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 | 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 Conges-
tion Encountered (CE). Does not affect Non ECN-Capable Trans-
port (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 fil-
ter.
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. Try-
ing to use more than one of them in a single command is considered un-
defined behavior; pipe multiple commands together instead.
SEE ALSO
tc(8), tc-u32(8), tc-pedit(8)
iproute2 21 Sep 2016 skbmod action in tc(8)
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