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reverse_proxy

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

Syntax

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

	# load balancing
	lb_policy       <name> [<options...>]
	lb_retries      <retries>
	lb_try_duration <duration>
	lb_try_interval <interval>
	lb_retry_match  <request-matcher>

	# 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>
	request_buffers <size>
	response_buffers <size>

	# request/header manipulation
	trusted_proxies [private_ranges] <ranges...>
	header_up   [+|-]<field> [<value|regexp> [<replacement>]]
	header_down [+|-]<field> [<value|regexp> [<replacement>]]
	method <method>
	rewrite <to>

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

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

		# special directives only available in handle_response
		copy_response [<matcher>] [<status>] {
			status <status>
		}
		copy_response_headers [<matcher>] {
			include <fields...>
			exclude <fields...>
		}
	}
}

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.
  • dynamic configures a dynamic upstreams module. This allows getting the list of upstreams dynamically for every request. See dynamic upstreams below for a description of standard dynamic upstream modules. Dynamic upstreams are retrieved at every proxy loop iteration (i.e. potentially multiple times per request if load balancing retries are enabled) and will be preferred over static upstreams. If an error occurs, the proxy will fall back to using any statically-configured upstreams.

Upstream addresses

Static upstream addresses can take the form of a URL that contains only scheme and host/port, or a conventional Caddy network address. 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
  • unix+h2c//var/grpc.sock

By default, connections are made to the upstream over plaintext HTTP. When using the URL form, a scheme can be used to set some transport defaults as a shorthand.

  • Using https:// as the scheme will use the http transport with tls enabled.

    Additionally, you may need to override the Host header such that it matches the TLS SNI value, which is used by servers for routing and certificate selection. See the HTTPS section below for more details.

  • Using h2c:// as the scheme will use the http transport with HTTP versions set to allow cleartext HTTP/2 connections.

  • Using http:// as the scheme is identical to having omitted the scheme, since HTTP is already the default. This syntax is included for symmetry with the other scheme shortcuts.

Schemes cannot be mixed, since they modify the common transport configuration (a TLS-enabled transport cannot carry both HTTPS and plaintext HTTP). Any explicit transport configuration will not be overwritten, and omitting schemes or using other ports will not assume a particular transport.

When using the network address form, the network type is specified as a prefix to the upstream address. This cannot be combined with a URL scheme. As a special case, unix+h2c/ is supported as a shortcut for the unix/ network plus the same effects as the h2c:// scheme.

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 dynamically static, meaning that potentially many different backends act as a single, static upstream in terms of health checks and load balancing. We recommend using a dynamic upstreams module instead, if possible.

Dynamic upstreams

Caddy's reverse proxy comes standard with some dynamic upstream modules. Note that using dynamic upstreams has implications for load balancing and health checks, depending on specific policy configuration: active health checks do not run for dynamic upstreams; and load balancing and passive health checks are best served if the list of upstreams is relatively stable and consistent (especially with round-robin). Ideally, dynamic upstream modules only return healthy, usable backends.

SRV

Retrieves upstreams from SRV DNS records.

	dynamic srv [<full_name>] {
		service   <service>
		proto     <proto>
		name      <name>
		refresh   <interval>
		resolvers <ip...>
		dial_timeout        <duration>
		dial_fallback_delay <duration>
	}
  • <full_name> is the full domain name of the record to look up (i.e. _service._proto.name).
  • service is the service component of the full name.
  • proto is the protocol component of the full name. Either tcp or udp.
  • name is the name component. Or, if service and proto are empty, the full domain name to query.
  • refresh is how often to refresh cached results. Default: 1m
  • resolvers is the list of DNS resolvers to override system resolvers.
  • dial_timeout is the timeout for dialing the query.
  • dial_fallback_delay is how long to wait before spawning an RFC 6555 Fast Fallback connection. Default: 300ms
A/AAAA

Retrieves upstreams from A/AAAA DNS records.

	dynamic a [<name> <port>] {
		name      <name>
		port      <port>
		refresh   <interval>
		resolvers <ip...>
		dial_timeout        <duration>
		dial_fallback_delay <duration>
	}
  • name is the domain name to query.
  • port is the port to use for the backend.
  • refresh is how often to refresh cached results. Default: 1m
  • resolvers is the list of DNS resolvers to override system resolvers.
  • dial_timeout is the timeout for dialing the query.
  • dial_fallback_delay is how long to wait before spawning an RFC 6555 Fast Fallback connection. Default: 300ms
Multi

Append the results of multiple dynamic upstream modules. Useful if you want redundant sources of upstreams, for example: a primary cluster of SRVs backed up by a secondary cluster of SRVs.

	dynamic multi {
		<source> [...]
	}
  • <source> is the name of the module for the dynamic upstreams, followed by its configuration. More than one may be specified.

