Configuring Backends

Manual section


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 = "";

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

.host = "";

The TCP port number can be specied as part of .host as a bove or separately using the .port attribute:

.port = 8081;

Attribute .path

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

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

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 = "Host:";

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 .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 .probe

Please see VCL-probe.



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.