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---
layout: "docs"
page_title: "Configuration"
sidebar_current: "docs-agent-config"
description: |-
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.
---
# 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 in the
[Reloadable Configuration](#reloadable-configuration) section. The
[reload command](/docs/commands/reload.html) 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. 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.
* `-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. 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 ending in ".json" as configuration files
in alphabetical order using the same merge routine as the `config-file`
option above. 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. Additional, 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 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.
* `-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 will continue to
retry the join indefinitely.
* `-join-wan` - Address of another wan agent to join upon starting up. This can be
specified multiple times to specify multiple agents that are on the WAN 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 will continue to
retry the join -wan indefinitely.
* `-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](/docs/upgrading.html).
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
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](/docs/agent/checks.html) and
[service configuration](/docs/agent/services.html) respectively. The service and check
definitions support being updated during a reload.
#### Example Configuration File
```javascript
{
"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 work on the
clients, it must be set on them too (to forward properly). Also, if we want
to enhance the ACL support for other features like service discovery,
enforcement might move to the edges, so it's best to just set the
`acl_datacenter` on all the 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" 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_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_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_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.
<br><br>
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.
<br><br>
The permissions of the socket file are tunable via the `unix_sockets` config
construct.
<br><br>
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.
<br><br>
The following keys are valid:
* `dns` - The DNS server. Defaults to `client_addr`
* `http` - The HTTP API. Defaults to `client_addr`
* `rpc` - The RPC endpoint. Defaults to `client_addr`
* `advertise_addr` - Equivalent to the `-advertise` 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 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 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 performed. See this guide on [DNS caching](/docs/guides/dns-cache.html).
The following sub-keys are available:
* `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.
* `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.
* `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` - 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.
* `enable_debug` - When set, enables some additional debugging features. Currently,
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 agents authenticity.
Must be provided along with the `cert_file`.
* `http_api_response_headers` - This object allows adding HTTP header response fields to
the HTTP API responses. For example, the following config can be used to enable CORS on
the HTTP API endpoints:
```javascript
{
"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 using their servers.
* `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 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.
* `skip_leave_on_interrupt` - This is the similar to`leave_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.
* `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 various 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 various runtime information. This streams via
TCP and can only be used with statsite.
* `syslog_facility` - When `enable_syslog` is provided, this controls which
facility messages are sent to. 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:
<br>
* `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 ID's.
* `mode` - The permission bits to set on the file.
<br>
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
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.
* `watches` - Watches is a list of watch specifications.
These allow an external process to be automatically invoked when a particular
data view is updated. See the [watch documentation](/docs/agent/watches.html) for
more documentation. Watches can be modified when the configuration is reloaded.
## Ports Used
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.
* 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
<a id="reloadable-configuration"></a>
Reloading configuration does not reload all configuration items. The
items which are reloaded include:
* Log level
* Checks
* Services
* Watches
* HTTP Client Address
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