add dnsmasq and example config for it

would be good to move more of this into a module, but that
doesn't sit well with the (potential) ability to run more than one
dnsmasq service, as modules are singletons

tests/pppoe/configuration.nix

@@ -1,6 +1,6 @@
-{ config, pkgs, ... } :
+{ config, pkgs, lib, ... } :
 let
-  inherit (pkgs.liminix.networking) interface address pppoe route;
+  inherit (pkgs.liminix.networking) interface address pppoe route dnsmasq;
   inherit (pkgs.liminix.services) oneshot longrun bundle target output;
 in rec {
   services.loopback =
@@ -13,6 +13,10 @@ in rec {
       ];
     };
 
+  services.lan4 =
+    let iface = interface { type = "hardware"; device = "eth1";};
+    in address iface { family = "inet4"; address ="192.168.19.1"; prefixLength = 24;};
+
   kernel.config = {
     "IKCONFIG_PROC" = "y";
     "PPP" = "y";
@@ -53,14 +57,29 @@ in rec {
       dependencies = [iface];
     };
 
+  users.dnsmasq = {
+    uid = 51; gid= 51; gecos = "DNS/DHCP service user";
+    dir = "/run/dnsmasq";
+    shell = "/bin/false";
+  };
+  groups.dnsmasq = {
+    gid = 51; usernames = ["dnsmasq"];
+  };
+  services.dns =
+    dnsmasq {
+      interface = services.lan4;
+      ranges = ["192.168.19.10,192.168.19.253"];
+      domain = "fake.liminix.org";
+    };
+
   services.default = target {
     name = "default";
     contents = with services; [
       loopback
       defaultroute4
       packet_forwarding
+      dns
     ];
   };
-
   defaultProfile.packages = [ pkgs.hello ] ;
 }

tests/pppoe/run.sh

@@ -23,3 +23,9 @@ fi
 
 ../../scripts/run-qemu.sh --background foo.sock result/vmlinux result/squashfs
 nix-shell -p expect --run "expect getaddress.expect"
+
+set -o pipefail
+response=$(nix-shell -p python3Packages.scapy --run 'python ./test-dhcp-service.py' )
+
+echo "$response"
+echo "$response" | nix-shell -p jq --run "jq -e 'select((.router ==  \"192.168.19.1\") and (.server_id==\"192.168.19.1\"))'"

tests/pppoe/test-dhcp-service.py

@@ -0,0 +1,88 @@
+# forge packets for testing liminix and send them via the qemu udp
+# multicast socket interface
+
+MCAST_GRP = '230.0.0.1'
+MCAST_PORT = 1235
+MULTICAST_TTL = 2
+
+TIMEOUT = 10                    # seconds
+
+from warnings import filterwarnings
+filterwarnings("ignore")
+
+import random
+import binascii
+import socket
+import time
+import json
+
+from builtins import bytes, bytearray
+
+from scapy.all import Ether, IP, UDP, BOOTP, DHCP, sendp, send, raw
+
+class JSONEncoderWithBytes(json.JSONEncoder):
+    def default(self, obj):
+        if isinstance(obj, (bytes, bytearray)):
+            return obj.decode('utf-8')
+        return json.JSONEncoder.default(self, obj)
+
+
+def dhcp_option(pkt, label):
+    if pkt.haslayer(DHCP):
+        for i in pkt[DHCP].options:
+            l, v = i
+            if l == label:
+                return v
+    return None
+
+def is_dhcp_offer(pkt):
+    val = dhcp_option(pkt, 'message-type')
+    return (val == 2)
+
+
+
+def mac_to_bytes(mac_addr: str) -> bytes:
+    """ Converts a MAC address string to bytes.
+    """
+    return int(mac_addr.replace(":", ""), 16).to_bytes(6, "big")
+
+
+client_mac = "01:02:03:04:05:06"
+discover = (
+    Ether(dst="ff:ff:ff:ff:ff:ff") /
+    IP(src="0.0.0.0", dst="255.255.255.255") /
+    UDP(sport=68, dport=67) /
+    BOOTP(
+        chaddr=mac_to_bytes(client_mac),
+        xid=random.randint(1, 2**32-1),
+    ) /
+    DHCP(options=[("message-type", "discover"), "end"])
+)
+payload = raw(discover)
+
+sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
+sock.settimeout(TIMEOUT)
+sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, MULTICAST_TTL)
+
+sock.bind((MCAST_GRP, MCAST_PORT))
+host = socket.gethostbyname(socket.gethostname())
+sock.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_IF, socket.inet_aton(host))
+sock.setsockopt(socket.SOL_IP, socket.IP_ADD_MEMBERSHIP,
+                socket.inet_aton(MCAST_GRP) + socket.inet_aton(host))
+
+endtime = time.time() + TIMEOUT
+sock.sendto(payload, (MCAST_GRP, MCAST_PORT))
+
+while time.time() < endtime:
+  try:
+    data, addr = sock.recvfrom(1024)
+  except socket.error:
+    print('Exception')
+  else:
+    reply = Ether(data)
+    if is_dhcp_offer(reply):
+        opts = dict([o for o in reply[DHCP].options if type(o) is tuple])
+        print(json.dumps(opts, cls=JSONEncoderWithBytes))
+        exit(0)
+exit(1)