inbound, outbound And ifindex Match No Packets If Linux Live Capture Falls Back To Userland Filtering

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"inbound", "outbound" and "ifindex" match no packets if Linux live capture falls back to userland filtering

Understanding the Issue

When performing live packet capture on a Linux system, the kernel is responsible for filtering packets based on a specified filter expression. However, if the filter expression is too complex or contains certain keywords, the kernel may refuse to process it, and the filtering will fall back to userland. In this case, the keywords "inbound", "outbound", and "ifindex" are known to cause this issue.

Steps to Reproduce the Issue

To reproduce this issue, follow these steps:

Step 1: Start a Background Ping

Start a background ping to the localhost using the following command:

ping localhost &

This will create a continuous stream of ICMP packets on the loopback interface (lo).

Step 2: Create a Long Filter Expression

Create a very long filter expression by writing the following code into a file (interface index 1 is the lo interface):

(ifindex 1 || inbound || outbound) &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
... (repeat 2000 times)
link[0] >= 0

This filter expression is designed to match any packets on the lo interface.

Step 3: Use the Filter with Tcpdump

Use the filter expression with tcpdump to capture packets on the lo interface:

tcpdump -n -c 2 -i lo -F long_filter.txt

This command will capture 2 packets on the lo interface using the filter expression in long_filter.txt.

Step 4: Observe the Output

Observe the output of tcpdump. Since the filter expression is optimized, tcpdump should capture 2 packets, and the kernel should process the filtering correctly.

Step 5: Repeat with Unoptimized Filter

Repeat the same steps, but this time use the unoptimized filter expression:

(ifindex 1 || inbound || outbound) &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
... (repeat 2000 times)
link[0] >= 0

However, this time, use the -O option with tcpdump to disable kernel filtering:

tcpdump -nO -c 2 -i lo -F long_filter.txt

This command will attempt to capture packets on the lo interface using the unoptimized filter expression.

Step 6: Observe the Output

Observe the output of tcpdump. Since the filter expression is unoptimized, the kernel will refuse to process it, and libpcap will filter the packets in userland. In this case, tcpdump should capture no packets, and the output will indicate that the kernel filter failed.

Understanding the Root Cause

The root cause of this issue lies in the way libpcap handles filter expressions. When the kernel refuses to process a filter expression, libpcap falls back to userland filtering. However, the auxiliary data structure used by libpcap does not include the required items for the keywords "inbound", "outbound", and "ifindex". As a result, these keywords are not processed correctly, and the filtering fails.

Conclusion

In conclusion, the issue of "inbound", "outbound", and "ifindex" matching no packets when Linux live capture falls back to userland filtering is a known problem. It is caused by the way libpcap handles filter expressions and the auxiliary data structure used by libpcap. To avoid this issue, it is recommended to use optimized filter expressions and to avoid using the keywords "inbound", "outbound", and "ifindex" in filter expressions.
Q&A: "inbound", "outbound" and "ifindex" match no packets if Linux live capture falls back to userland filtering

Q: What is the issue with "inbound", "outbound", and "ifindex" in Linux live capture?

A: The issue arises when the kernel refuses to process a filter expression, and libpcap falls back to userland filtering. In this case, the auxiliary data structure used by libpcap does not include the required items for the keywords "inbound", "outbound", and "ifindex". As a result, these keywords are not processed correctly, and the filtering fails.

Q: What are the steps to reproduce this issue?

A: To reproduce this issue, follow these steps:

  1. Start a background ping to the localhost using the command ping localhost &.
  2. Create a very long filter expression by writing the following code into a file (interface index 1 is the lo interface):
(ifindex 1 || inbound || outbound) &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
link[0] >= 0 &&
... (repeat 2000 times)
link[0] >= 0
  1. Use the filter expression with tcpdump to capture packets on the lo interface:
tcpdump -n -c 2 -i lo -F long_filter.txt
  1. Repeat the same steps, but this time use the unoptimized filter expression and disable kernel filtering with the -O option:
tcpdump -nO -c 2 -i lo -F long_filter.txt

Q: What is the expected output when using the optimized filter expression?

A: When using the optimized filter expression, tcpdump should capture 2 packets on the lo interface, and the kernel should process the filtering correctly.

Q: What is the expected output when using the unoptimized filter expression?

A: When using the unoptimized filter expression, the kernel will refuse to process it, and libpcap will filter the packets in userland. In this case, tcpdump should capture no packets, and the output will indicate that the kernel filter failed.

Q: How can I avoid this issue?

A: To avoid this issue, it is recommended to use optimized filter expressions and to avoid using the keywords "inbound", "outbound", and "ifindex" in filter expressions.

Q: What is the root cause of this issue?

A: The root cause of this issue lies in the way libpcap handles filter expressions and the auxiliary data structure used by libpcap. The auxiliary data structure does not include the required items for the keywords "inbound", "outbound", and "ifindex", which results in incorrect filtering.

Q: Is this issue specific to Linux?

A: No, this issue is not specific to Linux. It can occur on any platform that uses libpcap for packet capture.

Q: Can I use a workaround to avoid this issue?

A: Yes, you can use a workaround by avoiding the use of the keywords "inbound", "outbound", and "ifindex" in filter expressions. Alternatively, you can use a different packet capture tool that does not rely on libpcap for filtering.