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---
layout: "docs"
page_title: "Configuration"
sidebar_current: "docs-agent-config"
---
# 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.
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When loading configuration, Consul loads the configuration from files
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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.
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Consul also supports reloading of configuration when it receives the
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SIGHUP signal. Not all changes are respected, but those that are
are documented below. The [reload command](/docs/commands/reload.html)
can also be used to trigger a configuration reload.
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## 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.
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* `-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,
because otherwise consistency cannot be guaranteed if 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 `-bootstrap` flag.
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* `-bind` - The address that should be bound to for internal cluster communications.
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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.
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* `-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.
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* `-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.
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* `-config-dir` - A directory of configuration files to load. Consul will
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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.
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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.
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* `-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.
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* `-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.
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* `-protocol` - The Consul protocol version to use. This defaults to the latest
version. This should be set only when [upgrading](/docs/upgrading.html).
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You can view the protocol versions supported by Consul by running `consul -v`.
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* `-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.
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* `-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
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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.
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* `-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.
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* `-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.
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* `-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.
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## Configuration Files
In addition to the command-line options, configuration can be put into
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files. This may be easier in certain situations, for example when Consul is
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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.
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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.
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They are documented seperately under [check configuration](/docs/agent/checks.html) and
[service configuration](/docs/agent/services.html) respectively. The service and check
definitions support being updated during a reload.
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#### Example Configuration File
<pre class="prettyprint lang-json">
{
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"datacenter": "east-aws",
"data_dir": "/opt/consul",
"log_level": "INFO",
"node_name": "foobar",
"server": true
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}
</pre>
#### Configuration Key Reference
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* `bootstrap` - Equivalent to the `-bootstrap` command-line flag.
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* `bootstrap_expect` - Equivalent to the `-bootstrap-expect` command-line flag.
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* `bind_addr` - Equivalent to the `-bind` command-line flag.
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* `client_addr` - Equivalent to the `-client` command-line flag.
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* `datacenter` - Equivalent to the `-dc` command-line flag.
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* `data_dir` - Equivalent to the `-data-dir` command-line flag.
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* `log_level` - Equivalent to the `-log-level` command-line flag.
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* `node_name` - Equivalent to the `-node` command-line flag.
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* `protocol` - Equivalent to the `-protocol` command-line flag.
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* `server` - Equivalent to the `-server` command-line flag.
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* `ui_dir` - Equivalent to the `-ui-dir` command-line flag.
* `advertise_addr` - Equivalent to the `-advertise` command-line flag.
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* `enable_syslog` - Equivalent to the `-syslog` command-line flag.
* `rejoin_after_leave` - Equivalent to the `-rejoin` command-line flag.
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* `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.
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* `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`.
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* `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".
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* `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.
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* `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](/docs/guides/dns-cache.html).
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.
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* `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`.
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* `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:
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* `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.
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* `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.
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* `skip_leave_on_interrupt` - This is the similar to`leave_on_terminate` but
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only affects interrupt handling. By default, an interrupt causes Consul to
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gracefully leave, but setting this to true disables that. Defaults to false.
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Interrupts are usually from a Control-C from a shell.
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* `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.
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* `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.
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* `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.
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* `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.
* `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 parameter. When not provided, the empty token is used
which maps to the 'anonymous' ACL policy.
* `acl_master_token` - Only used for servers in the `acl_datacenter`. This token
will be created if it does not exist with management level permissions. It allows
operators to bootstrap the ACL system with a token ID that is well-known.
* `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 whilelist: any operation not
specifically allowed is blocked.
* `acl_down_policy` - Either "allow", "deny" or "extend-cache" which 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_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.
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## Ports Used
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Consul requires up to 5 different ports to work properly, some requiring
TCP, UDP, or both protocols. Below we document the requirements for each
port.
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* Server RPC (Default 8300). This is used by servers to handle incoming
requests from other agents. TCP only.
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* Serf LAN (Default 8301). This is used to handle gossip in the LAN.
Required by all agents, TCP and UDP.
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* Serf WAN( Default 8302). This is used by servers to gossip over the
WAN to other servers. TCP and UDP.
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* 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.
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