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A healthchecker for Anycasted Services

Project Description
A healthchecker for Anycasted services.


anycast-healthchecker monitors a service by doing periodic health checks and based on the result instructs Bird daemon to either advertise or withdraw the route to reach the monitored service. As a result Bird will only advertise routes for healthy services. Routes for IPv4 and IPv6 addresses are supported.

Bird must be configured in a certain way to interface properly with anycast-healthchecker. The configuration is detailed later in this document.

anycast-healthchecker is a Python program, which uses the daemon library to implement a well-behaved Unix daemon process and threading to run multiple service checks in parallel.

What is Anycast

Anycast is a network address scheme where traffic from a sender has more than one potential receivers, but only one of them receives it. Routing protocols, decide which one of the potential receivers will actually receive traffic based on the topology of the network. The main attribute which contributes to the decision is the cost of the network path between a sender and a receiver.

Cost is a protocol specific (usually integer) value that only has meaning within a protocol that is used as a metric of distance. Routing protocols provide default values for common topologies (BGP associates the cost of a path with the number of autonomous systems between the sender and the receiver, OSPF calculates the default cost based on the bandwidth of links), but its main use is to allow administrative control over traffic flow by specifying cost according to business needs.

The nearest receiver to a sender always receives the traffic and this only changes if something changes on the network, another receiver with best path to the sender appears or current receiver disappears. If multiple receivers have the same distance from the sender more than one of the receivers might receive traffic, based on further details of the network configuration.

The three drawings below exhibit how traffic is routed between a sender and multiple potential receivers when something changes on network. In this example BGP routing protocol is used:

These potential receivers use BGP or OSPF by running an Internet Routing daemon, Bird or Quagga, to simultaneously announce the same destination IP address from different places on the network. Due to the nature of Anycast receivers can be located on any network across a global network infrastructure.

Anycast doesn’t balance traffic as only one receiver attracts traffic from senders. For instance, if there are two receivers which announce the same destination IP address in certain location, traffic will be distributed across those two receivers unevenly as senders can be spread across the network in an uneven way.

Anycast is being used as a mechanism to switch traffic between and within data-centers for the following main reasons:

  • the switch of traffic occurs without the need to enforce a change to clients

In case of loss of a service in one location, traffic to that location will be switched to another data-center without any manual intervention and most importantly without pushing a change to clients which you don’t always control.

  • the switch happens within few milliseconds

The same technology can be used for balancing traffic using Equal-Cost Multi-Path.

ECMP routing is a network technology where traffic can be routed over multiple paths. In the context of routing protocols, path is the route a packet has to take in order to be delivered to a destination. Because these multiple paths have the same cost, traffic is balanced across them.

This provides the possibility to perform load-balancing of traffic across multiple servers. Routers are the devices which perform load-balancing of traffic and most of them use a deterministic way to select the server based on the following four properties of IP packets:

  • source IP
  • source PORT
  • destination IP
  • destination PORT

Each unique combination of values for those four properties is called network flow. For each different network flow a different destination server is selected so traffic is evenly balanced across all servers. These servers run an Internet Routing daemon in the same way as with Anycast case but with the major difference that all servers receive traffic.

The main characteristic of this type of load-balancing is that is stateless. Router balances traffic to a destination IP address based on the quadruple network flow without the need to understand and inspect protocols above Layer 3. As a result it is very cheap in terms of resources and very fast at the same time. This is commonly advertised as traffic balancing at wire-speed.

anycast-healthchecker can be utilized in Anycast and ECMP environments.

How anycast-healthchecker works

The current release of anycast-healthchecker supports only the Bird daemon, which you have to configure in a specific way. Thus, it is mandatory to explain very briefly how Bird handles advertisements for routes.

Bird maintains a routing information base (RIB) and various protocols import/export routes to/from it. The diagram below illustrates how Bird advertises routes for IPs assigned to the loopback interface to the rest of the network using BGP protocol. Bird can also import routes learned via BGP/OSPF protocols, but this part of the routing process is irrelevant to the functionality of anycast-healthchecker.

A route is always associated with a service which runs locally on the box. The Anycasted service is a daemon (HAProxy, Nginx, Bind etc) which processes incoming traffic and listens to an IP (Anycast Service Address) for which a route exists in the RIB and advertised by Bird.

