On Sun, Nov 14, 2021 at 10:40:28PM +0100, na...@poczta.fm wrote:
> Hi misc@,
>
> While testing the U-blox MPCI-L210 LTE modem on OpenBSD -current
> (the modem is attached as a cdce(4) device, the router mode is enabled;
> other modes are also available), I noticed that the automatic configuration
> via DHCP works correctly, but the MAC address assigned to the corresponding
> cdce0 network interface is different from the MAC address reported by
> lsusb -v. Unfortunately, due to this mismatch, incoming Ethernet frames
> having the actual MAC address of the device set as the destination address
> in the header are dropped by the kernel in the ether_input() function
> (/src/sys/net/if_ethersubr.c).
>
> Based on an analysis of the source code (/sys/dev/usb/if_cdce.c) and the
> results of the subsequent experiments, I was able to confirm that the cdce
> driver is unable to get the MAC address of the device (in particular,
> the usbd_get_string_desc() function returns USBD_STALLED instead of
> USBD_NORMAL_COMPLETION), and as a result, a semi-random fake Ethernet
> address is set:
>
> -----------------------------------------------------------------------------
> 306 if (!ethd || usbd_get_string_desc(sc->cdce_udev, ethd->iMacAddress, 0,
> 307 &eaddr_str, &len)) {
> 308 ether_fakeaddr(ifp); // <---- Set a semi-random MAC address
> 309 } else {
> -----------------------------------------------------------------------------
Can you find out what happens on Linux?
Their cdc_ether driver seems to be doing exactly the same thing. So with
this driver you would also end up with a random MAC if the device fails
to respond to a mac address query. In this case I have no idea what else
could be done apart from using the workaround you have already found.
But Linux has more CDC sub-drivers than we do, and runs some devices in
specialized modes.
Perhaps Linux performs a mode switch and uses a different driver for e.g.
MBIM mode? If that is the case then we could change our drivers to try to
switch this device into a mode where it works better.
