reverse_proxy

Proxies requests to one or more backends with configurable transport, load balancing, health checking, header manipulation, and buffering options.

• Syntax
• Upstreams
• Load balancing
  • Active health checks
  • Passive health checks
• Streaming
• Headers
• Transports
  • The http transport
  • The fastcgi transport
• Intercepting responses
• Examples

Syntax

reverse_proxy [<matcher>] [<upstreams...>] {
    # backends
    to <upstreams...>
	...

    # load balancing
    lb_policy       <name> [<options...>]
    lb_try_duration <duration>
    lb_try_interval <interval>

    # active health checking
    health_uri      <uri>
    health_port     <port>
    health_interval <interval>
    health_timeout  <duration>
    health_status   <status>
    health_body     <regexp>
	health_headers {
		<field> [<values...>]
	}

    # passive health checking
    fail_duration     <duration>
    max_fails         <num>
    unhealthy_status  <status>
    unhealthy_latency <duration>
    unhealthy_request_count <num>

    # streaming
    flush_interval <duration>
    buffer_requests
	buffer_responses
	max_buffer_size <size>

    # header manipulation
    header_up   [+|-]<field> [<value|regexp> [<replacement>]]
    header_down [+|-]<field> [<value|regexp> [<replacement>]]

    # round trip
    transport <name> {
        ...
    }

	# optionally intercept responses from upstream
	@name {
		status <code...>
		header <field> [<value>]
	}
	handle_response [<matcher>] [status_code] {
		<directives...>
	}
}

Upstreams

• <upstreams...> is a list of upstreams (backends) to which to proxy.
• to is an alternate way to specify the list of upstreams, one (or more) per line.

Upstream addresses can take the form of a conventional Caddy network address or a URL that contains only scheme and host/port, with a special exception that the scheme may be prefixed by srv+ to enable SRV DNS record lookups for load balancing. Valid examples:

• localhost:4000
• 127.0.0.1:4000
• http://localhost:4000
• https://example.com
• h2c://127.0.0.1
• example.com
• unix//var/php.sock
• srv+http://internal.service.consul
• srv+https://internal.service.consul

Note: Schemes cannot be mixed, since they modify the common transport configuration (a TLS-enabled transport cannot carry both HTTPS and plaintext HTTP). Specifying ports 80 and 443 are the same as specifying the HTTP and HTTPS schemes, respectively. Any explicit transport configuration will not be overwritten, and omitting schemes or using other ports will not assume a particular transport.

Additionally, upstream addresses cannot contain paths or query strings, as that would imply simultaneous rewriting the request while proxying, which behavior is not defined or supported. You may use the rewrite directive should you need this.

If the address is not a URL (i.e. does not have a scheme), then placeholders can be used, but this makes the upstream dynamic, meaning that the potentially many different backends act as one upstream in terms of health checks and load balancing.

When proxying over HTTPS, you may need to override the Host header (which by default, retains the value from the original request) such that the Host header matches the TLS SNI value, which is used by servers for routing and certificate selection. See the Headers section below for more details.

Load balancing

Load balancing is used whenever more than one upstream is defined.

• lb_policy is the name of the load balancing policy, along with any options. Default: random. Can be:

  • random - randomly choose an upstream
  • random_choose <n> - selects two or more upstreams randomly, then chooses one with least load (n is usually 2)
  • first - choose first available upstream, from the order they are defined in the config
  • round_robin - iterate each upstream in turn
  • least_conn - choose upstream with fewest number of current requests
  • ip_hash - map client IP to sticky upstream
  • uri_hash - map URI to sticky upstream
  • header [field] - map request header to sticky upstream
  • cookie [<name> [<secret>]] - based on the given cookie (default name is lb if not specified), which value is hashed; optionally with a secret for HMAC-SHA256

• lb_try_duration is a duration value that defines how long to try selecting available backends for each request if the next available host is down. By default, this retry is disabled. Clients will wait for up to this long while the load balancer tries to find an available upstream host.

• lb_try_interval is a duration value that defines how long to wait between selecting the next host from the pool. Default is 250ms. Only relevant when a request to an upstream host fails. Be aware that setting this to 0 with a non-zero lb_try_duration can cause the CPU to spin if all backends are down and latency is very low.

