This specific issue was reported as point 1. in the description of https://issues.redhat.com/browse/RHEL-7306, and it becomes rather relevant now because of https://issues.redhat.com/browse/RHEL-45241: libvirt documentation states that the "address" attribute from an "ip" element inside a "user" (network) interface is used by QEMU (via libslirp) as address in the guest.

However, that's not the case: if the user use a network identifier (say, 169.254.0.0, with the subnet mask being 255.255.0.0) in the "address" attribute, libslirp uses this "address" (not a usable IP address) as prefix, rather, and replaces it with 169.254.2.15.

passt, however, doesn't: the address will be configured as 169.254.0.0. So, if a user takes the (wrong) libslirp example and changes the back-end to passt, the address will actually change, and unexpectedly so.

This should be fixed by changing the example in the documentation, or clarifying this aspect.