net: add a sysctl to reflect the fwmark on replies

Kernel-originated IP packets that have no user socket associated
with them (e.g., ICMP errors and echo replies, TCP RSTs, etc.)
are emitted with a mark of zero. Add a sysctl to make them have
the same mark as the packet they are replying to.

This allows an administrator that wishes to do so to use
mark-based routing, firewalling, etc. for these replies by
marking the original packets inbound.

Tested using user-mode linux:
 - ICMP/ICMPv6 echo replies and errors.
 - TCP RST packets (IPv4 and IPv6).

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -231,6 +231,9 @@ void ipfrag_init(void);
 
 void ip_static_sysctl_init(void);
 
+#define IP4_REPLY_MARK(net, mark) \
+	((net)->ipv4.sysctl_fwmark_reflect ? (mark) : 0)
+
 static inline bool ip_is_fragment(const struct iphdr *iph)
 {
 	return (iph->frag_off & htons(IP_MF | IP_OFFSET)) != 0;

include/net/ipv6.h

@@ -113,6 +113,9 @@ struct frag_hdr {
 #define	IP6_MF		0x0001
 #define	IP6_OFFSET	0xFFF8
 
+#define IP6_REPLY_MARK(net, mark) \
+	((net)->ipv6.sysctl.fwmark_reflect ? (mark) : 0)
+
 #include <net/sock.h>
 
 /* sysctls */

include/net/netns/ipv4.h

@@ -77,6 +77,8 @@ struct netns_ipv4 {
 	int sysctl_ip_no_pmtu_disc;
 	int sysctl_ip_fwd_use_pmtu;
 
+	int sysctl_fwmark_reflect;
+
 	struct ping_group_range ping_group_range;
 
 	atomic_t dev_addr_genid;

include/net/netns/ipv6.h

@@ -30,6 +30,7 @@ struct netns_sysctl_ipv6 {
 	int flowlabel_consistency;
 	int icmpv6_time;
 	int anycast_src_echo_reply;
+	int fwmark_reflect;
 };
 
 struct netns_ipv6 {