xapi/nm: Send non-empty dns to networkd when using IPv6 autoconf #6586

psafont · 2025-07-11T11:12:28Z

Because Autoconf is not DHCP, networkd uses the dns value to write to
resolv.conf. This is done on ocaml/networkd/bin/network_server.ml line 745
This allows to have non-empty resolv.conf when using IPv6 autoconf.

This mismatch was caused byt technical debt that cause this check to be duplicated, the second removes it so
xapi decides the instances where these values are written and networkd follows that decision.

I've tested new isntallations using IPv4 and couldn't see any regression, two users have tested the patch and now the DNS entries don't get overriden when using IPv6 in Autoconf mode: xcp-ng/xcp#641 (comment)

I'd like Rob to review this approach in any case

Because Autoconf is not DHCP, networkd uses the dns value to write to resolv.conf. This is done on ocaml/networkd/bin/network_server.ml line 745 This allows to have non-empty resolv.conf when using IPv6 autoconf. Signed-off-by: Pau Ruiz Safont <[email protected]>

Previously both xapi and networkd had to inspect the IP configuration to decide whether the DNS values should be persistend into /etc/resolv.conf. This actually lead to a mismatch in them. Instead use an option value for DNS that simply means that if there's a value, it must be persisted. Now xapi decides the instances where these values are written. Signed-off-by: Pau Ruiz Safont <[email protected]>

Signed-off-by: Pau Ruiz Safont <[email protected]>

minglumlu · 2025-07-30T07:56:36Z

From the discussion, especially xcp-ng/xcp#641 (comment)
I understand we still need to handle a case of SLAAC + RDNSS. The networkd will need to capture the DNS server addresses from the Router Advertisements messages and write them into /etc/resolv.conf file.

psafont · 2025-07-30T08:33:19Z

I understand we still need to handle a case of SLAAC + RDNSS. The networkd will need to capture the DNS server addresses from the Router Advertisements messages and write them into /etc/resolv.conf file.

Yes, this patch fixes the case where DNS is configured using IPv4 mechanisms, but not written to /etc/resolv.conf because IPv6 is configured in Autoconf mode.

I'm working on a design to make configuration of IP and DNS independent from each other under IPv6 mode, but that will take longer to get done. In any case RDNSS needs to be implemented independently

minglumlu · 2025-08-01T05:00:05Z

I'd like Rob to review this approach in any case

Rob is in vacation. He will be back in 2 weeks.

but not written to /etc/resolv.conf because IPv6 is configured in Autoconf mode

Without these DNS server addresses, would the applications not be able to find DNS server to resolve their DNS name? XS doesn't have resolved running in dom0? Or I'm missing something. Anyway this could be resolved in your design.

psafont · 2025-08-01T08:58:47Z

Without these DNS server addresses, would the applications not be able to find DNS server to resolve their DNS name? XS doesn't have resolved running in dom0? Or I'm missing something. Anyway this could be resolved in your design.

The glibc function gethostbynameuses the resolv.conf file to know what IP to send the DNS requests to. So having the empty file means that some processes are unsable to send DNS requests if the file is empty.

This is an independent matter from the new design, the new design is meant to configure the IP addresses (yes, plural) of each PIF independently from its DNS configuration.

0lini · 2025-08-01T17:40:13Z

An example for something that doesn't work when DNS is empty, is yum update.

robhoes · 2025-08-19T14:57:26Z

ocaml/xapi-idl/network/network_interface.ml

  ; ipv6_gateway: Unix.inet_addr option [@default None]
  ; ipv4_routes: ipv4_route_t list [@default []]
-  ; dns: Unix.inet_addr list * string list [@default [], []]
+  ; dns: (Unix.inet_addr list * string list) option [@default None]


Can we assume that changing to an option type here does not change the expected JSON format? That is, existing dns fields in networkd.db (written before applying this patch) will be parsed correctly?

Will check, interestingly I didn't spot any issue, nor users reported it.

I modified the network_db executable to parse a custom networkdb file, on top of this PR, and extracted the networkd.db from a couple hosts to test this. I couldn't find any issue parsing the file:

$ dune exec ocaml/networkd/bin_db/networkd_db.exe -- -db networkd.old_dhcp.db -bridge xenbr1 -iface xenbr1 interfaces=eth1 mode=dhcp

$ dune exec ocaml/networkd/bin_db/networkd_db.exe -- -db networkd.old_static.db -bridge xenbr0 -iface xenbr0 interfaces=eth0 mode=static ipaddr=192.168.178.14 netmask=255.255.255.0 gateway=192.168.178.1 dns=192.168.178.64

Great, thanks for checking!

