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 X.Y;” 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.

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 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");
}

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.

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.

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 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.

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.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

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

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

Readable from: vcl_backend_response, vcl_backend_error

The entire backend response HTTP data structure

beresp.age

Type: DURATION

Readable from: vcl_backend_response, vcl_backend_error

The age of the object.

beresp.backend

Type: BACKEND

Readable from: vcl_backend_response, vcl_backend_error

This is the backend we fetched from. If bereq.backend was set to a director, this will be the backend selected by the director.

beresp.backend.ip

Type: IP

Readable from: vcl_backend_response, vcl_backend_error

IP of the backend this response was fetched from.

beresp.backend.name

Type: STRING

Readable from: vcl_backend_response, vcl_backend_error

Name of the backend this response was fetched from.

beresp.body

Type: BODY

Writable from: vcl_backend_error

The response body.

beresp.do_esi

Type: BOOL

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Boolean. ESI-process the object after fetching it. Defaults to false. Set it to true to parse the object for ESI directives. Will only be honored if req.esi is true.

beresp.do_gunzip

Type: BOOL

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Boolean. Unzip the object before storing it in the cache. Defaults to false.

beresp.do_gzip

Type: BOOL

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Boolean. Gzip the object before storing it. Defaults to false. When http_gzip_support is on Varnish will request already compressed content from the backend and as such compression in Varnish is not needed.

beresp.do_stream

Type: BOOL

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Deliver the object to the client while fetching the whole object into varnish. For uncacheable objects, storage for parts of the body which have been sent to the client may get freed early, depending on the storage engine used.

beresp.grace

Type: DURATION

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Set to a period to enable grace.

beresp.http.

Type: HEADER

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

The corresponding HTTP header.

beresp.keep

Type: DURATION

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Set to a period to enable conditional backend requests.

The keep time is cache lifetime in addition to the ttl.

Objects with ttl expired but with keep time left may be used to issue conditional (If-Modified-Since / If-None-Match) requests to the backend to refresh them.

beresp.proto

Type: STRING

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

The HTTP protocol version used the backend replied with.

beresp.reason

Type: STRING

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

The HTTP status message returned by the server.

beresp.status

Type: INT

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

The HTTP status code returned by the server.

beresp.storage_hint

Type: STRING

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Hint to Varnish that you want to save this object to a particular storage backend.

beresp.ttl

Type: DURATION

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

The object’s remaining time to live, in seconds.

beresp.uncacheable

Type: BOOL

Readable from: vcl_backend_response, vcl_backend_error

Writable from: vcl_backend_response, vcl_backend_error

Inherited from bereq.uncacheable, see there.

Setting this variable makes the object uncacheable, which may get stored as a hit-for-pass object in the cache.

Clearing the variable has no effect and will log the warning “Ignoring attempt to reset beresp.uncacheable”.

beresp.was_304

Type: BOOL

Readable from: vcl_backend_response, vcl_backend_error

Boolean. If this is a successful 304 response to a backend conditional request refreshing an existing cache object.

client¶

client.identity

Type: STRING

Readable from: client

Writable from: client

Identification of the client, used to load balance in the client director. Defaults to the client’s IP address.

client.ip

Type: IP

Readable from: client, backend

The client’s IP address.

local¶

local.ip

Type: IP

Readable from: client, backend

The IP address of the local end of the TCP connection.

now¶

now

Type: TIME

Readable from: all

The current time, in seconds since the epoch. When used in string context it returns a formatted string.

obj¶

obj.age

Type: DURATION

Readable from: vcl_hit, vcl_deliver

The age of the object.

obj.grace

Type: DURATION

Readable from: vcl_hit, vcl_deliver

The object’s remaining grace period in seconds.

obj.hits

Type: INT

Readable from: vcl_hit, vcl_deliver

The count of cache-hits on this object. A value of 0 indicates a cache miss.

obj.http.

Type: HEADER

Readable from: vcl_hit

The corresponding HTTP header.

obj.keep

Type: DURATION

Readable from: vcl_hit, vcl_deliver

The object’s remaining keep period in seconds.

obj.proto

Type: STRING

Readable from: vcl_hit

The HTTP protocol version used when the object was retrieved.

obj.reason

Type: STRING

Readable from: vcl_hit

The HTTP status message returned by the server.

obj.status

Type: INT

Readable from: vcl_hit

The HTTP status code returned by the server.

obj.ttl

Type: DURATION

Readable from: vcl_hit, vcl_deliver

The object’s remaining time to live, in seconds.

obj.uncacheable

Type: BOOL

Readable from: vcl_deliver

Whether the object is uncacheable (pass or hit-for-pass).

remote¶

remote.ip

Type: IP

Readable from: client, backend

The IP address of the other end of the TCP connection. This can either be the clients IP, or the outgoing IP of a proxy server.

req¶

req

Type: HTTP

Readable from: client

The entire request HTTP data structure

req.backend_hint

Type: BACKEND

Readable from: client

Writable from: client

Set bereq.backend to this if we attempt to fetch.

