1. Reload edited Varnish configs

Varnish caching rules live in /etc/varnish folder. The config entry point (main file) is /etc/varnish/default.vcl. The daemon itself (ports, etc.) is configured by /etc/defaults/varnish.

Varnish is controlled by an utility program varnishadm.You can use it in a console mode or issue direct command evaluations (think shell, MySQL client). On Ubuntu / Debian default installation varnishadm command as is is enough to control Varnish. However, on custom setup, you might need to guide it to a special console port or point it to a secret file.

Varnish config load is 2 stage process:

• Parse and load cfg file to a Varnish memory and give it a handle you can later refer to it
• Activate config by handle (only possible if step 1 success)

Below is an one liner shell script which generates a random handle and uses it to load the config if the config parses successfully.

HANDLE=varnish-cfg-$RANDOM ; \
  varnishadm vcl.load $HANDLE /etc/varnish/default.vcl && \
  varnishadm vcl.use $HANDLE

2. Purging Varnish cache from command line

Another useful snippet is to purge all Varnish cache from command line (invalidate all the cache):

varnishadm "ban.url ."  # Matches all URLs

Note: Command is purge.url in Varnish 2.x.

The cache is kept as shared memorymapped file in /var/lib/varnish/$INSTANCE/varnish_storage.bin. When Varnish is running it should map 1 GB (default) of your virtual memory to this file (as seen in ps, top).

You could also ban by a hostname:

varnishadm "ban req.http.host == opensourcehacker.com"

Here is a shell transcript where we observe that ban works as intended using wget utility.

# Go to /tmp because wget leaves files around
cd /tmp

# 1st load: uncached file, one X-Varnish stamp
wget -S http://opensourcehacker.com/wp-content/uploads/2011/08/Untitled-41.jpg

--2013-02-06 20:02:18--  http://opensourcehacker.com/wp-content/uploads/2011/08/Untitled-41.jpg
Resolving opensourcehacker.com (opensourcehacker.com)... 188.40.123.220
Connecting to opensourcehacker.com (opensourcehacker.com)|188.40.123.220|:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 200 OK
  Server: Apache/2.2.22 (Ubuntu)
  Last-Modified: Sun, 14 Aug 2011 22:55:01 GMT
  ETag: "2000893-108ec-4aa7f09555b40"
  Cache-Control: max-age=3600
  Expires: Wed, 06 Feb 2013 23:02:19 GMT
  Content-Type: image/jpeg
  Content-Length: 67820
  Accept-Ranges: bytes
  Date: Wed, 06 Feb 2013 22:02:19 GMT
  X-Varnish: 705602514

# 2st load: cached file, two X-Varnish stamps
wget -S http://opensourcehacker.com/wp-content/uploads/2011/08/Untitled-41.jpg
--2013-02-06 20:02:21--  http://opensourcehacker.com/wp-content/uploads/2011/08/Untitled-41.jpg
Resolving opensourcehacker.com (opensourcehacker.com)... 188.40.123.220
Connecting to opensourcehacker.com (opensourcehacker.com)|188.40.123.220|:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 200 OK
  Server: Apache/2.2.22 (Ubuntu)
  Last-Modified: Sun, 14 Aug 2011 22:55:01 GMT
  ETag: "2000893-108ec-4aa7f09555b40"
  Cache-Control: max-age=3600
  Expires: Wed, 06 Feb 2013 23:02:19 GMT
  Content-Type: image/jpeg
  Content-Length: 67820
  Accept-Ranges: bytes
  Date: Wed, 06 Feb 2013 22:02:22 GMT
  X-Varnish: 705602515 705602514

# Purge
varnishadm "ban.url ."

# It's non-cached again
wget -S http://opensourcehacker.com/wp-content/uploads/2011/08/Untitled-41.jpg
--2013-02-06 20:02:34--  http://opensourcehacker.com/wp-content/uploads/2011/08/Untitled-41.jpg
Resolving opensourcehacker.com (opensourcehacker.com)... 188.40.123.220
Connecting to opensourcehacker.com (opensourcehacker.com)|188.40.123.220|:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 200 OK
  Server: Apache/2.2.22 (Ubuntu)
  Last-Modified: Sun, 14 Aug 2011 22:55:01 GMT
  ETag: "2000893-108ec-4aa7f09555b40"
  Cache-Control: max-age=3600
  Expires: Wed, 06 Feb 2013 23:02:35 GMT
  Content-Type: image/jpeg
  Content-Length: 67820
  Accept-Ranges: bytes
  Date: Wed, 06 Feb 2013 22:02:35 GMT
  X-Varnish: 705602516

3. Restart Varnish on Ubuntu

This forces config flush, not sure about whether cache file storage gets reset(?).

service varnish restart