Dns проверил, так и так стоит гугловский 8.8.8.8. Синаптик не пробовал поскольку это сервер и доступ есть только к консоли. теперь про выполнение команд, получается следующее
root@server:~# apt update
Hit:1 http://ports.ubuntu.com xenial InRelease
Hit:2 http://ports.ubuntu.com xenial-security InRelease
Hit:3 http://ports.ubuntu.com xenial-updates InRelease
Hit:4 http://ports.ubuntu.com xenial-backports InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
182 packages can be upgraded. Run 'apt list --upgradable' to see them.
root@server:~# apt install tmux
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
libevent-2.0-5 libutempter0
The following NEW packages will be installed:
libevent-2.0-5 libutempter0 tmux
0 upgraded, 3 newly installed, 0 to remove and 182 not upgraded.
Need to get 297 kB of archives.
After this operation, 613 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Abort.