---
name: "tc-csum"
section: "8"
platform: "linux"
package: "iproute2"
version: "7.3.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/tc-csum"
---

# tc-csum(8)

checksum update action

## NAME

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

The **csum** action triggers checksum recalculation of specified packet headers. It is commonly used to fix incorrect checksums after the **pedit** action has modified the packet content.

## OPTIONS

- *TARGET*

  Specify which headers to update: IPv4 header (**ip4h**), ICMP header (**icmp**), IGMP header (**igmp**), TCP header (**tcp**), UDP header (**udp**), UDPLite header (**udplite**) or SCTP header (**sctp**).

- **SWEETS**

  These are merely syntactic sugar and ignored internally.

## EXAMPLES

The following performs stateless NAT for incoming packets from 192.0.2.100 to new destination 198.51.100.1. Assuming these are UDP packets, both IP and UDP checksums have to be recalculated:

```
# 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/8.md), [**tc-pedit**(8)](/tc-pedit/8.md)