED 2
+#define IP_NAT_RANGE_PROTO_RANDOM 4 /* add randomness to "port" selection */
 
 /* NAT sequence number modifications */
 struct ip_nat_seq {
diff --git a/net/ipv4/netfilter/ip_nat_core.c b/net/ipv4/netfilter/ip_nat_core.c
index 1741d55..2aa9131 100644
--- a/net/ipv4/netfilter/ip_nat_core.c
+++ b/net/ipv4/netfilter/ip_nat_core.c
@@ -262,8 +262,9 @@ get_unique_tuple(struct ip_conntrack_tuple *tuple,
 	if (maniptype == IP_NAT_MANIP_SRC) {
 		if (find_appropriate_src(orig_tuple, tuple, range)) {
 			DEBUGP("get_unique_tuple: Found current src map\n");
-			if (!ip_nat_used_tuple(tuple, conntrack))
-				return;
+			if (!(range->flags & IP_NAT_RANGE_PROTO_RANDOM))
+				if (!ip_nat_used_tuple(tuple, conntrack))
+					return;
 		}
 	}
 
@@ -277,6 +278,13 @@ get_unique_tuple(struct ip_conntrack_tuple *tuple,
 
 	proto = ip_nat_proto_find_get(orig_tuple->dst.protonum);
 
+	/* Change protocol info to have some randomization */
+	if (range->flags & IP_NAT_RANGE_PROTO_RANDOM) {
+		proto->unique_tuple(tuple, range, maniptype, conntrack);
+		ip_nat_proto_put(proto);
+		return;
+	}
+
 	/* Only bother mapping if it's not already in range and unique */
 	if ((!(range->flags & IP_NAT_RANGE_PROTO_SPECIFIED)
 	     || proto->in_range(tuple, maniptype, &range->min, &range->max))
diff --git a/net/ipv4/netfilter/ip_nat_proto_tcp.c b/net/ipv4/netfilter/ip_nat_proto_tcp.c
index 35d1d90..0879678 100644
--- a/net/ipv4/netfilter/ip_nat_proto_tcp.c
+++ b/net/ipv4/netfilter/ip_nat_proto_tcp.c
@@ -8,6 +8,7 @@
 
 #include <linux/types.h>
 #include <linux/init.h>
+#include <linux/random.h>
 #include <linux/netfilter.h>
 #include <linux/ip.h>
 #include <linux/tcp.h>
@@ -75,6 +76,13 @@ tcp_unique_tuple(struct ip_conntrack_tuple *tuple,
 		range_size = ntohs(range->max.tcp.port) - min + 1;
 	}
 
+	/* Start from random port to avoid prediction */
+	if (range->flags & IP_NAT_RANGE_PROTO_RANDOM)
+		port = secure_ipv4_port_ephemeral(tuple->src.ip, tuple->dst.ip,
+						  maniptype == IP_NAT_MANIP_SRC
+						  ? tuple->dst.u.tcp.port
+						  : tuple->src.u.tcp.port);
+
 	for (i = 0; i < range_size; i++, port++) {
 		*portptr = htons(min + port % range_size);
 		if (!ip_nat_used_tuple(tuple, conntrack)) {
diff --git a/net/ipv4/netfilter/ip_nat_proto_udp.c b/net/ipv4/netfilter/ip_nat_proto_udp.c
index 802ceb7..f061cb7 100644
--- a/net/ipv4/netfilter/ip_nat_proto_udp.c
+++ b/net/ipv4/netfilter/ip_nat_proto_udp.c
@@ -8,6 +8,7 @@
 
 #include <linux/types.h>
 #include <linux/init.h>
+#include <linux/random.h>
 #include <linux/netfilter.h>
 #include <linux/ip.h>
 #include <linux/udp.h>
@@ -74,6 +75,13 @@ udp_unique_tuple(struct ip_conntrack_tuple *tuple,
 		range_size = ntohs(range->max.udp.port) - min + 1;
 	}
 
+	/* Start from random port to avoid prediction */
+	if (range->flags & IP_NAT_RANGE_PROTO_RANDOM)
+		port = secure_ipv4_port_ephemeral(tuple->src.ip, tuple->dst.ip,
+						  maniptype == IP_NAT_MANIP_SRC
+						  ? tuple->dst.u.udp.port
+						  : tuple->src.u.udp.port);
+
 	for (i = 0; i < range_size; i++, port++) {
 		*portptr = htons(min + port % range_size);
 		if (!ip_nat_used_tuple(tuple, conntrack))
diff --git a/net/ipv4/netfilter/ip_nat_rule.c b/net/ipv4/netfilter/ip_nat_rule.c
index 699eef7..5a9b52f 100644
--- a/net/ipv4/netfilter/ip_nat_rule.c
+++ b/net/ipv4/netfilter/ip_nat_rule.c
@@ -201,6 +201,10 @@ static int ipt_dnat_checkentry(const char *tablename,
 		printk("DNAT: multiple ranges no longer supported\n");
 		return 0;
 	}
+	if (mr->range[0].flags & IP_NAT_RANGE_PROTO_RANDOM) {
+		printk("DNAT: port randomization not supported\n");
+		return 0;
+	}
 	return 1;
 }