netfilter: add user-space connection tracking helper infrastructure

There are good reasons to supports helpers in user-space instead:

* Rapid connection tracking helper development, as developing code
  in user-space is usually faster.

* Reliability: A buggy helper does not crash the kernel. Moreover,
  we can monitor the helper process and restart it in case of problems.

* Security: Avoid complex string matching and mangling in kernel-space
  running in privileged mode. Going further, we can even think about
  running user-space helpers as a non-root process.

* Extensibility: It allows the development of very specific helpers (most
  likely non-standard proprietary protocols) that are very likely not to be
  accepted for mainline inclusion in the form of kernel-space connection
  tracking helpers.

This patch adds the infrastructure to allow the implementation of
user-space conntrack helpers by means of the new nfnetlink subsystem
`nfnetlink_cthelper' and the existing queueing infrastructure
(nfnetlink_queue).

I had to add the new hook NF_IP6_PRI_CONNTRACK_HELPER to register
ipv[4|6]_helper which results from splitting ipv[4|6]_confirm into
two pieces. This change is required not to break NAT sequence
adjustment and conntrack confirmation for traffic that is enqueued
to our user-space conntrack helpers.

Basic operation, in a few steps:

1) Register user-space helper by means of `nfct':

 nfct helper add ftp inet tcp

 [ It must be a valid existing helper supported by conntrack-tools ]

2) Add rules to enable the FTP user-space helper which is
   used to track traffic going to TCP port 21.

For locally generated packets:

 iptables -I OUTPUT -t raw -p tcp --dport 21 -j CT --helper ftp

For non-locally generated packets:

 iptables -I PREROUTING -t raw -p tcp --dport 21 -j CT --helper ftp

3) Run the test conntrackd in helper mode (see example files under
   doc/helper/conntrackd.conf

 conntrackd

4) Generate FTP traffic going, if everything is OK, then conntrackd
   should create expectations (you can check that with `conntrack':

 conntrack -E expect

    [NEW] 301 proto=6 src=192.168.1.136 dst=130.89.148.12 sport=0 dport=54037 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=192.168.1.136 master-dst=130.89.148.12 sport=57127 dport=21 class=0 helper=ftp
[DESTROY] 301 proto=6 src=192.168.1.136 dst=130.89.148.12 sport=0 dport=54037 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=192.168.1.136 master-dst=130.89.148.12 sport=57127 dport=21 class=0 helper=ftp

This confirms that our test helper is receiving packets including the
conntrack information, and adding expectations in kernel-space.

The user-space helper can also store its private tracking information
in the conntrack structure in the kernel via the CTA_HELP_INFO. The
kernel will consider this a binary blob whose layout is unknown. This
information will be included in the information that is transfered
to user-space via glue code that integrates nfnetlink_queue and
ctnetlink.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

6 files changed, 71 insertions, 1 deletions

diff --git a/include/linux/netfilter/Kbuild b/include/linux/netfilter/Kbuild
index 1697036336b..874ae8f2706 100644
--- a/include/linux/netfilter/Kbuild
+++ b/include/linux/netfilter/Kbuild
@@ -10,6 +10,7 @@ header-y += nfnetlink.h
 header-y += nfnetlink_acct.h
 header-y += nfnetlink_compat.h
 header-y += nfnetlink_conntrack.h
+header-y += nfnetlink_cthelper.h
 header-y += nfnetlink_cttimeout.h
 header-y += nfnetlink_log.h
 header-y += nfnetlink_queue.h
diff --git a/include/linux/netfilter/nfnetlink.h b/include/linux/netfilter/nfnetlink.h
index a1048c1587d..18341cdb244 100644
--- a/include/linux/netfilter/nfnetlink.h
+++ b/include/linux/netfilter/nfnetlink.h
@@ -50,7 +50,8 @@ struct nfgenmsg {
 #define NFNL_SUBSYS_IPSET		6
 #define NFNL_SUBSYS_ACCT		7
 #define NFNL_SUBSYS_CTNETLINK_TIMEOUT	8
-#define NFNL_SUBSYS_COUNT		9
+#define NFNL_SUBSYS_CTHELPER		9
+#define NFNL_SUBSYS_COUNT		10
 
