libvirt's udev interface backend (RHEL 9 and later, Debian 12) only reads
interfaces, so the create form and the start, stop and delete buttons
only failed there. The pages ask the host with an invalid definition,
which defines nothing: a backend without define refuses it as not
supported. There they show the interface state and a note instead, and
posts change nothing; the host's devices, needed only by the create
form, are not read. Any other answer keeps the pages as they were.
The form required a bridge member for an ethernet interface, which does
not use one. When the host's network devices could not be read, the error
was swallowed and every creation failed with "the device is not on this
host"; the page now shows the real error and does not try.
The IPv4 address and gateway were required even for DHCP or no IPv4, so
such an interface could not be created; malformed addresses passed and a
missing prefix crashed the XML builder. They are validated like the IPv6
fields now (netmask notation accepted, IPv6 scope ids rejected), and a
static address without a gateway gets no empty route. Interface names
follow the Linux rule (up to 15 letters, digits, . _ -). A bridge member,
or the device an ethernet interface configures, must be a device of the
host (the page no longer falls back to made-up names), a bridge cannot
contain itself, and needs STP and a non-negative delay.