…onf (xapi-project#6586)" This reverts commit 05e6317, reversing changes made to 1fbdaae.

robhoes · 2025-09-01T13:46:51Z

Unfortunately, this introduced a problem on update when the mode is DHCP. In that case, the old db file has

  "dns": [
    [],
    []
  ],

which the updated xcp-networkd reads a Some []. And because xcp-networkd no longer conditionally applies the dns setting based it on the mode, it clears resolv.conf even though the mode is DHCP. And it does that after starting dhclient, removing any DNS config set through DHCP.

psafont · 2025-09-01T14:54:09Z

Ah, that's unfortunate. we can revert the second commit, Can Gaby open the PR? I'll be back to work next week

…onf (xapi-project#6586)" This reverts commit 05e6317, reversing changes made to 1fbdaae.

…onf (xapi-project#6586)" This reverts commit 05e6317, reversing changes made to 1fbdaae. Signed-off-by: Gabriel Buica <[email protected]>

@psafont

This reverts @psafont's PR, #6586, this causing the `/etc/resolve.conf` to be overwritten during update, after the dhclient writes it. This happens because `set_dns` no longer checks the mode before applying the DNS config, This reverts commit 05e6317, reversing changes made to 1fbdaae.

Signed-off-by: Pau Ruiz Safont <[email protected]>

Because Autoconf is not DHCP, networkd uses the dns value to write to resolv.conf. This is done on ocaml/networkd/bin/network_server.ml line 745 This allows to have non-empty resolv.conf when using IPv6 autoconf. xapi-idl/network: Remove code duplication for DNS persistence decisions: Previously both xapi and networkd had to inspect the IP configuration to decide whether the DNS values should be persistend into /etc/resolv.conf. This actually lead to a mismatch in them. Instead use an option value for DNS that simply means that if there's a value, it must be persisted. Now xapi decides the instances where these values are written. Treat a couple of empty lists as a lack of value to avoid writing empty resolv.conf files. This can happen when updating a host from previous versions, which use empty lists when using DHCP. Tested manually by installing a version with this change and restarting the toolstack. The file is kept intact, unlike the previous version of the change that did not take into account the update behaviour. This is PR fixed version of #6586

psafont added 2 commits July 11, 2025 11:28

psafont requested a review from robhoes July 11, 2025 11:12

lindig approved these changes Jul 11, 2025

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psafont added a commit to psafont/xapi that referenced this pull request Jul 11, 2025

8.3: backport of xapi-project/xen-api#6586

a1cf3eb

Signed-off-by: Pau Ruiz Safont <[email protected]>

psafont added a commit to psafont/xapi that referenced this pull request Jul 11, 2025

8.3: backport of xapi-project/xen-api#6586

33c5745

Signed-off-by: Pau Ruiz Safont <[email protected]>

psafont added a commit to psafont/xapi that referenced this pull request Jul 11, 2025

8.3: backport of xapi-project/xen-api#6586

d32108e

Signed-off-by: Pau Ruiz Safont <[email protected]>

psafont marked this pull request as ready for review July 21, 2025 08:40

robhoes reviewed Aug 19, 2025

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robhoes approved these changes Aug 20, 2025

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psafont added this pull request to the merge queue Aug 20, 2025

Merged via the queue into xapi-project:master with commit 05e6317 Aug 20, 2025
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psafont deleted the dev/pau/host-ipv6 branch August 20, 2025 10:07

GabrielBuica added a commit to GabrielBuica/xen-api that referenced this pull request Aug 29, 2025

Revert "xapi/nm: Send non-empty dns to networkd when using IPv6 autoc…

c2258d6

…onf (xapi-project#6586)" This reverts commit 05e6317, reversing changes made to 1fbdaae.

GabrielBuica added a commit to GabrielBuica/xen-api that referenced this pull request Sep 1, 2025

Revert "xapi/nm: Send non-empty dns to networkd when using IPv6 autoc…

638b883

…onf (xapi-project#6586)" This reverts commit 05e6317, reversing changes made to 1fbdaae.

GabrielBuica mentioned this pull request Sep 1, 2025

CA-416532: Revert #6586, overwrites /etc/resolve.conf #6643

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psafont added a commit to xcp-ng-rpms/xapi that referenced this pull request Sep 10, 2025

8.3: backport of xapi-project/xen-api#6586

d76cca8

Signed-off-by: Pau Ruiz Safont <[email protected]>

psafont added a commit to xcp-ng-rpms/xapi that referenced this pull request Sep 11, 2025

8.3: backport of xapi-project/xen-api#6586

d58ad1e

Signed-off-by: Pau Ruiz Safont <[email protected]>

psafont mentioned this pull request Sep 11, 2025

xapi/nm: Send non-empty dns to networkd when using IPv6 autoconf #6664

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