[Bug 188990] [xen] dhclient(8) does not accept DHCP Offer on FreeBSD 10 as Xen DomU Guest

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=188990

Zhenlei Huang <zlei@FreeBSD.org> changed:

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                 CC|                            |zlei@FreeBSD.org

--- Comment #25 from Zhenlei Huang <zlei@FreeBSD.org> ---
(In reply to Roger Pau Monné from comment #24)

I spent sometime diagnosing this issue. From the code how kernel process
inbound UDP packets and how BPF works, I may conclude this issue is caused by a
combination of
 1. Dom0 Virtual NIC does not really calculate the checksum of outbound UDP
packets ( if TXCSUM is enabled, which cheat the Dom0 kernel not do soft
checksum calculatation  ), and only some *flags* are set on the internal
presentation of packets, but the packets' on-wire checksum field is not filled
with correct value.
 2. DomU Virtual NIC' RXCSUM does not matter. If the internal presentation of
UDP packets have *flags* then those packets are marked valid. If not then it
make no difference between re-calculating in driver and kernel.
 3. DomU guest (FreeBSD) kernel will check and drop inbound UDP packets with
invalid checksum. 
 4. The BPF was hooked into driver part, it does not care checksums of packets
and pass them as is to consumers such as DHCP client. And BPF consumers will
have to re-calculate the checksum by themself.
 5. Unfortunately BPF consumer (DHCP client) would definitely got wrong
checksum and it is not the fault of BPF consumers.

So it sound like a design defect of BPF, which does not have a way to pass
checksum (offload) information to its consumers.

@Roger Pau Monné
I bet disabling DomU Virtual NIC' RXCSUM feature will not help.

I think it is more a hack to workaround the problem in BPF consumers' code. The
right approach should be disable Dom0 Virtual NIC's TXCSUM offload feature
(large impact), or using firewall rules (on Dom0 side) to calculate and fill
UDP checksum of outbound packets from DHCP server application.

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