TC-CSUM(8)

✨ NAME ✨

Checksum action in tc(8) 🎀

csum - checksum update action 💖

💅 SYNOPSIS 💅

tc ... action csum UPDATE

UPDATE := TARGET [ UPDATE ]

TARGET := { ip4h | icmp | igmp | tcp | udp | udplite | sctp | SWEETS }

SWEETS := { and | or | + }

🦋 DESCRIPTION 🦋

okay so the csum action is basically your checksum fixer-upper 🧚 it recalculates checksums for the packet headers you pick. it's giving "clean up after yourself" energy, and it's super commonly used to repair wrong checksums after the pedit action has messed with the packet contents 😭🔥

🌸 OPTIONS 🌸

• TARGET — pick which header(s) get updated ✨ your choices are the IPv4 header (ip4h), the ICMP header (icmp), the IGMP header (igmp), the TCP header (tcp), the UDP header (udp), the UDPLite header (udplite) or the SCTP header (sctp) 💜
• SWEETS — these are just and, or and + 🍓 purely syntactic sugar, so they're literally ignored internally. no cap 💀

🌈 EXAMPLES 🌈

okay this one is iconic 👑 it does stateless NAT for incoming packets coming from 192.0.2.100 and sends them to the new destination 198.51.100.1. assuming these are UDP packets, both the IP and UDP checksums need recalculating, so we slap csum ip and udp on the end 💅

# tc qdisc add dev eth0 ingress handle ffff:
# tc filter add dev eth0 prio 1 protocol ip parent ffff: \
     u32 match ip src 192.0.2.100/32 flowid :1 \
     action pedit munge ip dst set 198.51.100.1 pipe \
     csum ip and udp

🧚 SEE ALSO 🧚

tc(8), tc-pedit(8) 🦋

bestie that's the whole tea 🫶 this page is from iproute2, last updated 11 Jan 2015, and it's all about the Checksum action in tc(8) 🌸💖 stay fabulous and keep your checksums fresh ✨👑