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.

    For policies that involve hashing, the highest-random-weight (HRW) algorithm is used to ensure that a client or request with the same hash key is mapped to the same upstream, even if the list of upstreams change.

    • random randomly chooses an upstream

    • random_choose <n> selects two or more upstreams randomly, then chooses one with least load (n is usually 2)

    • first chooses the first available upstream, from the order they are defined in the config

    • round_robin iterates each upstream in turn

    • least_conn choose upstream with fewest number of current requests; if more than one host has the least number of requests, then one of those hosts is chosen at random

    • ip_hash maps the client IP to a sticky upstream

    • uri_hash maps the request URI (path and query) to a sticky upstream

    • header [field] maps a request header to a sticky upstream, by hashing the header value; if the specified header field is not present, a random upstream is selected

    • cookie [<name> [<secret>]] on the first request from a client (when there's no cookie), a random upstream is selected, and a Set-Cookie header is added to the response (default cookie name is lb if not specified). The cookie value is the upstream dial address of the chosen upstream, hashed with HMAC-SHA256 (using <secret> as the shared secret, empty string if not specified).

      On subsequent requests where the cookie is present, the cookie value will be mapped to the same upstream if it's available; if not available or not found, a new random upstream is selected and the cookie is added to the response.

      If you wish to use a particular upstream for debugging purposes, you may hash the upstream address with the secret, and set the cookie in your HTTP client (browser or otherwise). For example, with PHP, you could run the following to compute the cookie value, where 10.1.0.10:8080 is the address of one of your upstreams, and secret is your configured secret.

      echo hash_hmac('sha256', '10.1.0.10:8080', 'secret');
      // cdd96966817dd14a99f47ee17451464f29998da170814a16b483e4c1ff4c48cf
      

      You can set the cookie in your browser via the Javascript console, for example to set the cookie named lb:

      document.cookie = "lb=cdd96966817dd14a99f47ee17451464f29998da170814a16b483e4c1ff4c48cf";
      
  • lb_retries is how many times to retry selecting available backends for each request if the next available host is down. By default, retries are disabled (zero).

    If lb_try_duration is also configured, then retries may stop early if the duration is reached. In other words, the retry duration takes precedence over the retry count.

  • 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, retries are disabled (zero duration).

    Clients will wait for up to this long while the load balancer tries to find an available upstream host. A reasonable starting point might be 5s since the HTTP transport's default dial timeout is 3s, so that should allow for at least one retry if the first selected upstream cannot be reached; but feel free to experiment to find the right balance for your usecase.

  • 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.

  • lb_retry_match restricts with which requests retries are allowed. A request must match this condition in order to be retried if the connection to the upstream succeded but the subsequent round-trip failed. If the connection to the upstream failed, a retry is always allowed. By default, only GET requests are retried.

    The syntax for this option is the same as for named request matchers, but without the @name. If you only need a single matcher, you may configure it on the same line. For multiple matchers, a block is necessary.

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; the default is 0 (off). A reasonable starting point might be 30s to balance error rates with responsiveness when bringing an unhealthy upstream back online; but feel free to experiment to find the right balance for your usecase.

  • max_fails is the maximum number of failed requests within fail_duration 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. In other words, if a particular backend is currently handling this many requests, then it's considered "overloaded" and other backends will be preferred instead.

    This should be a reasonably large number; configuring this means that the proxy will have a limit of unhealthy_request_count × upstreams_count total simultaneous requests, and any requests after that point will result in an error due to no upstreams being available.

