NAT action in tc(8) Linux NAT action in tc(8)
NAME
nat - stateless native address translation action
SYNOPSIS
tc ... action nat DIRECTION OLD NEW
DIRECTION := { ingress | egress }
OLD := IPV4_ADDR_SPEC
NEW := IPV4_ADDR_SPEC
IPV4_ADDR_SPEC := { default | any | all | in_addr[/{prefix|netmask}]
DESCRIPTION
The nat action allows to perform NAT without the overhead of conntrack, which is desirable if the number of
flows or addresses to perform NAT on is large. This action is best used in combination with the u32 filter to
allow for efficient lookups of a large number of stateless NAT rules in constant time.
OPTIONS
ingress
Translate destination addresses, i.e. perform DNAT.
egress Translate source addresses, i.e. perform SNAT.
OLD Specifies addresses which should be translated.
NEW Specifies addresses which OLD should be translated into.
NOTES
The accepted address format in OLD and NEW is quite flexible. It may either consist of one of the keywords
default, any or all, representing the all-zero IP address or a combination of IP address and netmask or prefix
length separated by a slash (/) sign. In any case, the mask (or prefix length) value of OLD is used for NEW as
well so that a one-to-one mapping of addresses is assured.
Address translation is done using a combination of binary operations. First, the original (source or destina‐
tion) address is matched against the value of OLD. If the original address fits, the new address is created
by taking the leading bits from NEW (defined by the netmask of OLD) and taking the remaining bits from the
original address.
There is rudimental support for upper layer protocols, namely TCP, UDP and ICMP. While for the first two only
checksum recalculation is performed, the action also takes care of embedded IP headers in ICMP packets by
translating the respective address therein, too.
SEE ALSO
tc(8)
iproute2 12 Jan 2015 NAT action in tc(8)
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