how to add http headers to a packet sniffed using scapy

Question

I am trying to sniff an out going http packet using scapy, add a few new http headers in it and send it ahead. The intention here is to only insert new headers while keeping

Answer 1

If I understand correctly, the problem you're having is that you want to update an existing HTTP request with a new header. What you want is to update a string in place, which Python can't do directly (strings are immutable).

So what you should do is take the HTTP header:

old_hdr = pkt[Raw] or old_hdr = pkt[TCP].payload

and manipulate it like a string:

new_hdr = 'New Header: value'
hdr = old_hdr.split('\r\n') # This is a crappy hack. Parsing HTTP headers
hdr.insert(new_hdr, 2)      # is a [solved problem][1].
send_hdr = '\r\n'.join(hdr)
pkt[TCP].payload = send_hdr

If you find checksums are not updating, delete them before sending the packet:

del pkt[TCP].chksum

and Scapy will put them back for you, with the right values.

Edit: I just noticed that my link is fail. Here is how to parse HTTP headers.