skbmod action in tc(8) Linux skbmod action in tc(8)

# [NAME](#NAME)

skbmod - user-friendly packet editor action

# [SYNOPSIS](#SYNOPSIS)

**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** }

# [DESCRIPTION](#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. Action must be one of **set**, **swap** and **ecn**. **set** and **swap** only affect Ethernet packets, while **ecn** only affects IP packets.

# [OPTIONS](#OPTIONS)

[**dmac DMAC**](#dmac)

Change the destination mac to the specified address.

[**smac SMAC**](#smac)

Change the source mac to the specified address.

[**etype ETYPE**](#etype)

Change the ethertype to the specified value.

[**mac**](#mac)

Used to swap mac addresses.

[**ecn**](#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*](#CONTROL)

The following keywords allow one to control how the tree of qdisc, classes, filters and actions is further traversed after this action.

[**reclassify**](#reclassify)

Restart with the first filter in the current list.

[**pipe**](#pipe)

Continue with the next action attached to the same filter.

[**drop**](#drop)

[**shot**](#shot)

Drop the packet.

[**continue**](#continue)

Continue classification with the next filter in line.

[**pass**](#pass)

Finish classification process and return to calling qdisc for further packet processing. This is the default.

# [EXAMPLES](#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
```

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.

# [SEE ALSO](#SEE_ALSO)

[**tc**(8)](/man/os/iproute2/tc.8), [**tc-u32**(8)](/man/os/iproute2/tc-u32.8), [**tc-pedit**(8)](/man/os/iproute2/tc-pedit.8)

21 Sep 2016 iproute2

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Document: tc-skbmod(8) from iproute2 7.2.0-r1 (Chainguard OS)
Source: https://kernel.googlesource.com/pub/scm/network/iproute2/iproute2