Using an easier and safer way to update dhcpd configuration

main.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 
 import os
+import fileinput
 
 from pathlib import Path
 from typing import Annotated
@@ -17,11 +18,11 @@ app = FastAPI()
 
 security = HTTPBasic()
 
-PATH = '/etc/'
-# PATH = './'
-USB_DHCPD_FILENAME = 'dhcpcd-backup.conf'
-ON_FILENAME = 'dhcpcd-backup-on.conf'
-OFF_FILENAME = 'dhcpcd-backup-off.conf'
+USB_DHCPD_FILENAME = '/etc/dhcpcd-backup.conf'
+#USB_DHCPD_FILENAME = './dhcpcd-backup.conf'
+AIR_INTERFACE_NAME = 'usb0'
+METRIC_ON = 50
+METRIC_OFF = 300
 
 
 class Config:
@@ -92,8 +93,14 @@ async def set_status(username: Annotated[str, Depends(get_username)], status):
 
 
 def is_usb_on():
-    real_filename = os.path.basename(os.path.realpath(PATH + USB_DHCPD_FILENAME))
-    return real_filename == ON_FILENAME
+    with open(USB_DHCPD_FILENAME, 'r') as dhcpd_file:
+        right_interface = False
+        for line in dhcpd_file:
+            if line.startswith('interface'):
+                right_interface = line.strip().split(' ')[-1] == AIR_INTERFACE_NAME
+
+            if right_interface and 'metric' in line:
+                return line.strip().split(' ')[-1] == str(METRIC_ON)
 
 
 def switch_status(turn_on):
@@ -106,9 +113,15 @@ def switch_status(turn_on):
 
 
 def update_dhcp_conf(turn_on):
-    os.remove(PATH + USB_DHCPD_FILENAME)
-    source_file = ON_FILENAME if turn_on else OFF_FILENAME
-    os.symlink(PATH + source_file, PATH + USB_DHCPD_FILENAME)
+    right_interface = False
+    for line in fileinput.input(USB_DHCPD_FILENAME, inplace=True):
+        if line.startswith('interface'):
+            right_interface = line.strip().split(' ')[-1] == AIR_INTERFACE_NAME
+
+        if right_interface and 'metric' in line:
+            print("\tmetric {}\n".format(METRIC_ON if turn_on else METRIC_OFF), end='')
+        else:
+            print(line, end='')
 
 
 def update_usb_modem(turn_on):