Demystifying TCP Flow Counting with Wireshark Dissector
Problem: You need to analyze network traffic and understand the number of active TCP connections (flows) at any given time.
Rephrased: Imagine you're watching a busy street. You want to know how many cars are currently moving along the road, but instead of cars, you're interested in TCP conversations happening on your network. Wireshark can help you do this, and we'll learn how to build a custom tool to count those conversations.
Scenario: You're a network administrator and need to monitor the health of your network. You suspect there might be an issue with TCP connections. To confirm, you need to be able to count the number of active TCP flows.
Solution: Wireshark provides a powerful framework called dissectors that allow you to analyze network traffic in detail. We can write a simple dissector to count the number of unique TCP flows by identifying the source and destination IP addresses and ports of each connection.
Original Code:
/*
* Wireshark dissector for counting TCP flows
*/
#include "epan/packet.h"
#include "epan/prefs.h"
#include "epan/proto.h"
#include "epan/wmem.h"
static void dissect_tcp_flow_counter(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data) {
// Extract source and destination IP addresses and ports
guint32 src_ip = tvb_get_ipv4(tvb, pinfo->net_src);
guint32 dst_ip = tvb_get_ipv4(tvb, pinfo->net_dst);
guint16 src_port = tvb_get_ntohs(tvb, pinfo->net_src);
guint16 dst_port = tvb_get_ntohs(tvb, pinfo->net_dst);
// Create a unique identifier for each flow
gchar flow_key[64];
g_snprintf(flow_key, sizeof(flow_key), "%u.%u.%u.%u:%u - %u.%u.%u.%u:%u",
((src_ip >> 24) & 0xFF), ((src_ip >> 16) & 0xFF), ((src_ip >> 8) & 0xFF), (src_ip & 0xFF), src_port,
((dst_ip >> 24) & 0xFF), ((dst_ip >> 16) & 0xFF), ((dst_ip >> 8) & 0xFF), (dst_ip & 0xFF), dst_port);
// Check if the flow has been seen before
g_hash_table_insert(data, flow_key, gpointer_to_int(1));
// Add a field to the packet info to identify the flow
pinfo->private_data = flow_key;
}
static void dissect_tcp_flow_counter_init(const char *module_name, proto_plugin *plugin, void *user_data) {
// Register the dissector with the appropriate protocol
plugin->register_dissector("tcp", dissect_tcp_flow_counter, plugin);
// Initialize a hash table to store flow identifiers
plugin->user_data = g_hash_table_new(g_str_hash, g_str_equal);
}
static void dissect_tcp_flow_counter_exit(const char *module_name, proto_plugin *plugin, void *user_data) {
// Clean up the hash table
g_hash_table_destroy((GHashTable *)plugin->user_data);
}
static proto_plugin tcp_flow_counter_plugin = {
.name = "tcp_flow_counter",
.init = dissect_tcp_flow_counter_init,
.exit = dissect_tcp_flow_counter_exit
};
void plugin_register_tcp_flow_counter(void) {
register_plugin(&tcp_flow_counter_plugin);
}
Explanation:
-
Dissector Function (
dissect_tcp_flow_counter):- This function is called for each TCP packet captured by Wireshark.
- It extracts source and destination IP addresses and ports from the packet.
- It then generates a unique identifier for the TCP flow by combining the source and destination IP addresses and ports.
- This identifier is stored in a hash table to keep track of unique flows.
- Finally, it sets the flow identifier as the
private_datafield of the packet info for future reference.
-
Initialization Function (
dissect_tcp_flow_counter_init):- This function is called when the plugin is loaded.
- It registers the dissector with the "tcp" protocol. This means that the
dissect_tcp_flow_counterfunction will be called whenever Wireshark encounters a TCP packet. - It initializes a hash table to store unique flow identifiers.
-
Exit Function (
dissect_tcp_flow_counter_exit):- This function is called when the plugin is unloaded.
- It cleans up the hash table by destroying it.
-
Plugin Registration:
- This code registers the plugin with Wireshark.
Key Insights:
- Dissectors are powerful: Wireshark's dissector framework allows you to manipulate and analyze network packets at a granular level.
- Hash tables for efficiency: Using a hash table to store flow identifiers allows for fast lookups and ensures that we only count a flow once.
- Flexibility: This dissector can be easily modified to count other types of network flows, like UDP or even custom protocols.
Benefits for Readers:
- Real-world application: Learn how to build a practical tool for analyzing network traffic.
- Code example: Provides a working code snippet that you can modify and adapt to your needs.
- Detailed explanation: Covers the core concepts of Wireshark dissectors and the rationale behind the code.
Additional Value:
- Debugging tools: You can extend this dissector to provide even more insights, such as tracking the number of packets in each flow or the duration of each flow.
- Performance analysis: The number of TCP flows can be a good indicator of network health. A sudden spike in flow count might indicate an attack or a network malfunction.
- Customizing your network analysis: You can use this dissector as a starting point to develop your own specialized network analysis tools.
References:
Conclusion:
This article demonstrates how to write a Wireshark dissector to count the number of TCP flows in a network capture. By understanding the basic principles of dissector development and utilizing the power of hash tables, you can create valuable tools for network analysis and monitoring.
