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";

       The TCP port number can be specified as part of .host as above  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: 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 .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(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)