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)