Reintroduce the IP rule&route for OAI UE through setup_ue_ipv4_route()

These rules & route are necessary to properly send packets on oaitun_ue1
interface:

1. This forces the packets coming from a subnet different than the UE's
   subnet to go back through oaitun_ue1 rather than via what the default
   route defined on the system (e.g., important if ping from the
   internet arrives).

2. On machines setting net.ipv4.conf.oaitun_ue1.rp_filter=1 (e.g.
   RHEL), this prevents that source filtering for packets coming back is
   applied and those packets be dropped. By default, many hosts have
   rp_filter=2, so no strict source filtering is applied, and it would
   work; on others, this rule prevents source filter dropping.

openair2/RRC/NAS/nas_config.c

@@ -29,6 +29,7 @@
 
 #include "nas_config.h"
 #include "common/utils/LOG/log.h"
+#include "common/utils/system.h"
 
 /*
  * \brief set a genneric interface parameter
@@ -107,3 +108,29 @@ bool nas_config(int interface_id, const char *ip, const char *ifpref)
   close(sock_fd);
   return success;
 }
+
+void setup_ue_ipv4_route(int interface_id, const char *ipv4, const char *ifpref)
+{
+  int table_id = interface_id - 1 + 10000;
+  char interfaceName[IFNAMSIZ];
+  snprintf(interfaceName, sizeof(interfaceName), "%s%d", ifpref, interface_id);
+
+  char command_line[500];
+  int res = sprintf(command_line,
+                    "ip rule add from %s/32 table %d && "
+                    "ip rule add to %s/32 table %d && "
+                    "ip route add default dev %s table %d",
+                    ipv4,
+                    table_id,
+                    ipv4,
+                    table_id,
+                    interfaceName,
+                    table_id);
+
+  if (res < 0) {
+    LOG_E(UTIL, "Could not create ip rule/route commands string\n");
+    return;
+  }
+  background_system(command_line);
+}
+

openair2/RRC/NAS/nas_config.h

@@ -40,4 +40,18 @@
  */
 bool nas_config(int interface_id, const char *ip, const char *ifprefix);
 
+/*!
+ * \brief Setup a IPv4 rule in table (interface_id - 1 + 10000) and route to
+ * force packets coming into interface back through it, and workaround
+ * net.ipv4.conf.all.rp_filter=2 (strict source filtering would filter out
+ * responses of packets going out through interface to another IP address not
+ * in same subnet).
+ * \param[in] interface_id number of this interface, prepended after interface
+ * name
+ * \param[in] ipv4 IPv4 address of the UE
+ * \param[in] ifprefix interface name prefix to which an interface number will
+ * be appended
+ */
+void setup_ue_ipv4_route(int interface_id, const char *ipv4, const char *ifpref);
+
 #endif /*NAS_CONFIG_H_*/