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docs	Configuration	docs-agent-config	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. Defaults are specified with their descriptions.

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. Defaults are specified with their descriptions.

When loading configuration, Consul loads the configuration from files and directories in lexical order. For example, configuration file basic_config.json will be processed before extra_config.js. Configuration specified later will be merged into configuration specified earlier. In most cases, "merge" means that the later version will override the earlier. In some cases, such as event handlers, merging appends the handlers to the existing configuration. The exact merging behavior is specified for each option below.

Consul also supports reloading configuration when it receives the SIGHUP signal. Not all changes are respected, but those that are are documented below in the Reloadable Configuration section. 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. 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.

• -atlas - This flag enables Atlas integration. It is used to provide the Atlas infrastructure name and the SCADA connection. This enables Atlas features such as the dashboard and node auto joining.

• -atlas-join - When set, enables auto-join via Atlas. Atlas will track the most recent members to join the infrastructure named by -atlas and automatically join them on start. For servers, the LAN and WAN pool are both joined.

• -atlas-token - Provides the Atlas API authentication token. This can also be provided using the ATLAS_TOKEN environment variable. Required for use with Atlas.

• -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. 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; otherwise, consistency cannot be guaranteed as multiple nodes are able to self-elect. It is not recommended to use this flag after a cluster has been bootstrapped.

• -bootstrap-expect - This flag provides the number of expected servers in the datacenter. Either this value should not be provided or the value must agree with other servers in the cluster. When provided, Consul waits until the specified number of servers are available and then bootstraps the cluster. This allows an initial leader to be elected automatically. This cannot be used in conjunction with the legacy -bootstrap flag.

• -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 the same port for both. If you have any firewalls, be sure to allow both protocols.

• -client - The address to which Consul will bind client interfaces, including 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. 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. During a config merge, single-value keys (string, int, bool) will simply have their values replaced while list types will be appended together.

• -config-dir - A directory of configuration files to load. Consul will load all files in this directory with the suffix ".json". The load order is alphabetical, and the the same merge routine is used as with the config-file option above. For more information on the format of the configuration files, see the Configuration Files section.

• -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 cluster state. Additionally, the directory must support the use of filesystem locking, meaning some types of mounted folders (e.g. VirtualBox shared folders) may not be suitable.

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

• -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.

• -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. The provided key is automatically persisted to the data directory and loaded automatically whenever the agent is restarted. This means that to encrypt Consul's gossip protocol, this option only needs to be provided once on each agent's initial startup sequence. If it is provided after Consul has been initialized with an encryption key, then the provided key is ignored and a warning will be displayed.

• -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.

• -retry-join - Similar to -join but allows retrying a join if the first attempt fails. This is useful for cases where we know the address will become available eventually.

• -retry-interval - Time to wait between join attempts. Defaults to 30s.

• -retry-max - The maximum number of -join attempts to be made before exiting with return code 1. By default, this is set to 0 which is interpreted as infinite retries.

• -join-wan - Address of another wan agent to join upon starting up. This can be specified multiple times to specify multiple WAN 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-wan any nodes when it starts up.

• -retry-join-wan - Similar to retry-join but allows retrying a wan join if the first attempt fails. This is useful for cases where we know the address will become available eventually.

• -retry-interval-wan - Time to wait between -join-wan attempts. Defaults to 30s.

• -retry-max-wan - The maximum number of -join-wan attempts to be made before exiting with return code 1. By default, this is set to 0 which is interpreted as infinite retries.

• -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", and "err". Note that you can always connect to an agent via consul monitor and use any log level. Also, 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.

• -pid-file - This flag provides the file path for the agent to store its PID. This is useful for sending signals (for example, SIGINT to close the agent or SIGHUP to update check definit

• -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.

• -recursor - Specifies the address of an upstream DNS server. This option may be provided multiple times, and is functionally equivalent to the recursors configuration option.

• -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 the directory containing the Web UI resources for Consul. This must be provided to enable the Web UI. The directory must be readable to the agent.

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 as 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 separately 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,
  "watches": [
    {
        "type": "checks",
        "handler": "/usr/bin/health-check-handler.sh"
    }
  ]
}

Configuration Key Reference

• acl_datacenter - Only used by servers. This designates the datacenter which is authoritative for ACL information. It must be provided to enable ACLs. All servers and datacenters must agree on the ACL datacenter. Setting it on the servers is all you need for enforcement, but for the APIs to forward properly from the clients, it must be set on them too. Future changes may move enforcement to the edges, so it's best to just set acl_datacenter on all nodes.

• acl_default_policy - Either "allow" or "deny"; defaults to "allow". The default policy controls the behavior of a token when there is no matching rule. In "allow" mode, ACLs are a blacklist: any operation not specifically prohibited is allowed. In "deny" mode, ACLs are a whitelist: any operation not specifically allowed is blocked.

• acl_down_policy - Either "allow", "deny" or "extend-cache"; "extend-cache" is the default. In the case that the policy for a token cannot be read from the acl_datacenter or leader node, the down policy is applied. In "allow" mode, all actions are permitted, "deny" restricts all operations, and "extend-cache" allows any cached ACLs to be used, ignoring their TTL values. If a non-cached ACL is used, "extend-cache" acts like "deny".

• acl_master_token - Only used for servers in the acl_datacenter. This token will be created with management-level permissions if it does not exist. It allows operators to bootstrap the ACL system with a token ID that is well-known.
  
  Note that the acl_master_token is only installed when a server acquires cluster leadership. If you would like to install or change the acl_master_token, set the new value for acl_master_token in the configuration for all servers. Once this is done, restart the current leader to force a leader election.

• acl_token - When provided, the agent will use this token when making requests to the Consul servers. Clients can override this token on a per-request basis by providing the "?token" query parameter. When not provided, the empty token, which maps to the 'anonymous' ACL policy, is used.

• acl_ttl - Used to control Time-To-Live caching of ACLs. By default, this is 30 seconds. This setting has a major performance impact: reducing it will cause more frequent refreshes while increasing it reduces the number of caches. However, because the caches are not actively invalidated, ACL policy may be stale up to the TTL value.

• addresses - This is a nested object that allows setting bind addresses.
  
  Both rpc and http support binding to Unix domain sockets. A socket can be specified in the form unix:///path/to/socket. A new domain socket will be created at the given path. If the specified file path already exists, Consul will attempt to clear the file and create the domain socket in its place.
  
  The permissions of the socket file are tunable via the unix_sockets config construct.
  
  When running Consul agent commands against Unix socket interfaces, use the -rpc-addr or -http-addr arguments to specify the path to the socket. You can also place the desired values in CONSUL_RPC_ADDR and CONSUL_HTTP_ADDR environment variables. For TCP addresses, these should be in the form ip:port.
  
  The following keys are valid:

  • dns - The DNS server. Defaults to client_addr
  • http - The HTTP API. Defaults to client_addr
  • https - The HTTPS API. Defaults to client_addr
  • rpc - The RPC endpoint. Defaults to client_addr

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

• atlas_acl_token When provided, any requests made by Atlas will use this ACL token unless explicitly overriden. When not provided the acl_token is used. This can be set to 'anonymous' to reduce permission below that of acl_token.

• atlas_infrastructure Equivalent to the -atlas command-line flag.

• atlas_join Equivalent to the -atlas-join command-line flag.

• atlas_token Equivalent to the -atlas-token command-line flag.

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

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

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

• ca_file This provides a 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 file path to a PEM-encoded certificate. The certificate is provided to clients or servers to verify the agent's authenticity. It must be provided along with key_file.

• check_update_interval This interval controls how often check output from checks in a steady state is synchronized 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 deferring 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 synchronized immediately. To disable this behavior, set the value to "0s".

• 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.

• disable_anonymous_signature Disables providing an anonymous signature for de-duplication with the update check. See disable_update_check.

• disable_remote_exec Disables support for remote execution. When set to true, the agent will ignore any incoming remote exec requests.

• disable_update_check Disables automatic checking for security bulletins and new version releases.

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

  • allow_stale - Enables a stale query for DNS information. This allows any Consul server, rather than only the leader, to service the request. 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, providing stronger consistency but less throughput and higher latency.

  • max_stale When allow_stale is specified, this is used to limit how stale results are allowed to be. By default, this is set to "5s": 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.

  • node_ttl By default, this is "0s", so all node lookups are served with a 0 TTL value. DNS caching for node lookups can be enabled by setting this value. 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 used when there is no specific policy available for a service. By default, all services are served with a 0 TTL value. DNS caching for service lookups can be enabled by setting this value.

  • enable_truncate If set to true, a UDP DNS query that would return more than 3 records will set the truncated flag, indicating to clients that they should re-query using TCP to get the full set of records.

  • only_passing If set to true, any nodes whose healthchecks are not passing will be excluded from DNS results. By default (or if set to false), only nodes whose healthchecks are failing as critical will be excluded.

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

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

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

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

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

• http_api_response_headers This object allows adding headers to the HTTP API responses. For example, the following config can be used to enable CORS on the HTTP API endpoints:

    {
      "http_api_response_headers": {
          "Access-Control-Allow-Origin": "*"
      }
    }
  

• 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.

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

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

• 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.
  • https - The HTTPS API, -1 to disable. Default -1 (disabled).
  • 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.

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

• recursor Provides a single recursor address. This has been deprecated, and the value is appended to the recursors list for backwards compatibility.

• recursors This flag provides addresses of upstream DNS servers that are 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.

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

• retry_join Equivalent to the -retry-join command-line flag. Takes a list of addresses to attempt joining every retry_interval until at least one -join works.

• retry_interval Equivalent to the -retry-interval command-line flag.

• retry_join_wan Equivalent to the -retry-join-wan command-line flag. Takes a list of addresses to attempt joining to WAN every retry_interval_wan until at least one -join-wan works.

• retry_interval_wan Equivalent to the -retry-interval-wan command-line flag.

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

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

• session_ttl_min The minimum allowed session TTL. This ensures sessions are not created with TTL's shorter than the specified limit. It is recommended to keep this limit at or above the default to encourage clients to send infrequent heartbeats. Defaults to 10s.

• skip_leave_on_interrupt This is similar to leave_on_terminate but only affects interrupt handling. By default, an interrupt (such as hitting Control-C in a shell) causes Consul to gracefully leave. Setting this to true disables that. Defaults to false.

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

• start_join_wan An array of strings specifying addresses of WAN nodes to -join-wan upon startup.

• statsd_addr This provides the address of a statsd instance. If provided, Consul will send various telemetry information to that instance for aggregation. This can be used to capture runtime information. This sends UDP packets only and can be used with statsd or statsite.

• 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 runtime information. This streams via TCP and can only be used with statsite.

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

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

• unix_sockets - This allows tuning the ownership and permissions of the Unix domain socket files created by Consul. Domain sockets are only used if the HTTP or RPC addresses are configured with the unix:// prefix. The following options are valid within this construct and apply globally to all sockets created by Consul:
  

  • user - The name or ID of the user who will own the socket file.
  • group - The group ID ownership of the socket file. Note that this option currently only supports numeric IDs.
  • mode - The permission bits to set on the file.
    It is important to note that this option may have different effects on different operating systems. Linux generally observes socket file permissions while many BSD variants ignore permissions on the socket file itself. It is important to test this feature on your specific distribution. This feature is currently not functional on Windows hosts.

• 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 applies to both server RPC and to the HTTPS API. Note: to enable the HTTPS API, you must define an HTTPS port via the ports configuration. By default, HTTPS is disabled.

• verify_outgoing - If set to true, Consul requires that all outgoing connections make use of TLS and that the server provides 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.

• watches - Watches is a list of watch specifications which allow an external process to be automatically invoked when a particular data view is updated. See the watch documentation for more detail. Watches can be modified when the configuration is reloaded.

Ports Used

Consul requires up to 5 different ports to work properly, some on TCP, UDP, or both protocols. Below we document the requirements for each port.

• Server RPC (Default 8300). This is used by servers to handle incoming requests from other agents. TCP only.

• Serf LAN (Default 8301). This is used to handle gossip in the LAN. Required by all agents. TCP and UDP.

• Serf WAN (Default 8302). This is used by servers to gossip over the WAN to other servers. TCP and UDP.

• CLI RPC (Default 8400). This is used by all agents to handle RPC from the CLI. TCP only.

• HTTP API (Default 8500). This is used by clients to talk to the HTTP API. TCP only.

• DNS Interface (Default 8600). Used to resolve DNS queries. TCP and UDP.

Reloadable Configuration

Reloading configuration does not reload all configuration items. The items which are reloaded include:

• Log level
• Checks
• Services
• Watches
• HTTP Client Address