[varnish] add a few words about vmods

varnish/content.md

@@ -13,21 +13,45 @@ Varnish is an HTTP accelerator designed for content-heavy dynamic web sites as w
 Create a `default.vcl` file:
 
 ```vcl
-vcl 4.0;
+# specify the VCL syntax version to use
+vcl 4.1;
 
-backend default {
-  .host = "www.nytimes.com:80";
+# import vmod_dynamic for better backend name resolution
+import dynamic;
+
+# we won't use any static backend, but Varnish still need a default one
+backend default none;
+
+# set up a dynamic director
+# for more info, see https://github.com/nigoroll/libvmod-dynamic/blob/master/src/vmod_dynamic.vcc
+sub vcl_init {
+        new d = dynamic.director(port = "80");
+}
+
+sub vcl_recv {
+	# force the host header to match the backend (not all backends need it,
+	# but example.com does)
+	set req.http.host = "example.com";
+	# set the backend
+	set req.backend_hint = d.backend("example.com");
 }
 ```
 
 Then run:
 
 ```console
-# we need both a configuration file at /etc/varnish/default.vcl
-# and our workdir to be mounted as tmpfs to avoid disk I/O
-$ docker run -v /path/to/default.vcl:/etc/varnish/default.vcl:ro --tmpfs /var/lib/varnish/varnishd:exec %%IMAGE%%
+# we need the configuration file at /etc/varnish/default.vcl,
+# our workdir has to be mounted as tmpfs to avoid disk I/O,
+# and we'll use port 8080 to talk to our container (internally listening on 80)
+$ docker run \
+	-v /path/to/default.vcl:/etc/varnish/default.vcl:ro \
+	--tmpfs /var/lib/varnish/varnishd:exec \
+	-p 8080:80 \
+	%%IMAGE%%
 ```
 
+From there, you can visit `localhost:8080` in your browser and see the example.com homepage.
+
 Alternatively, a simple `Dockerfile` can be used to generate a new image that includes the necessary `default.vcl` (which is a much cleaner solution than the bind mount above):
 
 ```dockerfile
@@ -39,7 +63,7 @@ COPY default.vcl /etc/varnish/
 Place this file in the same directory as your `default.vcl`, run `docker build -t my-varnish .`, then start your container:
 
 ```console
-$ docker --tmpfs /var/lib/varnish/varnishd:exec my-varnish
+$ docker --tmpfs /var/lib/varnish/varnishd:exec -p 8080:80 my-varnish
 ```
 
 ## Reloading the configuration
@@ -64,14 +88,14 @@ docker run varnish varnishreload -h
 By default, the containers will use a cache size of 100MB, which is usually a bit too small, but you can quickly set it through the `VARNISH_SIZE` environment variable:
 
 ```console
-$ docker run --tmpfs /var/lib/varnish/varnishd:exec -e VARNISH_SIZE=2G %%IMAGE%%
+$ docker run --tmpfs /var/lib/varnish/varnishd:exec -p 8080:80 -e VARNISH_SIZE=2G %%IMAGE%%
 ```
 
 Additionally, you can add arguments to `docker run` after `%%IMAGE%%`, if the first one starts with a `-`, they will be appendend to the [default command](https://github.com/varnish/docker-varnish/blob/master/docker-varnish-entrypoint#L8):
 
 ```console
 # extend the default keep period
-$ docker run --tmpfs /var/lib/varnish/varnishd:exec -e VARNISH_SIZE=2G %%IMAGE%% -p default_keep=300
+$ docker run --tmpfs /var/lib/varnish/varnishd:exec -p 8080:80 -e VARNISH_SIZE=2G %%IMAGE%% -p default_keep=300
 ```
 
 If your first argument after `%%IMAGE%%` doesn't start with `-`, it will be interpreted as a command to override the default one:
@@ -84,13 +108,9 @@ $ docker run %%IMAGE%% varnishd -?
 $ docker run %%IMAGE%% varnishd -x parameter
 
 # run the server with your own parameters (don't forget -F to not daemonize)
-$ docker run %%IMAGE%% varnishd -a :8080 -b 127.0.0.1:8181 -t 600 -p feature=+http2
+$ docker run %%IMAGE%% varnishd -F -a :8080 -b 127.0.0.1:8181 -t 600 -p feature=+http2
 ```
 
-## Exposing the port
-
-```console
-$ docker run --name my-running-varnish --tmpfs /var/lib/varnish/varnishd:exec -d -p 8080:80 my-varnish
-```
+## vmods (since 7.1)
 
-Then you can hit `http://localhost:8080` or `http://host-ip:8080` in your browser.
+As mentioned above, you can use [vmod_dynamic](https://github.com/nigoroll/libvmod-dynamic) for backend resolution. The [varnish-modules](https://github.com/varnish/varnish-modules) collection is also included in the image. All the documentation regarding usage and syntax can be found in the [src/](https://github.com/varnish/varnish-modules/tree/master/src) directory of the repository.