Cannot capture loopback traffic (lo/127.0.0.1) with --input-raw on Ubuntu 6.8 Kernel

[antonioribeiro]

gor version: 1.3.0

Environment:

• Operating System: Ubuntu (running 6.8 kernel)
• Kernel Version: 6.8.0-52-generic #53-Ubuntu SMP PREEMPT_DYNAMIC Sat Jan 11 00:06:25 UTC 2025 x86_64
• Architecture: x86_64
• Environment Type: Cloud VM (e.g., DigitalOcean, based on earlier context)

Description of the Issue:

We are attempting to use gor --input-raw to capture unencrypted HTTP traffic flowing over the loopback interface (lo/127.0.0.1) on our server. Specifically, traffic from Caddy proxying to PHP-FPM (configured to listen on TCP 127.0.0.1:9000).

Despite confirming that traffic is flowing correctly on the target loopback port and interface and that the underlying libpcap library is functional for loopback capture using other tools, gor fails to capture any packets and does not create the specified output file or print anything to stdout when configured to do so.

Steps to Reproduce (on the affected server environment):

1. Ensure PHP-FPM is configured to listen on 127.0.0.1:9000 (TCP) on the loopback interface.
   (Confirmed this setup is active and listening via lsof -i :9000 | grep LISTEN)

2. Configure a web server (like Caddy) on the same machine to proxy requests to this PHP-FPM instance at 127.0.0.1:9000.

3. Ensure traffic is hitting the web server externally, resulting in requests being successfully proxied to 127.0.0.1:9000 internally.
   (Confirmed by Caddy access logs showing HTTP 200 status responses from PHP-FPM for PHP requests, and telnet 127.0.0.1 9000 working).

4. Run gor with --input-raw targeting the loopback address and port, and outputting to a file (or stdout):

   # Example using file output, run as root or with sudo
   sudo gor --input-raw 127.0.0.1:9000 --output-file /tmp/capture.gor -verbose 9

   (Also tried --input-raw localhost:9000, --output-stdout, removing -append, and targeting different directories like user's home and /var/log).

5. Send requests to the web server while gor is running.

6. Observe the output file location or stdout.

Observed Behavior:

• gor starts and prints initialization info (Interface: lo, BPF Filter: ((tcp dst port 9000) and (dst host 127.0.0.1 or dst host ::1)), Version 1.3.0).
• The gor process successfully starts and stays running (Sl+ state confirmed by ps aux).
• The specified output file (/tmp/capture.gor in this example, or files in other attempted directories) is never created.
• No captured data is printed to stdout when --output-stdout is used.
• No error messages are printed by gor to its own output during execution or upon Ctrl+C.

Expected Behavior:

gor should capture the TCP traffic flowing to 127.0.0.1:9000 on the loopback interface and write the captured requests (extracted HTTP from the FastCGI stream) to the specified output.

Diagnostic Findings (Crucial Information):

1. PHP-FPM is listening on 127.0.0.1:9000: Confirmed by lsof -i :9000.
2. Caddy is successfully connecting and communicating with PHP-FPM on 127.0.0.1:9000: Confirmed by Caddy access logs showing HTTP 200 statuses for requests proxied to PHP-FPM, and telnet 127.0.0.1 9000 successfully connects.
3. Traffic is visible and capturable on the loopback interface targeting port 9000 at the libpcap level by other tools: Confirmed by sudo tcpdump -i lo 'tcp port 9000' -nn -vvvv successfully showing the TCP handshake and data packets flowing between Caddy's ephemeral port and 127.0.0.1:9000 when requests are sent. This proves libpcap works on lo on this system.
4. Manual file creation works with the user running sudo in /tmp and other directories where gor was unable to create the output file.
5. A simpler loopback capture test with nc also failed with gor:
   • Running nc -l 8080 and sending traffic with echo ... | nc 127.0.0.1 8080 confirmed traffic arrived at 8080.
   • Running sudo gor --input-raw :8080 --output-stdout -v 9 simultaneously with the nc test showed no output from gor, confirming a failure to capture/output any loopback traffic, not just FastCGI, on this system using gor.
6. System logs show no errors related to gor or pcap: sudo journalctl -f (watched during gor run and request sending) shows no relevant denial or error messages from gor's PID, pcap, AppArmor, SELinux (checked status, seem inactive or not logging denials here), or kernel.

Conclusion from Diagnostics:

The issue is not with traffic flow, basic libpcap functionality on lo, or simple file permissions. Given that tcpdump -i lo works while gor --input-raw lo fails even for simple nc traffic and fails to create any output file despite the process running, it strongly suggests a specific incompatibility, bug, or environment interaction within the gor 1.3.0 binary on this Ubuntu kernel version (6.8.0-52-generic) that prevents it from capturing or processing loopback traffic correctly and/or writing any output.

Any insights or suggestions for further debugging steps would be greatly appreciated.

[dosubot]

Hello @antonioribeiro! I'm Dosu, here to help you tackle bugs, answer your questions, and assist you on your journey to becoming a contributor. While you wait for a human maintainer, feel free to share more details or ask anything!

GoReplay relies on libpcap for capturing network traffic, and it seems that libpcap is functioning correctly on your system since tcpdump can capture loopback traffic. However, GoReplay might have specific issues or configurations that need to be addressed for loopback traffic capture.

Here are a few suggestions to troubleshoot and potentially resolve the issue:

1. Check GoReplay Version: Ensure you are using the latest version of GoReplay, as there might be bug fixes or improvements related to loopback traffic capture in newer releases.

2. Verbose and Debug Logging: Use the --verbose and --debug flags to increase the logging level and gather more detailed information about what GoReplay is doing. This might provide insights into why it is not capturing traffic or creating output files ^[1].

