skbmod action in tc(8)    (21 Sep 2016)    skbmod action in tc(8)

     NAME
          skbmod - user-friendly packet editor action

     SYNOPSIS
          tc ... action skbmod { set SETTABLE |  swap SWAPPABLE  } [
                  CONTROL ] [ index INDEX ]

          SETTABLE :=   [ dmac DMAC ]   [ smac SMAC ]   [ etype ETYPE
                  ]

          SWAPPABLE :=  mac

          CONTROL := { reclassify | pipe | drop |

     DESCRIPTION
          The skbmod action is intended as a usability upgrade to the
          existing pedit action. Instead of having to manually edit
          8-, 16-, or 32-bit chunks of an ethernet header, skbmod
          allows complete substitution of supported elements.

     OPTIONS
          dmac DMAC
               Change the destination mac to the specified address.

          smac SMAC
               Change the source mac to the specified address.

          etype ETYPE
               Change the ethertype to the specified value.

          mac  Used to swap mac addresses.

          CONTROL
               The following keywords allow to control how the tree of
               qdisc, classes, filters and actions is further tra-
               versed after this action.

               reclassify
                    Restart with the first filter in the current list.

               pipe Continue with the next action attached to the same
                    filter.

               drop shot Drop the packet.

               continue
                    Continue classification with the next filter in
                    line.

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     skbmod action in tc(8)    (21 Sep 2016)    skbmod action in tc(8)

               pass Finish classification process and return to call-
                    ing qdisc for further packet processing. This is
                    the default.

     EXAMPLES
          To start, observe the following filter with a pedit action:

               tc filter add dev eth1 parent 1: protocol ip prio 10 \
                    u32 match ip protocol 1 0xff flowid 1:2 \
                    action pedit munge offset -14 u8 set 0x02 \
                    munge offset -13 u8 set 0x15 \
                    munge offset -12 u8 set 0x15 \
                    munge offset -11 u8 set 0x15 \
                    munge offset -10 u16 set 0x1515 \
                    pipe

          Using the skbmod action, this command can be simplified to:

               tc filter add dev eth1 parent 1: protocol ip prio 10 \
                    u32 match ip protocol 1 0xff flowid 1:2 \
                    action skbmod set dmac 02:15:15:15:15:15 \
                    pipe

          Complexity will increase if source mac and ethertype are
          also being edited as part of the action. If all three fields
          are to be changed with skbmod:

               tc filter add dev eth5 parent 1: protocol ip prio 10 \
                    u32 match ip protocol 1 0xff flowid 1:2 \
                    action skbmod \
                    set etype 0xBEEF \
                    set dmac 02:12:13:14:15:16 \
                    set smac 02:22:23:24:25:26

          Finally, swap the destination and source mac addresses in
          the header:

               tc filter add dev eth3 parent 1: protocol ip prio 10 \
                    u32 match ip protocol 1 0xff flowid 1:2 \
                    action skbmod \
                    swap mac

          However, trying to set and swap in a single skbmod command
          will cause undefined behavior.

     SEE ALSO
          tc(8), tc-u32(8), tc-pedit(8)

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