tc-skbmod

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 outstanding 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 executed, if they are present.