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_*/

openair3/NAS/COMMON/ESM/MSG/PduSessionEstablishmentAccept.c

@@ -119,6 +119,7 @@ void capture_pdu_session_establishment_accept_msg(uint8_t *buffer, uint32_t msg_
                    addr->pdu_addr_oct3,
                    addr->pdu_addr_oct4);
           nas_config(1, ip, "oaitun_ue");
+          setup_ue_ipv4_route(1, ip, "oaitun_ue");
           LOG_T(NAS, "PDU SESSION ESTABLISHMENT ACCEPT - Received UE IP: %s\n", ip);
         } else {
           curPtr += psea_msg.pdu_addr_ie.pdu_length;

openair3/NAS/NR_UE/nr_nas_msg_sim.c

@@ -846,6 +846,7 @@ void decodeDownlinkNASTransport(as_nas_info_t *initialNasMsg, uint8_t * pdu_buff
     sprintf(ip, "%d.%d.%d.%d", *(ip_p), *(ip_p + 1), *(ip_p + 2), *(ip_p + 3));
     LOG_A(NAS, "Received PDU Session Establishment Accept\n");
     nas_config(1, ip, "oaitun_ue");
+    setup_ue_ipv4_route(1, ip, "oaitun_ue");
   } else {
     LOG_E(NAS, "Received unexpected message in DLinformationTransfer %d\n", msg_type);
   }
@@ -1432,6 +1433,7 @@ void *nas_nrue(void *args_p)
                   snprintf(ip, sizeof(ip), "%d.%d.%d.%d", *(ip_p), *(ip_p + 1), *(ip_p + 2), *(ip_p + 3));
                   LOG_I(NAS, "Received PDU Session Establishment Accept, UE IP: %s\n", ip);
                   nas_config(1, ip, "oaitun_ue");
+                  setup_ue_ipv4_route(1, ip, "oaitun_ue");
                   break;
                 }
               }