Add IP and UDP header to OFFER attempt

It d oesn't quite work, but it looks convincing.  UDP checksum may
need to be added, and beyond that explore changes to the payload since
it is likely not enough DHCP was provided.

confluent_server/confluent/discovery/protocols/pxe.py

@@ -32,6 +32,19 @@ import struct
 
 libc = ctypes.CDLL(ctypes.util.find_library('c'))
 
+iphdr = b'\x45\x00\x00\x00\x00\x00\x00\x00\x40\x11\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff'
+constiphdrsum = b'\x85\x11'
+udphdr = b'\x00\x43\x00\x44\x00\x00\x00\x00'
+
+def _ipsum(data):
+    currsum = 0
+    for datum in struct.unpack('!' + 'H' * (len(data) // 2), data):
+        currsum += datum
+        if currsum >> 16:
+            currsum &= 0xffff
+            currsum += 1
+    return currsum
+
 class sockaddr_ll(ctypes.Structure):
     _fields_ = [('sll_family', ctypes.c_ushort),
                 ('sll_protocol', ctypes.c_ushort),
@@ -337,6 +350,10 @@ def check_reply(node, info, packet, sock, cfg, reqview):
         return
     reply = bytearray(1024)
     repview = memoryview(reply)
+    repview[:20] = iphdr
+    repview[12:16] = myipn
+    repview[20:28] = udphdr
+    repview = repview[28:]
     repview[0] = 2
     repview[1:10] = reqview[1:10] # duplicate txid, hwlen, and others
     repview[10] = 0x80  # always set broadcast
@@ -355,6 +372,12 @@ def check_reply(node, info, packet, sock, cfg, reqview):
     repview[245:249] = myipn
     repview[249:255] = b'\x33\x04\x00\x00\x00\xf0'
     repview[255] = 0xff  # end of options, should always be last byte
+    repview = memoryview(reply)
+    pktlen = struct.pack('!H', 256 + 28)  # ip+udp = 28
+    repview[2:4] = pktlen
+    curripsum = ~(_ipsum(constiphdrsum + pktlen + myipn)) & 0xffff
+    repview[10:12] = struct.pack('!H', curripsum)
+    repview[24:26] = struct.pack('!H', 256 + 8)
     tsock = socket.socket(socket.AF_PACKET, socket.SOCK_DGRAM,
                           socket.htons(0x800))
     targ = sockaddr_ll()
@@ -365,8 +388,9 @@ def check_reply(node, info, packet, sock, cfg, reqview):
     targ.sll_halen = reqview[2]
     targ.sll_protocol = socket.htons(0x800)
     targ.sll_ifindex = info['netinfo']['ifidx']
-    pkt = ctypes.byref((ctypes.c_char * 256).from_buffer(repview))
-    sendto(tsock.fileno(), pkt, 256, 0, ctypes.byref(targ),
+    pkt = ctypes.byref((ctypes.c_char * 284).from_buffer(repview))
+
+    sendto(tsock.fileno(), pkt, 284, 0, ctypes.byref(targ),
            ctypes.sizeof(targ))
     print('Thinking about reply to {0}'.format(node))