man VCL (1): Varnish Configuration Language

DESCRIPTION

The VCL language is a small domain-specific language designed to be used to describe request handling and document caching policies for Varnish Cache.

When a new configuration is loaded, the varnishd management process translates the VCL code to C and compiles it to a shared object which is then loaded into the server process.

This document focuses on the syntax of the VCL language. For a full description of syntax and semantics, with ample examples, please see the online documentation at https://www.varnish-cache.org/docs/ .

Starting with Varnish 4.0, each VCL file must start by declaring its version with a special "vcl 4.0;" marker at the top of the file.

Operators

The following operators are available in VCL:

=: Assignment operator.
==: Comparison.
~: Match. Can either be used with regular expressions or ACLs.
!: Negation.
&&: Logical and.
||: Logical or.

Conditionals

VCL has if and else statements. Nested logic can be implemented with the elseif statement. (elsif/elif/else if is equivalent.)

Note that are no loops or iterators of any kind in VCL.

Strings, booleans, time, duration and integers

These are the data types in Varnish. You can set or unset these.

Example:

set req.http.User-Agent = "unknown";
unset req.http.Range;

Strings

Basic strings are enclosed in double quotes (" ... "), and may not contain newlines. Long strings are enclosed in {" ... "}. They may contain any character including single double quotes ("), newline and other control characters except for the NUL (0x00) character.

Booleans

Booleans can be either true or false.

Time

VCL has time. The function now returns a time. A duration can be added to a time to make another time. In string context they return a formatted string.

Durations

Durations are defined by a number and a designation. The number can be a real so 1.5w is allowed.

ms: milliseconds
s: seconds
m: minutes
h: hours
d: days
w: weeks
y: years

Integers

Certain fields are integers, used as expected. In string context they return a string.

Real numbers

VCL understands real numbers. As with integers, when used in a string context they will return a string.

Regular Expressions

Varnish uses Perl-compatible regular expressions (PCRE). For a complete description please see the pcre(3) man page.

To send flags to the PCRE engine, such as to do case insensitive matching, add the flag within parens following a question mark, like this:

# If host is NOT example dot com..
if (req.http.host !~ "(?i)example.com$") {
    ...
}

Include statement

To include a VCL file in another file use the include keyword:

include "foo.vcl";

Import statement

The import statement is used to load Varnish Modules (VMODs.)

Example:

import std;
sub vcl_recv {
    std.log("foo");
}

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 {
    .attribute = "value";
}

The only mandatory attribute is host. The attributes will inherit their defaults from the global parameters. The following attributes are available:

host (mandatory): The host to be used. IP address or a hostname that resolves to a single IP address.
port: The port on the backend that Varnish should connect to.
host_header: A host header to add.
connect_timeout: Timeout for connections.
first_byte_timeout: Timeout for first byte.
between_bytes_timeout: Timeout between bytes.
probe: Attach a probe to the backend. See Probes.
max_connections: Maximum number of open connections towards this backend. If Varnish reaches the maximum Varnish it will start failing connections.
Backends can be used with directors. Please see the vmod_directors(3) man page for more information.

Probes

Probes will query the backend for status on a regular basis and mark the backend as down it they fail. A probe is defined as this::

probe name {
     .attribute = "value";
}

There are no mandatory options. These are the options you can set:

url: The URL to query. Defaults to "/".
request: Specify a full HTTP request using multiple strings. .request will have rn automatically inserted after every string. If specified,
expected_response: The expected HTTP response code. Defaults to 200.
timeout: The timeout for the probe. Default is 2s.
interval: How often the probe is run. Default is 5s.
initial: How many of the polls in .window are considered good when Varnish starts. Defaults to the value of threshold - 1. In this case, the backend starts as sick and requires one single poll to be considered healthy.
window: How many of the latest polls we examine to determine backend health. Defaults to 8.
threshold: How many of the polls in .window must have succeeded for us to consider the backend healthy. Defaults to 3.

Access Control List (ACL)

An Access Control List (ACL) declaration creates and initialises a named access control list which can later be used to match client addresses:

acl localnetwork {
    "localhost";    # myself
    "192.0.2.0"/24; # and everyone on the local network
    ! "192.0.2.23"; # except for the dial-in router
}

If an ACL entry specifies a host name which Varnish is unable to resolve, it will match any address it is compared to. Consequently, if it is preceded by a negation mark, it will reject any address it is compared to, which may not be what you intended. If the entry is enclosed in parentheses, however, it will simply be ignored.

To match an IP address against an ACL, simply use the match operator:

if (client.ip ~ localnetwork) {
    return (pipe);
}

VCL objects

A VCL object can be made with the new keyword.

Example:

sub vcl_init {
    new b = directors.round_robin()
    b.add_backend(node1);
}

Subroutines

A subroutine is used to group code for legibility or reusability:

sub pipe_if_local {
    if (client.ip ~ localnetwork) {
        return (pipe);
    }
}

Subroutines in VCL do not take arguments, nor do they return values. The built in subroutines all have names beginning with vcl_, which is reserved.

To call a subroutine, use the call keyword followed by the subroutine's name:

sub vcl_recv {
    call pipe_if_local;
}

Return statements

The ongoing vcl_* subroutine execution ends when a return(action) statement is made.

The action specifies how execution should proceed. The context defines which actions are available.

Multiple subroutines

If multiple subroutines with the name of one of the built-in ones are defined, they are concatenated in the order in which they appear in the source.

The built-in VCL distributed with Varnish will be implicitly concatenated when the VCL is compiled.

Variables

In VCL you have access to certain variable objects. These contain requests and responses currently being worked on. What variables are available depends on context.

bereq

bereq Type: HTTP

Readable from: backend

The entire backend request HTTP data structure

bereq.backend Type: BACKEND

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

bereq.between_bytes_timeout Type: DURATION

Readable from: backend

Writable from: backend

The time in seconds to wait between each received byte from the backend. Not available in pipe mode.

bereq.connect_timeout Type: DURATION

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

The time in seconds to wait for a backend connection.

bereq.first_byte_timeout Type: DURATION

Readable from: backend

Writable from: backend

The time in seconds to wait for the first byte from the backend. Not available in pipe mode.

bereq.http. Type: HEADER

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

The corresponding HTTP header.

bereq.method Type: STRING

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

The request type (e.g. "GET", "HEAD").

bereq.proto Type: STRING

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

The HTTP protocol version used to talk to the server.

bereq.retries Type: INT

Readable from: backend

A count of how many times this request has been retried.

bereq.uncacheable Type: BOOL

Readable from: backend

Writable from: backend

bereq.url Type: STRING

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

The requested URL.

bereq.xid Type: STRING

Readable from: backend

Unique ID of this request.