Since newer versions of Ubuntu like 20, you will find there is no longer dynagen and that the dynamips provided is faulty and will segfault each time:
Cisco Router Simulation Platform (version 0.2.14-amd64/Linux stable)
Copyright (c) 2005-2011 Christophe Fillot.
Build date: Apr 3 2018 12:20:29
Local UUID: 3c1c0b7f-2fab-4fda-b40b-74841d1bcfe0
Instance ID set to 1.
netio_tap_create: unable to open TAP device tap1 (No such fi........