Implemented external IP resolution for interface discovery announcer

RNS/Discovery.py

@@ -2,6 +2,7 @@ import os
 import RNS
 import time
 import threading
+import subprocess
 from .vendor import umsgpack as msgpack
 
 NAME            = 0xFF
@@ -102,8 +103,24 @@ class InterfaceAnnouncer():
                      LONGITUDE:      interface.discovery_longitude,
                      HEIGHT:         interface.discovery_height}
 
+            reachable_on = self.sanitize(interface.reachable_on)
+            if not RNS.vendor.platformutils.is_windows():
+                try:
+                    exec_path = os.path.expanduser(reachable_on)
+                    if os.path.isfile(exec_path) and os.access(exec_path, os.X_OK):
+                        RNS.log(f"Evaluating reachable_on from executable at {exec_path}", RNS.LOG_DEBUG)
+                        exec_result = subprocess.run([exec_path], stdout=subprocess.PIPE)
+                        exec_stdout = exec_result.stdout.decode("utf-8")
+                        if exec_result.returncode != 0: raise ValueError("Non-zero exit code from subprocess")
+                        reachable_on = self.sanitize(exec_stdout)
+
+                except Exception as e:
+                    RNS.log(f"Error while getting reachable_on from executable at {interface.reachable_on}: {e}", RNS.LOG_ERROR)
+                    RNS.log(f"Aborting discovery announce", RNS.LOG_ERROR)
+                    return None
+
             if interface_type in ["BackboneInterface", "TCPServerInterface"]:
-                info[REACHABLE_ON]    = self.sanitize(interface.reachable_on)
+                info[REACHABLE_ON]    = reachable_on
                 info[PORT]            = interface.bind_port
 
             if interface_type == "I2PInterface" and interface.connectable and interface.b32: