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Configuration

The agent has various configuration options that can be specified via the command-line or via configuration files. All of the configuration options are completely optional and their defaults will be specified with their descriptions.

When loading configuration, Consul loads the configuration from files and directories in the order specified. Configuration specified later will be merged into configuration specified earlier. In most cases, "merge" means that the later version will override the earlier. But in some cases, such as event handlers, merging just appends the handlers. The exact merging behavior will be specified.

Consul also supports reloading of configuration when it receives the SIGHUP signal. Not all changes are respected, but those that are are documented below. The reload command can also be used to trigger a configuration reload.

Command-line Options

The options below are all specified on the command-line.

• -advertise - The advertise address is used to change the address that we advertise to other nodes in the cluster. By default, the -bind address is advertised. However, in some cases, there may be a routable address that cannot be bound to. This flag enables gossiping a different address to support this. If this address is not routable, the node will be in a constant flapping state, as other nodes will treat the non-routability as a failure.

• -bootstrap - This flag is used to control if a server is in "bootstrap" mode. It is important that no more than one server per datacenter be running in this mode. The initial server must be in bootstrap mode. Technically, a server in bootstrap mode is allowed to self-elect as the Raft leader. It is important that only a single node is in this mode, because otherwise consistency cannot be guaranteed if multiple nodes are able to self-elect. Once there are multiple servers in a datacenter, it is generally a good idea to disable bootstrap mode on all of them.

• -bind - The address that should be bound to for internal cluster communications. This is an IP address that should be reachable by all other nodes in the cluster. By default this is "0.0.0.0", meaning Consul will use the first available private IP address. Consul uses both TCP and UDP and use the same port for both, so if you have any firewalls be sure to allow both protocols.

• -client - The address that Consul will bind to client interfaces. This includes the HTTP, DNS, and RPC servers. By default this is "127.0.0.1" allowing only loopback connections. The RPC address is used by other Consul commands, such as consul members, in order to query a running Consul agent.

• -config-file - A configuration file to load. For more information on the format of this file, read the "Configuration Files" section below. This option can be specified multiple times to load multiple configuration files. If it is specified multiple times, configuration files loaded later will merge with configuration files loaded earlier, with the later values overriding the earlier values.

• -config-dir - A directory of configuration files to load. Consul will load all files in this directory ending in ".json" as configuration files in alphabetical order. For more information on the format of the configuration files, see the "Configuration Files" section below.

• -data-dir - This flag provides a data directory for the agent to store state. This is required for all agents. The directory should be durable across reboots. This is especially critical for agents that are running in server mode, as they must be able to persist the cluster state.

• -dc - This flag controls the datacenter the agent is running in. If not provided it defaults to "dc1". Consul has first class support for multiple data centers but it relies on proper configuration. Nodes in the same datacenter should be on a single LAN.

• -join - Address of another agent to join upon starting up. This can be specified multiple times to specify multiple agents to join. If Consul is unable to join with any of the specified addresses, agent startup will fail. By default, the agent won't join any nodes when it starts up.

• -log-level - The level of logging to show after the Consul agent has started. This defaults to "info". The available log levels are "trace", "debug", "info", "warn", "err". This is the log level that will be shown for the agent output, but note you can always connect via consul monitor to an agent at any log level. The log level can be changed during a config reload.

• -node - The name of this node in the cluster. This must be unique within the cluster. By default this is the hostname of the machine.

• -protocol - The Consul protocol version to use. This defaults to the latest version. This should be set only when upgrading. You can view the protocol versions supported by Consul by running consul -v.

• -rejoin - When provided Consul will ignore a previous leave and attempt to rejoin the cluster when starting. By default, Consul treats leave as a permanent intent, and does not attempt to join the cluster again when starting. This flag allows the previous state to be used to rejoin the cluster.

• -server - This flag is used to control if an agent is in server or client mode. When provided, an agent will act as a Consul server. Each Consul cluster must have at least one server, and ideally no more than 5 per datacenter. All servers participate in the Raft consensus algorithm, to ensure that transactions occur in a consistent, linearizable manner. Transactions modify cluster state, which is maintained on all server nodes to ensure availability in the case of node failure. Server nodes also participate in a WAN gossip pool with server nodes in other datacenters. Servers act as gateways to other datacenters and forward traffic as appropriate.

• -syslog - This flag enables logging to syslog. This is only supported on Linux and OSX. It will result in an error if provided on Windows.

• -ui-dir - This flag provides a the directory containing the Web UI resources for Consul. This must be provided to enable the Web UI. Directory must be readable.

• -pid-file - This flag provides the file path for the agent to store it's PID. This is useful for sending signals to the agent, such as SIGINT to close it or SIGHUP to update check definitions.

Configuration Files

In addition to the command-line options, configuration can be put into files. This may be easier in certain situations, for example when Consul is being configured using a configuration management system.

The configuration files are JSON formatted, making them easily readable and editable by both humans and computers. The configuration is formatted at a single JSON object with configuration within it.

Configuration files are used for more than just setting up the agent, they are also used to provide check and service definitions. These are used to announce the availability of system servers to the rest of the cluster. They are documented seperately under check configuration and service configuration respectively. The service and check definitions support being updated during a reload.

Example Configuration File

{
  "datacenter": "east-aws",
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "foobar",
  "server": true
}

Configuration Key Reference

• bootstrap - Equivalent to the -bootstrap command-line flag.

