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