NAME
VCL - Varnish Configuration Language
DESCRIPTION
The VCL language is a small domain-specific language designed to be used
to define request handling and document caching policies for the Varnish
HTTP accelerator.
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 dynamically linked into the server process.
Syntax
The VCL syntax is very simple, and deliberately similar to C and Perl.
Blocks are delimited by curly braces, statements end with semicolons, and
comments may be written as in C, C++ or Perl according to your own
preferences.
In addition to the C-like assignment (=), comparison (==) and boolean (!,
&& and ||) operators, VCL supports regular expression and ACL matching
using the ~ operator.
Unlike C and Perl, the backslash (\) character has no special meaning in
strings in VCL, which use the (%xx) escape mechanism just like URLs, so
it can be freely used in regular expressions without doubling.
Strings are concatenated by just putting them one after each other
without any operator in between.
Assignments are introduced with the set keyword. There are no user-
defined variables; values can only be assigned to variables attached to
backend, request or document objects. Most of these are typed, and the
values assigned to them must have a compatible unit suffix.
VCL has if tests, but no loops.
The contents of another VCL file may be inserted at any point in the code
by using the include keyword followed by the name of the other file as a
quoted string.
Backend declarations
A backend declaration creates and initializes a named backend object:
backend www {
.host = "www.example.com";
.port = "http";
}
The backend object can later be used to select a backend at request time:
if (req.http.host ~ "^(www.)?example.com$") {
set req.backend = www;
}
To avoid overloading backend servers, .max_connections can be set to
limit the maximum number of concurrent backend connections.
The timeout parameters can be overridden in the backend declaration. The
timeout parameters are .connect_timeout for the time to wait for a
backend connection, .first_byte_timeout for the time to wait for the
first byte from the backend and .between_bytes_timeout for time to wait
between each received byte.
These can be set in the declaration like this:
backend www {
.host = "www.example.com";
.port = "http";
.connect_timeout = 1s;
.first_byte_timeout = 5s;
.between_bytes_timeout = 2s;
}
To mark a backend as unhealthy after number of items have been added to
it's saintmode list .saintmode_threshold can be set to the maximum list
size. Setting a value of 0 disables saintmode checking entirely for that
backend. The value in the backend declaration overrides the parameter.
Directors
Directors choose from different backends based on health status and a
per-director algorithm. There currently exists a round-robin and a
random director.
Directors are defined using:
director b2 random {
.retries = 5;
{
/* We can refer to named backends */
.backend = b1;
.weight = 7;
}
{
/* Or define them inline */
.backend = {
.host = "fs2";
}
.weight = 3;
}
}
The random director
The random director takes one per-director option .retries. This
specifies how many tries it will use to find a working backend. The
default is the same as the number of backends defined for the director.
There is also a per-backend option: weight which defines the portion of
traffic to send to the particular backend.
The round-robin director
The round-robin does not take any options.
Backend probes
Backends can be probed to see whether they should be considered healthy
or not. The return status can also be checked by using
req.backend.healthy .window is how many of the latest polls we examine,
while .threshold is how many of those must have succeeded for us to
consider the backend healthy. .initial is how many of the probes are
considered good when Varnish starts - defaults to the same amount as the
threshold.
backend www {
.host = "www.example.com";
.port = "http";
.probe = {
.url = "/test.jpg";
.timeout = 0.3 s;
.window = 8;
.threshold = 3;
.initial = 3;
}
}
It is also possible to specify the raw HTTP request.
backend www {
.host = "www.example.com";
.port = "http";
.probe = {
# NB: \r\n automatically inserted after each string!
.request =
"GET / HTTP/1.1"
"Host: www.foo.bar"
"Connection: close";
}
}
ACLs
An ACL declaration creates and initializes a named access control list
which can later be used to match client addresses:
acl local {
"localhost"; /* myself */
"192.0.2.0"/24; /* and everyone on the local network */
! "192.0.2.23"; /* except for the dialin 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 ~ local) {
pipe;
}
Grace
If the backend takes a long time to generate an object there is a risk of
a thread pile up. In order to prevent this you can enable grace. This
allows varnish to serve an expired version of the object while a fresh
object is being generated by the backend.
