tc-skbmod - Man Page

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)

tc-actions(8).