Provided by: varnish_7.5.0-3_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.
+, -, *, /, %
Basic math on numerical values.
+=, -=, *=, /=
Assign and increment/decrement/multiply/divide operator.
For strings, += appends.
(, ) Evaluate separately.
==, !=, <, >, <=, >=
Comparisons
~, !~ Match / non-match. Can either be used with regular expressions or ACLs.
! Negation.
&& / ||
Logical and/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.
Variables
VCL does most of the work by examining, set'ing and unset'ing variables:
if (req.url == "/mistyped_url.html") {
set req.url = "/correct_url.html";
unset req.http.cookie;
}
There are obvious limitations to what can be done, for instance it makes no sense to unset
req.url; - a request must have some kind of URL to be valid, and likewise trying to
manipulate a backend response when there is none (yet) makes no sense. The VCL compiler
will detect such errors.
Variables have types. Most of them a STRINGS, and anything in VCL can be turned into a
STRING, but some variables have types like DURATION, IP etc.
When setting a such variables, the right hand side of the equal sign must have the correct
variables type, you cannot assign a STRING to a variable of type NUMBER, even if the
string is "42".
Explicit conversion functions are available in vmod_std(3).
For the complete album of VCL variables see: vcl-var(7).
Strings
Basic strings are enclosed in double quotes "...", and may not contain newlines. Long
strings are enclosed in {"..."} or """...""". 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 unset. This allows checks of the type if
(req.http.opthdr) {} to test if a header exists, even if it is empty, whereas if
(req.http.opthdr == "") {} does not distinguish if the header does not exist or if it is
empty.
Backend types will evaluate 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 notion of the current time, which is kept consistent during VCL
subroutine invocations, so during the execution of a VCL state subroutine (vcl_* {}),
including all user-defined subroutines being called, now always returns the same value.
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
In string context they return a string with their value rounded to 3 decimal places and
excluding the unit, e.g. 1.500.
Integers
Certain fields are integers, used as expected. In string context they return a string,
e.g. 1234.
Real numbers
VCL understands real numbers. In string context they return a string with their value
rounded to 3 decimal places, e.g. 3.142.
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";
Optionally, the include keyword can take a +glob flag to include all files matching a glob
pattern:
include +glob "example.org/*.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.
*/
}
Backends and health probes
Please see vcl-backend(7) and vcl-probe(7)
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 if the host name cannot be
resolved.
To match an IP address against an ACL, simply use the match operator:
if (client.ip ~ localnetwork) {
return (pipe);
}
ACLs have feature flags which can be set or cleared for each ACL individually:
• +log - Emit a Acl record in VSL to tell if a match was found or not.
• +table - Implement the ACL with a table instead of compiled code. This runs a little
bit slower, but compiles large ACLs much faster.
• -pedantic - Allow masks to cover non-zero host-bits. This allows the following to work:
acl foo -pedantic +log {
"firewall.example.com" / 24;
}
However, if the name resolves to both IPv4 and IPv6 you will still get an error.
• +fold - Fold ACL supernets and adjacent networks.
With this parameter set to on, ACLs are optimized in that subnets contained in other
entries are skipped (e.g. if 1.2.3.0/24 is part of the ACL, an entry for 1.2.3.128/25
will not be added) and adjacent entries get folded (e.g. if both 1.2.3.0/25 and
1.2.3.128/25 are added, they will be folded to 1.2.3.0/24).
Skip and fold operations on VCL entries are output as warnings during VCL compilation as
entries from the VCL are processed in order.
Logging under the VCL_acl tag can change with this parameter enabled: Matches on skipped
subnet entries are now logged as matches on the respective supernet entry. Matches on
folded entries are logged with a shorter netmask which might not be contained in the
original ACL as defined in VCL. Such log entries are marked by fixed: folded.
Negated ACL entries are never folded.
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.
It is possible to exit a subroutine that is not part of the built-in ones using a simple
return statement without specifying an action. It exits the subroutine without
transitioning to a different state:
sub filter_cookies {
if (!req.http.cookie) {
return;
}
# complex cookie filtering
}
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.
Functions
The following built-in functions are available:
ban(STRING)
Deprecated. See std.ban().
The ban() function is identical to std.ban(), but does not provide error reporting.
hash_data(input)
Adds an input to the hash input. In the built-in VCL hash_data() is called on the host
and URL of the request. Available in vcl_hash.
synthetic(STRING)
Prepare a synthetic response body containing the STRING. Available in vcl_synth and
vcl_backend_error.
Identical to set resp.body / set beresp.body.
regsub(str, regex, sub)
Returns a copy of str with the first occurrence of the regular expression regex
replaced with sub. Within sub, \0 (which can also be spelled \&) is replaced with the
entire matched string, and \n is replaced with the contents of subgroup n in the
matched string.
regsuball(str, regex, sub)
As regsub(), but this replaces all occurrences.
For converting or casting VCL values between data types use the functions available in the
std VMOD.
VERSIONING
Multiple versions of the VCL syntax can coexist within certain constraints.
The VCL syntax version at the start of VCL file specified with -f sets the hard limit that
cannot be exceeded anywhere, and it selects the appropriate version of the builtin VCL.
That means that you can never include vcl 9.1; from vcl 8.7;, but the opposite may be
possible, to the extent the compiler supports it.
Files pulled in via include do not need to have a vcl X.Y; but it may be a good idea to do
it anyway, to not have surprises in the future. The syntax version set in an included
file only applies to that file and any files it includes - unless these set their own VCL
syntax version.
The version of Varnish this file belongs to supports syntax 4.0 and 4.1.
EXAMPLES
For examples, please see the online documentation.
SEE ALSO
• varnishd(1)
• vcl-backend(7)
• vcl-probe(7)
• vcl-step(7)
• vcl-var(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) 2006-2015 Varnish Software AS
VCL(7)