Re: IPv6 networking problems in 14.3

> On 15. Sep 2025, at 14:59, Karl Denninger <[email protected]> wrote:
> 
> Hmmmmm.... just came in via git pull:
> commit ffd956a3918cd5e64c8850eb77247428a29f7221
> Author: Michael Tuexen <[email protected]>
> Date:   Wed Sep 10 17:13:35 2025 +0200
> 
>     dhclient: improve UDP checksum handling
> 
>     When sending UDP packets:
>     * compute the checksum in the correct order. This only has an impact
>       if the length of the payload is odd.
>     * don't send packet with a checksum of zero, use 0xffff instead as
>       required.
>     When receiving UDP packets:
>     * don't do any computations when the checksum is zero.
>     * compute the checksum in the correct order. This only has an impact
>       if the length of the payload is odd.
>     * when computing the checksum, store the pseudo header checksum
>     * if the checksum is computed as zero, use 0xffff instead.
>     * also accept packets, when the checksum in the packet is the pseudo
>       header checksum.
>     The last point fixes a problem when the DHCP client runs in a VM,
>     the DHCP server runs on the host serving the VM and the network
>     interface supports transmit checksum offloading. Since dhclient
>     doesn't use UDP sockets but bpf devices to read the packets, the
>     checksum will be incorrect and only contain the checksum of the
>     pseudo header.
> 
> This could potentially apply to other bpf-using things -- which includes dhcpcd.  And you have tso/lro turned on.
Hi Karl,

this is true. Do we have an dhcpd in-tree? Or are you aware of other in-tree
programs which use UDP via bpf and not via the socket interface?

Best regards
Michael
> It is a patch to dhclient, not dhcpcd but does the same issue potentially apply?
> On 9/14/2025 13:00, Karl Denninger wrote:
>> On 9/14/2025 12:38, Chris Ross wrote:
>>> 
>>>> On Sep 14, 2025, at 12:29, Karl Denninger <[email protected]> wrote:
>>>> Rolling this around in my head some more..... what is the underlying interface?
>>>> I ask because I saw this happen with "re" driver interfaces (both IPv4 and 6) where it would not get an ARP map and thus couldn't see anything at all on the outside - there were enough other screwball things going on with the "re" driver (timeouts and similar) that I tossed that and now run on ix and a couple of SFP+ transceivers which has been entirely-stable (although igb also appears to work as I've gotten my hands on a box with a couple of those and tested that too.)
>>>> 
>>> In my case it’s an ix. Connected to a 1gbps switch interface, but an ix interface. And, the same hardware that was doing this fine a few months ago.
>>> 
>>> vlan0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
>>> options=4600703<RXCSUM,TXCSUM,TSO4,TSO6,LRO,RXCSUM_IPV6,TXCSUM_IPV6,MEXTPG>
>>> ether a4:53:0e:79:b9:82
>>> inet A.B.C.D netmask 0xffffff00 broadcast A.B.C.255
>>> inet6 fe80::6e8:e675:f359:3465%vlan0 prefixlen 64 scopeid 0x4
>>> groups: vlan
>>> vlan: 6 vlanproto: 802.1q vlanpcp: 0 parent interface: ix0
>>> media: Ethernet autoselect (1000baseT <full-duplex>)
>>> status: active
>>> nd6 options=1<PERFORMNUD>
>>> 
>>> ix0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
>>> options=4e53fbb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,TSO6,LRO,WOL_UCAST,WOL_MCAST,WOL_MAGIC,VLAN_HWFILTER,VLAN_HWTSO,RXCSUM_IPV6,TXCSUM_IPV6,HWSTATS,MEXTPG>
>>> ether a4:53:0e:79:b9:82
>>> media: Ethernet autoselect (1000baseT <full-duplex>)
>>> status: active
>>> nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
>>> 
>>> 
>> If you find anything a note back here would be greatly excellent.  I do note that per the various notes from long ago I have both tso and lro turned off (but I use ipfw, which is where that apparently comes from) on the outside interface -- but I doubt that is involved as I did try with it on and it didn't change anything.
>> 
>> -- 
>> Karl Denninger
>> [email protected]
>> The Market Ticker
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> -- 
> Karl Denninger
> [email protected]
> The Market Ticker
> [S/MIME encrypted email preferred]