skbmod - user-friendly packet editor action

       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.  Action must be one of set, swap and ecn.  set and swap only affect Ethernet packets,
       while ecn only affects IP packets.

       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

       To swap the destination and source mac addresses in the Ethernet 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

       Finally, to mark the CE codepoint in the IP header for ECN Capable Transport (ECT) packets:

              tc filter add dev eth0 parent 1: protocol ip prio 10 \
                   u32 match ip protocol 1 0xff flowid 1:2 \
                   action skbmod \
                   ecn

       Only one of set, swap and ecn shall be used in a single command.  Trying to use more than one of them  in
       a single command is considered undefined behavior; pipe multiple commands together instead.

       skbmod - user-friendly packet editor action

dmacDMAC
              Change the destination mac to the specified address.

       smacSMAC
              Change the source mac to the specified address.

       etypeETYPE
              Change the ethertype to the specified value.

       mac    Used to swap mac addresses.

       ecn    Used  to  mark  ECN  Capable  Transport (ECT) IP packets as Congestion Encountered (CE).  Does not
              affect Non ECN-Capable Transport (Non-ECT) packets.

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

              reclassify
                     Restart with the first filter in the current list.

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

              dropshot   Drop the packet.

              continue
                     Continue classification with the next filter in line.

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

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

iproute2                                           21 Sep 2016                            skbmodactionintc(8)

tc ... actionskbmod { setSETTABLE | swapSWAPPABLE  | ecn } [ CONTROL ] [ indexINDEX ]

       SETTABLE :=  [ dmacDMAC ]  [ smacSMAC ]  [ etypeETYPE ]

       SWAPPABLE := macCONTROL := { reclassify | pipe | drop | shot | continue | pass }