• bind_addr - Equivalent to the -bind command-line flag.

• client_addr - Equivalent to the -client command-line flag.

• datacenter - Equivalent to the -dc command-line flag.

• data_dir - Equivalent to the -data-dir command-line flag.

• log_level - Equivalent to the -log-level command-line flag.

• node_name - Equivalent to the -node command-line flag.

• protocol - Equivalent to the -protocol command-line flag.

• server - Equivalent to the -server command-line flag.

• ui_dir - Equivalent to the -ui-dir command-line flag.

• advertise_addr - Equivalent to the -advertise command-line flag.

• enable_syslog - Equivalent to the -syslog command-line flag.

• rejoin_after_leave - Equivalent to the -rejoin command-line flag.

• ca_file - This provides a the file path to a PEM encoded certificate authority. The certificate authority is used to check the authenticity of client and server connections with the appropriate verify_incoming or verify_outgoing flags.

• cert_file - This provides a the file path to a PEM encoded certificate. The certificate is provided to clients or servers to verify the agents authenticity. Must be provided along with the key_file.

• check_update_interval - This interval controls how often check output from checks in a steady state is syncronized with the server. By default, this is set to 5 minutes ("5m"). Many checks which are in a steady state produce slightly different output per run (timestamps, etc) which cause constant writes. This configuration allows defering the sync of check output for a given interval to reduce write pressure. If a check ever changes state, the new state and associated output is syncronized immediately. To disable this behavior, set the value to "0s".

• domain - By default, Consul responds to DNS queries in the "consul." domain. This flag can be used to change that domain. All queries in this domain are assumed to be handled by Consul, and will not be recursively resolved.

• dns_config - This object allows a number of sub-keys to be set which can tune how DNS queries are perfomed. See this guide on DNS caching. The following sub-keys are available:

  • node_ttl - By default, this is "0s", which means all node lookups are served with a 0 TTL value. This can be set to allow node lookups to set a TTL value, which enables DNS caching. This should be specified with the "s" suffix for second, or "m" for minute.

  • service_ttl - This is a sub-object, which allows for setting a TTL on service lookups with a per-service policy. The "*" wildcard service can be specified and is used when there is no specific policy available for a service. By default, all services are served with a 0 TTL value. Setting this enables DNS caching.

  • allow_stale - Enables a stale query for DNS information. This allows any Consul server to service the request, instead of only the leader. The advantage of this is you get linear read scalability with Consul servers. By default, this is false, meaning all requests are serviced by the leader. This provides stronger consistency but with less throughput and higher latency.

  • max_stale - When allow_stale is specified, this is used to limit how stale of a result will be used. By default, this is set to "5s", which means if a Consul server is more than 5 seconds behind the leader, the query will be re-evaluated on the leader to get more up-to-date results.

• enable_debug - When set, enables some additional debugging features. Currently, only used to set the runtime profiling HTTP endpoints.

• encrypt - Specifies the secret key to use for encryption of Consul network traffic. This key must be 16-bytes that are base64 encoded. The easiest way to create an encryption key is to use consul keygen. All nodes within a cluster must share the same encryption key to communicate.

• key_file - This provides a the file path to a PEM encoded private key. The key is used with the certificate to verify the agents authenticity. Must be provided along with the cert_file.

• leave_on_terminate - If enabled, when the agent receives a TERM signal, it will send a Leave message to the rest of the cluster and gracefully leave. Defaults to false.

• ports - This is a nested object that allows setting the bind ports for the following keys:

  • dns - The DNS server, -1 to disable. Default 8600.
  • http - The HTTP api, -1 to disable. Default 8500.
  • rpc - The RPC endpoint. Default 8400.
  • serf_lan - The Serf LAN port. Default 8301.
  • serf_wan - The Serf WAN port. Default 8302.
  • server - Server RPC address. Default 8300.

• recursor - This flag provides an address of an upstream DNS server that is used to recursively resolve queries if they are not inside the service domain for consul. For example, a node can use Consul directly as a DNS server, and if the record is outside of the "consul." domain, the query will be resolved upstream using this server.

• skip_leave_on_interrupt - This is the similar toleave_on_terminate but only affects interrupt handling. By default, an interrupt causes Consul to gracefully leave, but setting this to true disables that. Defaults to false. Interrupts are usually from a Control-C from a shell.

• start_join - An array of strings specifying addresses of nodes to join upon startup.

• server_name - When give, this overrides the node_name for the TLS certificate. It can be used to ensure that the certificate name matches the hostname we declare.

• statsite_addr - This provides the address of a statsite instance. If provided Consul will stream various telemetry information to that instance for aggregation. This can be used to capture various runtime information.

• syslog_facility - When enable_syslog is provided, this controls which facility messages are sent to. By default, LOCAL0 will be used.

• verify_incoming - If set to True, Consul requires that all incoming connections make use of TLS, and that the client provides a certificate signed by the Certificate Authority from the ca_file. By default, this is false, and Consul will not enforce the use of TLS or verify a client's authenticity. This only applies to Consul servers, since a client never has an incoming connection.

• verify_outgoing - If set to True, Consul requires that all outgoing connections make use of TLS, and that the server provide a certificate that is signed by the Certificate Authority from the ca_file. By default, this is false, and Consul will not make use of TLS for outgoing connections. This applies to clients and servers, as both will make outgoing connections.