 #ifdef __KERNEL__
 
diff --git a/include/linux/netfilter/nfnetlink_cthelper.h b/include/linux/netfilter/nfnetlink_cthelper.h
new file mode 100644
index 00000000000..33659f6fad3
--- /dev/null
+++ b/include/linux/netfilter/nfnetlink_cthelper.h
@@ -0,0 +1,55 @@
+#ifndef _NFNL_CTHELPER_H_
+#define _NFNL_CTHELPER_H_
+
+#define NFCT_HELPER_STATUS_DISABLED	0
+#define NFCT_HELPER_STATUS_ENABLED	1
+
+enum nfnl_acct_msg_types {
+	NFNL_MSG_CTHELPER_NEW,
+	NFNL_MSG_CTHELPER_GET,
+	NFNL_MSG_CTHELPER_DEL,
+	NFNL_MSG_CTHELPER_MAX
+};
+
+enum nfnl_cthelper_type {
+	NFCTH_UNSPEC,
+	NFCTH_NAME,
+	NFCTH_TUPLE,
+	NFCTH_QUEUE_NUM,
+	NFCTH_POLICY,
+	NFCTH_PRIV_DATA_LEN,
+	NFCTH_STATUS,
+	__NFCTH_MAX
+};
+#define NFCTH_MAX (__NFCTH_MAX - 1)
+
+enum nfnl_cthelper_policy_type {
+	NFCTH_POLICY_SET_UNSPEC,
+	NFCTH_POLICY_SET_NUM,
+	NFCTH_POLICY_SET,
+	NFCTH_POLICY_SET1	= NFCTH_POLICY_SET,
+	NFCTH_POLICY_SET2,
+	NFCTH_POLICY_SET3,
+	NFCTH_POLICY_SET4,
+	__NFCTH_POLICY_SET_MAX
+};
+#define NFCTH_POLICY_SET_MAX (__NFCTH_POLICY_SET_MAX - 1)
+
+enum nfnl_cthelper_pol_type {
+	NFCTH_POLICY_UNSPEC,
+	NFCTH_POLICY_NAME,
+	NFCTH_POLICY_EXPECT_MAX,
+	NFCTH_POLICY_EXPECT_TIMEOUT,
+	__NFCTH_POLICY_MAX
+};
+#define NFCTH_POLICY_MAX (__NFCTH_POLICY_MAX - 1)
+
+enum nfnl_cthelper_tuple_type {
+	NFCTH_TUPLE_UNSPEC,
+	NFCTH_TUPLE_L3PROTONUM,
+	NFCTH_TUPLE_L4PROTONUM,
+	__NFCTH_TUPLE_MAX,
+};
+#define NFCTH_TUPLE_MAX (__NFCTH_TUPLE_MAX - 1)
+
+#endif /* _NFNL_CTHELPER_H */
diff --git a/include/linux/netfilter_ipv4.h b/include/linux/netfilter_ipv4.h
index fa0946c549d..e2b12801378 100644
--- a/include/linux/netfilter_ipv4.h
+++ b/include/linux/netfilter_ipv4.h
@@ -66,6 +66,7 @@ enum nf_ip_hook_priorities {
 	NF_IP_PRI_SECURITY = 50,
 	NF_IP_PRI_NAT_SRC = 100,
 	NF_IP_PRI_SELINUX_LAST = 225,
+	NF_IP_PRI_CONNTRACK_HELPER = 300,
 	NF_IP_PRI_CONNTRACK_CONFIRM = INT_MAX,
 	NF_IP_PRI_LAST = INT_MAX,
 };
diff --git a/include/linux/netfilter_ipv6.h b/include/linux/netfilter_ipv6.h
index 57c025127f1..7c8a513ce7a 100644
--- a/include/linux/netfilter_ipv6.h
+++ b/include/linux/netfilter_ipv6.h
@@ -71,6 +71,7 @@ enum nf_ip6_hook_priorities {
 	NF_IP6_PRI_SECURITY = 50,
 	NF_IP6_PRI_NAT_SRC = 100,
 	NF_IP6_PRI_SELINUX_LAST = 225,
+	NF_IP6_PRI_CONNTRACK_HELPER = 300,
 	NF_IP6_PRI_LAST = INT_MAX,
 };
 
diff --git a/include/net/netfilter/nf_conntrack_helper.h b/include/net/netfilter/nf_conntrack_helper.h
index 84b24c3a383..9aad956d100 100644
--- a/include/net/netfilter/nf_conntrack_helper.h
+++ b/include/net/netfilter/nf_conntrack_helper.h
@@ -15,6 +15,11 @@
 
 struct module;
 
+enum nf_ct_helper_flags {
+	NF_CT_HELPER_F_USERSPACE	= (1 << 0),
+	NF_CT_HELPER_F_CONFIGURED	= (1 << 1),
+};
+
 #define NF_CT_HELPER_NAME_LEN	16
 
 struct nf_conntrack_helper {
@@ -42,6 +47,9 @@ struct nf_conntrack_helper {
 	int (*from_nlattr)(struct nlattr *attr, struct nf_conn *ct);
 	int (*to_nlattr)(struct sk_buff *skb, const struct nf_conn *ct);
 	unsigned int expect_class_max;
+
+	unsigned int flags;
+	unsigned int queue_num;		/* For user-space helpers. */
 };
 
 extern struct nf_conntrack_helper *
@@ -96,4 +104,7 @@ nf_ct_helper_expectfn_find_by_name(const char *name);
 struct nf_ct_helper_expectfn *
 nf_ct_helper_expectfn_find_by_symbol(const void *symbol);
 
+extern struct hlist_head *nf_ct_helper_hash;
+extern unsigned int nf_ct_helper_hsize;
+
 #endif /*_NF_CONNTRACK_HELPER_H*/