Add support for connection tracking helper/ALGs.

This patch adds support for specifying a "helper" or ALG to assist
connection tracking for protocols that consist of multiple streams.
Initially, only support for FTP is included.

Below is an example set of flows to allow FTP control connections from
port 1->2 to establish active data connections in the reverse direction:

    table=0,priority=1,action=drop
    table=0,arp,action=normal
    table=0,in_port=1,tcp,action=ct(alg=ftp,commit),2
    table=0,in_port=2,tcp,ct_state=-trk,action=ct(table=1)
    table=1,in_port=2,tcp,ct_state=+trk+est,action=1
    table=1,in_port=2,tcp,ct_state=+trk+rel,action=ct(commit),1

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

lib/ofp-parse.c

@@ -21,6 +21,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <stdlib.h>
+#include <netinet/in.h>
 
 #include "byte-order.h"
 #include "dynamic-string.h"
@@ -168,6 +169,20 @@ str_to_ip(const char *str, ovs_be32 *ip)
     return NULL;
 }
 
+/* Parses 'str' as a conntrack helper into 'alg'.
+ *
+ * Returns NULL if successful, otherwise a malloc()'d string describing the
+ * error.  The caller is responsible for freeing the returned string. */
+char * OVS_WARN_UNUSED_RESULT
+str_to_connhelper(const char *str, uint16_t *alg)
+{
+    if (!strcmp(str, "ftp")) {
+        *alg = IPPORT_FTP;
+        return NULL;
+    }
+    return xasprintf("invalid conntrack helper \"%s\"", str);
+}
+
 struct protocol {
     const char *name;
     uint16_t dl_type;