Master tcpdump: Capture, Filter, and Analyze Network Traffic Like a Pro

Introduction

tcpdump is a powerful network packet capture tool that can dump traffic on a network, allowing you to intercept and analyze data packets. Mastering tcpdump helps you troubleshoot packet loss, retransmissions, and database link calls.

Using tcpdump

Syntax

<code>Usage: tcpdump [-aAdDefhIJKlLnNOpqRStuUvxX] [ -B size ] [ -c count ]
        [ -C file_size ] [ -E algo:secret ] [ -F file ] [ -G seconds ]
        [ -i interface ] [ -j tstamptype ] [ -M secret ]
        [ -Q|-P in|out|inout ]
        [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
        [ -W filecount ] [ -y datalinktype ] [ -z command ]
        [ -Z user ] [ expression ]</code>

The most important part is the expression , which filters traffic by type, direction, or protocol.

Three expression types are supported:

Type : host, net, port (default is host)

Direction : src, dst and their combinations

Proto : tcp, udp, icmp, arp, etc.

Common expressions can be combined with parentheses (escaped when needed).

Parameters

Example command:

tcpdump tcp -i bond0 -tttt -s 0 -c 100 and dst port ! 22 and src net 10.10.1.0/24 -w 20190131.tcpdump

Explanation:

<code>(1) tcp: protocol filter
(2) -i bond0: capture on interface bond0
(3) -tttt: timestamp format "YYYY-MM-DD HH:MM:SS.ssssss"
(4) -s 0: capture full packet length
(5) -c 100: stop after 100 packets
(6) dst port ! 22: exclude destination port 22
(7) src net 10.10.1.0/24: source network filter
(8) -w 20190131.tcpdump: write output to file for later analysis</code>

Other useful parameters:

<code>-D: list all interfaces
-n: disable name resolution (show IPs only)
-X: display packet contents in hex and ASCII
-A: display packet contents as ASCII
-v, -vv, -vvv: increase verbosity
</code>

Packet Analysis

Capture command example:

tcpdump -i bond0 -n -s 0 port 3312 -c 10 -tttt

Key fields in the output:

Timestamp (e.g., 2019-02-02 10:46:13.447563)

Protocol name (IP, TCP, etc.)

Source and destination IP/port (e.g., 10.215.20.13.appman-server > 10.9.51.13.25569)

Flags: S=SYN, P=PUSH, F=FIN, .=no flag, etc.

Sequence and acknowledgment numbers

Window size (controls flow control)

Length (payload size)

Case Study: Analyzing MySQL Interaction

Collect packets with:

<code># tcpdump -i bond0 -n -s 0 port 3320 -tttt -A -w /tmp/20190202.tcpdump
tcpdump: listening on bond0, link-type EN10MB (Ethernet), capture size 65535 bytes
32 packets captured
36 packets received by filter
0 packets dropped by kernel</code>

Open the resulting file in Wireshark, filter by the MySQL protocol, and inspect SQL statements such as SELECT and SHOW TABLES . By comparing timestamps of relevant packets (e.g., packets 26 and 27), you can calculate query execution time for performance analysis.

Recommended Reading

Official tcpdump website

tcpdump tutorial on Zhihu

Tcpdump in database practice

Learning tcpdump on Linux