I'm trying to install varnish on ubuntu 16.04,

I read several article none are working.
From what I read, since ubuntu 15.04, the way of configuring varnish has changed (because of systemd).

Now on I've got a real mess wich don't work :

/etc/default/varnish :

DAEMON_OPTS="-a :80 
 -T localhost:6082 
 -f /etc/varnish/default.vcl 
 -S /etc/varnish/secret 
 -s malloc,256m"

/etc/varnish/default.vcl (normally it points to a host pointing to 127.0.0 and port 8080, but for debuging purpose I modified it to an external domain)
vcl 4.0;

# Default backend definition. Set this to point to your content server.
backend default 
 .host = "www.varnish-cache.org"; 
 .port = "80";

/etc/apache2/ports.conf

Listen 8080

grep -R 'ExecStart=/usr/sbin/varnishd' /etc/

/etc/systemd/system/varnish.service:ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m
/etc/systemd/system/varnish.service.d/customexec.conf:ExecStart=/usr/sbin/varnishd -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m
/etc/systemd/system/multi-user.target.wants/varnish.service:ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m

/lib/systemd/system/varnish.service :

 GNU nano 2.5.3 Fichier : /lib/systemd/system/varnish.service 

[Unit]
Description=Varnish HTTP accelerator
Documentation=https://www.varnish-cache.org/docs/4.1/ man:varnishd

[Service]
Type=simple
LimitNOFILE=131072
LimitMEMLOCK=82000
ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m
ExecReload=/usr/share/varnish/reload-vcl
ProtectSystem=full
ProtectHome=true
PrivateTmp=true
PrivateDevices=true

[Install]
WantedBy=multi-user.target

service --status-all | grep varnish

 [ - ] varnish
 [ + ] varnishlog
 [ + ] varnishncsa

after a

sudo service varnish stop
sudo service varnish start

The varnish service is not listening on http://127.0.0.1:80/, before a reboot, it listend on http://127.0.0.1:6081/ but it don't work no more ... I don't know what to do more...

EDIT : after a reboot, nothing works,

if I do :
systemctl status varnish

● varnish.service - Varnish HTTP accelerator
 Loaded: loaded (/etc/systemd/system/varnish.service; enabled; vendor preset: enabled)
 Drop-In: /etc/systemd/system/varnish.service.d
 └─customexec.conf
 Active: inactive (dead) since jeu. 2017-01-05 14:48:09 CET; 1s ago
 Docs: https://www.varnish-cache.org/docs/4.1/
 man:varnishd
 Process: 5077 ExecStart=/usr/sbin/varnishd -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m (code=exited, status=0/SUCCESS)
 Main PID: 5077 (code=exited, status=0/SUCCESS)

janv. 05 14:48:09 xubuntu-16 systemd[1]: Started Varnish HTTP accelerator.

service --status-all | grep varnish

 [ - ] varnish
 [ - ] varnishlog
 [ - ] varnishncsa

if I sudo : varnishd -d -f /etc/varnish/default.vcl, then start, everything works fine... until i quit the cli

solved thanks to @Gerald Schneider response. I post the steps I had to do :

sudo apt remove varnish
sudo apt-get purge varnish
# I manually remove the 3 files in created in /etc/systemd/system/*
sudo apt install varnish
sudo nano /lib/systemd/system/varnish.service # put the rigth conf
sudo nano /etc/varnish/default.vcl #put the rigth conf
sudo systemctl daemon-reload
sudo service varnish restart

and everything works fine!
the magic is in the /lib/systemd/system/varnish.service file, other online resources that I found made me make think it is elsewhere, so beware of the online (outdated) tutorials!

You need to change the varnish start parameters in the systemd service definition as well. You could edit the line starting with ExecStart in the service defintion file:

sudo vi /lib/systemd/system/varnish.service

Modifying this file however, has the disadvantage that it will not be updated in future updates of the package. Alternatively, as suggested in the comments, you could create a systemd drop in file, which is the preferred way of adding settings to systemd definitions.

# create the drop in directory
sudo mkdir /etc/systemd/system/varnish.service.d
# create the drop in file. The name is irrelevant, as long as it ends in .conf
sudo vi /etc/systemd/system/varnish.service.d/mysettings.conf

Here you only need to add the settings you want do change, everything else will be loaded from the default definition file.

Example:

[Service]
ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m

_{This is the default line, change it as you need}

Afterwards, tell systemctl to reload it's config files and to restart the service

sudo systemctl daemon-reload
sudo service varnish restart

Varnish should now listen on port 80.

Note that the drop-in should have an empty ExecStart=
Otherwise you will get an error starting the service (duplicate ExecStart)

sudo mkdir /etc/systemd/system/varnish.service.d
sudo nano /etc/systemd/system/varnish.service.d/varnishd.conf

With

[Service]
ExecStart=
ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a :80 -T <YOUR WEBSERVER IP>:8081 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m