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skbmod action in tc

Section: Linux (8)
Updated: 21 Sep 2016
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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.