Show Module



Display TCP flows like tcpdump. Features are connection or flow highlighting (up to 256 colors for supported terminals like rxvt-unicode-256color) or powerful matching capabilities.


											specify the number of displayed ID's
											specify if "connection" or "flow" should be colored
-m MATCH, --match=MATCH
											if statment is true the string is color in red
-s, --suppress        don't display other packets
-n, --number          number the packets


Highlight different connections or different flows. If several connections are captured then the connection options provide a quick way to see how many connections are captured and when. If one flow is captured then this is a method to see TX and RX directions of the flow. Used to highlight only packets there the provided filter evaluate to true. Filter can be: Only useful in combination with match. Instead of coloring packets red (which is the default), only matched packets are displayed. All other packets are not displayed. Show in first row the absolute packet number. This can be used to correlate one packet with the packet in wireshark or tcpdump.


Highlight different flows

$ captcp show -d flow capture.pcap

The next example and image illustrate the default options to differentiate connections.

$ captcp show capture.pcap

Matching can be really powerfull. We start with the hello world example:

$ captcp show --match True trace.pcap

This will match all packets, thus all packets are red colored. Another usefull option is --suppress: only packets are displayed where the match evalutated to true. This can be used to count special packets. As an example: to display only packets with TCP SACK (selective acknowledgements) you can filter for it and suppress all other packets

$ captcp show --match "sackblocks" --suppress trace.pcap

Highlight packet where the TCP SYN and ACK flag is set:

$ captcp show --match "syn_flag and ack_flag" trace.pcap

Show packets where the Maximum Segment Size is less then 1000:

$ captcp show --match "mss and mss < 1000" trace.pcap

It is important to note that MSS must be checked before the values can be compared. A packet where no MSS is present (the MSS option is only set at the Three Way Handshake) the mss value is False. So a plain statement like "mss < 1400" will evalulate to "False < 100" which is true in Python. Therefore the additional check is required.

Captcp Show Output on urxvt

Show all packets with SACK blocks and where the first TCP SACK block (left-edge) starts with a number less then the acknowledgenumber. Normally this is not superfluous, but sometimes DUP-ACKs are cached and retransmitted and the SACK information is not updated.

$ captcp show -m "sackblocks and len(sackblocks) > 0 and sackblocks[0][0] < ack" --suppress --number trace2.pcap

The following match keywords are valid and can be used

Keyword Description Type Example
sip Source IP address, IPv4 or IPv6 str sip == "" or sip == ""
dip Destination IP address, IPv4 or IPv6 str r'192\.168.*', dip) and r'.*\.2$', dip)
seq TCP Sequence Number int seq > 23233
ack TCP Acknowledgement number int ack > 1000
urp TCP Urgend Data pointer int urp == 1000000
sum TCP Checksum int sum != 0
sport TCP Source Port int sport == 80 and dport != 80
dport TCP Destination Port int dport in (80, 443) or dport in range(1000, 2000)
ack_flag Acknowledgement Flag bool ack_flag == False
syn_flag Synchronization Flag bool ack_flag and syn_flag
urg_flag Urgend Pointer Flag bool urg_flag == True and ack_flag
psh_flag Push Flag bool psh_flag and not ack_flag
fin_flag FIN Flag, Type bool syn_flag and fin_flag
rst_flag Reset Flag bool rst_flag and not fin_flag
ece_flag ECE (ECN) Flag bool ece_flag and cwr_flag
cwr_flag CWR Flag bool not cwr_flag
mss Maximum Segement Size (MSS) int mss < 1400
wsc Window Scaled Option int wsc == 0
tsval TCP Timestamp TSVAL value int tsval == 0 and tsecr == 0
tsecr TCP Timestamp TSECR value int tsval != 0 and tsecr != 0
sackok Selective Acknowledgements OK flag, Used during Three Way Handshake bool sackok
sackblocks If SACK is transmitted list of list sackblocks and len(sackblocks) &tg; 2