netfilter: add SYNPROXY core/target

Add a SYNPROXY for netfilter. The code is split into two parts, the synproxy
core with common functions and an address family specific target.

The SYNPROXY receives the connection request from the client, responds with
a SYN/ACK containing a SYN cookie and announcing a zero window and checks
whether the final ACK from the client contains a valid cookie.

It then establishes a connection to the original destination and, if
successful, sends a window update to the client with the window size
announced by the server.

Support for timestamps, SACK, window scaling and MSS options can be
statically configured as target parameters if the features of the server
are known. If timestamps are used, the timestamp value sent back to
the client in the SYN/ACK will be different from the real timestamp of
the server. In order to now break PAWS, the timestamps are translated in
the direction server->client.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Tested-by: Martin Topholm <mph@one.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 77 insertions, 0 deletions

diff --git a/include/net/netfilter/nf_conntrack_synproxy.h b/include/net/netfilter/nf_conntrack_synproxy.h
new file mode 100644
index 000000000000..806f54a290d6
--- /dev/null
+++ b/include/net/netfilter/nf_conntrack_synproxy.h
@@ -0,0 +1,77 @@
+#ifndef _NF_CONNTRACK_SYNPROXY_H
+#define _NF_CONNTRACK_SYNPROXY_H
+
+#include <net/netns/generic.h>
+
+struct nf_conn_synproxy {
+	u32	isn;
+	u32	its;
+	u32	tsoff;
+};
+
+static inline struct nf_conn_synproxy *nfct_synproxy(const struct nf_conn *ct)
+{
+#if IS_ENABLED(CONFIG_NETFILTER_SYNPROXY)
+	return nf_ct_ext_find(ct, NF_CT_EXT_SYNPROXY);
+#else
+	return NULL;
+#endif
+}
+
+static inline struct nf_conn_synproxy *nfct_synproxy_ext_add(struct nf_conn *ct)
+{
+#if IS_ENABLED(CONFIG_NETFILTER_SYNPROXY)
+	return nf_ct_ext_add(ct, NF_CT_EXT_SYNPROXY, GFP_ATOMIC);
+#else
+	return NULL;
+#endif
+}
+
+struct synproxy_stats {
+	unsigned int			syn_received;
+	unsigned int			cookie_invalid;
+	unsigned int			cookie_valid;
+	unsigned int			cookie_retrans;
+	unsigned int			conn_reopened;
+};
+
+struct synproxy_net {
+	struct nf_conn			*tmpl;
+	struct synproxy_stats __percpu	*stats;
+};
+
+extern int synproxy_net_id;
+static inline struct synproxy_net *synproxy_pernet(struct net *net)
+{
+	return net_generic(net, synproxy_net_id);
+}
+
+struct synproxy_options {
+	u8				options;
+	u8				wscale;
+	u16				mss;
+	u32				tsval;
+	u32				tsecr;
+};
+
+struct tcphdr;
+struct xt_synproxy_info;
+extern void synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
+				   const struct tcphdr *th,
+				   struct synproxy_options *opts);
+extern unsigned int synproxy_options_size(const struct synproxy_options *opts);
+extern void synproxy_build_options(struct tcphdr *th,
+				   const struct synproxy_options *opts);
+
+extern void synproxy_init_timestamp_cookie(const struct xt_synproxy_info *info,
+					   struct synproxy_options *opts);
+extern void synproxy_check_timestamp_cookie(struct synproxy_options *opts);
+
+extern unsigned int synproxy_tstamp_adjust(struct sk_buff *skb,
+					   unsigned int protoff,
+					   struct tcphdr *th,
+					   struct nf_conn *ct,
+					   enum ip_conntrack_info ctinfo,
+					   const struct nf_conn_synproxy *synproxy);
+
+#endif /* _NF_CONNTRACK_SYNPROXY_H */