The following vcl code will make Varnish serve expired objects. All
object will be kept up to two minutes past their expiration time or a
fresh object is generated.
sub vcl_recv {
set req.grace = 2m;
}
sub vcl_fetch {
set obj.grace = 2m;
}
Functions
The following built-in functions are available:
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 regsuball() but this replaces all occurrences.
purge_url(regex)
Purge all objects in cache whose URLs match regex.
Subroutines
A subroutine is used to group code for legibility or reusability:
sub pipe_if_local {
if (client.ip ~ local) {
pipe;
}
}
Subroutines in VCL do not take arguments, nor do they return values.
To call a subroutine, use the call keyword followed by the subroutine's
name:
call pipe_if_local;
There are a number of special subroutines which hook into the Varnish
workflow. These subroutines may inspect and manipulate HTTP headers and
various other aspects of each request, and to a certain extent decide how
the request should be handled. Each subroutine terminates by calling one
of a small number of keywords which indicates the desired outcome.
vcl_recv
Called at the beginning of a request, after the complete request
has been received and parsed. Its purpose is to decide whether
or not to serve the request, how to do it, and, if applicable,
which backend to use.
The vcl_recv subroutine may terminate with one of the following
keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
pass Switch to pass mode. Control will eventually pass to
vcl_pass.
pipe Switch to pipe mode. Control will eventually pass to
vcl_pipe.
lookup Look up the requested object in the cache. Control will
eventually pass to vcl_hit or vcl_miss, depending on
whether the object is in the cache.
vcl_pipe
Called upon entering pipe mode. In this mode, the request is
passed on to the backend, and any further data from either client
or backend is passed on unaltered until either end closes the
connection.
The vcl_pipe subroutine may terminate with one of the following
keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
pipe Proceed with pipe mode.
vcl_pass
Called upon entering pass mode. In this mode, the request is
passed on to the backend, and the backend's response is passed on
to the client, but is not entered into the cache. Subsequent
requests submitted over the same client connection are handled
normally.
The vcl_pass subroutine may terminate with one of the following
keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
pass Proceed with pass mode.
vcl_hash
Use req.hash += req.http.Cookie or similar to include the Cookie
HTTP header in the hash string. The vcl_hash subroutine may
terminate with one of the following keywords:
hash Proceed.
vcl_hit
Called after a cache lookup if the requested document was found
in the cache.
The vcl_hit subroutine may terminate with one of the following
keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
pass Switch to pass mode. Control will eventually pass to
vcl_pass.
deliver
Deliver the cached object to the client. Control will
eventually pass to vcl_deliver.
vcl_miss
Called after a cache lookup if the requested document was not
found in the cache. Its purpose is to decide whether or not to
attempt to retrieve the document from the backend, and which
backend to use.
The vcl_miss subroutine may terminate with one of the following
keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
pass Switch to pass mode. Control will eventually pass to
vcl_pass.
fetch Retrieve the requested object from the backend. Control
will eventually pass to vcl_fetch.
vcl_fetch
Called after a document has been successfully retrieved from the
backend.
The vcl_fetch subroutine may terminate with one of the following
keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
pass Switch to pass mode. Control will eventually pass to
vcl_pass.
deliver
Possibly insert the object into the cache, then deliver
it to the client. Control will eventually pass to
vcl_deliver.
esi ESI-process the document which has just been fetched.
vcl_deliver
Called before a cached object is delivered to the client.
The vcl_deliver subroutine may terminate with one of the
following keywords:
error code [reason]
Return the specified error code to the client and abandon
the request.
deliver
Deliver the object to the client.
