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skbmod action in tc(8) Linux 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 | 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.
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. The swap mac directive is performed after any outstand-
ing D/SMAC changes.
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
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
As mentioned above, the swap action will occur after any smac/dmac substitutions are exe-
cuted, if they are present.
SEE ALSO
tc(8), tc-u32(8), tc-pedit(8)
iproute2 21 Sep 2016 skbmod action in tc(8)
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