Adding ethernet padding to packet using scapy

Question

I need to write a tool that can take in a pcap file and rewrite all packets shorter than 60 bytes and apply the ethernet padding to them. This is to avoid having the NIC alter the number of bytes sent when I run tcpreplay.

When I try this segment of code I can see that scapy can detect a padding field in some packets in my file.

pkts = rdpcap(infile)
for pkt in pkts:
    if len(pkt) = 60:
        print pkt.show()

This returns the following output for a packet with padding.

###[ Ethernet ]###                               
  dst       = fe:ff:20:00:01:00                  
  src       = 00:00:01:00:00:00                  
  type      = 0x800                              
###[ IP ]###                                     
     version   = 4L                              
     ihl       = 5L                              
     tos       = 0x0                             
     len       = 40                              
     id        = 3929                            
     flags     = DF                              
     frag      = 0L                              
     ttl       = 128                             
     proto     = tcp                             
     chksum    = 0xa438                          
     src       = 145.254.160.237                 
     dst       = 216.239.59.99                   
     \options   \                                
###[ TCP ]###                                    
        sport     = 3371                         
        dport     = www                          
        seq       = 918692089                    
        ack       = 778787258                    
        dataofs   = 5L                           
        reserved  = 0L                           
        flags     = A                            
        window    = 8760                         
        chksum    = 0x5902                       
        urgptr    = 0                            
        options   = {}                           
###[ Padding ]###                                
           load      = '\x00\x00\x00\x00\x00\x00'

Then I tried altering my code to write to the padding field for shorter packets.

pkts = rdpcap(infile)
for pkt in pkts:
    if len(pkt) < 60:
        print pkt.show()
        pad_len = 60 - len(pkt)
        pkt[Padding].load = '\x00' * pad_len
    if len(pkt) == 60:
        print pkt.show()

However this returns an error

Traceback (most recent call last):                                              
  File "rewrite_pcap.py", line 18, in <module>                                  
    pkt[Padding].load = '\x00' * pad_len                                        
  File "/usr/lib/pymodules/python2.7/scapy/packet.py", line 758, in __getitem__ 
    raise IndexError("Layer [%s] not found" % lname)                            
IndexError: Layer [Padding] not found                                           

How can I use scapy to append some padding data to the end of a packet?

Answer 1

So this turned out to be simpler than I expected. I just needed to initialize a Padding object first then add it to my packet.

pkts = rdpcap(infile)
for pkt in pkts:
    print len(pkt)
    if len(pkt) < 60:
        pad_len = 60 - len(pkt)
        pad = Padding()
        pad.load = '\x00' * pad_len
        pkt = pkt/pad
    if len(pkt) == 60:
        print pkt.show()