Linux network namespace support for HAProxy
===========================================

HAProxy supports proxying between Linux network namespaces. This
feature can be used, for example, in a multi-tenant networking
environment to proxy between different networks. HAProxy can also act
as a front-end proxy for non namespace-aware services.

The proxy protocol has been extended to support transferring the
namespace information, so the originating namespace information can be
kept. This is useful when chaining multiple proxies and services.

To enable Linux namespace support, compile HAProxy with the `USE_NS=1`
make option.


## Setting up namespaces on Linux

To create network namespaces, use the 'ip netns' command. See the
manual page ip-netns(8) for details.

Make sure that the file descriptors representing the network namespace
are located under `/var/run/netns`.

For example, you can create a network namespace and assign one of the
networking interfaces to the new namespace:

```
$ ip netns add netns1
$ ip link set eth7 netns netns1
```


## Listing namespaces in the configuration file

HAProxy uses namespaces explicitly listed in its configuration file.
If you are not using namespace information received through the proxy
protocol, this usually means that you must specify namespaces for
listeners and servers in the configuration file with the 'namespace'
keyword.

However, if you're using the namespace information received through
the proxy protocol to determine the namespace of servers (see
'namespace * below'), you have to explicitly list all allowed
namespaces in the namespace_list section of your configuration file:

```
namespace_list
    namespace netns1
    namespace netns2
```


## Namespace information flow

The haproxy process always runs in the namespace it was started on.
This is the default namespace.

The bind addresses of listeners can have their namespace specified in
the configuration file. Unless specified, sockets associated with
listener bind addresses are created in the default namespace. For
example, this creates a listener in the netns2 namespace:

```
frontend f_example
	bind 192.168.1.1:80 namespace netns2
	default_backend http
```

Each client connection is associated with its source namespace. By
default, this is the namespace of the bind socket it arrived on, but
can be overridden by information received through the proxy protocol.
Proxy protocol v2 supports transferring namespace information, so if
it is enabled for the listener, it can override the associated
namespace of the connection.

Servers can have their namespaces specified in the configuration file
with the 'namespace' keyword:

```
backend b_example
	server s1 192.168.1.100:80 namespace netns2
```

If no namespace is set for a server, it is assumed that it is in the
default namespace. When specified, outbound sockets to the server are
created in the network namespace configured. To create the outbound
(server) connection in the namespace associated with the client, use
the '*' namespace. This is especially useful when using the
destination address and namespace received from the proxy protocol.

```
frontend f_example
	bind 192.168.1.1:9990 accept-proxy
	default_backend b_example

backend b_example
	mode tcp
	source 0.0.0.0 usesrc clientip
	server snodes * namespace *
```

If HAProxy is configured to send proxy protocol v2 headers to the
server, the outgoing header will always contain the namespace
associated with the client connection, not the namespace configured
for the server.