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 .probe
¶
Please see VCL-probe.
SEE ALSO¶
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