If one of these subroutines is left undefined or terminates without
reaching a handling decision, control will be handed over to the builtin
default. See the EXAMPLES section for a listing of the default code.
Multiple subroutines
If multiple subroutines with the same name are defined, they are
concatenated in the order in which the appear in the source.
Example:
# in file "main.vcl"
include "backends.vcl";
include "purge.vcl";
# in file "backends.vcl"
sub vcl_recv {
if (req.http.host ~ "example.com") {
set req.backend = foo;
} elsif (req.http.host ~ "example.org") {
set req.backend = bar;
}
}
# in file "purge.vcl"
sub vcl_recv {
if (client.ip ~ admin_network) {
if (req.http.Cache-Control ~ "no-cache") {
purge_url(req.url);
}
}
}
The builtin default subroutines are implicitly appended in this way.
Variables
Although subroutines take no arguments, the necessary information is made
available to the handler subroutines through global variables.
The following variables are always available:
now The current time, in seconds since the epoch.
The following variables are available in backend declarations:
.host
Host name or IP address of a backend.
.port
Service name or port number of a backend.
The following variables are available while processing a request:
client.ip
The client's IP address.
server.hostname
The host name of the server.
server.identity
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
The IP address of the socket on which the client connection was
received.
server.port
The port number of the socket on which the client connection was
received.
req.request
The request type (e.g. "GET", "HEAD").
req.url
The requested URL.
req.proto
The HTTP protocol version used by the client.
req.backend
The backend to use to service the request.
req.backend.healthy
Whether the backend is healthy or not.
req.http.header
The corresponding HTTP header.
The following variables are available while preparing a backend request
(either for a cache miss or for pass or pipe mode):
bereq.request
The request type (e.g. "GET", "HEAD").
bereq.url
The requested URL.
bereq.proto
The HTTP protocol version used to talk to the server.
bereq.http.header
The corresponding HTTP header.
bereq.connect_timeout
The time in seconds to wait for a backend connection.
bereq.first_byte_timeout
The time in seconds to wait for the first byte from the backend.
Not available in pipe mode.
bereq.between_bytes_timeout
The time in seconds to wait between each received byte from the
backend. Not available in pipe mode.
The following variables are available after the requested object has been
retrieved from cache or from the backend:
obj.proto
The HTTP protocol version used when the object was retrieved.
obj.status
The HTTP status code returned by the server.
obj.response
The HTTP status message returned by the server.
obj.cacheable
True if the request resulted in a cacheable response. A response
is considered cacheable if it is valid (see above), and the HTTP
status code is 200, 203, 300, 301, 302, 404 or 410.
obj.ttl
The object's remaining time to live, in seconds.
obj.lastuse
The approximate time elapsed since the object was last requests, in
seconds.
obj.hits
The approximate number of times the object has been delivered. A
value of 0 indicates a cache miss.
The following variables are available while determining the hash key of
an object:
req.hash
The hash key used to refer to an object in the cache. Used when
both reading from and writing to the cache.
The following variables are available while preparing a response to the
client:
resp.proto
The HTTP protocol version to use for the response.
resp.status
The HTTP status code that will be returned.
resp.response
The HTTP status message that will be returned.
resp.http.header
The corresponding HTTP header.
Values may be assigned to variables using the set keyword:
sub vcl_recv {
# Normalize the Host: header
if (req.http.host ~ "^(www.)?example.com$") {
set req.http.host = "www.example.com";
}
}
HTTP headers can be removed entirely using the remove keyword:
sub vcl_fetch {
# Don't cache cookies
remove obj.http.Set-Cookie;
}
EXAMPLES
The following code is the equivalent of the default configuration with
the backend address set to "backend.example.com" and no backend port
specified.
