What is Mergecap and What Does it Do?

Mergecap is a command-line tool used to merge multiple packet capture files (e.g., .pcap, .pcapng) into a single file. It is often used in scenarios such as network traffic analysis, security research, and network troubleshooting. By bringing together packets captured at different times or in different network segments, it offers a more comprehensive analysis capability.

Key Points:

Mergecap is part of the Wireshark packet analysis software.
It can merge multiple capture files in a sequential manner.
It can convert different file formats (e.g., .pcap to .pcapng).
You can select the packets to be merged with filtering options.

In What Situations is Mergecap Used?

Mergecap has a wide range of uses. Here are some common scenarios:

Distributed Network Monitoring: If you are running multiple packet capture tools (e.g., tcpdump, Wireshark) at different points in your network, you can use Mergecap to merge these captures.
Network Analysis Over Time: You can merge packets captured at different times to analyze network events that occurred over a specific period.
Security Incident Investigation: When investigating a suspicious security incident, combining packet capture data from different sources can help you understand the scope of the incident.
Network Performance Troubleshooting: To diagnose network performance issues, you can merge packets captured from different network segments to identify problematic points.
Data Backup and Archiving: If you regularly back up packet capture files, you can merge these files to create a more manageable archive.

Real-Life Example: A company wants to analyze network traffic in different branches. Packet capture is performed with Wireshark in each branch. Later, these captured files are merged with Mergecap to perform a central analysis and identify potential security vulnerabilities or performance issues in the network.

How to Use Mergecap? Basic Commands and Options

Mergecap is quite simple to use. The basic command structure is as follows:

mergecap [options] -w <output_file> <input_files>

Here:

[options]: These are various options that change the behavior of Mergecap (e.g., specifying the file format, applying a filter).
-w <output_file>: Specifies the name of the output file to which the merged packets will be written.
<input_files>: This is the list of packet capture files to be merged. Multiple files can be specified.

Commonly Used Options:

Option	Description
`-F <file_type>`	Specifies the format of the output file (e.g., pcap, pcapng).
`-s <snaplen>`	Specifies the maximum size (snaplen) of packets to capture.
`-T <file_type>`	Specifies the type of input files (automatically detected by default).
`-v`	Provides verbose output (verbose mode).
`-w <output_file>`	Specifies the name of the output file.

Example Commands:

mergecap -w birlesik.pcap dosya1.pcap dosya2.pcap dosya3.pcap: Merges dosya1.pcap, dosya2.pcap, and dosya3.pcap files into a file named birlesik.pcap.
mergecap -F pcapng -w birlesik.pcapng dosya1.pcap dosya2.pcap: Merges dosya1.pcap and dosya2.pcap files into a .pcapng format file named birlesik.pcapng.
mergecap -v -w birlesik.pcap dosya*.pcap: Merges all .pcap files in the same directory into a file named birlesik.pcap and provides verbose output.

Step-by-Step Process:

Open the command line.
Navigate to the directory where Mergecap is installed, or add the path of Mergecap to the system variables.
Using one of the example commands above, specify the files you want to merge and the name of the output file.
Run the command. Mergecap will merge the specified files and write them to the output file.

How to Filter with Mergecap?

Mergecap does not have the feature to filter directly. However, filtering can be performed by using it in conjunction with Wireshark's editcap tool. First, you can filter each file separately with editcap, and then merge them with Mergecap.

Example Scenario: You only want to merge packets coming from or going to a specific IP address.

Step 1: Filter each file with editcap.
```
editcap -r dosya1.pcap filtered_file1.pcap "ip.addr == 192.168.1.100"
editcap -r dosya2.pcap filtered_file2.pcap "ip.addr == 192.168.1.100"
```
These commands separate packets with the IP address 192.168.1.100 from the dosya1.pcap and dosya2.pcap files and write them to the filtered_file1.pcap and filtered_file2.pcap files. The -r option reads the input file, applies the filter, and writes it to the output file.
Step 2: Merge the filtered files with Mergecap.
```
mergecap -w combined_filtered.pcap filtered_file1.pcap filtered_file2.pcap
```
This command merges the filtered_file1.pcap and filtered_file2.pcap files into a file named combined_filtered.pcap.

Alternative Method: You can also filter using Wireshark's interface. Open the files with Wireshark, apply the desired filter, and then save the filtered packets to a new file. You can then merge these filtered files with Mergecap.

Different Packet Capture Formats and Mergecap Compatibility

Mergecap supports various packet capture formats. The most common formats are:

PCAP (.pcap): The most commonly used packet capture format. It is supported by Wireshark, tcpdump, and many other network analysis tools.
PCAP Next Generation (.pcapng): An improved version of PCAP. It offers more metadata and advanced features.
Libpcap Savefile Format: A variation of the PCAP format.

Mergecap usually automatically detects and converts different formats. However, in some cases, it may be necessary to specify the output format with the -F option.

Format Compatibility Table:

Input Format	Output Format (with `-F` option)	Support Status
PCAP (.pcap)	PCAP (.pcap), PCAPNG (.pcapng)	Full Support
PCAPNG (.pcapng)	PCAP (.pcap), PCAPNG (.pcapng)	Full Support
Libpcap Savefile Format	PCAP (.pcap), PCAPNG (.pcapng)	Full Support

Important Note: In rare cases, Mergecap may not support some special or old formats. In this case, it may be necessary to convert the files to a more common format with Wireshark or another tool.

Merging Large Files with Mergecap: Performance and Optimization

Performance issues may occur when merging large packet capture files. Here are some tips to improve performance:

Hardware Resources: Use sufficient RAM and a fast storage unit. When merging large files, Mergecap can hold a significant amount of data in memory.
File Format: The PCAPNG format may require more storage space and slow down the merging process because it contains more metadata than PCAP. Use the PCAP format if possible.
Disk I/O Optimization: You can improve disk I/O performance by placing input and output files on different physical disks.
Parallel Processing: Mergecap does not support parallel processing. However, you can split the files into parts and merge them separately, and then merge these parts again (a complex method).

Case Study: Performance issues are experienced when merging 10 PCAP files of 10GB each. As a solution, the files are merged on an SSD disk and the amount of RAM is increased. This significantly reduces the merging time.

Additional Recommendations:

Reduce the file size by filtering unnecessary packets (with editcap).
Run the merging process in the background (for example, with the nohup command).
Identify bottlenecks by monitoring system resources (for example, with the top or htop commands).

Mergecap Troubleshooting: Common Errors and Solutions

You may encounter some errors when using Mergecap. Here are common errors and solutions:

"Invalid capture file format" error: This error indicates that Mergecap does not recognize the format of the input file. As a solution, specify the file type with the -T option or open the file with Wireshark and save it in a different format.
"Not enough memory" error: This error indicates that Mergecap does not have enough memory for the merging process. As a solution, close unused applications, increase the amount of RAM, or merge the files by dividing them into smaller parts.
"File not found" error: This error indicates that Mergecap cannot find the specified input file. Make sure the file name is correct and the file is in the correct directory.
Merging process is too slow: This problem may be due to insufficient hardware resources or the file size being too large. Apply the performance optimization tips above.

Example Error Scenario: The user is trying to merge file1.pcap and file2.pcapng files, but receives an "Invalid capture file format" error. As a solution, the command is corrected as follows: mergecap -T pcap -w birlesik.pcap file1.pcap file2.pcapng. This command tells Mergecap that the input files are in PCAP format (with the -T pcap option). However, since file2 is actually in pcapng format, this command will not work correctly. The correct solution is to open the file with Wireshark, save it in pcap format, and then merge it.

Additional Tips: