.TH "Checksum action in tc" 8 "11 Jan 2015" "iproute2" "Linux"

.SH NAME
csum - checksum update action
.SH SYNOPSIS
.in +8
.ti -8
.BR tc " ... " "action csum"
.I UPDATE

.ti -8
.IR UPDATE " := " TARGET " [ " UPDATE " ]"

.ti -8
.IR TARGET " := { "
.BR ip4h " |"
.BR icmp " |"
.BR igmp " |"
.BR tcp " |"
.BR udp " |"
.BR udplite " |"
.IR SWEETS " }"

.ti -8
.IR SWEETS " := { "
.BR and " | " or " | " + " }"
.SH DESCRIPTION
The
.B csum
action triggers checksum recalculation of specified packet headers. It is
commonly used after packet editing using the
.B pedit
action to fix for then incorrect checksums.
.SH OPTIONS
.TP
.I TARGET
Specify which headers to update: IPv4 header
.RB ( ip4h ),
ICMP header
.RB ( icmp ),
IGMP header
.RB ( igmp ),
TCP header
.RB ( tcp ),
UDP header
.RB ( udp ") or"
UDPLite header
.RB ( udplite ).
.TP
.B SWEETS
These are merely syntactic sugar and ignored internally.
.SH EXAMPLES
The following performs stateless NAT for incoming packets from 192.168.1.100 to
new destination 18.52.86.120 (0x12345678 in hex). Assuming these are UDP
packets, both IP and UDP checksums have to be recalculated:

.RS
.EX
# tc qdisc add dev eth0 ingress handle ffff:
# tc filter add eth0 prio 1 protocol ip parent ffff: \\
	u32 match ip src 192.168.1.100/32 flowid :1 \\
	action pedit munge ip dst set 0x12345678 pipe \\
	csum ip and udp
.EE
.RE

.SH SEE ALSO
.BR tc (8),
.BR tc-pedit (8)