network/ovn: LXD does not create records for reverse DNS.

[fnordahl]

The OVN DNS interception supports providing answers for both forward and reverse DNS lookups on instance names.

However, LXD does currently not populate the database with reverse DNS records.

This causes issues for software that rely on correct reverse DNS lookups to determine FQDN.

Something like this could potentially fix it:

diff --git a/lxd/network/openvswitch/ovn.go b/lxd/network/openvswitch/ovn.go
index d15d508253..ff9879d8ae 100644
--- a/lxd/network/openvswitch/ovn.go
+++ b/lxd/network/openvswitch/ovn.go
@@ -4,6 +4,7 @@ import (
        "context"
        "fmt"
        "net"
+       "netip"
        "os"
        "strconv"
        "strings"
@@ -1377,6 +1378,7 @@ func (o *OVN) LogicalSwitchPortSetDNS(switchName OVNSwitch, portName OVNSwitchPo
 
        // Only include DNS name record if IPs supplied.
        if len(dnsIPs) > 0 {
+               lowerDnsName := strings.ToLower(dnsName)
                var dnsIPsStr strings.Builder
                for i, dnsIP := range dnsIPs {
                        if i > 0 {
@@ -1386,7 +1388,17 @@ func (o *OVN) LogicalSwitchPortSetDNS(switchName OVNSwitch, portName OVNSwitchPo
                        dnsIPsStr.WriteString(dnsIP.String())
                }
 
-               cmdArgs = append(cmdArgs, fmt.Sprintf(`records={"%s"="%s"}`, strings.ToLower(dnsName), dnsIPsStr.String()))
+               cmdArgs = append(cmdArgs, fmt.Sprintf(`records={"%s"="%s"`, lowerDnsName, dnsIPsStr.String()))
+
+               for i, dnsIP = range dnsIPs {
+                       addr, ok := netip.AddrFromSlice(dnsIP)
+                       if addr.Is6() {
+                               cmdArgs = append(cmdArgs, fmt.Sprintf(`,"%s"="%s"`, reverse6(dnsIP.String(), lowerDnsName)))
+                       } else {
+                               cmdArgs = append(cmdArgs, fmt.Sprintf(`,"%s"="%s"`, reverse(dnsIP.String(), lowerDnsName)))
+                       }
+               }
+               cmdArgs = append(cmdArgs, "}")
        }
 
        dnsUUID = strings.TrimSpace(dnsUUID)

[tomponline]

Thanks @fnordahl indeed I have tested this with a manually patched OVN dns record entry and it works.

We just need to come up with an implementation (or use an external one) for reverse and reverse6 functions.

[fnordahl]

They are already in https://github.com/canonical/lxd/blob/main/shared/dnsutil/dnsutil.go :)

[tomponline]

They are already in https://github.com/canonical/lxd/blob/main/shared/dnsutil/dnsutil.go :)

Excellent, and its my own function from 6 years ago :)