Streaming

By default, the proxy partially buffers the response 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 (typically -1) suggests "low-latency mode" which disables response buffering completely and flushes immediately after each write to the client, and does not cancel the request to the backend even if the client disconnects early. This option is ignored and responses are flushed immediately to the client if one of the following applies from the response:

    • Content-Type: text/event-stream
    • Content-Length is unknown
    • HTTP/2 on both sides of the proxy, Content-Length is unknown, and Accept-Encoding is either not set or is "identity"
  • request_buffers will cause the proxy to read up to <size> amount of bytes from the 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). This accepts all size formats supported by go-humanize.

  • response_buffers will cause the proxy to read up to <size> amount of bytes from the response body to be read into a buffer before being returned 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. This accepts all size formats supported by go-humanize.

Headers

The proxy can manipulate headers between itself and the backend:

  • header_up sets, adds (with the + prefix), deletes (with the - prefix), or performs a replacement (by using two arguments, a search and replacement) in a request header going upstream to the backend.

  • header_down sets, adds (with the + prefix), deletes (with the - prefix), or performs a replacement (by using two arguments, a search and replacement) in a response header coming downstream from the backend.

For example, to set a request header, overwriting any existing values:

header_up Some-Header "the value"

To add a response header; note that there can be multiple values for a header field:

header_down +Some-Header "first value"
header_down +Some-Header "second value"

To delete a request header, preventing it from reaching the backend:

header_up -Some-Header

To delete all matching request, using a suffix match:

header_up -Some-*

To perform a regular expression replacement on a request header:

header_up Some-Header "^prefix-([A-Za-z0-9]*)$" "replaced-$1-suffix"

The regular expression language used is RE2, included in Go. See the RE2 syntax reference and the Go regexp syntax overview. The replacement string is expanded, allowing use of captured values, for example $1 being the first capture group.

Defaults

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

For these X-Forwarded-* headers, by default, the proxy will ignore their values from incoming requests, to prevent spoofing.

If Caddy is not the first server being connected to by your clients (for example when a CDN is in front of Caddy), you may configure trusted_proxies with a list of IP ranges (CIDRs) from which incoming requests are trusted to have sent good values for these headers.

It is recommended that you configure this via the servers > trusted_proxies global option so that this applies to all proxy handlers in your server, without repetition.

Additionally, when using the http transport, the Accept-Encoding: gzip header will be set, if it is missing in the request from the client. This allows the upstream to serve compressed content if it can. This behavior can be disabled with compression off on the transport.

HTTPS

Since (most) headers retain their original value when being proxied, it is often necessary to override the Host header with the configured upstream address when proxying to HTTPS, such that the Host header matches the TLS ServerName value. For example:

reverse_proxy https://example.com {
	header_up Host {upstream_hostport}
}

Rewrites

By default, Caddy performs the upstream request with the same HTTP method and URI as the incoming request, unless a rewrite was performed in the middleware chain before it reaches reverse_proxy.

Before proxying it, the request is cloned; this ensures that any modifications done to the request during the handler do not leak to other handlers. This is useful in situations where the handling needs to continue after the proxy.

In addition to header manipulations, the request's method and URI may be changed before it is sent to the upstream:

  • method changes the HTTP method of the cloned request. If the method is changed to GET or HEAD, then the incoming request's body will not be sent upstream by this handler. This is useful if you wish to allow a different handler to consume the request body.
  • rewrite changes the URI (path and query) of the cloned request. This is similar to the rewrite directive, except that it doesn't persist the rewrite past the scope of this handler.

These rewrites are often useful for a pattern like "pre-check requests", where a request is sent to another server to help make a decision on how to continue handling the current request.

For example, the request could be sent to an authentication gateway to decide whether the request was from an authenticated user (e.g. the request has a session cookie) and should continue, or should instead be redirected to a login page. For this pattern, Caddy provides a shortcut directive forward_auth to skip most of the config boilerplate.

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>
	resolvers <ip...>
	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 <server_name>
	tls_renegotiation <level>
	tls_except_ports <ports...>
	keepalive [off|<duration>]
	keepalive_interval <interval>
	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. It accepts all formats supported by go-humanize. Default: 4KiB.

  • write_buffer is the size of the write buffer in bytes. It accepts all formats supported by go-humanize. Default: 4KiB.

  • max_response_header is the maximum amount of bytes to read from response headers. It accepts all formats supported by go-humanize. Default: 10MiB.

