TC-SKBMOD(8)

✨ skbmod action in tc(8) ✨

okay bestie, this is the manual page for skbmod, the action that makes packet editing way less painful 🎀

💖 NAME

skbmod - user-friendly packet editor action

🧚 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

so the skbmod action is basically a glow-up of the existing pedit action 💅 instead of hand-editing 8-, 16-, or 32-bit chunks of an ethernet header like it's 1999, skbmod lets you swap out whole supported elements in one go 🫶

you pick one of three actions: set, swap, or ecn. set and swap only touch Ethernet packets, while ecn only touches IP packets 🌸

🎀 OPTIONS

• dmac DMAC — change the destination mac to whatever address you give it 💜
• smac SMAC — change the source mac to the specified address 🍓
• etype ETYPE — change the ethertype to the specified value ✨
• mac — swaps the mac addresses 🔁
• ecn — marks ECN Capable Transport (ECT) IP packets as Congestion Encountered (CE) 🌈 it does not touch Non ECN-Capable Transport (Non-ECT) packets, so those stay exactly as they are 🙈
• CONTROL — these keywords decide how the tree of qdisc, classes, filters and actions gets traversed after this action runs 🧭
  • reclassify — restart with the first filter in the current list 🔄
  • pipe — continue with the next action attached to the same filter ➡️
  • drop or shot — drop the packet 💀
  • continue — continue classification with the next filter in line 🏃‍♀️
  • pass — finish classification and return to the calling qdisc for further packet processing. this is the default 👑

💅 EXAMPLES

okay so first, let's look at a filter using the 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

with skbmod, that whole mess becomes:

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

it gets a little more involved if you also edit the source mac and ethertype. if all three fields are changing 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

and 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

⚠️ bestie, listen up: only one of set, swap and ecn can be used in a single command. trying to use more than one in the same command is undefined behavior 💀 if you need multiple, pipe multiple commands together instead 🫶

🌈 SEE ALSO

tc(8), tc-u32(8), tc-pedit(8) 🦋

that's a wrap, bestie 💖 this page is from iproute2, dated 21 Sep 2016, titled skbmod action in tc(8) ✨🎀