[Networkmanager] NetworkManager link local IPv4 address based on last 2 octets of ethernet MAC address

[manjunatha srinivasan]

Hi  Thomas Haller

Thanks for your valuable information and clarification. I tested the
behaviour of a change in IPv4 address at device side, when another device
is in  link local network for the same static IPv4 address.

Regards
Manjunatha Srinivasan N


On Thu, 19 Oct 2023 at 18:04, Thomas Haller <thaller at redhat.com> wrote:

> On Thu, 2023-10-19 at 17:48 +0530, manjunatha srinivasan wrote:
> > Hi
> > I have 2 following ethernet profiles at /etc/NetworkManager/system-
> > connections:
>
> btw, you can also enable LL additionally with DHCP. Then DHCP is
> attempted in the background, while you always have a LL address. See
> "ipv4.link-local" setting in `man nm-settings`. That is IMO better than
> two profiles.
>
> > One for dynamic ip configuration and other for link local as fallback
> > when DHCP server is not reachable. As expected when DHCP server is
> > not reachable, fall back IPv4 address happens e.g. 169.254.105.20
> > with subnet mask as 255.255.0.0 by the NetworkManager.
> > This particular IP address is always the same which is expected one.
> > Please let me know how this constant IPv4 address is decided by the
> > NetworkManager.
>
> IPv4LL is documented by RFC
> https://datatracker.ietf.org/doc/html/rfc3927#section-2.1
>
> The code is here:
>
> https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/94ead251ded1ed7d3a1ea9953f041708daf3787f/src/core/nm-l3-ipv4ll.c#L382
>
> It basically hashes the MAC address and a generation counter.
>
> > Also let me know if there is any configuration option in
> > NetworkManager to change last 2 octets (i.e.105.20) represented in
> > the decimal value of last 2 values of ethernet MAC hex address.
>
> The address couldn't be fixed, because there is always the possibility
> that another host on your network already picked this address. It falls
> back to a retry via the generation counter.
>
> In any case, there is no option in NetworkManager (or RFC) to influence
> the IPv4LL address in a more immediate way.
>
> >
> > Ethernet (eth0) method based on DHCP:
> > [connection]
> > id=my-eth0
> > uuid=8466b654-0183-4900-a2d6-26a9e6af275e
> > type=ethernet
> > autoconnect-priority=1
> > interface-name=eth0
> > timestamp=1697699281
> >
> > [ethernet]
> >
> > [ipv4]
> > dhcp-timeout=10
> > method=auto
> >
> > [ipv6]
> > addr-gen-mode=stable-privacy
> > method=auto
> > ra-timeout=10
> >
> > [proxy]
> > -------------------
> > Ethernet (eth0) method based on link-local:
> > root at FXR90D5FB72:/etc/NetworkManager/system-connections# cat  rfid-
> > ll-eth0.nmconnection
> > [connection]
> > id=ll-eth0
> > uuid=f07f43e6-59c8-428f-bc04-55a0e581e621
> > type=ethernet
> > interface-name=eth0
> >
> > [ethernet]
> >
> > [ipv4]
> > method=link-local
> >
> > [ipv6]
> > addr-gen-mode=stable-privacy
> > method=auto
> >
> > [proxy]
> > ------
> >
> > When DHCP request fails, the IPv4 address fallback to
> >
> >
> >
> > Thanks & Regards
> > Manjunatha Srinivasan N
>
>
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