req.can_gzip

Type: BOOL

Readable from: client

Does the client accept the gzip transfer encoding.

req.esi

Type: BOOL

Readable from: client

Writable from: client

Boolean. Set to false to disable ESI processing regardless of any value in beresp.do_esi. Defaults to true. This variable is subject to change in future versions, you should avoid using it.

req.esi_level

Type: INT

Readable from: client

A count of how many levels of ESI requests we’re currently at.

req.hash_always_miss

Type: BOOL

Readable from: vcl_recv

Writable from: vcl_recv

Force a cache miss for this request. If set to true Varnish will disregard any existing objects and always (re)fetch from the backend.

req.hash_ignore_busy

Type: BOOL

Readable from: vcl_recv

Writable from: vcl_recv

Ignore any busy object during cache lookup. You would want to do this if you have two server looking up content from each other to avoid potential deadlocks.

req.http.

Type: HEADER

Readable from: client

Writable from: client

The corresponding HTTP header.

req.method

Type: STRING

Readable from: client

Writable from: client

The request type (e.g. “GET”, “HEAD”).

req.proto

Type: STRING

Readable from: client

Writable from: client

The HTTP protocol version used by the client.

req.restarts

Type: INT

Readable from: client

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

req.ttl

Type: DURATION

Readable from: client

Writable from: client

req.url

Type: STRING

Readable from: client

Writable from: client

The requested URL.

req.xid

Type: STRING

Readable from: client

Unique ID of this request.

req_top¶

req_top.http.

Type: HEADER

Readable from: client

HTTP headers of the top-level request in a tree of ESI requests. Identical to req.http. in non-ESI requests.

req_top.method

Type: STRING

Readable from: client

The request method of the top-level request in a tree of ESI requests. (e.g. “GET”, “HEAD”). Identical to req.method in non-ESI requests.

req_top.proto

Type: STRING

Readable from: client

HTTP protocol version of the top-level request in a tree of ESI requests. Identical to req.proto in non-ESI requests.

req_top.url

Type: STRING

Readable from: client

The requested URL of the top-level request in a tree of ESI requests. Identical to req.url in non-ESI requests.

resp¶

resp

Type: HTTP

Readable from: vcl_deliver, vcl_synth

The entire response HTTP data structure.

resp.body

Type: BODY

Writable from: vcl_synth

The response body.

resp.http.

Type: HEADER

Readable from: vcl_deliver, vcl_synth

Writable from: vcl_deliver, vcl_synth

The corresponding HTTP header.

resp.is_streaming

Type: BOOL

Readable from: vcl_deliver, vcl_synth

Returns true when the response will be streamed from the backend.

resp.proto

Type: STRING

Readable from: vcl_deliver, vcl_synth

Writable from: vcl_deliver, vcl_synth

The HTTP protocol version to use for the response.

resp.reason

Type: STRING

Readable from: vcl_deliver, vcl_synth

Writable from: vcl_deliver, vcl_synth

The HTTP status message that will be returned.

resp.status

Type: INT

Readable from: vcl_deliver, vcl_synth

Writable from: vcl_deliver, vcl_synth

The HTTP status code that will be returned.

Assigning a HTTP standardized code to resp.status will also set resp.reason to the corresponding status message.

server¶

server.hostname

Type: STRING

Readable from: all

The host name of the server.

server.identity

Type: STRING

Readable from: all

The identity of the server, as set by the -i parameter. If the -i parameter is not passed to varnishd, server.identity will be set to the name of the instance, as specified by the -n parameter.

server.ip

Type: IP

Readable from: client, backend

The IP address of the socket on which the client connection was received.

storage¶

storage.<name>.free_space

Type: BYTES

Readable from: client, backend

Free space available in the named stevedore. Only available for the malloc stevedore.

storage.<name>.used_space

Type: BYTES

Readable from: client, backend

Used space in the named stevedore. Only available for the malloc stevedore.

storage.<name>.happy

Type: BOOL

Readable from: client, backend

Health status for the named stevedore. Not available in any of the current stevedores.

Functions¶

The following built-in functions are available:

ban(expression): Invalidates all objects in cache that match the expression with the ban mechanism.
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.

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 only.

EXAMPLES¶

For examples, please see the online documentation.

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 possibly others.

COPYRIGHT¶

This document is licensed under the same license as Varnish itself. See LICENSE for details.

VCL¶

Varnish Configuration Language¶

DESCRIPTION¶

Operators¶

Conditionals¶

Strings, booleans, time, duration, integers and real numbers¶

Strings¶

Booleans¶

Time¶

Durations¶

Integers¶

Real numbers¶

Regular Expressions¶

Include statement¶

Import statement¶

Comments¶

Backend definition¶

Probes¶

Access Control List (ACL)¶

VCL objects¶

Subroutines¶

Return statements¶

Multiple subroutines¶

Variables¶

bereq¶

beresp¶

client¶

local¶

now¶

obj¶

remote¶

req¶

req_top¶

resp¶

server¶

storage¶

Functions¶

Versioning¶

EXAMPLES¶

SEE ALSO¶

HISTORY¶

COPYRIGHT¶