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Operating system and processes run as processes on the host kernel
Hardware is emulated
- Disks - files in the host filesystem
- Terminals and serial lines - xterms, pseudo-terminals, ttys, file descriptors,
- Network devices - TUN/TAP, ethertap, and slip, multicast and switch/hub daemon
Runs as normal user, no kernel mods needed
Since UML is a port to Linux, it runs as a set of processes on the host.
All hardware is virtual. Devices are constructed from resources on the host. Block devices live in files, consoles and serial lines use a number of character devices, and there are a number of host network interfaces that can be used by the UML network driver.
It is also non-privileged, normally running as a normal user, although a setuid helper is needed in order to set up an network interface on the host.
No modifications to the host kernel are needed.