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Date:	Tue, 01 Dec 2015 12:22:22 +0100
From:	Hannes Frederic Sowa <>
To:	Bjørn Mork <>,
Subject: Re: [RFC] ipv6: use a random ifid for headerless devices


On Mon, Nov 30, 2015, at 12:55, Bjørn Mork wrote:
> Generating a random ifid for devices with no L2 header
> at all, allowing such devices to take part in IPv6
> autoconfiguration. The tuntap driver is one example of
> a driver where such an ifid would be useful.
> Note that as there is no persistence, new addresses
> will be generated every time an interface is brought up:
>  # ip -6 addr show dev tun0
>  8: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 state
>  UNKNOWN qlen 500
>      inet6 fe80::eef2:111c:f270:92ba/64 scope link
>         valid_lft forever preferred_lft forever
>  # ip link set tun0 down
>  # ip link set tun0 up
>  # ip -6 addr show dev tun0
>  8: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 state
>  UNKNOWN qlen 500
>      inet6 fe80::eec0:48d0:6b52:8835/64 scope link
>         valid_lft forever preferred_lft forever
> Signed-off-by: Bjørn Mork <>
> ---
> I'm planning raw-ip support for the qmi_wwan driver.  And
> the feedback from primary users (ModemManager++) is that
> a headerless netdev is preferred over a fake ethernet
> device. The current plan is to model this after 'tun'
> devices, using ARPHRD_NONE as type.
> But these devices will need an IPv6 link local address for
> full SLAAC support.  I am therefore wondering if an approach
> like this patch will be acceptable, or if I should look for
> some other solution?

I see no problem with the patch as it eases operating those devices. I
would also suggest storing the ifid in the inet6_dev so it does only
change during device creation and destruction. Otherwise I would
recommend to use stable privacy addresses to generate the link local
addresses. EUI-48 based LL creation should hopefully not be used anymore

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