conntrackd: cthelper: Add new SLP helper

Service Location Protocol (SLP) uses multicast requests for DA (Directory
agent) and SA (Service agent) discovery. Replies to these requests are
unicast and their source address does not match destination address of the
request so that we need a conntrack helper. A kernel helper was submitted
back in 2013 but was rejected as userspace helper infrastructure is
preferred. This adds an SLP helper to conntrackd.

As the function of SLP helper is the same as what existing mDNS helper
does, src/helpers/slp.c is essentially just a copy of src/helpers/mdns.c,
except for the default timeout and example usage. As with mDNS helper,
there is no NAT support for the time being as that would probably require
kernel side changes and certainly further study (and could possibly work
only for source NAT).

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

2 files changed, 92 insertions, 0 deletions

diff --git a/src/helpers/Makefile.am b/src/helpers/Makefile.am
index 51e2841..58c9ad0 100644
--- a/src/helpers/Makefile.am
+++ b/src/helpers/Makefile.am
@@ -8,6 +8,7 @@ pkglib_LTLIBRARIES = ct_helper_amanda.la \
 		     ct_helper_tftp.la	\
 		     ct_helper_tns.la	\
 		     ct_helper_sane.la	\
+		     ct_helper_slp.la	\
 		     ct_helper_ssdp.la
 
 HELPER_LDFLAGS = -avoid-version -module $(LIBNETFILTER_CONNTRACK_LIBS) @LAZY_LDFLAGS@
@@ -45,6 +46,10 @@ ct_helper_sane_la_SOURCES = sane.c
 ct_helper_sane_la_LDFLAGS = $(HELPER_LDFLAGS)
 ct_helper_sane_la_CFLAGS = $(HELPER_CFLAGS)
 
+ct_helper_slp_la_SOURCES = slp.c
+ct_helper_slp_la_LDFLAGS = $(HELPER_LDFLAGS)
+ct_helper_slp_la_CFLAGS = $(HELPER_CFLAGS)
+
 ct_helper_ssdp_la_SOURCES = ssdp.c
 ct_helper_ssdp_la_LDFLAGS = $(HELPER_LDFLAGS)
 ct_helper_ssdp_la_CFLAGS = $(HELPER_CFLAGS)
diff --git a/src/helpers/slp.c b/src/helpers/slp.c
new file mode 100644
index 0000000..b8339d6
--- /dev/null
+++ b/src/helpers/slp.c
@@ -0,0 +1,87 @@
+/*
+ * This helper creates and expectation to allow unicast replies to multicast
+ * requests (RFC2608 section 6.1). While the destination address of the
+ * outcoming request is known, the reply can come from any unicast address so
+ * that we need to allow replies from any source address. Default expectation]
+ * timeout is set one second longer than default CONFIG_MC_MAX from RFC2608
+ * section 13.
+ *
+ * Example usage:
+ *
+ *     nfct add helper slp inet udp
+ *     iptables -t raw -A OUTPUT -m addrtype --dst-type MULTICAST \
+ *         -p udp --dport 427 -j CT --helper slp
+ *     iptables -t raw -A OUTPUT -m addrtype --dst-type BROADCAST \
+ *         -p udp --dport 427 -j CT --helper slp
+ *     iptables -t filter -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED \
+ *         -j ACCEPT
+ *
+ * Requires Linux 3.12 or higher. NAT is unsupported.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include "conntrackd.h"
+#include "helper.h"
+#include "myct.h"
+#include "log.h"
+
+#include <libnetfilter_conntrack/libnetfilter_conntrack.h>
+#include <linux/netfilter.h>
+
+static int slp_helper_cb(struct pkt_buff *pkt, uint32_t protoff,
+			 struct myct *myct, uint32_t ctinfo)
+{
+	struct nf_expect *exp;
+	int dir = CTINFO2DIR(ctinfo);
+	union nfct_attr_grp_addr saddr;
+	uint16_t sport, dport;
+
+	exp = nfexp_new();
+	if (!exp) {
+		pr_debug("conntrack_slp: failed to allocate expectation\n");
+		return NF_ACCEPT;
+	}
+
+	cthelper_get_addr_src(myct->ct, dir, &saddr);
+	cthelper_get_port_src(myct->ct, dir, &sport);
+	cthelper_get_port_src(myct->ct, !dir, &dport);
+
+	if (cthelper_expect_init(exp,
+				 myct->ct,
+				 0 /* class */,
+				 NULL /* saddr */,
+				 &saddr /* daddr */,
+				 IPPROTO_UDP,
+				 &dport /* sport */,
+				 &sport /* dport */,
+				 NF_CT_EXPECT_PERMANENT)) {
+		pr_debug("conntrack_slp: failed to init expectation\n");
+		nfexp_destroy(exp);
+		return NF_ACCEPT;
+	}
+
+	myct->exp = exp;
+	return NF_ACCEPT;
+}
+
+static struct ctd_helper slp_helper = {
+	.name		= "slp",
+	.l4proto	= IPPROTO_UDP,
+	.priv_data_len	= 0,
+	.cb		= slp_helper_cb,
+	.policy		= {
+		[0] = {
+			.name		= "slp",
+			.expect_max	= 8,
+			.expect_timeout	= 16, /* default CONFIG_MC_MAX + 1 */
+		},
+	},
+};
+
+static void __attribute__ ((constructor)) slp_init(void)
+{
+	helper_register(&slp_helper);
+}