Provided by: varnish_7.7.0-1_amd64 
NAME
VCL-backends - Configuring Backends
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(1).
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(1) 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(1) 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 <https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt>
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
<https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt> .
Implementation detail:
If .via = <proxy> is used, a PROXY2 <https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-
protocol.txt>
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
<https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt>
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.
.authority can not be an IP address.
A colon and anything following (signifying a port) is removed from the 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
<https://raw.githubusercontent.com/haproxy/haproxy/master/doc/proxy-protocol.txt>
preamble.
ATTRIBUTE .PROBE
Please see vcl-probe(7).
SEE ALSO
• varnishd(1)
• vcl(7)
• vcl-probe(7)
• vmod_directors(3)
• vmod_std(3)
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
VCL-BACKENDS(7)