  • dial_timeout is the maximum duration to wait when connecting to the upstream socket. Default: 3s.

  • dial_fallback_delay is the maximum duration to wait before spawning an RFC 6555 Fast Fallback connection. A negative value disables this. Default: 300ms.

  • response_header_timeout is the maximum duration to wait for reading response headers from the upstream. Default: No timeout.

  • expect_continue_timeout is the maximum duration to wait for the upstreams's first response headers after fully writing the request headers if the request has the header Expect: 100-continue. Default: No timeout.

  • read_timeout is the maximum duration to wait for the next read from the backend. Default: No timeout.

  • write_timeout is the maximum duration to wait for the next writes to the backend. Default: No timeout.

  • resolvers is a list of DNS resolvers to override system resolvers.

  • tls uses HTTPS with the backend. This will be enabled automatically if you specify backends using the https:// scheme or port :443, or if any of the below tls_* options are configured.

  • 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 TLS handshake verification, making the connection insecure and vulnerable to man-in-the-middle attacks. Do not use in production.

  • tls_timeout is the maximum duration to wait for the TLS handshake to complete. Default: No timeout.

  • 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 server name used when verifying the certificate received in the TLS handshake. By default, this will use the upstream address' host part.

    You only need to override this if your upstream address does not match the certificate the upstream is likely to use. For example if the upstream address is an IP address, then you would need to configure this to the hostname being served by the upstream server.

    A request placeholder may be used, in which case a clone of the HTTP transport config will be used on every request, which may incur a performance penalty.

  • tls_renegotiation sets the TLS renegotiation level. TLS renegotiation is the act of performing subsequent handshakes after the first. The level may be one of:

    • never (the default) disables renegotiation.
    • once allows a remote server to request renegotiation once per connection.
    • freely allows a remote server to repeatedly request renegotiation.
  • tls_except_ports when TLS is enabled, if the upstream target uses one of the given ports, TLS will be disabled for those connections. This may be useful when configuring dynamic upstreams, where some upstreams expect HTTP and others expect HTTPS requests.

  • keepalive is either off or a duration value that specifies how long to keep connections open (timeout). Default: 2m.

  • keepalive_interval is the duration between liveness probes. Default: 30s.

  • keepalive_idle_conns defines the maximum number of connections to keep alive. Default: No limit.

  • 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. Default: No limit.

The fastcgi transport

transport fastcgi {
	root  <path>
	split <at>
	env   <key> <value>
	resolve_root_symlink
	dial_timeout  <duration>
	read_timeout  <duration>
	write_timeout <duration>
	capture_stderr
}
  • 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: 3s.

  • 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.

  • capture_stderr enables capturing and logging of any messages sent by the upstream fastcgi server on stderr. Logging is done at WARN level by default. If the response has a 4xx or 5xx status, then the ERROR level will be used instead. By default, stderr is ignored.

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 a response handler is invoked, 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. If the route does not write a response, then request handling will continue with any handlers that are ordered after this reverse_proxy.

  • @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.
  • replace_status simply changes the status code of response when matched by the given matcher.
  • 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.

Additionally, inside handle_response, two special handler directives may be used:

  • copy_response copies the response body received from the backend back to the client. Optionally allows changing the status code of the response while doing so. This directive is ordered before respond.
  • copy_response_headers copies the response headers from the backend to the client, optionally including OR excluding a list of headers fields (cannot specify both include and exclude). This directive is ordered after header.

Three placeholders will be made available within the handle_response routes:

  • {rp.status_code} The status code from the backend's response.
  • {rp.status_text} The status text from the backend's response.
  • {rp.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

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
		response_header_timeout 30s
	}
}

Reverse proxy to an HTTPS endpoint:

reverse_proxy https://example.com {
	header_up Host {upstream_hostport}
}

Strip a path prefix before proxying:

handle_path /prefix/* {
	reverse_proxy localhost:9000
}

Replace a path prefix before proxying:

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

X-Accel-Redirect support, i.e. serving static files as requested by the proxy upstream:

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

Get backends dynamically from A/AAAA record DNS queries:

reverse_proxy {
	dynamic a example.com 9000
}

Get backends dynamically from SRV record DNS queries:

reverse_proxy {
	dynamic srv _api._tcp.example.com
}