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docs: Improve & link cnat doc

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Change-Id: Ib985ed6a644ae3f4c330bf6a27dc69c49a489a2f
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
This commit is contained in:
Nathan Skrzypczak
2020-09-04 16:27:20 +02:00
committed by Damjan Marion
parent e697caf9f8
commit 24179ee1a4
3 changed files with 81 additions and 13 deletions
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docs/gettingstarted/developers/cnat.rst Symbolic link
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../../../src/plugins/cnat/cnat.rst

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docs/gettingstarted/developers/index.rst
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@ -41,3 +41,4 @@ The Developers section covers the following areas:
punt
quic_plugin
cross_compile_macos.rst
cnat

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src/plugins/cnat/cnat.rst
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@ -17,27 +17,45 @@ translations of the packets destined to them
Terminology & Usage
___________________
Setting up the NAT will consist in the creation of a translation
that has several backends. A translation is 3-tuple containing :
Setting up the NAT will consist in the creation of a ``translation``
that has several backends. A ``translation`` is 3-tuple containing :
a fully qualified IP address a port and a protocol. All packets
destined to it (ip, port) will then choose one of the backends,
and follow its rewrite rules.
A backend consists of four rewrites components (source & destination
A ``backend`` consists of four rewrites components (source & destination
address, source & destination port) that shall be applied to packets
on the way in, and reverted on the way back.
Backends are equally load-balanced with a flow hash. The choice
of a backend for a flow will trigger the creation of a NAT session,
of a ``backend`` for a flow will trigger the creation of a NAT ``session``,
that will store the packet rewrite to do and the one to undo
until the flow is reset or a timeout is reached
A ``session`` is a fully resolved 9-tuple of ``src_ip, src_port, dest_ip, dest_port, proto``
to match incoming packets, and their new attributes ``new_src_ip, new_src_port, new_dest_ip, new_dest_port``. It allows for ``backend`` stickyness and a fast-path for established connections.
These ``sessions`` expire after 30s for regular ``sessions`` and 1h for estabished
TCP connections. These can be changed in vpp's configuration file
.. code-block:: console
cnat {
session-max-age 60
tcp-max-age 3600
}
Traffic is matched by inserting FIB entries, that are represented
by a ``client``. These maintain a refcount of the number of ``sessions``
and/or ``translations`` depending on them and be cleaned up when
all have gone.
Translating Addresses
---------------------
In this example, all packets destined to 30.0.0.2:80 will be
rewritten so that their destination IP is 20.0.0.1 and destination
port 8080. Here 30.0.0.2 has to be a virtual IP, it cannot be
In this example, all packets destined to ``30.0.0.2:80`` will be
rewritten so that their destination IP is ``20.0.0.1`` and destination
port ``8080``. Here ``30.0.0.2`` has to be a virtual IP, it cannot be
assigned to an interface
.. code-block:: console
@ -45,9 +63,9 @@ assigned to an interface
cnat translation add proto TCP vip 30.0.0.2 80 to ->20.0.0.1 8080
If 30.0.0.2 is the address of an interface, we can use the following
If ``30.0.0.2`` is the address of an interface, we can use the following
to do the same translation, and additionnaly change the source.
address with 1.2.3.4
address with ``1.2.3.4``
.. code-block:: console
@ -67,15 +85,63 @@ SourceNATing outgoing traffic
A independant part of the plugin allows changing the source address
of outgoing traffic on a per-interface basis.
In the following example, all traffic comming from ``tap0`` and NOT
going to ``20.0.0.0/24`` will be source NAT-ed with ``30.0.0.1``.
On the way back the translation will be undone.
NB: ``30.0.0.1`` should be and address known to the FIB (e.g. the
address assigned to an interface)
.. code-block:: console
cnat snat with 30::1
cnat snat exclude 20::/100
ex_ctl _calico_master cnat snat exclude 10::/100
ex_ctl _calico_master set interface feature tap0 ip6-cnat-snat arc ip6-unicast
cnat snat with 30.0.0.1
cnat snat exclude 20.0.0.0/24
set interface feature tap0 ip4-cnat-snat arc ip4-unicast
Other parameters
----------------
In vpp's startup file, you can also configure the bihash sizes for
* the translation bihash ``(proto, port) -> translation``
* the session bihash ``src_ip, src_port, dest_ip, dest_port, proto -> new_src_ip, new_src_port, new_dest_ip, new_dest_port``
* the snat bihash for searching ``snat exclude`` prefixes
.. code-block:: console
cnat {
translation-db-memory 64K
translation-db-buckets 1024
session-db-memory 1M
session-db-buckets 1024
snat-db-memory 64M
snat-db-buckets 1024
}
Extending the NAT
_________________
This plugin is built to be extensible. For now two NAT types are defined, ``cnat_node_vip.c`` and ``cnat_node_snat.c``. They both inherit from ``cnat_node.h`` which provides :
* Session lookup : ``rv`` will be set to ``0`` if a session was found
* Translation primitives ``cnat_translation_ip4`` based on sessions
* A session creation primitive ``cnat_session_create``
Creating a session will also create a reverse session (for matching return traffic),
and call a NAT node back that will perform the translation.
Known limitations
_________________
This plugin is still under developpment, it lacks the following features :
* Load balancing doesn't support parametric probabilities
* VRFs aren't supported. All rules apply to fib table 0 only
* Programmatic session handling (deletion, lifetime updates) aren't supported
* ICMP is not yet supported
* Traffic matching is only done based on ``(proto, dst_addr, dst_port)`` source matching isn't supported
* Statistics & session tracking are still rudimentary.