[mythtv-users] directly connecting HDHR to spare LAN port

Stephen Worthington stephen_agent at jsw.gen.nz
Tue Jun 14 14:37:43 UTC 2022

On Tue, 14 Jun 2022 15:52:27 +0200, you wrote:

>Hi Jim,
>On Mon, 13 Jun 2022 at 21:07, James Abernathy <jfabernathy at gmail.com> wrote:
>> I have a NUC with  2 RJ45 ports on the back. One is 2.5Gb and the other is
>> 1Gb.  Is there a simple way to connect a HDHomeRun Connect tuner to one of
>> these ports so it would work with Mythtv and eliminate any record problems
>> that I'm having from being network related??
>On my production system I have a separate network card with a direct cable
>connection to a HDHomeRun without any configuration whatsoever. No fixed IP
>addresses, no DHCP, no routing, not anything and it just works. Of course,
>then the HDHomeRun cannot be accessed by anything else in the network but
>that does definitely eliminate interference from other devices on the

That is likely working because modern Linux kernels will do link local
IPv4 addresses in the 169.254. 0.0/16 address block automatically if
there is no other IPv4 address assigned.  Since the HDHR also has a
modern Linux kernel (or equivalent), it will self assign an address in
that block and be able to talk to the MythTV box via the MythTV box's
self assigned link-local address.  Then on top of that, there is a
discovery protocol for finding HDHR devices, which MythTV's HDHR code
will be using to find any HDHR devices on any ports of the MythTV box.
It will be sending out a packet saying "hey, any HDHR out there,
please reply with your device ID and IP address" and using the replies
to match to its configured HDHR tuners.

Since the same 169.254. 0.0/16 block will be in use on all subnets for
link-local addresses, it is not possible to route packets using those
addresses - they can only be used on the local subnet.

The 169.254. 0.0/16 link-local IPv4 addresses are a relatively new
feature of IPv4 - it is possible to run devices with ancient kernels
where they do not work.  I am not sure of the timeline, but I suspect
that they came about as a result of link-local addresses being a
fundamental part of the IPv6 specification, and how useful they are
there.  The current draft RFC (RFC3927) is dated 2005, but there may
be older versions of the RFC.

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