Active health checks

Active health checks perform health checking in the background on a timer:

• health_uri is the URI path (and optional query) for active health checks.
• health_port is the port to use for active health checks, if different from the upstream's port.
• health_interval is a duration value that defines how often to perform active health checks.
• health_timeout is a duration value that defines how long to wait for a reply before marking the backend as down.
• health_status is the HTTP status code to expect from a healthy backend. Can be a 3-digit status code, or a status code class ending in xx. For example: 200 (which is the default), or 2xx.
• health_body is a substring or regular expression to match on the response body of an active health check. If the backend does not return a matching body, it will be marked as down.
• health_headers allows specifying headers to set on the active health check requests. This is useful if you need to change the Host header, or if you need to provide some authentication to your backend as part of your health checks.

Passive health checks

Passive health checks happen inline with actual proxied requests:

• fail_duration is a duration value that defines how long to remember a failed request. A duration > 0 enables passive health checking.
• max_fails is the maximum number of failed requests within fail_timeout that are needed before considering a backend to be down; must be >= 1; default is 1.
• unhealthy_status counts a request as failed if the response comes back with one of these status codes. Can be a 3-digit status code or a status code class ending in xx, for example: 404 or 5xx.
• unhealthy_latency is a duration value that counts a request as failed if it takes this long to get a response.
• unhealthy_request_count is the permissible number of simultaneous requests to a backend before marking it as down.

Streaming

The proxy buffers responses by default for wire efficiency:

• flush_interval is a duration value that adjusts how often Caddy should flush the response buffer to the client. By default, no periodic flushing is done. A negative value disables response buffering, and flushes immediately after each write to the client. This option is ignored when the upstream's response is recognized as a streaming response, or if its content length is -1; for such responses, writes are flushed to the client immediately.
• buffer_requests will cause the proxy to read the entire request body into a buffer before sending it upstream. This is very inefficient and should only be done if the upstream requires reading request bodies without delay (which is something the upstream application should fix).
• buffer_responses will cause the entire response body to be read and buffered in memory before being proxied to the client. This should be avoided if at all possible for performance reasons, but could be useful if the backend has tighter memory constraints.
• max_buffer_size if body buffering is enabled, this sets the maximum size of the buffers used for the requests and responses. This accepts all size formats supported by go-humanize.

Headers

The proxy can manipulate headers between itself and the backend:

• header_up Sets, adds, removes, or performs a replacement in a request header going upstream to the backend.
• header_down Sets, adds, removes, or performs a replacement in a response header coming downstream from the backend.

Defaults

By default, Caddy passes thru incoming headers to the backend—including the Host header—without modifications, with two exceptions:

• It adds or augments the X-Forwarded-For header field.
• It sets the X-Forwarded-Proto header field.

HTTPS

For HTTPS upstreams, since the Host header retains its original value, it is typically necessary to override the header with the configured upstream address, such that the Host header matches the TLS SNI value. A X-Forwarded-Host header may also be added to inform the upstream of the original Host header's value. For example:

reverse_proxy https://example.com {
	header_up Host {upstream_hostport}
	header_up X-Forwarded-Host {host}
}

Transports

Caddy's proxy transport is pluggable:

• transport defines how to communicate with the backend. Default is http.

The http transport

transport http {
	read_buffer             <size>
	write_buffer            <size>
	max_response_header     <size>
	dial_timeout            <duration>
	dial_fallback_delay     <duration>
	response_header_timeout <duration>
	expect_continue_timeout <duration>
	tls
	tls_client_auth <automate_name> | <cert_file> <key_file>
	tls_insecure_skip_verify
	tls_timeout <duration>
	tls_trusted_ca_certs <pem_files...>
    tls_server_name <sni>
	keepalive [off|<duration>]
	keepalive_idle_conns <max_count>
    keepalive_idle_conns_per_host <count>
    versions <versions...>
    compression off
    max_conns_per_host <count>
}