As it is exhibited in the above diagram a route is advertised only when:

  1. The IP is assigned to the loopback interface.
  2. direct protocol from Bird imports a route for that IP in the RIB.
  3. BGP/OSPF protocols export that route from the RIB to a network peer.

The route associated with the Anycasted service must be either advertised or withdrawn based on the health of the service, otherwise traffic will always be routed to the local node regardless of the status of the service.

Bird provides filtering capabilities with the help of a simple programming language. A filter can be used to either accept or reject routes before they are exported from the RIB to the network.

A list of IP prefixes (<IP>/<prefix length>) is stored in a text file. IP prefixes that are not included in the list are filtered-out and they do not get exported from the RIB to the network. The white-list text file is sourced by Bird upon startup, reload and reconfiguration. The following diagram illustrates how this technique works:

This configuration logic allows a separate process to update the list by adding or removing IP prefixes and trigger a reconfiguration of Bird in order to advertise or withdraw routes. anycast-healthchecker is that separate process. It monitors Anycasted services and based on the status of the health checks updates the list of IP prefixes.

Bird does not allow the definition of a list with no elements and when that happens Bird will emit an error and refuses to start. Because of this anycast-healthchecker makes sure that there is always an IP prefix in the list, see dummy_ip_prefix and dummy_ip6_prefix settings in Daemon section.

Configuring anycast-healthchecker

Because anycast-healthchecker is very much tied in with Bird daemon, the configuration of Bird is explained first. Next, the configuration of anycast-healthchecker (including the configuration for the health checks) is covered and finally the options for invoking the program from the command line will be described.

IPv6 support

IPv4 and IPv6 addresses are supported by Bird Internet Routing Daemon project by providing a different daemon per IP protocol version, bird for IPv4 and bird6 for IPv6. This implies that configuration files are split as well. So, you can’t define IPv6 addresses in a configuration, which is sourced by the IPv4 daemon.

Bird configuration

The logic described in How anycast-healthchecker works can be accomplished by configuring:

  1. an include statement to source other configuration files in bird.conf
  2. a function, match_route, as an export filter for the routing protocol (BGP or OSPF)
  3. a list of IP prefixes for routes which allowed to be exported by Bird

anycast-healthchecker does not install any of the files mentioned in this section.


The most important parts are the lines include "/etc/bird.d/*.conf"; and export where match_route();. The former statement causes inclusion of files and the latter forces all routes to pass from the match_route function before they are exported. BGP protocol is used in the below bird configuration example but OSPF protocol can be used as well:

include "/etc/bird.d/*.conf";
protocol device {
    scan time 10;
protocol direct direct1 {
    interface "lo";
        export none;
        import all;
template bgp bgp_peers {
    import none;
    export where match_route();
    local as 64815;
protocol bgp BGP1 from bgp_peers {
    disabled no;
    neighbor as 64814;


match-route.conf file configures the match_route function, which performs the white and black listing of IP prefixes by looking up the IP prefix of the route in a list and accepts the export if it finds a matching entry:

function match_route()
    return net ~ ACAST_PS_ADVERTISE;

This the equivalent function for IPv6:

function match_route6()
    return net ~ ACAST6_PS_ADVERTISE;


anycast-prefixes.conf file defines a list of IP prefixes which is stored in a variable with the name ACAST_PS_ADVERTISE. The name of the variable can be anything meaningful but bird_variable setting must be changed accordingly in order for anycast-healthchecker to modify it.


anycast-healthchecker daemon removes IP prefixes from the list for which a service check is not configured. But, the IP prefix set in dummy_ip_prefix does not need a service check configuration.

This the equivalent list for IPv6 prefixes:


anycast-healthchecker creates anycast-prefixes.conf file for both IP versions upon startup if those file don’t exist. After the launch no other process(es) should modify those files.

Use daemon settings bird_conf and bird6_conf to control the location of the files.

With the default settings those files are located under /var/lib/anycast-healthchecker and /var/lib/anycast-healthchecker/6 Administrators must create those 2 directories with permissions 755 and user/group ownership to the account under which the daemon runs.

