Re: FreeBSD dhcp failing with UDP checksum errors

On Mon, Oct 14, 2024 at 10:46:22AM -0400, Laine Stump wrote: > On 10/14/24 5:35 AM, Richard W.M. Jones wrote: > >On Mon, Oct 14, 2024 at 09:52:13AM +0100, Daniel P. Berrangé wrote: > >> > >>Urgh, I wonder if this is fallout from switching to NFT instead of iptables. > > > >I can list the firewall rules if you tell me what I'm looking for ... > > > >>IIUC, the NFT kernel maintainers didn't implement for checksum fixup rules, > >>since they believe that all modern distros would have long ago fixed their > >>bugs wrt mangled checksums. > > That's the first thing that came to my mind too - maybe RHEL5 > *isn't* the only guest OS that has this problem. (I certainly hope > that isn't the case :-/) > > There are two ways to test out this theory: > > 1) change the setting of "firewall_backend" in > /etc/libvirt/network.conf to "iptables" and restart virtnetworkd > > (if that does work, then switch back to nftables, restart > virtnetworkd, and test again just to make sure the issue wasn't > caused by some out-of-place rule) I changed the setting between nftables and iptables a few times and I can confirm that your theory seems to be correct. iptables => "5 bad udp checksums in 5 packets" message is NOT seen FreeBSD gets an immediate DHCPOFFER and boots quickly with network nftables => FreeBSD sends 5 DHCPDISCOVER messages "5 bad udp checksums in 5 packets" reappears FreeBSD does NOT see DHCPOFFER, although it does seem to remember the offer from the previous boot, so it does get a network connection in the end. > or > > 2) tell qemu to setup the virtio-net device to do its packet > processing in userspace rather than the kernel. You do this by > adding > > <driver name='qemu'/> > > to the <interface> section. This also works (with nftables). > >If I understand the trace correctly, the bad checksum originates on > >the Linux host (the reply sent by dnsmasq). > > I need to try it again to verify, but my recollection is that (when > you're using virtio-net with default settings) the checksums of DHCP > packets in one direction or the other *always* show up in tcpdump as > having bad checksums, but they still end up getting to the other end > with a proper checksum. Sometime in the distant past I *may have* > had it explained to me why this happens, but I don't recall now. > Anyway, I'm just saying this so that you know the validity of the > UDP checksum shouldn't be used as an indicator of whether or not > things are "working". I have to say I also don't really understand what's happening here. Isn't the Linux host sending DHCPOFFER? Why doesn't it set the UDP checksum correctly and/or why would tcpdump report it wrongly if it is setting it?