ISC DHCPv4 static mapping with domain starting with dot breaks Unbound DNS

[dseven]

See also: https://forum.opnsense.org/index.php?topic=45457.0

Reproducible in v25.1. Unknown if this was ever not an issue.

The OPNsense Web UI allows addition of an ISC DHCPv4 static mapping with a domain name starting with a dot ("."). The resulting Unbound DNS config (assuming "Register DHCP Static Mappings" is enabled) looks like:

local-data-ptr: "192.168.1.254 fakehostname..fakedomain"
local-data: "fakehostname..fakedomain IN A 192.168.1.254"

which results in Unbound failing to start:

<27>1 2025-01-30T16:00:08+00:00 opntest1.localdomain unbound 8416 - [meta sequenceId="24"] [8416:0] error: error parsing local-data at 15 'fakehostname..fakedomain IN A 192.168.1.254': Empty label
<27>1 2025-01-30T16:00:08+00:00 opntest1.localdomain unbound 8416 - [meta sequenceId="25"] [8416:0] error: Bad local-data RR fakehostname..fakedomain IN A 192.168.1.254
<26>1 2025-01-30T16:00:08+00:00 opntest1.localdomain unbound 8416 - [meta sequenceId="26"] [8416:0] fatal error: Could not set up local zones

To Reproduce

Steps to reproduce the behavior:

1. Enable "Register DHCP Static Mappings" for Unbound DNS
2. Create an ISC DHCPv4 static mapping (for any DHCP-enabled interface) with a fake MAC address (e.g. "aa:bb:cc:dd:ee:ff"), hostname "fakehostname" and domain ".fakedomain"
3. (try to) restart the Unbound DNS service (it will fail)
4. Observe error in resolver log

Note: It appears that neither ISC DHCPv6 static mappings nor Kea reservations allow specification of a domain name, so this issue seems to pertain to ISC DHCPv4 only.

Expected behavior

Perhaps OPNsense should strip any leading dots from domain names when combining them with hostnames to produce DNS records, which would allow Unbound to start.

If that's not feasible, the UI should refuse to create a static mapping where the domain starts with a dot.

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