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.

 

SEE ALSO

tc(8), tc-u32(8), tc-pedit(8)


 

Index

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
EXAMPLES
SEE ALSO
LinuxReviews : manual page archive : man8