In order for Bird daemon to load them using the include statement in the main Bird configuration (bird.conf), a link for each file must be created under /etc/bird.d directory. Administrators must also create those two links. Here is an example from a production server:

% ls -ls /etc/bird.d/anycast-prefixes.conf
4 lrwxrwxrwx 1 root root 105 Dec  2 16:08 /etc/bird.d/anycast-prefixes.conf ->

% ls -ls /etc/bird.d/6/anycast-prefixes.conf
4 lrwxrwxrwx 1 root root 107 Jan 10 10:33 /etc/bird.d/6/anycast-prefixes.conf
-> /var/lib/anycast-healthchecker/6/anycast-prefixes.conf

Configuring the daemon

anycast-healthchecker uses the popular INI format for its configuration files. This is an example configuration file for the daemon (/etc/anycast-healthchecker.conf):

interface            = lo

pidfile              = /var/run/anycast-healthchecker/
ipv4                 = true
ipv6                 = false
bird_conf            = /var/lib/anycast-healthchecker/anycast-prefixes.conf
bird6_conf           = /var/lib/anycast-healthchecker/6/anycast-prefixes.conf
bird_variable        = ACAST_PS_ADVERTISE
bird6_variable       = ACAST6_PS_ADVERTISE
bird_reconfigure_cmd = sudo /usr/sbin/birdc configure
bird6_reconfigure_cmd = sudo /usr/sbin/birdc6 configure
dummy_ip_prefix      =
dummy_ip6_prefix     = 2001:db8::1/128
bird_keep_changes    = false
bird6_keep_changes   = false
bird_changes_counter = 128
bird6_changes_counter = 128
purge_ip_prefixes    = false
loglevel             = debug
log_maxbytes         = 104857600
log_backups          = 8
log_file             = /var/log/anycast-healthchecker/anycast-healthchecker.log
stderr_file          = /var/log/anycast-healthchecker/stderr.log
stdout_file          = /var/log/anycast-healthchecker/stdout.log

Above settings are used as defaults when daemon is launched without a configuration file. The daemon does not need to run as root as long as it has sufficient privileges to modify the Bird configuration set in bird_conf or bird6_conf, and trigger a reconfiguration of Bird by running the command configured in bird_reconfigure_cmd or bird6_reconfigure_cmd. In the above example sudo is used for that purpose (sudoers file has been modified for that purpose).

DEFAULT section

Below are the default settings for all service checks, see Configuring checks for services for an explanation of the parameters. Settings in this section can be overwritten in other sections.


Daemon section

Settings for anycast-healthchecker daemon

  • pidfile Defaults to /var/run/anycast-healthchecker/

File to store the process id of the daemon. The parent directory must be created prior the initial launch.

  • ipv4 Defaults to true

true enables IPv4 support and false disables it. NOTE: Daemon will not start if IPv4 support is disabled while there is an service check configured for IPv4 prefix.

  • ipv6 Defaults to false

true enables IPv6 support and false disables it NOTE: Daemon will not start if IPv6 support is disabled while there is an service check configured for IPv6 prefix.

  • bird_conf Defaults to /var/lib/anycast-healthchecker/anycast-prefixes.conf

File with the list of IPv4 prefixes allowed to be exported. If this file is a symbolic link then the destination and the link itself must be on the same mounted filesystem.

  • bird6_conf Defaults to /var/lib/anycast-healthchecker/6/anycast-prefixes.conf

File with the list of IPv6 prefixes allowed to be exported. If this file is a symbolic link then the destination and the link itself must be on the same mounted filesystem.

  • bird_variable Defaults to ACAST_PS_ADVERTISE

The name of the list defined in bird_conf

  • bird6_variable Defaults to ACAST6_PS_ADVERTISE

The name of the list defined in bird6_conf

  • bird_reconfigure_cmd Defaults to sudo /usr/sbin/birdc configure

Command to trigger a reconfiguration of IPv4 Bird daemon

  • bird6_reconfigure_cmd Defaults to sudo /usr/sbin/birdc6 configure

Command to trigger a reconfiguration of IPv6 Bird daemon

  • dummy_ip_prefix Defaults to

An IP prefix in the form <IP>/<prefix length> which will be always available in the list defined by bird_variable to avoid having an empty list. The dummy_ip_prefix must not be used by any service or assigned to the interface set with interface or configured anywhere on the network as anycast-healthchecker does not perform any checks for it.

  • dummy_ip6_prefix Defaults to 2001:db8::1/128

An IPv6 prefix in the form <IPv6>/<prefix length> which will be always available in the list defined by bird6_variable to avoid having an empty list. The dummy_ip6_prefix must not be used by any service or assigned to the interface set with interface or configured anywhere on the network as anycast-healthchecker does not perform any checks for it.

