tc-skbmod (8)
NAME
skbmod - user-friendly packet editor actionSYNOPSIS
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.