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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | SEE ALSO | COLOPHON |
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skbmod action in tc(8) Linux skbmod action in tc(8)
skbmod - user-friendly packet editor action
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 }
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.
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.
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.
tc(8), tc-u32(8), tc-pedit(8)
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 21 Sep 2016 skbmod action in tc(8)
Pages that refer to this page: tc-actions(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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