For working exam-mode we need to block direct internet access by firewall.
Users have to use squid-proxy on firewall, which can be disabled for exam-users.
To allow VM-traffic (anonymous user), we use a local squid server with users
kerberos-ticket to authenticate on the parent squid.
When using VMs on teacherdevices offsite, the local squid has to use direct internet access.
So we need two squid configs. When switching between offsite and onsite,
the squid has to be restartet with corresponding config.
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