pwru (packet, where are you?)

pwru is an eBPF-based tool for tracing network packets in the Linux kernel with advanced filtering capabilities. It allows fine-grained introspection of kernel state to facilitate debugging network connectivity issues.

The following example shows where the packets of a curl request are dropped after installing an IP tables rule:

Quick start with Vagrant

If you have Vagrant installed, you can run the above example with the following commands.

1. In a terminal (terminal 1), bring up the Vagrant box:

   $ vagrant up

   This will take a few minutes to download and provision the box.

2. Connect to the Vagrant box:

   $ vagrant ssh

3. Build pwru:

   $ cd /pwru
   $ make

4. Run pwru:

   $ sudo ./pwru --filter-dst-ip=1.1.1.1 --filter-dst-port=80 --filter-proto=tcp --output-tuple

5. In a new terminal (terminal 2), connect to the Vagrant box:

   $ vagrant ssh

6. In terminal 2, run curl to generate some traffic to 1.1.1.1:

   $ curl 1.1.1.1

   Observe the output of pwru in terminal 1.

7. In terminal 2, add an iptables rule to block traffic to 1.1.1.1:

   $ sudo iptables -t filter -I OUTPUT 1 -m tcp --proto tcp --dst 1.1.1.1/32 -j DROP

8. In terminal 2, run curl to generate some traffic to 1.1.1.1:

   $ curl 1.1.1.1

   Observe the output of pwru in terminal 1.

9. To clean up, press Ctrl+C to terminate pwru in terminal 1, exit both shells, and run:

   $ vagrant destroy

Running

Requirements

pwru requires >= 5.5 kernel to run. For --output-skb >= 5.9 kernel is required.

The following kernel configuration is required.

Option	Note
CONFIG_DEBUG_INFO_BTF=y	Available since >= 5.3
CONFIG_KPROBES=y
CONFIG_PERF_EVENTS=y
CONFIG_BPF=y
CONFIG_BPF_SYSCALL=y

You can use zgrep $OPTION /proc/config.gz to validate whether option is enabled.

Downloading

You can download the statically linked executable for x86_64 arch which includes the eBPF bytecode from the release page.

Usage

Usage of ./pwru:
      --filter-dst-ip string        filter destination IP addr
      --filter-dst-port uint16      filter destination port
      --filter-func string          filter kernel functions to be probed by name (exact match, supports RE2 regular expression)
      --filter-mark uint32          filter skb mark
      --filter-netns uint32         filter netns inode
      --filter-proto string         filter L4 protocol (tcp, udp, icmp)
      --filter-src-ip string        filter source IP addr
      --filter-src-port uint16      filter source port
      --output-limit-lines uint     exit the program after the number of events has been received/printed
      --output-meta                 print skb metadata
      --output-relative-timestamp   print relative timestamp per skb
      --output-skb                  print skb
      --output-stack                print stack
      --output-tuple                print L4 tuple

If multiple filters are specified, all of them have to match in order for a packet to be traced.

The --filter-func switch does an exact match on function names i.e. --filter-func=foo only matches foo(); for a wildcarded match, try --filter-func=".*foo.*" instead.

Developing

Dependencies

• Go >= 1.16
• LLVM/clang >= 1.12

Building

make

Alternatively, you can build and run in the Docker container:

docker build -t pwru .
docker run --privileged -it pwru [filter1] [filtern]

Contributing

pwru is an open source project licensed under GPLv2. Everybody is welcome to contribute. Contributors are required to follow the Contributor Covenant Code of Conduct and must adhere to the Developer Certificate of Origin by adding a Signed-off-by line to their commit messages.

Logo Credits

The detective gopher is based on the Go gopher designed by Renee French.