A scapy script for pulling the sequence number out of a UDP multicast packet payload. Used for troubleshooting packet captures of applications which claim to be losing traffic due to missing sequence.
usage: mcsd.py [-h] [-s SOURCE] [-d DEST] [-S SPORT] [-D DPORT] [-o OFFSET]
[-l LENGTH] [-r]
filename
positional arguments:
filename pcap filename to read
optional arguments:
-h, --help show this help message and exit
-s SOURCE, --source SOURCE
filter on source IP address
-d DEST, --dest DEST filter on destination IP address
-S SPORT, --sport SPORT
filter on source port number
-D DPORT, --dport DPORT
filter on destination port number
-o OFFSET, --offset OFFSET
offset into packet to start reading
-l LENGTH, --length LENGTH
length of characters to read
-r, --raw raw mode. only print payload data
Fields displayed are:
frame src IP → dst IP sport → dport payload
A capture file with multiple streams:
$ mcsd.py mcpackets.pcap
1 10.0.0.1 → 239.0.0.1 65535 → 9001 00000001
2 10.0.0.2 → 239.0.0.2 1111 → 9999 00000002
3 10.0.0.1 → 239.0.0.1 65535 → 9001 00000003
4 10.0.0.2 → 239.0.0.2 1111 → 9999 00000004
5 10.0.0.1 → 239.0.0.1 65535 → 9001 00000005
6 10.0.0.2 → 239.0.0.2 1111 → 9999 00000006
7 10.0.0.1 → 239.0.0.1 65535 → 9001 00000007
8 10.0.0.2 → 239.0.0.2 1111 → 9999 00000008
9 10.0.0.1 → 239.0.0.1 65535 → 9001 00000009
10 10.0.0.2 → 239.0.0.2 1111 → 9999 00000010
11 10.0.0.1 → 239.0.0.1 65535 → 9001 00000011
12 10.0.0.2 → 239.0.0.2 1111 → 9999 00000012
13 10.0.0.1 → 239.0.0.1 65535 → 9001 00000013
14 10.0.0.2 → 239.0.0.2 1111 → 9999 00000014
15 10.0.0.1 → 239.0.0.1 65535 → 9001 00000015
16 10.0.0.2 → 239.0.0.2 1111 → 9999 00000016
17 10.0.0.1 → 239.0.0.1 65535 → 9001 00000017
18 10.0.0.2 → 239.0.0.2 1111 → 9999 00000018
19 10.0.0.1 → 239.0.0.1 65535 → 9001 00000019
20 10.0.0.2 → 239.0.0.2 1111 → 9999 00000020
Filtering on source IP:
$ mcsd.py -s 10.0.0.1 mcpackets.pcap
1 10.0.0.1 → 239.0.0.1 65535 → 9001 00000001
3 10.0.0.1 → 239.0.0.1 65535 → 9001 00000003
5 10.0.0.1 → 239.0.0.1 65535 → 9001 00000005
7 10.0.0.1 → 239.0.0.1 65535 → 9001 00000007
9 10.0.0.1 → 239.0.0.1 65535 → 9001 00000009
11 10.0.0.1 → 239.0.0.1 65535 → 9001 00000011
13 10.0.0.1 → 239.0.0.1 65535 → 9001 00000013
15 10.0.0.1 → 239.0.0.1 65535 → 9001 00000015
17 10.0.0.1 → 239.0.0.1 65535 → 9001 00000017
19 10.0.0.1 → 239.0.0.1 65535 → 9001 00000019
Filtering on destination port:
$ mcsd.py -D 9999 mcpackets.pcap
2 10.0.0.2 → 239.0.0.2 1111 → 9999 00000002
4 10.0.0.2 → 239.0.0.2 1111 → 9999 00000004
6 10.0.0.2 → 239.0.0.2 1111 → 9999 00000006
8 10.0.0.2 → 239.0.0.2 1111 → 9999 00000008
10 10.0.0.2 → 239.0.0.2 1111 → 9999 00000010
12 10.0.0.2 → 239.0.0.2 1111 → 9999 00000012
14 10.0.0.2 → 239.0.0.2 1111 → 9999 00000014
16 10.0.0.2 → 239.0.0.2 1111 → 9999 00000016
18 10.0.0.2 → 239.0.0.2 1111 → 9999 00000018
20 10.0.0.2 → 239.0.0.2 1111 → 9999 00000020
Raw mode, just displaying payload:
$ mcsd.py -D 9999 -r mcpackets.pcap
00000002
00000004
00000006
00000008
00000010
00000012
00000014
00000016
00000018
00000020
Reading further ahead into the packet:
$ mcsd.py -D 9999 -r -o 4 -l 4 mcpackets.pcap
0002
0004
0006
0008
0010
0012
0014
0016
0018
0020
Text processing tools can be used to iterate over printed sequence numbers and identify missing data.
The exact processing depends on the format of the packet data.
An example using awk to find missing sequence numbers which don't increase by 1, starting at 1, using the above sample data:
$ mcsd.py -D 9999 mcpackets.pcap | awk '$NF!=p+1{print p+1"-"$NF-1}{p=$NF}'
1-1
3-3
5-5
7-7
9-9
11-11
13-13
15-15
17-17
19-19
This output describes that we are missing sequence number 1, 3, 5, and so on.
- Python 3 (tested on v3.6.8)
- scapy (tested on v2.4.2)
Put mcsd.py somewhere in $PATH, eg:
mkdir -p ~/bin
wget -O ~/bin/mcsd.py https://raw.githubusercontent.com/superjamie/mcsd/master/mcsd.py
chmod +x ~/bin/mcsd.py
Jamie Bainbridge - [email protected]
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