ISC DHCPv6-BIND9 DDNS update problem

[Glenn Satchell]

Hi Mirsad,

I think offloading the checksum is not a problem, it's just that tcpdump 
sees the packet before it gets into the network interface card where the 
checksum is calculated. I think if you turn that back on it will make no 
difference.

This explains it quite well: 
https://serverfault.com/questions/776253/how-does-tcp-offload-nic-cause-the-tcp-checksum-to-be-invalid

regards,
Glenn

On 2022-06-09 19:58, Mirsad Goran Todorovac wrote:

> P.P.S.
> 
> I have turned off NIC checksum offloading by `ethtool -K eth1 rx off tx 
> off`. Now the UDP checksum should be calculated in the kernel (slower).
> 
> 11:54:40.438248 IP6 (hlim 1, next-header UDP (17) payload length: 103) 
> fe80::9418:9a22:54b8:743f.546 > ff02::1:2.547: [udp sum ok] dhcp6 
> solicit (xid=a1f102 (elapsed-time 0) (client-ID hwaddr/time type 1 time 
> 499890753 f0761c5b0dd5) (IA_NA IAID:338441082 T1:0 T2:0) (Client-FQDN) 
> (vendor-class) (option-request DNS-search-list DNS-server 
> vendor-specific-info Client-FQDN))
> 11:54:40.438928 IP6 (flowlabel 0x82364, hlim 64, next-header UDP (17) 
> payload length: 159) fe80::f21f:afff:fef1:420a.547 > 
> fe80::9418:9a22:54b8:743f.546: [udp sum ok] dhcp6 advertise (xid=a1f102 
> (IA_NA IAID:338441082 T1:3600 T2:7200 (IA_ADDR 2001:b68:2:2800::10:139d 
> pltime:604800 vltime:3600)) (client-ID hwaddr/time type 1 time 
> 499890753 f0761c5b0dd5) (server-ID hwaddr/time type 1 time 707489786 
> f01faff1420a) (preference 255) (DNS-search-list local.alu.hr. alu.hr.) 
> (DNS-server 2001:b68:2:2800::3 2001:b68:c:2::70:0))
> 11:54:41.445113 IP6 (hlim 1, next-header UDP (17) payload length: 103) 
> fe80::9418:9a22:54b8:743f.546 > ff02::1:2.547: [udp sum ok] dhcp6 
> solicit (xid=a1f102 (elapsed-time 100) (client-ID hwaddr/time type 1 
> time 499890753 f0761c5b0dd5) (IA_NA IAID:338441082 T1:0 T2:0) 
> (Client-FQDN) (vendor-class) (option-request DNS-search-list DNS-server 
> vendor-specific-info Client-FQDN))
> 
> Now the checksums are OK, however the server still doesn't receive 
> Request or Confirm message from the client.
> I'm pretty much out of ideas.
> 
> Mirsad

> --
> Mirsad Todorovac
> CARNet system engineer
> Faculty of Graphic Arts | Academy of Fine Arts
> University of Zagreb
> Republic of Croatia, the European Union
> --
> CARNet sistem inženjer
> Grafički fakultet | Akademija likovnih umjetnosti
> Sveučilište u Zagrebu