[Bug 2076398] Re: Tcpdump utility captures incorrect packets on VLAN interface when using SLL2

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** Tags added: patch

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Title:
  Tcpdump utility captures incorrect packets on VLAN interface when
  using SLL2

Status in libpcap package in Ubuntu:
  New
Status in libpcap source package in Focal:
  In Progress
Status in libpcap source package in Jammy:
  In Progress

Bug description:
  [Impact]
  When using the `tcpdump -i any` command with SLL2, bogus packets may be captured.
  This issue arises due to differing offsets used for inserting VLAN tags across various data link types.

  [Fix]
  An upstream commit has been made to address and fix this issue.
  commit 4bfca3682e5aeabe05b4406daf00c9abcc36c571
  Author:     Guy Harris <gharris at sonic.net>
  Date: Sat Apr 9 16:55:20 2022 -0700

      linux: set handlep->vlan_offset if the linktype is changed.

      The change to the linktype might change the offset at which to insert
      VLAN tags (or change it to -1, meaning "don't insert VLAN tags").

      This should fix issue #1105.

  [Test Plan]
  1. Set up a VLAN interface:
  # ip link add link eth0 eth0.24 type vlan id 24
  # ifconfig eth0.24 1.0.24.1/24
  2. Generate traffic for packet sniffing:
  # ping -n 1.0.24.3 > /dev/null 2>&1 &
  3. Use tcpdump to capture packets and verify that the error does not occur.
  # tcpdump -nn -i any -Q out not tcp and not udp
  tcpdump: data link type LINUX_SLL2
  tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
  listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
  06:43:28.360947 eth0.24 Out ARP, Request who-has 1.0.24.3 tell 1.0.24.1, length 28
  06:43:28.360949 eth0  Out ARP, Unknown Hardware (12318) (len 0), Unknown Protocol (0x0000) (len 1), length 32

  [Where problems could occur]
  The patch introduced a step to set the VLAN offset when configuring the data link type.
  This change primarily affects the handling of SLL2 and should not impact other data link types.

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