Time-based Packet Capture with tcpdump: Difference between revisions
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[[Category:General Linux]] | [[Category:General Linux]] | ||
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A common challenge with `tcpdump` is that its built-in file rotation (`-G`) can be unreliable; the process may exit or fail due to permission errors. This occurs because, by default, `tcpdump` drops its root privileges after starting, which interferes with its ability to manage files in protected directories. | A common challenge with `tcpdump` is that its built-in file rotation (`-G`) can be unreliable; the process may exit or fail due to permission errors. This occurs because, by default, `tcpdump` drops its root privileges after starting, which interferes with its ability to manage files in protected directories. | ||
You can solve this without a wrapper script by using the `-Z` option to specify which user `tcpdump` should run as. | You can solve this without a wrapper script by using the `-Z` option to specify which user `tcpdump` should run as after it has initialized the capture. | ||
== The Command == | == The Command == | ||
This one-liner command runs `tcpdump` with time-based rotation and explicitly tells it to use the `root` user for file operations. This ensures it has the necessary permissions to continuously overwrite the capture file. | This one-liner command runs `tcpdump` with time-based rotation and explicitly tells it to use the `root` user for file operations. This ensures it has the necessary permissions to continuously overwrite the capture file every 30 minutes. | ||
<syntaxhighlight lang="bash"> | <syntaxhighlight lang="bash"> | ||
tcpdump -i any -w /tmp/capture.pcap -G 1800 -W 1 -Z root 'host hostname.asd.local and port 12345' | |||
</syntaxhighlight> | </syntaxhighlight> | ||
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* <code>-i any</code>: Listen on all network interfaces. | * <code>-i any</code>: Listen on all network interfaces. | ||
* <code>-w /tmp/capture.pcap</code>: Writes the raw packet output to a file. | * <code>-w /tmp/capture.pcap</code>: Writes the raw packet output to a file. | ||
* <code>-G | * <code>-G 1800</code>: Sets the rotation interval. It triggers a file rotation every 1800 seconds (30 minutes). | ||
* <code>-W 1</code>: Limits the number of capture files to one, ensuring the same file is overwritten. | * <code>-W 1</code>: Limits the number of capture files to one, ensuring the same file is overwritten. | ||
* <code>-Z root</code>: This is the crucial flag. It tells `tcpdump` to change its user to `root` after opening the capture device. Since it's running as root, it will have no problem writing or overwriting the file in `/tmp/` during rotation. | * <code>-Z root</code>: This is the crucial flag. It tells `tcpdump` to change its user to `root` after opening the capture device. Since it's running as root, it will have no problem writing or overwriting the file in `/tmp/` during rotation. | ||
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== Why This Method Works == | == Why This Method Works == | ||
This approach directly addresses the privilege-dropping issue. By forcing `tcpdump` to operate as the `root` user with `-Z root`, you ensure that when the | This approach directly addresses the privilege-dropping issue. By forcing `tcpdump` to operate as the `root` user with `-Z root`, you ensure that when the 30-minute rotation occurs, the process still has sufficient permissions to manage the capture file. This prevents both the "Permission denied" error and the premature termination of the process, resulting in a stable, continuous capture loop. | ||
== Running as a Background Process == | == Running as a Background Process == | ||
To run this capture continuously, you can send it to the background with `nohup` and `&`. | To run this capture continuously, you can send it to the background with `nohup` and `&`. | ||
<syntaxhighlight lang="bash"> | <syntaxhighlight lang="bash"> | ||
nohup | nohup tcpdump -i any -w /tmp/capture.pcap -G 1800 -W 1 -Z root 'host hostname.asd.local and port 12345' & | ||
</syntaxhighlight> | </syntaxhighlight> | ||
To stop the capture, find its process ID with `pgrep tcpdump` and use ` | To stop the capture, find its process ID with `pgrep tcpdump` and use `kill <PID>`. Note that because the process and the file it creates are owned by root, you will need `sudo` to manage them. | ||
Latest revision as of 10:41, 27 August 2025
Creating a Continuous Packet Capture without a Script
A common challenge with `tcpdump` is that its built-in file rotation (`-G`) can be unreliable; the process may exit or fail due to permission errors. This occurs because, by default, `tcpdump` drops its root privileges after starting, which interferes with its ability to manage files in protected directories.
You can solve this without a wrapper script by using the `-Z` option to specify which user `tcpdump` should run as after it has initialized the capture.
The Command
This one-liner command runs `tcpdump` with time-based rotation and explicitly tells it to use the `root` user for file operations. This ensures it has the necessary permissions to continuously overwrite the capture file every 30 minutes.
tcpdump -i any -w /tmp/capture.pcap -G 1800 -W 1 -Z root 'host hostname.asd.local and port 12345'
Parameter Breakdown
-i any: Listen on all network interfaces.-w /tmp/capture.pcap: Writes the raw packet output to a file.-G 1800: Sets the rotation interval. It triggers a file rotation every 1800 seconds (30 minutes).-W 1: Limits the number of capture files to one, ensuring the same file is overwritten.-Z root: This is the crucial flag. It tells `tcpdump` to change its user to `root` after opening the capture device. Since it's running as root, it will have no problem writing or overwriting the file in `/tmp/` during rotation.'host ... and port ...': The filter expression to capture only the traffic you need.
Why This Method Works
This approach directly addresses the privilege-dropping issue. By forcing `tcpdump` to operate as the `root` user with `-Z root`, you ensure that when the 30-minute rotation occurs, the process still has sufficient permissions to manage the capture file. This prevents both the "Permission denied" error and the premature termination of the process, resulting in a stable, continuous capture loop.
Running as a Background Process
To run this capture continuously, you can send it to the background with `nohup` and `&`.
nohup tcpdump -i any -w /tmp/capture.pcap -G 1800 -W 1 -Z root 'host hostname.asd.local and port 12345' &
To stop the capture, find its process ID with `pgrep tcpdump` and use `kill <PID>`. Note that because the process and the file it creates are owned by root, you will need `sudo` to manage them.