Extract Ethernet, IP header, TCP and payload from socket recv Python

Question

How can I extract Extract Ethernet, IP header, TCP and payload from socket.recv in Python Right now I can obtain information above using socket.recvfrom():

    packet = s.recvfrom(NETWORK_MAX_SIZE)
    packet = packet[0] 
    #parse ethernet header
    eth_length = 14
    eth_header = packet[:eth_length]
    eth = unpack('!6s6sH' , eth_header)
    eth_protocol = socket.ntohs(eth[2])
    t = iph_length + eth_length
    tcp_header = packet[t:t+20]
    #now unpack them :)
    tcph = unpack('!HHLLBBHHH' , tcp_header)
    source_port = tcph[0]
    dest_port = tcph[1]
    sequence = tcph[2]
    acknowledgement = tcph[3]
    doff_reserved = tcph[4]
    tcph_length = doff_reserved >> 4
    h_size = eth_length + iph_length + tcph_length * 4
    data_size = len(packet) - h_size
    #get data from the packet
    data = packet[h_size:]

Reference: http://www.binarytides.com/python-packet-sniffer-code-linux/

When I use same function by a fragmented TCP packet and call socket.recv() I get errors when unpacking tcpheader.

Thanks

The TCP header isn't always 20 bytes- you have to inspect the header for the data offset value to know where the header ends- FYI — tMC
– tMC, Commented Nov 13, 2013 at 18:30

gogasca · Accepted Answer · 2013-11-13 22:54:07Z

2

Thanks I recognized that socket.recv() returns a type str and socket.recvfrom() returns a tuple type, hence for socket.recv() I omitted packet = packet[0]. I will update code to handle TCP header to be variable.

answered Nov 13, 2013 at 22:54

gogasca

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