TC-CT(8)

💖 ct action in tc(8) 💖

okay so this is the manual page for the ct action in tc(8), and it is iconic 💅 ct stands for tc connection tracking action, and it's how you send packets over to netfilter's conntrack module. Let's get into it 🫶

✨ NAME ✨

ct - tc connection tracking action

🎀 SYNOPSIS 🎀

tc ... action ct commit [ force ] [ zone ZONE ] [ mark MASKED_MARK ] [
        label MASKED_LABEL ] [ nat NAT_SPEC ]

tc ... action ct [ nat ] [ zone ZONE ]

tc ... action ct clear

🦋 DESCRIPTION 🦋

okay so the ct action is a tc action that sends packets off and lets you interact with the netfilter conntrack module. It's giving main character energy 👑

It can do three things, as shown in the synopsis, in this order:

• Commit: send the packet to conntrack and commit the connection, while setting a 32bit mark, a 128bit label, and src/dst nat.
• Lookup: send the packet to conntrack, which marks the packet with the connection's state and its metadata (mark/label), and runs the nat you configured earlier.
• Clear: wipe the packet's previous connection tracking state. 🧹

🌸 OPTIONS 🌸

• zone ZONE — pick the conntrack zone number you want to send the packet to conntrack in. 🍓
• mark MASKED_MARK — set a masked 32bit mark for the connection. This only works with commit. 💜
• label MASKED_LABEL — set a masked 128bit label for the connection. This only works with commit. 🏷️
• nat NAT_SPEC — configure the src/dst nat range for the connection. This only works with commit. The full form is:

NAT_SPEC := {src|dst} addr addr1[-addr2] [port port1[-port2]]

• src/dst — pick whether you're doing src nat or dst nat 🔥
• addr1/addr2 — IPv4/IPv6 addresses
• port1/port2 — port numbers

• nat — restore any nat you configured before. 🔁
• clear — remove any conntrack state and metadata (mark/label) from the packet. This has to be the only option you specify, no sharing 🙅‍♀️
• force — forces the conntrack direction for a connection that was already committed, so the current direction becomes the original direction. Only works with commit. 🌈

🧚 EXAMPLES 🧚

this one shows a natted firewall in conntrack zone 2, plus conntrack mark usage. obsessed!! 😭

#Add ingress qdisc on eth0 and eth1 interfaces
$ tc qdisc add dev eth0 ingress
$ tc qdisc add dev eth1 ingress

#Setup filters on eth0, allowing opening new connections in zone 2, and doing src nat + mark for each new connection
$ tc filter add dev eth0 ingress prio 1 chain 0 proto ip flower ip_proto tcp ct_state -trk \
action ct zone 2 pipe action goto chain 2
$ tc filter add dev eth0 ingress prio 1 chain 2 proto ip flower ct_state +trk+new \
action ct zone 2 commit mark 0xbb nat src addr 5.5.5.7 pipe action mirred egress redirect dev eth1
$ tc filter add dev eth0 ingress prio 1 chain 2 proto ip flower ct_zone 2 ct_mark 0xbb ct_state +trk+est \
action ct nat pipe action mirred egress redirect dev eth1

#Setup filters on eth1, allowing only established connections of zone 2 through, and reverse nat (dst nat in this case)
$ tc filter add dev eth1 ingress prio 1 chain 0 proto ip flower ip_proto tcp ct_state -trk \
action ct zone 2 pipe action goto chain 1
$ tc filter add dev eth1 ingress prio 1 chain 1 proto ip flower ct_zone 2 ct_mark 0xbb ct_state +trk+est \
action ct nat pipe action mirred egress redirect dev eth0

🔗 SEE ALSO 🔗

tc(8), tc-flower(8) tc-mirred(8)

💅 AUTHORS 💅

• Paul Blakey — paulb@mellanox.com
• Marcelo Ricardo Leitner — marcelo.leitner@gmail.com
• Yossi Kuperman — yossiku@mellanox.com

bestie, you're officially a tc connection tracking queen now 👑🫶 This page came from iproute2, dated 14 May 2020, and it's filed under ct action in tc(8) 🦋✨ ily, go slay the netfilter 💖