On devices with read-only rootfs, e.g. mobile phones, nic device number (wlan<N>) may increase every time disabled and re-enabled. To be more precisely, this happens only on devices when disabling a nic removes the corresponding driver.

"/lib/udev/rules.d/75-persistent-net-generator.rules" checks whether NAME attribute has been assigned to wlan<N> device: if yes, skip all the followed steps, or, call to "/lib/udev/write_net_rules" to generate a persistent device name rule file. That persistent file should be created under "/etc/udev/rules.d" and named "70-persistent-net.rules", so it guarantees NAME attribute should be assigned if available before being read. However, when rootfs was previously mounted as read-only, a file "/run/udev/tmp-ruiles--70-persistent-net.rules" is created instead. This temporary file is supposed to be moved back into "/etc/udev/rules.d" by a systemd service udev-finish right after the system finishes start-up chaos. Again, if rootfs is still mounted as read-only, this move will certainly fail. One last important thing, /run/udev is _NOT_ included in udev rules inclusion paths, so any rules written here will not be taken into account when processing uevents.

So, when wlan0 is probed for the first time on a device with read-only rootfs, udev creates "/run/udev/tmp-ruiles--70-persistent-net.rules" and inserts one rule for it. When wlan0 is disabled and re-enabled, since "/run/udev/tmp-ruiles--70-persistent-net.rules" is not taken into account, its NAME attribute will not be set, and udev recognize it as a new nic and tries to write another rule for it again. However, in this time, "wlan0" has been taken in the previously written temporary rules file, so "wlan1" is chosen instead, and an exactly the same matching rule (except for NAME= part) is appended to "/run/udev/tmp-ruiles--70-persistent-net.rules". When the device is again disabled and re-enabled, "wlan2" will be assigned. And so on ....