title: Low Performance with Scapy
body: <p>I'm creating a script that sends to Eth0 all traffic from Tap0, and sends to Tap0 all traffic from Eth0. After finding many examples online, i managed to make it work. The issue i have is that the performance is very low.</p>
  
  <p>Pinging between 2 VMs without using the script, takes less than 1ms. With the script it takes ~15ms.</p>
  
  <p>When i send a 10 MB file from a VM to another using scp, the avg. transfer rate is 12 Mbps without the script. With the script it goes down to less than 1 Mbps.</p>
  
  <p>I know that Python is not actually the fastest language to deal with network traffic, but is it that slow?</p>
  
  <p>Is there a way to optimize this code?</p>
  
  <p>My VMs are Ubuntu 10.04 32 bits.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>import os,sys,getopt,struct,re,string,logging
  
  from socket import *
  from fcntl  import ioctl
  from select import select
  
  from scapy.all import *
  
  TUNSETIFF = 0x400454ca
  IFF_TAP   = 0x0002
  TUNMODE   = IFF_TAP
  
  ETH_IFACE  = "eth0"
  TAP_IFACE = "tap0"
  
  conf.iface = ETH_IFACE
  
  # Here we capture frames on ETH0
  s = conf.L2listen(iface = ETH_IFACE)
  
  # Open /dev/net/tun in TAP (ether) mode (create TAP0)
  f = os.open("/dev/net/tun", os.O_RDWR)
  ifs = ioctl(f, TUNSETIFF, struct.pack("16sH", "tap%d", TUNMODE))
  
  
  # Speed optimization so Scapy does not have to parse payloads
  Ether.payload_guess=[]
  
  os.system("ifconfig eth0 0.0.0.0")
  os.system("ifconfig tap0 192.168.40.107")
  os.system("ifconfig tap0 down")
  os.system("ifconfig tap0 hw ether 00:0c:29:7a:52:c4")
  os.system("ifconfig tap0 up")
  
  eth_hwaddr = get_if_hwaddr('eth0')
  
  while 1:
   r = select([f,s],[],[])[0] #Monitor f(TAP0) and s(ETH0) at the same time to see if a frame came in.
  
   #Frames from TAP0
   if f in r:  #If TAP0 received a frame
    # tuntap frame max. size is 1522 (ethernet, see RFC3580) + 4
    tap_frame = os.read(f,1526)
    tap_rcvd_frame = Ether(tap_frame[4:]) 
    sendp(tap_rcvd_frame,verbose=0) #Send frame to ETH0
  
   #Frames from ETH0
   if s in r: #If ETH0 received a frame   
    eth_frame = s.recv(1522)
    if eth_frame.src != eth_hwaddr:           
     # Add Tun/Tap header to frame, convert to string and send. "\x00\x00\x00\x00" is a requirement when writing to tap interfaces. It is an identifier for the Kernel.
     eth_sent_frame = "\x00\x00\x00\x00" + str(eth_frame)     
     os.write(f, eth_sent_frame) #Send frame to TAP0
  </code></pre>