backend default {
.host = "backend.example.com";
.port = "http";
}
sub vcl_recv {
if (req.http.x-forwarded-for) {
set req.http.X-Forwarded-For =
req.http.X-Forwarded-For ", " client.ip;
} else {
set req.http.X-Forwarded-For = client.ip;
}
if (req.request != "GET" &&
req.request != "HEAD" &&
req.request != "PUT" &&
req.request != "POST" &&
req.request != "TRACE" &&
req.request != "OPTIONS" &&
req.request != "DELETE") {
/* Non-RFC2616 or CONNECT which is weird. */
return (pipe);
}
if (req.request != "GET" && req.request != "HEAD") {
/* We only deal with GET and HEAD by default */
return (pass);
}
if (req.http.Authorization || req.http.Cookie) {
/* Not cacheable by default */
return (pass);
}
return (lookup);
}
sub vcl_pipe {
# Note that only the first request to the backend will have
# X-Forwarded-For set. If you use X-Forwarded-For and want to
# have it set for all requests, make sure to have:
# set req.http.connection = "close";
# here. It is not set by default as it might break some broken web
# applications, like IIS with NTLM authentication.
return (pipe);
}
sub vcl_pass {
return (pass);
}
sub vcl_hash {
set req.hash += req.url;
if (req.http.host) {
set req.hash += req.http.host;
} else {
set req.hash += server.ip;
}
return (hash);
}
sub vcl_hit {
if (!obj.cacheable) {
return (pass);
}
return (deliver);
}
sub vcl_miss {
return (fetch);
}
sub vcl_fetch {
if (!beresp.cacheable) {
return (pass);
}
if (beresp.http.Set-Cookie) {
return (pass);
}
return (deliver);
}
sub vcl_deliver {
return (deliver);
}
sub vcl_error {
set obj.http.Content-Type = "text/html; charset=utf-8";
synthetic {"
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<title>"} obj.status " " obj.response {"</title>
</head>
<body>
<h1>Error "} obj.status " " obj.response {"</h1>
<p>"} obj.response {"</p>
<h3>Guru Meditation:</h3>
<p>XID: "} req.xid {"</p>
<hr>
<p>Varnish cache server</p>
</body>
</html>
"};
return (deliver);
}
The following example shows how to support multiple sites running on
separate backends in the same Varnish instance, by selecting backends
based on the request URL.
backend www {
.host = "www.example.com";
.port = "80";
}
backend images {
.host = "images.example.com";
.port = "80";
}
sub vcl_recv {
if (req.http.host ~ "^(www.)?example.com$") {
set req.http.host = "www.example.com";
set req.backend = www;
} elsif (req.http.host ~ "^images.example.com$") {
set req.backend = images;
} else {
error 404 "Unknown virtual host";
}
}
The following snippet demonstrates how to force a minimum TTL for all
documents. Note that this is not the same as setting the default_ttl
run-time parameter, as that only affects document for which the backend
did not specify a TTL.
sub vcl_fetch {
if (obj.ttl < 120s) {
set obj.ttl = 120s;
}
}
The following snippet demonstrates how to force Varnish to cache
documents even when cookies are present.
sub vcl_recv {
if (req.request == "GET" && req.http.cookie) {
lookup;
}
}
sub vcl_fetch {
if (obj.http.Set-Cookie) {
deliver;
}
}
The following code implements the HTTP PURGE method as used by Squid for
object invalidation:
acl purge {
"localhost";
"192.0.2.1"/24;
}
sub vcl_recv {
if (req.request == "PURGE") {
if (!client.ip ~ purge) {
error 405 "Not allowed.";
}
lookup;
}
}
sub vcl_hit {
if (req.request == "PURGE") {
set obj.ttl = 0s;
error 200 "Purged.";
}
}
sub vcl_miss {
if (req.request == "PURGE") {
error 404 "Not in cache.";
}
}
SEE ALSO
varnishd(1)
HISTORY
The VCL language was developed by Poul-Henning Kamp <phk@phk.freebsd.dk>
in cooperation with Verdens Gang AS and Linpro AS. This manual page was
written by Dag-Erling Smorgrav <des@des.no>.