Re: [PATCH v3] net/af_packet: add option to ignore outgoing packets

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue,  6 Oct 2026 07:42:28 +0100
Frank Dressler <frank@dressler.pro> wrote:

> By default, AF_PACKET delivers both incoming and outgoing packets.
> This might surprise applications that expect rte_eth_rx_burst() to
> return only incoming traffic.
> 
> This patch adds the devarg ignore_outgoing=<0|1> to enable
> PACKET_IGNORE_OUTGOING. When set, the kernel drops outgoing frames.
> The default is 0 to keep the existing behavior.
> 
> Signed-off-by: Frank Dressler <frank@dressler.pro>
> ---
> v3:
> - drop #ifdef PACKET_IGNORE_OUTGOING (uapi since 4.20; DPDK requires kernel >= 5.4)
> - fail the test if ignore_outgoing=1 cannot create a port
> - drop kernel >= 4.20 notes from docs and release notes
> v2:
> - use PACKET_IGNORE_OUTGOING / ignore_outgoing= instead of capture_dir
> 
> ---

AI spotted issue with multiple qpairs.

[PATCH v3] net/af_packet: add option to ignore outgoing packets

Builds clean with -Dwerror=true. Test not run.

Error

1. ignore_outgoing=1 has no effect when qpairs > 1.

     rc = setsockopt(qsockfd, SOL_PACKET, PACKET_IGNORE_OUTGOING,
                     &ignore_outgoing, sizeof(ignore_outgoing));

   With more than one queue every socket joins a fanout group.
   fanout_add() removes the socket's own packet_type from ptype_all
   and registers the group's instead, and dev_queue_xmit_nit() only
   looks at the group's ignore_outgoing. The per-socket option is
   accepted and silently does nothing. The group needs
   PACKET_FANOUT_FLAG_IGNORE_OUTGOING (6.2) or'd into the flags half
   of fanout_arg.

   Kernels before 6.2 do not validate fanout flags; the bit is
   accepted and ignored there. Either keep the sll_pkttype check in
   eth_af_packet_rx() as a backstop, or document that
   ignore_outgoing with qpairs > 1 needs 6.2. Distro headers older
   than 6.2 lack the define, so provide a fallback #define.

Warning

2. Test only covers the single queue case and never sends an
   incoming frame.

     TEST_ASSERT(rx_on == 0, "Expected no packets with ignore_outgoing=1");

   A port that receives nothing at all passes this. Write frames to
   tap_fd as test_af_packet_loopback() does and check that the
   ignore_outgoing=1 port still receives them. Run the same check
   with qpairs=2; that would have caught item 1.

Info

3. Commit message:

     When set, the kernel drops outgoing frames.

   The frames are still transmitted; the socket just does not get a
   copy. The doc text has it right, use that wording.