# man > tc-skbmod(8)

_skbmod_ _action_ _in_ [_tc_(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown)                          Linux                         _skbmod_ _action_ _in_ [_tc_(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown)

## 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 com‐
       plete 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 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.

              **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 ac‐
       tion. 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)](https://www.chedong.com/phpMan.php/man/tc/8/markdown), [**tc-u32**(8)](https://www.chedong.com/phpMan.php/man/tc-u32/8/markdown), [**tc-pedit**(8)](https://www.chedong.com/phpMan.php/man/tc-pedit/8/markdown)

iproute2                                     21 Sep 2016                      _skbmod_ _action_ _in_ [_tc_(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown)