Fully working implementation

generate-dhcpd-conf

@@ -0,0 +1,83 @@
+#!/usr/bin/env python3
+
+import argparse
+import ipaddress
+import json
+import re
+import subprocess
+
+from typing import List
+
+DEFAULT_ROUTE = "default"
+NS_IP_RE = re.compile(r'^nameserver\s+(\S+)$')
+RESOLV_CONF_PATH = '/etc/resolv.conf'
+
+DHCP_CONF_TEMPLATE = """
+start {host_addr}
+end   {host_addr}
+
+# avoid dhcpd complaining that we have
+# too many addresses
+maxleases 1
+
+interface {dhcp_intf}
+
+option dns    {dns}
+option router {gateway}
+option subnet {subnet}
+"""
+
+def nameservers() -> List[str]:
+    """Returns the list of nameserver IPs in resolv.conf"""
+    result = []
+    with open(RESOLV_CONF_PATH) as resolv_f:
+        for line in resolv_f:
+            match = NS_IP_RE.match(line)
+            if match:
+                result.append(match.group(1))
+    return result
+
+def default_route(routes):
+    """Returns the host's default route"""
+    for route in routes:
+        if route['dst'] == DEFAULT_ROUTE:
+            return route
+    raise ValueError('no default route')
+
+def addr_of(addrs, dev : str) -> ipaddress.IPv4Interface:
+    """Finds and returns the IP address of `dev`"""
+    for addr in addrs:
+        if addr['ifname'] != dev:
+            continue
+        if len(addr['addr_info']) != 1:
+            raise ValueError('only exactly one address on dev is supported')
+        info = addr['addr_info'][0]
+        return ipaddress.IPv4Interface((info['local'], info['prefixlen']))
+    raise ValueError('dev {0} not found'.format(dev))
+
+def generate_conf(intf_name : str) -> str:
+    """Generates a dhcpd config. `intf_name` is the interface to listen on."""
+    with subprocess.Popen(['ip', '-json', 'route'],
+                          stdout=subprocess.PIPE) as proc:
+        routes = json.load(proc.stdout) 
+    with subprocess.Popen(['ip', '-json', 'addr'],
+                          stdout=subprocess.PIPE) as proc:
+        addrs = json.load(proc.stdout)
+    
+    droute = default_route(routes)
+    host_addr = addr_of(addrs, droute['dev'])
+
+    return DHCP_CONF_TEMPLATE.format(
+        dhcp_intf = intf_name,
+        dns = ' '.join(nameservers()),
+        host_addr = host_addr.ip,
+        gateway = droute['gateway'],
+        subnet = host_addr.network.netmask,
+    )
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+    parser.add_argument('intf_name')
+    args = parser.parse_args()
+
+    print(generate_conf(args.intf_name))