VCL

Varnish Configuration Language

Manual section:

7

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.

Identifiers

VCL language elements are generally written in ASCII.

Identifiers start with an ASCII alphabetic character and continue with any combination of alphabetic or numerical character, - or _.

To illustrate, e1xam_p-le is a valid identifier, 1try and a%remainder are not.

Character Sets

While identifiers can only consist of this subset of ASCII, strings can contain any bytes except NUL (zero, 0), which marks the end of the string. The Varnish Configuration Language itself is not concerned with the character encoding of strings. VCL code handling strings in different character sets needs to track encodings itself. VMODs exist to help with such tasks (e.g. iconv).

To illustrate, "🥬" is a valid string which, when interpreted as UTF-8, happens to represent code point U+1F96C (Leafy Green) but 🥬 is not a valid identifier.

Character Sets in HTTP

Not all valid strings in VCL are valid in HTTP elements other than the body (e.g. URLs and headers). For example, HTTP header field values can not contain control characters, see also the validate_headers feature.

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 - Varnish Standard Module.

For the complete album of VCL variables see: VCL-Variables.

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-backends and VCL-probe

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);
}

The object name (b in the example) needs to be a valid Identifier.

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 BOOL ban(STRING).

The ban() function is identical to BOOL ban(STRING), 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

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.