I have a nice netboot setup where we can PXEboot hosts to an automated installer. The last step ensures the puppet agent is running and pointed at the correct puppetmaster. The .preseed files are generated from an erb template that ends in the following:

[...]
<% if  @distcodename == "jessie" -%>
d-i preseed/late_command string \
echo -e 'DAEMON_OPTS="--server <%= @puppetmaster %>"' > /target/etc/default/puppet ; \
rm -f /target/var/lib/puppet/state/agent_disabled.lock
<% else -%>
d-i preseed/late_command string \
in-target sed -i '/\[main\]/a server = <%= @puppetmaster %>' /etc/puppet/puppet.conf ; \
in-target ln -s /lib/systemd/system/puppet.service /etc/systemd/system/multi-user.target.wants/puppet.service
<% end -%>

Older distribution versions allowed us to populate the --server option in /etc/default/puppet. This is addressed in the first part of the if clause.

Newer distribution versions use systemd, where this approach no longer works. In addition, the puppet agent is now disabled by default on new installs. The else clause in the second part address this case.