Merge pull request #1016 from sever-sever/reverse-proxy

Add load-balancing reverse-proxy documentation and examples

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+:lastproofread: 2023-01-27
+
+.. _load-balancing:
+
+WAN load balancing
+==================
+
+Outbound traffic can be balanced between two or more outbound interfaces.
+If a path fails, traffic is balanced across the remaining healthy paths,
+a recovered path is automatically added back to the routing table and used by 
+the load balancer. The load balancer automatically adds routes for each path to
+the routing table and balances traffic across the configured interfaces,
+determined by interface health and weight.
+
+
+In a minimal configuration, the following must be provided:
+
+ * an interface with a nexthop
+ * one rule with a LAN (inbound-interface) and the WAN (interface).
+
+Let's assume we have two DHCP WAN interfaces and one LAN (eth2):
+
+.. code-block:: none
+
+    set load-balancing wan interface-health eth0 nexthop 'dhcp'
+    set load-balancing wan interface-health eth1 nexthop 'dhcp'
+    set load-balancing wan rule 1 inbound-interface 'eth2'
+    set load-balancing wan rule 1 interface eth0
+    set load-balancing wan rule 1 interface eth1
+
+.. note::
+
+    WAN Load Balacing should not be used when dynamic routing protocol is
+    used/needed. This feature creates customized routing tables and firewall
+    rules, that makes it incompatible to use with routing protocols.
+
+Balancing Rules
+---------------
+
+Interfaces, their weight and the type of traffic to be balanced are defined in
+numbered balancing rule sets. The rule sets are executed in numerical order
+against outgoing packets. In case of a match the packet is sent through an
+interface specified in the matching rule. If a packet doesn't match any rule
+it is sent by using the system routing table. Rule numbers can't be changed.
+
+Create a load balancing rule, it can be a number between 1 and 9999:
+
+.. code-block:: none
+
+    vyos@vyos# set load-balancing wan rule 1
+    Possible completions:
+    description             Description for this rule
+    > destination           Destination
+    exclude                 Exclude packets matching this rule from wan load balance
+    failover                Enable failover for packets matching this rule from wan load balance
+    inbound-interface       Inbound interface name (e.g., "eth0") [REQUIRED]
+    +> interface            Interface name [REQUIRED]
+    > limit                 Enable packet limit for this rule
+    per-packet-balancing    Option to match traffic per-packet instead of the default, per-flow
+    protocol                Protocol to match
+    > source                Source information
+
+Interface weight
+****************
+
+Let's expand the example from above and add weight to the interfaces.
+The bandwidth from eth0 is larger than eth1. Per default, outbound traffic is
+distributed randomly across available interfaces. Weights can be assigned to
+interfaces to influence the balancing.
+
+.. code-block:: none
+
+    set load-balancing wan rule 1 interface eth0 weight 2
+    set load-balancing wan rule 1 interface eth1 weight 1
+
+66% of traffic is routed to eth0, eth1 gets 33% of traffic.
+
+Rate limit
+**********
+
+A packet rate limit can be set for a rule to apply the rule to traffic above or
+below a specified threshold. To configure the rate limiting use:
+
+.. code-block:: none
+
+    set load-balancing wan rule <rule> limit <parameter>
+
+* ``burst``: Number of packets allowed to overshoot the limit within ``period``.
+  Default 5.
+* ``period``: Time window for rate calculation. Possible values:
+  ``second`` (one second), ``minute`` (one minute), ``hour`` (one hour).
+  Default is ``second``.
+* ``rate``: Number of packets. Default 5.
+* ``threshold``: ``below`` or ``above`` the specified rate limit.
+
+Flow and packet-based balancing
+*******************************
+
+Outgoing traffic is balanced in a flow-based manner.
+A connection tracking table is used to track flows by their source address,
+destination address and port. Each flow is assigned to an interface according
+to the defined balancing rules and subsequent packets are sent through the
+same interface. This has the advantage that packets always arrive in order if
+links with different speeds are in use.
+
+Packet-based balancing can lead to a better balance across interfaces when out
+of order packets are no issue. Per-packet-based balancing can be set for a
+balancing rule with:
+
+.. code-block:: none
+
+    set load-balancing wan rule <rule> per-packet-balancing
+
+Exclude traffic
+***************
+
+To exclude traffic from load balancing, traffic matching an exclude rule is not
+balanced but routed through the system routing table instead:
+
+.. code-block:: none
+
+    set load-balancing wan rule <rule> exclude
+
+
+Health checks
+-------------
+
+The health of interfaces and paths assigned to the load balancer is
+periodically checked by sending ICMP packets (ping) to remote destinations,
+a TTL test or the execution of a user defined script. If an interface fails the
+health check it is removed from the load balancer's pool of interfaces.
+To enable health checking for an interface:
+
+.. code-block:: none
+
+    vyos@vyos# set load-balancing wan interface-health <interface>
+    Possible completions:
+    failure-count    Failure count
+    nexthop          Outbound interface nexthop address. Can be 'dhcp or ip address' [REQUIRED]
+    success-count    Success count
+    +> test          Rule number
+
+Specify nexthop on the path to the destination, ``ipv4-address`` can be set to
+``dhcp``
+
+.. code-block:: none
+
+    set load-balancing wan interface-health <interface> nexthop <ipv4-address>
+
+Set the number of health check failures before an interface is marked as
+unavailable, range for number is 1 to 10, default 1. Or set the number of
+successful health checks before an interface is added back to the interface
+pool, range for number is 1 to 10, default 1.