• read_buffer is the size of the read buffer in bytes.
• write_buffer is the size of the write buffer in bytes.
• max_response_header is the maximum amount of bytes to read from response headers.
• dial_timeout is how long to wait when connecting to the upstream socket. Accepts duration values.
• dial_fallback_delay is how long to wait before spawning an RFC 6555 Fast Fallback connection. A negative value disables this. Accepts duration values.
• response_header_timeout is how long to wait for reading response headers from the upstream. Accepts duration values.
• expect_continue_timeout is how long to wait for the upstreams's first response headers after fully writing the request headers if the request has the header Expect: 100-continue. Accepts duration values.
• tls uses HTTPS with the backend. This will be enabled automatically if you specify backends using the https:// scheme or port :443.
• tls_client_auth enables TLS client authentication one of two ways: (1) by specifying a domain name for which Caddy should obtain a certificate and keep it renewed, or (2) by specifying a certificate and key file to present for TLS client authentication with the backend.
• tls_insecure_skip_verify turns off security. Do not use in production.
• tls_timeout is a duration value that specifies how long to wait for the TLS handshake to complete.
• tls_trusted_ca_certs is a list of PEM files that specify CA public keys to trust when connecting to the backend.
• tls_server_name sets the ServerName (SNI) to put in the ClientHello; only needed if the remote server requires it.
• keepalive is either off or a duration value that specifies how long to keep connections open.
• keepalive_idle_conns defines the maximum number of connections to keep alive.
• keepalive_idle_conns_per_host if non-zero, controls the maximum idle (keep-alive) connections to keep per-host. Default: 32
• versions allows customizing which versions of HTTP to support. As a special case, "h2c" is a valid value which will enable cleartext HTTP/2 connections to the upstream (however, this is a non-standard feature that does not use Go's default HTTP transport, so it is exclusive of other features; subject to change or removal). Default: 1.1 2, or if scheme is h2c://, h2c 2
• compression can be used to disable compression to the backend by setting it to off.
• max_conns_per_host optionally limits the total number of connections per host, including connections in the dialing, active, and idle states. Has no limit by default.

The fastcgi transport

transport fastcgi {
	root  <path>
	split <at>
	env   <key> <value>
	resolve_root_symlink
	dial_timeout  <duration>
	read_timeout  <duration>
	write_timeout <duration>
}

• root is the root of the site. Default: {http.vars.root} or current working directory.
• split is where to split the path to get PATH_INFO at the end of the URI.
• env sets an extra environment variable to the given value. Can be specified more than once for multiple environment variables.
• resolve_root_symlink enables resolving the root directory to its actual value by evaluating a symbolic link, if one exists.
• dial_timeout is how long to wait when connecting to the upstream socket. Accepts duration values. Default: no timeout.
• read_timeout is how long to wait when reading from the FastCGI server. Accepts duration values. Default: no timeout.
• write_timeout is how long to wait when sending to the FastCGI server. Accepts duration values. Default: no timeout.

Intercepting responses

The reverse proxy can be configured to intercept responses from the backend. To facilitate this, response matchers can be defined (similar to the syntax for request matchers) and the first matching handle_response route will be invoked. When this happens, the response from the backend is not written to the client, and the configured handle_response route will be executed instead, and it is up to that route to write a response.

• @name is the name of a response matcher. As long as each response matcher has a unique name, multiple matchers can be defined. A response can be matched on the status code and presence or value of a response header.
• handle_response defines the route to execute when matched by the given matcher (or, if a matcher is omitted, all responses). The first matching block will be applied. Inside a handle_response block, any other directives can be used.

Three placeholders will be made available to the handle_response routes:

• {http.reverse_proxy.status_code} The status code from the backend's response.
• {http.reverse_proxy.status_text} The status text from the backend's response.
• {http.reverse_proxy.header.*} The headers from the backend's response.

Response matcher

Response matchers can be used to filter (or classify) responses by specific criteria.

status

status <code...>

By HTTP status code.

• <code...> is a list of HTTP status codes. Special cases are 2xx, 3xx, ... which match against all status codes in the range of 200-299, 300-399, ... respectively

header

See the header request matcher for the supported syntax.

Examples

Reverse proxy all requests to a local backend:

reverse_proxy localhost:9005

Load-balance all requests between 3 backends:

reverse_proxy node1:80 node2:80 node3:80

Same, but only requests within /api, and with header affinity:

reverse_proxy /api/* node1:80 node2:80 node3:80 {
	lb_policy header X-My-Header
}

Configure some transport options:

reverse_proxy localhost:8080 {
	transport http {
		dial_timeout 2s
		tls_timeout  2s
	}
}

Reverse proxy to an HTTPS endpoint:

reverse_proxy https://example.com {
	header_up Host {upstream_hostport}
	header_up X-Forwarded-Host {host}
}

Replace a path prefix before proxying:

handle_path /old-prefix/* {
	rewrite * /new-prefix{path}
	reverse_proxy localhost:9000
}

X-Accel-Redirect support:

reverse_proxy localhost:8080 {
	@accel header X-Accel-Redirect *
	handle_response @accel {
		root    * /path/to/private/files
		rewrite   {http.reverse_proxy.header.X-Accel-Redirect}
		file_server
	}
}

Custom error page for errors from upstream:

reverse_proxy localhost:8080 {
	@error status 500 503
	handle_response @error {
		root    * /path/to/error/pages
		rewrite * /{http.reverse_proxy.status_code}.html
		file_server
	}
}