3. Loopback Interface Handling: Verify that the loopback interface is correctly identified and not ignored by GoReplay. The setInterfaces method in GoReplay's code is responsible for setting the loopback index, which is crucial for capturing loopback traffic ^[2][3].

4. Permissions and Environment: Double-check that GoReplay has the necessary permissions to capture traffic and write files. Running GoReplay with sudo should typically suffice, but ensure there are no additional restrictions from AppArmor or SELinux, even if they seem inactive.

5. Alternative Capture Engines: If possible, try using different capture engines or configurations within GoReplay to see if they affect the ability to capture loopback traffic. The tests in the repository use capture.EnginePcap, which might be worth experimenting with ^[4].

6. Compatibility with Kernel Version: Although not explicitly documented, there might be compatibility issues with specific kernel versions. Testing on a different kernel version or environment, if feasible, could help determine if the issue is kernel-specific.

If these steps do not resolve the issue, it might be worth reviewing the GoReplay code related to loopback traffic capture or considering alternative tools for capturing loopback traffic on this specific environment.

To continue talking to Dosu, mention @dosu.

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[antonioribeiro]

@dosu Thank you for the helpful suggestions! We've continued troubleshooting based on your points and have found some crucial details.

First, we discovered a significant discrepancy: The gor_1.3.3_amd64.deb package downloaded from https://github.com/buger/goreplay/releases/download/1.3.3/gor_1.3.3_amd64.deb actually installs a gor binary that reports its version as v1.3.0 (gor --help output confirms this). This caused the initial confusion about the version being tested.

So, all our testing has consistently been performed using the binary installed by this specific .deb file, which identifies itself as gor version 1.3.0.

Regarding your specific suggestions and our findings with the binary from the 1.3.3 deb (identifying as gor v1.3.0):

1. Check GoReplay Version: As noted, the latest stable .deb installs a binary identifying as 1.3.0, so we couldn't easily test a true 1.3.3 using this method from the official releases.
2. Verbose and Debug Logging: We used -verbose 9 which is the highest verbosity level available on this version (we checked --help and could not find a separate --debug option). While gor initializes (showing interface lo, filter, etc.), it produces no further debug or error output related to capturing packets, processing, or writing, even when traffic is confirmed to be flowing on the target interface/port (as verified by tcpdump). The lack of debug output during the capture phase when traffic is present is part of the problem.
3. Loopback Interface Handling: This area remains a strong suspect. gor initializes correctly, but seems to fail after setup when trying to capture/process on lo.
4. Permissions and Environment: We ran with sudo. We verified manual file creation works in /tmp and other target directories. Crucially, system logs (journalctl, dmesg) showed no AppArmor, SELinux, permission, or file writing errors related to gor's process ID when it was running and attempting capture/output. This makes environment restrictions less likely as the primary cause, or indicates they are very specific/silent.
5. Alternative Capture Engines: We focused on --input-raw (libpcap) as the standard method. We did not explore potential alternative engine flags as --input-raw lo itself appears broken.
6. Compatibility with Kernel Version: This is still a likely contributing factor, especially combined with gor's specific libpcap usage on loopback.

Key Diagnostic Summary (Re-emphasized):

• We confirmed traffic is flowing on 127.0.0.1:9000 (lsof, Caddy logs, telnet).
• sudo tcpdump -i lo 'tcp port 9000' successfully captures this traffic, proving libpcap works on lo.
• sudo gor --input-raw 127.0.0.1:9000 --output-file ... fails to capture and does not create the output file.
• A simpler test: sudo gor --input-raw :8080 --output-stdout also fails to capture basic nc traffic on loopback, while nc -l 8080 successfully receives it.
• gor process stays running, but produces no output (file or stdout) and no system log errors.

Conclusion from further debugging:

The issue is a low-level problem with the binary identifying as gor version 1.3.0's --input-raw functionality specifically on the loopback interface (lo) on this Ubuntu kernel (6.8.0-52-generic). It fails to capture or process traffic correctly, and also fails to write output without logging errors, even in simple scenarios where tcpdump succeeds. Additionally, there is a separate bug where the official gor_1.3.3_amd64.deb package from the 1.3.3 release page contains this binary identifying as version 1.3.0.

Thank you again for the initial guidance!

[dosubot]

It seems you've identified a couple of critical issues: the version discrepancy in the GoReplay package and the failure to capture loopback traffic on your specific setup.

1. Version Discrepancy: The gor_1.3.3_amd64.deb package installing a binary that reports as version 1.3.0 is indeed problematic. This could be a packaging error where the wrong binary was included in the release. This might explain why you're encountering issues that could have been resolved in a later version.

2. Loopback Traffic Capture: Given that tcpdump can capture the traffic but GoReplay cannot, it suggests a specific issue with how GoReplay's --input-raw functionality interacts with the loopback interface on your kernel version. This might be due to a bug or an incompatibility with the kernel version or the way GoReplay handles loopback traffic.

To address these issues, consider the following steps:

• Build from Source: If possible, try building GoReplay from the latest source code on GitHub. This might help you bypass the version discrepancy and test with the actual latest codebase, which could include fixes not present in the binary you have.

• Alternative Capture Methods: While --input-raw is the standard method, you might want to explore other input methods or configurations that could bypass the issue, such as using a different capture engine if available.

• Kernel Compatibility: Testing on a different kernel version, if feasible, could help determine if the issue is specific to the kernel version you're using.

These steps might help you further isolate or resolve the issue. If building from source resolves the version discrepancy, it could also potentially address the loopback capture problem if it was fixed in a later commit.

Additionally, this issue seems quite complex, @buger, and your expertise would be invaluable to help resolve it.

To continue talking to Dosu, mention @dosu.

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