+
+.. code-block:: none
+
+    set load-balancing wan interface-health <interface> failure-count <number>
+    set load-balancing wan interface-health <interface> success-count <number>
+
+Each health check is configured in its own test, tests are numbered and
+processed in numeric order. For multi target health checking multiple tests
+can be defined:
+
+.. code-block:: none
+
+    vyos@vyos# set load-balancing wan interface-health eth1 test 0
+    Possible completions:
+    resp-time    Ping response time (seconds)
+    target       Health target address
+    test-script  Path to user defined script
+    ttl-limit    Ttl limit (hop count)
+    type         WLB test type
+
+* ``resp-time``: the maximum response time for ping in seconds.
+  Range 1...30, default 5
+* ``target``: the target to be sent ICMP packets to, address can be an IPv4
+  address or hostname
+* ``test-script``: A user defined script must return 0 to be considered
+  successful and non-zero to fail. Scripts are located in /config/scripts,
+  for different locations the full path needs to be provided
+* ``ttl-limit``: For the UDP TTL limit test the hop count limit must be
+  specified. The limit must be shorter than the path length, an ICMP time
+  expired message is needed to be returned for a successful test. default 1
+* ``type``: Specify the type of test. type can be ping, ttl or a user defined
+  script
+
+Source NAT rules
+----------------
+
+Per default, interfaces used in a load balancing pool replace the source IP
+of each outgoing packet with its own address to ensure that replies arrive on
+the same interface. This works through automatically generated source NAT (SNAT)
+rules, these rules are only applied to balanced traffic. In cases where this
+behaviour is not desired, the automatic generation of SNAT rules can be
+disabled:
+
+.. code-block:: none
+
+    set load-balancing wan disable-source-nat
+
+Sticky Connections
+------------------
+Inbound connections to a WAN interface can be improperly handled when the reply
+is sent back to the client.
+
+.. image:: /_static/images/sticky-connections.jpg
+   :width: 80%
+   :align: center
+
+
+Upon reception of an incoming packet, when a response is sent, it might be
+desired to ensure that it leaves from the same interface as the inbound one.
+This can be achieved by enabling sticky connections in the load balancing:
+
+.. code-block:: none
+
+    set load-balancing wan sticky-connections inbound
+
+Failover
+--------
+
+In failover mode, one interface is set to be the primary interface and other
+interfaces are secondary or spare. Instead of balancing traffic across all
+healthy interfaces, only the primary interface is used and in case of failure,
+a secondary interface selected from the pool of available interfaces takes over.
+The primary interface is selected based on its weight and health, others become
+secondary interfaces. Secondary interfaces to take over a failed primary
+interface are chosen from the load balancer's interface pool, depending
+on their weight and health. Interface roles can also be selected based on rule
+order by including interfaces in balancing rules and ordering those rules
+accordingly. To put the load balancer in failover mode, create a failover rule:
+
+.. code-block:: none
+
+    set load-balancing wan rule <number> failover
+
+Because existing sessions do not automatically fail over to a new path,
+the session table can be flushed on each connection state change:
+
+.. code-block:: none
+
+    set load-balancing wan flush-connections
+
+.. warning::
+
+    Flushing the session table will cause other connections to fall back from
+    flow-based to packet-based balancing until each flow is reestablished.
+
+Script execution
+----------------
+
+A script can be run when an interface state change occurs. Scripts are run
+from /config/scripts, for a different location specify the full path:
+
+.. code-block:: none
+
+    set load-balancing wan hook script-name
+
+Two environment variables are available:
+
+* ``WLB_INTERFACE_NAME=[interfacename]``: Interface to be monitored
+* ``WLB_INTERFACE_STATE=[ACTIVE|FAILED]``: Interface state
+
+.. warning::
+
+    Blocking call with no timeout. System will become unresponsive if script
+    does not return!
+
+Handling and monitoring
+-----------------------
+
+
+Show WAN load balancer information including test types and targets.
+A character at the start of each line depicts the state of the test
+
+* ``+`` successful
+* ``-`` failed
+* a blank indicates that no test has been carried out
+
+.. code-block:: none
+
+    vyos@vyos:~$ show wan-load-balance
+    Interface:  eth0
+    Status:  failed
+    Last Status Change:  Tue Jun 11 20:12:19 2019
+    -Test:  ping  Target:
+        Last Interface Success:  55s
+        Last Interface Failure:  0s
+        # Interface Failure(s):  5
+
+    Interface:  eth1
+    Status:  active
+    Last Status Change:  Tue Jun 11 20:06:42 2019
+    +Test:  ping  Target:
+        Last Interface Success:  0s
+        Last Interface Failure:  6m26s
+        # Interface Failure(s):  0
+
+Show connection data of load balanced traffic:
+
+.. code-block:: none
+
+    vyos@vyos:~$ show wan-load-balance connection
+    conntrack v1.4.2 (conntrack-tools): 3 flow entries have been shown.
+    Type    State           Src                     Dst                     Packets Bytes
+    tcp     TIME_WAIT       10.1.1.13:38040         203.0.113.2:80          203.0.113.2  192.168.188.71
+    udp                     10.1.1.13:41891         198.51.100.3:53         198.51.100.3 192.168.188.71
+    udp                     10.1.1.13:55437         198.51.100.3:53         198.51.100.3 192.168.188.71
+
+Restart
+*******
+
+.. code-block:: none
+
+    restart wan-load-balance