CLI - bossing Varnish around¶
varnishd is started, you can control it using the command line
The easiest way to do this, is using
varnishadm on the
same machine as
varnishd is running:
If you want to run
varnishadm from a remote system, you can do it
You can SSH into the
varnishd computer and run
ssh $http_front_end varnishadm help
But you can also configure
varnishd to accept remote CLI connections
(using the ‘-T’ and ‘-S’ arguments):
varnishd -T :6082 -S /etc/varnish_secret
And then on the remote system run
varnishadm -T $http_front_end -S /etc/copy_of_varnish_secret help
but as you can see, SSH is much more convenient.
If you run
varnishadm without arguments, it will read CLI commands from
stdin, if you give it arguments, it will treat those as the single
CLI command to execute.
The CLI always returns a status code to tell how it went: ‘200’ means OK, anything else means there were some kind of trouble.
varnishadm will exit with status 1 and print the status code on
standard error if it is not 200.
What can you do with the CLI¶
The CLI gives you almost total control over
varnishd some of the more important tasks you can perform are:
load/use/discard VCL programs
ban (invalidate) cache content
start/stop worker process
We will discuss each of these briefly below.
Load, use and discard VCL programs¶
All caching and policy decisions are made by VCL programs.
You can have multiple VCL programs loaded, but one of them is designated the “active” VCL program, and this is where all new requests start out.
To load new VCL program:
varnish> vcl.load some_name some_filename
Loading will read the VCL program from the file, and compile it. If the compilation fails, you will get an error messages:
.../mask is not numeric. ('input' Line 4 Pos 17) "192.168.2.0/24x", ----------------#################- Running VCC-compiler failed, exit 1 VCL compilation failed
If compilation succeeds, the VCL program is loaded, and you can now make it the active VCL, whenever you feel like it:
varnish> vcl.use some_name
If you find out that was a really bad idea, you can switch back to the previous VCL program again:
varnish> vcl.use old_name
The switch is instantaneous, all new requests will start using the VCL you activated right away. The requests currently being processed complete using whatever VCL they started with.
It is good idea to design an emergency-VCL before you need it, and always have it loaded, so you can switch to it with a single vcl.use command.
Ban cache content¶
Varnish offers “purges” to remove things from cache, provided that you know exactly what they are.
But sometimes it is useful to be able to throw things out of cache without having an exact list of what to throw out.
Imagine for instance that the company logo changed and now you need Varnish to stop serving the old logo out of the cache:
varnish> ban req.url ~ "logo.*[.]png"
should do that, and yes, that is a regular expression.
We call this “banning” because the objects are still in the cache, but they are banned from delivery.
Instead of checking each and every cached object right away, we test each object against the regular expression only if and when an HTTP request asks for it.
Banning stuff is much cheaper than restarting Varnish to get rid of wronly cached content.
Parameters can be set on the command line with the ‘-p’ argument, but they can also be examined and changed on the fly from the CLI:
varnish> param.show prefer_ipv6 200 prefer_ipv6 off [bool] Default is off Prefer IPv6 address when connecting to backends which have both IPv4 and IPv6 addresses. varnish> param.set prefer_ipv6 true 200
In general it is not a good idea to modify parameters unless you have a good reason, such as performance tuning or security configuration.
Most parameters will take effect instantly, or with a natural delay of some duration, but a few of them requires you to restart the child process before they take effect. This is always noted in the description of the parameter.
Starting and stopping the worker process¶
In general you should just leave the worker process running, but if you need to stop and/or start it, the obvious commands work:
If you start
varnishd with the ‘-d’ (debugging) argument, you will
always need to start the child process explicitly.
Should the child process die, the master process will automatically restart it, but you can disable that with the ‘auto_restart’ parameter.
The shell, the other CLI¶
Besides accessing the CLI via its interface or via
is the matter of actually running the
varnishd command line, usually
via a shell. See Security first for security concerns around the
varnishd command line. See also Syntax about the CLI
syntax and quoting pitfalls when using
The programs shipped with Varnish can expose their optstring in order
to help writing wrapper scripts, in particular to get an opportunity to
hook a task before a program daemonizes. With the exception of
varnishadm, you can write Shell wrappers for
varnishd using the
-x option and other programs using the
--optstring long option.
This way, when writing a wrapper script you don’t need to maintain the
optstring in sync when you only need a subset of the options, usually
optstring=$(varnishd -x optstring) while getopts "$optstring" opt do case $opt in n) # handle $OPTARG ;; # handle other options *) # ignore unneeded options ;; esac done varnishd "$@" # do something with the options
You can for example write a wrapper script that blocks until the shared
memory is ready or when the child is started if you need that kind of
synchronization. You can also prevent
varnishd from starting if the
-S option is inadvertently set to not challenge access to the CLI.