NAME

       skbmod - user-friendly packet editor action

SYNOPSIS

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

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

       SWAPPABLE := mac

       CONTROL := { reclassify | pipe | drop | shot | continue | pass }

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

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.

       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 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.

              drop
              shot   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.

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

       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.

SEE ALSO

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