Comment 4 for bug 1914740

------- Comment From <email address hidden> 2021-02-16 11:49 EDT-------
This morning we had a debug session with Asha and team.

The first issues were configuration problems. For documentation purposes:
- By default HiperSockets channels (CHPIDs) are isolated network segments, you cannot communicate between devices on different CHPIDs
- By default HiperSockets cannot be used for communications between different CECs
Network planning needs to be done accordingly, a simple way is e.g. : 1 IP subnet <-> 1 HiperSockets channel

Then we reproduced the following issue:
DHCP server and DHCP client both connected to the same HS channel via Layer 3 interfaces.
Requests are sent out via broadcast (tcpdump on client),
server sends responses (seen in journalctl on server)
responses never reach the client (tcpdump on client).

Sankar will attach detailed debug information.

As mentioned, last Friday I was successfully running DHCP between Ubunut 20.04 server and client
via Layer 2 HiperSockets.

Today I reproduced with Layer3 HiperSockets and it failed.

Remember that for HiperSockets Layer3 needs to be used on all participating interfaces!! (OSA is different)

Analysis:
See the "Device Drivers, Features, and Commands" https://www.ibm.com/developerworks/linux/linux390/documentation_dev.html
Chapter "Required options for using dhcpcd with layer3"

There it states "Run the DHCP client with an option that instructs the DHCP server to broadcast its response to the
client." Unfortunately I did not find such an option for netplan dhcp.
Deprecated workaround: add "always-broadcast on;" to /etc/dhcp/dhcpd.conf on the server side
(I tried this with my LPARs and it worked.)

Recommended solution: Use layer2 HiperSockets, if you want to use DHCP on HiperSockets and have only Linux'es connected.
(By default z/OS only supports layer3 HiperSockets, so all endpoints need to be Layer 3 HiperSockets, if a z/OS is
attached to this network segment)