NAT action in tc
Section: Linux (8)
Updated: 12 Jan 2015
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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 destination) 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)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- NOTES
-
- SEE ALSO
-
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