  • bird_keep_changes Defaults to false

Keep a history of changes for bird_conf file by copying it to a directory. During the startup of the daemon a directory with the name history is created under the directory where bird_conf file resides. The daemon has to have sufficient privileges to create that directory.

  • bird6_keep_changes Defaults to false

Keep a history of changes for bird6_conf file by copying it to a directory. During the startup of the daemon a directory with the name history is created under the directory where bird6_conf file resides. The daemon has to have sufficient privileges to create that directory. WARNING: When keeping a history of changes is enabled for both IP versions then configuration files set in bird_conf and bird6_conf settings must be stored on two different directories.

  • bird_changes_counter Defaults to 128

How many bird_conf files to keep in the history directory.

  • bird6_changes_counter Defaults to 128

How many bird6_conf files to keep in the history directory.

  • purge_ip_prefixes Defaults to false

Purge IP-Prefixes from configuration files set in bird_conf and bird6_conf on start-up which don’t have a service check associated with them.

NOTE: Those IP-Prefixes are always removed from the configuration files set in bird_conf and in bird6_conf settings when daemon updates those files. purge_ip_prefixes is considered only during start-up and was introduced in order to be compatible with the behavior of previous releases, which didn’t remove those IP-Prefixes on start-up.

  • loglevel Defaults to debug

Log level to use, possible values are: debug, info, warning, error, critical

  • log_file Defaults to /var/log/anycast-healthchecker/anycast-healthchecker.log

File to log messages to. The parent directory must be created prior the initial launch.

  • log_maxbytes Defaults to 104857600 (bytes)

Maximum size in bytes for log files

  • log_backups Defaults to 8

Number of old log files to maintain

  • stderr_file Defaults to /var/log/anycast-healthchecker/stderr.log

File to redirect standard error to. The parent directory must be created prior the initial launch.

  • stdout_file Defaults to /var/log/anycast-healthchecker/stdout.log

File to redirect standard output to. The parent directory must be created prior the initial launch.

JSON logging

anycast-healthchecker daemon can be configured to send logging messages over HTTP to a central place in addition to write them to log files. It builds a JSON blob with a specific data structure, which is not configurable at the moment.

The following settings can be added to the [daemon] section for enabling JSON logging.

  • json_logging Defaults to false

true enables JSON logging false disables it

  • http_server Unset by default

Server name to send JSON logging over HTTP protocol

  • http_server_port Unset by default

Port to connect

  • http_server_protocol Unset by default

HTTP protocol to use, either http or https

  • http_server_timeout Unset by default

How long to wait for the server to accept data before giving up, as a floating point number. Daemon sends JSON data over HTTP in blocking mode, which means that possible long delays sending JSON will make the health checks to be delayed as well. http_server_timeout accepts floating point numbers as value, which is passed to underlying requests module as a single timeout, which is applied to both the connect and the read timeouts.

Configuring checks for services

The configuration for a single service check is defined in one section. Here are few examples:

check_cmd         = /usr/bin/curl --fail --silent
check_interval    = 10
check_timeout     = 2
check_fail        = 2
check_rise        = 2
check_disabled    = false
on_disabled       = withdraw
ip_prefix         =

check_cmd         = /usr/bin/curl --fail 'http://[fd12:aba6:57db:ffff::1]:8888'
check_timeout     = 5
check_rise        = 2
check_fail        = 2
check_disabled    = false
on_disabled       = withdraw
ip_prefix         = fd12:aba6:57db:ffff::1/128
ip_check_disabled = false

The name of the section becomes the name of the service check and appears in the log files for easier searching of error/warning messages.

  • check_cmd Unset by default

The command to run to determine the status of the service based on the return code. Complex health checking should be wrapped in a script. When check command fails, the stdout and stderr appears in the log file.

  • check_interval Defaults to 2 (seconds)

How often to run the check

  • check_timeout Defaults to 2 (seconds)

Maximum time in seconds for the check command to complete. anycast-healthchecker will try kill the check if it doesn’t return after check_timeout seconds. If check_cmd runs under another user account (root) via sudo then it wouldn’t be killed. anycast-healthchecker could run as root to overcome this problem, but it is highly recommended to run it as normal user.

