Ubuntu Manpage: VCL - Varnish Configuration Language

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Provided by: varnish_5.2.1-1ubuntu0.1_amd64

NAME

       VCL - 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 vcl <major>.<minor>;
marker at the top of the file. See more about this under Versioning below.

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 are equivalent).

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

Strings, booleans, time, duration, integers and real numbers
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. In addition, in a boolean context some data types will evaluate to
true or false depending on their value.

String types will evaluate to false if they are empty; backend types will evalute to false if they don't
have a backend assigned; integer types will evaluate to false if their value is zero; duration types will
evaluate to false if their value is equal or less than zero.

Time
VCL has time. A duration can be added to a time to make another time. In string context they return a
formatted string in RFC1123 format, e.g. Sun, 06 Nov 1994 08:49:37 GMT.

The keyword now returns a time representing the current time in seconds since the Epoch.

Durations
Durations are defined by a number followed by a unit. The number can include a fractional part, e.g.
1.5s. The supported units are:

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");
}

Comments
Single lines of VCL can be commented out using // or #. Multi-line blocks can be commented out with
/*block*/.

Example:

sub vcl_recv {
// Single line of out-commented VCL.
# Another way of commenting out a single line.
/*
Multi-line block of commented-out VCL.
*/
}

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 to probes and regular backend requests if they have no such header.

.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

.proxy_header
The PROXY protocol version Varnish should use when connecting to this backend. Allowed values
are 1 and 2.

.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";
}

The probe named default is special and will be used for all backends which do not explicitly reference a
probe.

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 \r\n automatically
inserted after every string. If specified, .request will take precedence over .url.

.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 instantiated with the new keyword:

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

This is only available in vcl_init.

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

This is the backend or director we attempt to fetch from. When set to a director, reading this
variable returns an actual backend if the director has resolved immediately, or the director
otherwise. When used in string context, returns the name of the director or backend, respectively.

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.body
Type: BODY

Writable from: vcl_backend_fetch

The request body.

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.hash
Type: BLOB

Readable from: vcl_pipe, backend

The hash key of this request.

bereq.http.
Type: HEADER

Readable from: vcl_pipe, backend

Writable from: vcl_pipe, backend

The corresponding HTTP header.

bereq.is_bgfetch
Type: BOOL

Readable from: backend

True for background fetches.

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

Indicates whether this request is uncacheable due to a pass in the client side or a hit on an
hit-for-pass object.

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.

beresp
beresp
Type: HTTP