I am trying to generate pcap file from a tcpdump output, how can I generate those packets having sequence number as first:last?
Here is what my tcpdump input looks like:
tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
1509471560.944080 MAC1 > MAC2, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 23237, offset 0, flags [DF], proto TCP (6), length 60)
IP1.port > IP2.port: Flags [S], cksum 0x6d2f (incorrect -> 0x0b4a), seq 1127096708, win 65535, options [mss 1460,sackOK,TS val 817985 ecr 0,nop,wscale 6], length 0
1509471561.042855 MAC2 > MAC1, ethertype IPv4 (0x0800), length 58: (tos 0x0, ttl 64, id 3107, offset 0, flags [none], proto TCP (6), length 44)
IP2.port > IP1.port: Flags [S.], cksum 0x85d8 (correct), seq 449984001, ack 1127096709, win 65535, options [mss 1460], length 0
1509471561.044008 MAC1 > MAC2, ethertype IPv4 (0x0800), length 54: (tos 0x0, ttl 64, id 23238, offset 0, flags [DF], proto TCP (6), length 40)
IP1.port > IP2.port: Flags [.], cksum 0x6d1b (incorrect -> 0x9d95), seq 1, ack 1, win 65535, length 0
1509471561.046607 MAC1 > MAC2, ethertype IPv4 (0x0800), length 191: (tos 0x0, ttl 64, id 23239, offset 0, flags [DF], proto TCP (6), length 177)
IP1.port > IP2.port: Flags [P.], cksum 0x6da4 (incorrect -> 0x98df), seq 1:138, ack 1, win 65535, length 137
1509471914.089046 MAC1 > MAC2, ethertype IPv4 (0x0800), length 82: (tos 0x0, ttl 64, id 54304, offset 0, flags [DF], proto UDP (17), length 68)
Following is the code I have prepared to process the TCP packets:
from scapy.all import *
import secrets
def generatePcapfromText(inputtxt,output):
with open (inputtxt,encoding='cp850') as input:
framenum=0
for line in input:
if line[0].isdigit(): # line one
framenum += 1
frametime=float(line[:16])
srcmac= line[18:34]
dstmac= line[38:54]
ethertype = int(line[line.find('(')+1:line.find(')')], 16)
frameLen=int(line[line.find('length')+7:line.find(': (')])
frameTos=int(line[line.find('tos')+4:line.find(', ttl')],16)
frameTtl=int(line[line.find('ttl')+4:line.find(', id')])
frameId=int(line[line.find('id')+3:line.find(', offset')])
frameOffset=line[line.find('offset')+7:line.find(', flags')]
frameFlags=line[line.find('[')+1:line.find(']')]
protocol = line[line.find('proto')+6:line.rfind('(')-1]
ipLen = int(line[line.rfind('length')+6:line.rfind(')')])
if frameFlags == "none":
frameFlags = ""
ether = Ether(dst=dstmac, src=srcmac, type=ethertype)
elif len(line)>5:
if line[5].isdigit(): # line two
srcinfo = line[4:line.find ( '>' )]
dstinfo = line[line.find ( '>' ) + 2:line.find ( ':' )]
ipsrc = srcinfo[:srcinfo.rfind ( '.' )]
ipdst = dstinfo[:dstinfo.rfind ( '.' )]
srcport = int(srcinfo[srcinfo.rfind ( '.' ) + 1:])
dstport = int(dstinfo[dstinfo.rfind ( '.' ) + 1:])
ip = ether/IP(src=ipsrc, dst=ipdst, len=frameLen, tos=frameTos, ttl=frameTtl, id=frameId, flags=frameFlags, proto=protocol.lower())
if protocol == "TCP":
frameFlag = line[line.find ( '[' ) + 1:line.find ( ']' )]
frameFlag=frameFlag.replace(".","A")
cksum = int(line[line.find ( 'cksum' ) + 6:line.find ( '(' )],16)
if ", ack" in line:
seq_n = line[line.find ( ', seq' ) + 6:line.find ( ', ack' )]
ack_n = int(line[line.find ( 'ack' ) + 4:line.find ( ', win' )])
else:
seq_n = line[line.find ( ', seq' ) + 6:line.find ( ', win' )]
ack_n = 0
if "options" in line:
win = int(line[line.find ( 'win' ) + 4:line.find ( ', options' )])
options= line[line.find ( 'options' ) + 8:line.find ( ', length' )]
else:
win = int(line[line.find ( 'win' ) + 4:line.find ( ', length' )])
options="[]"
pktlen = int(line[line.find ( ', length' ) + 9:])
if ":" in seq_n:
# ???
else:
pkt = ip / TCP(sport=srcport, dport=dstport , flags=frameFlag, seq=int(seq_n), ack=ack_n, chksum=cksum, window=win) / secrets.token_hex(pktlen)
pkt.time = frametime
wrpcap(output, pkt, append=True)
As TCP in Scapy need an integer for the sequence number I cannot pass it first:last as the sequence number, so It needs some modification which I am not familiar with as It is my first time working with Scapy. I have mark where this modification should be done via #??? in the code above.
For my purpose it is important that the packets have the same timestamp as the tcpdump input, so I have set the packet timestamp via pkt.time=timestamp.
PS: You can find history behind this question here.