  • check_fail Defaults to 2

A service is considered DOWN after this many consecutive unsuccessful health checks

  • check_rise Defaults to 2

A service is considered HEALTHY after this many consecutive successful health checks

  • check_disabled Defaults to false

true disables the check, false enables it

  • on_disabled Defaults to withdraw

What to do when check is disabled, either withdraw or advertise

  • ip_prefix Unset by default

IP prefix associated with the service. It must be assigned to the interface set in interface parameter unless ip_check_disabled is set to true. Prefix length is optional and defaults to 32 for IPv4 addresses and to 128 for IPv6 addresses.

  • ip_check_disabled Defaults to false

true disables the assignment check of ip_prefix to the interface set in interface, false enables it.

If the check_cmd checks the availability of the service by sending a request to the Anycasted IP address then that request may be served by another node which advertises the same IP address on the network. This usually happens when the Anycasted IP address is not assigned to loopback or any other interface on the local node.

Therefore, it should be only enabled in environments where the network or the network configuration of the local node prevents the request from check_cmd to be forwarded to another node.

  • interface Defaults to lo

The name of the interface that ip_prefix is assigned to

Multiple sections may be combined in one file or provide one file per section. File must be stored under one directory and their name should use .conf as suffix (

Starting the daemon

Daemon CLI usage:

anycast-healthchecker --help
A simple healthchecker for Anycasted services.

    anycast-healthchecker [ -f <file> -c -p -P ] [ -d <directory> | -F <file> ]

    -f, --file=<file>          read settings for the daemon from <file>
                               [default: /etc/anycast-healthchecker.conf]
    -d, --dir=<dir>            read settings for service checks from files
                               under <dir> directory
                               [default: /etc/anycast-healthchecker.d]
    -F, --service-file=<file>  read <file> for settings of a single service
    -c, --check                perform a sanity check on configuration
    -p, --print                show default settings for daemon and service
    -P, --print-conf           show running configuration with default settings
    -v, --version              show version
    -h, --help                 show this screen

The daemon can be launched by supplying a configuration file and a directory with configuration files for service checks:

anycast-healthchecker -f ./anycast-healthchecker.conf -d ./anycast-healthchecker.d

At the root of the project there is System V init and a Systemd unit file for proper integration with OS startup tools.

Systemd and SysVinit integration

Under contrib/systemd and contrib/SysVinit directories there are the necessary Unit service and startup files which can be used to start the daemon on boot.

Nagios check

Under contrib/nagios directory there is a nagios plugin to check if daemon is up and if all threads are running.


Use pip:

pip install anycast-healthchecker

From Source:

sudo python install

Build (source) RPMs:

python clean --all; python bdist_rpm

Build a source archive for manual installation:

python sdist


  1. Bump version in anycast_healthchecker/

  2. Commit above change with:

    git commit -av -m'RELEASE 0.1.3 version'
  3. Create a signed tag, pbr will use this for the version number:

    git tag -s 0.1.3 -m 'bump release'
  4. Create the source distribution archive (the archive will be placed in the dist directory):

    python sdist
  5. pbr will update ChangeLog file and we want to squeeze them to the previous commit thus we run:

    git commit -av --amend
  6. Move current tag to the last commit:

    git tag -fs 0.1.3 -m 'bump release'
  7. Push changes:

    git push;git push --tags


I would love to hear what other people think about anycast_healthchecker and provide feedback. Please post your comments, bug reports and wishes on my issues page.


At the root of the project there is a script which you can use for testing purposes. It does the following:

  1. Creates the necessary directory structure under $PWD/var to store configuration and log files
  2. Generates configuration for the daemon and for 2 service checks
  3. Generates bird configuration(anycast-prefixes.conf)
  4. Installs anycast-healthchecker with python3 install
  5. Assigns 4 IPv4 addresses and 2 IPv6 addresses to loopback interface
  6. Checks if bird daemon runs but it does not try to start if it is down
  7. Starts the daemon as normal user and not as root

Requirements for running

  1. python3 installation
  2. A working python virtual environment, use the excellent tool virtualenvwrapper
  3. Bird installed and configured as it is mentioned in Bird configuration
  4. sudo access to run birdc configure and birdc6 configure
  5. sudo access to assign IPs on the loopback interface using ip tool


The following people have contributed to project with feedback, commits and code reviews

  • Károly Nagy (@charlesnagy)
  • Nick Demou (@ndemou)
  • Ralf Ertzinger (@alufu)


Apache 2.0


This program was originally developed for With approval from, the code was generalised and published as Open Source on github, for which the author would like to express his gratitude.

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