# VCL-backends

## Configuring Backends

Manual section:

  7

### Backend definition

A backend declaration creates and initialises a named backend object. A declaration start with the keyword `backend` followed by the name of the backend. The actual declaration is in curly brackets, in a key/value fashion.:

    backend name {
        .attribute1 = value;
        .attribute2 = value;
        [...]
    }

If there is a backend named `default` it will be used unless another backend is explicitly set. If no backend is named `default` the first backend in the VCL program becomes the default.

If you only use dynamic backends created by VMODs, an empty, always failing (503) backend can be specified:

    backend default none;

A backend must be specified with either a `.host` or a `.path` attribute, but not both. All other attributes have default values.

### Attribute `.host`

To specify a networked backend `.host` takes either a numeric IPv4/IPv6 address or a domain name which resolves to at most one IPv4 and one IPv6 address:

    .host = "127.0.0.1";

    .host = "[::1]:8080";

    .host = "example.com:8081";

    .host = "example.com:http";

### Attribute `.port`

The TCP port number or service name can be specified as part of `.host` as above or separately using the `.port` attribute:

    .port = "8081";

    .port = "http";

### Attribute `.path`

The absolute path to a Unix(4) domain socket of a local backend:

    .path = "/var/run/http.sock";

or, where available, `@` followed by the name of an abstract socket of a local backend:

    .path = "@mybackend";

A warning will be issued if the uds-socket does not exist when the VCL is loaded. This makes it possible to start the UDS-listening peer, or set the socket file’s permissions afterwards.

If the uds-socket socket does not exist or permissions deny access, connection attempts will fail.

### Attribute `.host_header`

A host header to add to probes and regular backend requests if they have no such header:

    .host_header = "example.com";

### Timeout Attributes

These attributes control how patient varnishd is during backend fetches:

    .connect_timeout = 1.4s;
    .first_byte_timeout = 20s;
    .between_bytes_timeout = 10s;

The default values comes parameters with the same names, see varnishd.

### Attribute `.max_connections`

Limit how many simultaneous connections varnish can open to the backend:

    .max_connections = 1000;

### Attribute `.wait_limit`

Maximum number of transactions that can queue waiting for a backend connection to become available:

    .wait_limit = 1000;

Note that this feature must be used with caution, as it can cause threads to pile up and increase response times.

Defaults to the varnishd backend_wait_limit parameter.

### Attribute `.wait_timeout`

When a transaction is queued waiting for a backend connection to become available (see .wait_limit), this is the time that the transaction will wait before giving up:

    .wait_timeout = 1s;

It is strongly advised to never set this higher than a couple of seconds.

Defaults to the varnishd backend_wait_timeout parameter.

### Attribute `.proxy_header`

Send a PROXY protocol header to the backend with the `client.ip` and `server.ip` values:

    .proxy_header = 2;

Legal values are one and two, depending which version of the PROXY protocol you want.

Notice this setting will lead to backend connections being used for a single request only (subject to future improvements). Thus, extra care should be taken to avoid running into failing backend connections with EADDRNOTAVAIL due to no local ports being available. Possible options are:

> - Use additional backend connections to extra IP addresses or TCP ports

> - Increase the number of available ports (Linux sysctl `net.ipv4.ip_local_port_range`)

> - Reuse backend connection ports early (Linux sysctl `net.ipv4.tcp_tw_reuse`)

### Attribute `.preamble`

Send a BLOB on all newly opened connections to the backend:

    .preamble = :SGVsbG8gV29ybGRcbgo=:;

### Attribute `.via`

Name of another proxy backend through which to make the connection to the destination backend using the PROXY2 protocol, for example:

    backend proxy {
      .path = "/path/to/proxy2_endpoint";
    }
    backend destination {
      .host = "1.2.3.4";
      .via = proxy;
    }

The proxy backend can also use a `.host`/`.port` definition rather than `.path`.

Use of the `.path` attribute for the destination backend is not supported.

The `.via` attribute is unrelated to `.proxy_header`. If both are used, a second header is sent as per `.proxy_header` specification.

Unless specified for destination, the `connect_timeout`, `first_byte_timeout` and `between_bytes_timeout` attributes are copied from proxy to destination.

As of this release, the proxy backend used with `.via` cannot be a director, it cannot itself use `.via` (error: Cannot stack .via backends) and the protocol is fixed to PROXY2.

Implementation detail:

If `.via = <proxy>` is used, a PROXY2 preamble is created with the destination backend’s address information as `dst_addr`/`dst_port` and, optionally, other TLV attributes. The connection is then made to the proxy backend’s endpoint (`path` or `host`/`port`). This is technically equivalent to specifying a `backend destination_via_proxy` with a `.preamble` attribute containing the appropriate PROXY2 preamble for the destination backend.

### Attribute `.authority`

The HTTP authority to use when connecting to this backend. If unset, `.host_header` or `.host` are used.

`.authority = ""` disables sending an authority.

As of this release, the attribute is only used by `.via` connections as a `PP2_TYPE_AUTHORITY` Type-Length-Value (TLV) in the PROXY2 preamble.

### Attribute `.probe`

Please see VCL-probe.

### SEE ALSO

- varnishd

- VCL

- VCL-probe

- VMOD directors - Varnish Directors Module

- VMOD std - Varnish Standard Module

### HISTORY

VCL was developed by Poul-Henning Kamp in cooperation with Verdens Gang AS, Redpill Linpro and Varnish Software. This manual page is written by Per Buer, Poul-Henning Kamp, Martin Blix Grydeland, Kristian Lyngstøl, Lasse Karstensen and others.

### COPYRIGHT

This document is licensed under the same license as Varnish itself. See LICENSE for details.

- Copyright (c) 2006 Verdens Gang AS

- Copyright (c) 2006-2021 Varnish Software AS