use random ID in IP header

ip_packet.nim

@@ -1,6 +1,7 @@
 from nativesockets import ntohs, ntohl, htons, htonl
 from net import IpAddress, IpAddressFamily, Port, `$`
 from posix import InAddr, inet_ntoa
+from random import randomize, rand
 import unittest
 
 type
@@ -134,6 +135,7 @@ proc tcpChecksum(buffer: string): uint16 =
   result = not checksum.uint16
 
 proc serialize*(packet: IpPacket): string =
+  randomize()
   case packet.protocol
   of tcp:
     result = newString(sizeof(Ip) + sizeof(Tcphdr))
@@ -146,7 +148,7 @@ proc serialize*(packet: IpPacket): string =
     ipHeader.ip_v = 4
     ipHeader.ip_tos = 0
     ipHeader.ip_len = htons(sizeof(Ip).uint16 + sizeof(TcpHdr).uint16)
-    ipHeader.ip_id = htons(54321) # FIXME = random number
+    ipHeader.ip_id = rand(cushort)
     ipHeader.ip_off = 0
     ipHeader.ip_ttl = packet.ipTTL
     ipHeader.ip_p = 6.cuchar