Fix DHCPv6 PXE boot: use option6:client-arch for architecture detection

The IPv6 section of the dnsmasq template used option:client-arch to
match client architecture, but this is a DHCPv4-only alias (option 93).
In dnsmasq, the "option:" prefix resolves via the DHCPv4 option table
(opttab) and the resulting match rule is added to daemon->dhcp_match,
which is never evaluated by the DHCPv6 server code (rfc3315.c only
iterates daemon->dhcp_match6). The directives were silently ignored.

The fix uses option6:client-arch, which correctly resolves to DHCPv6
option 61 (Client System Architecture Type, RFC 5970). Both EDK2/OVMF
and iPXE send this option in DHCPv6 SOLICIT messages.

Note: option6:client-arch requires dnsmasq to have option 61 in its
DHCPv6 option table (opttab6). Upstream dnsmasq is missing this entry,
but Red Hat has carried a downstream patch adding it since 2021
(dnsmasq-2.86-dhcpv6-client-arch.patch), which is included in the
CentOS Stream 9 base image used by this container.

Also replaces the impossible BIOS fallback (undionly.kpxe over IPv6)
with an x86_64 EFI fallback for PXE clients whose architecture could
not be determined.

Fixes: #916

Author: holmboe

ironic-config/dnsmasq.conf.j2

@@ -56,15 +56,19 @@ ra-param={{ env.PROVISIONING_INTERFACE }},0,0
 
 dhcp-vendorclass=set:pxe6,enterprise:343,PXEClient
 dhcp-userclass=set:ipxe6,iPXE
+# Architecture detection via DHCPv6 option 61 (RFC 5970).
+# Note: option:client-arch is a DHCPv4-only alias (option 93) and silently
+# adds rules to the DHCPv4 match list; use option6:client-arch for DHCPv6.
+dhcp-match=set:amd64,option6:client-arch,7
+dhcp-match=set:amd64,option6:client-arch,9
 # Client is (i)PXE booting on EFI machine
-dhcp-match=set:amd64,option:client-arch,7
-dhcp-match=set:amd64,option:client-arch,9
 dhcp-option=tag:pxe6,tag:amd64,option6:bootfile-url,tftp://{{ env.IRONIC_URL_HOST }}/snponly-x86_64.efi
 # Client is (i)PXE booting on arm64 EFI machine
-dhcp-match=set:arm64,option:client-arch,11
+dhcp-match=set:arm64,option6:client-arch,11
 dhcp-option=tag:pxe6,tag:arm64,option6:bootfile-url,tftp://{{ env.IRONIC_URL_HOST }}/snponly-arm64.efi
-# Client is running (i)PXE on BIOS machine
-dhcp-option=tag:!pxe6,option6:bootfile-url,tftp://{{ env.IRONIC_URL_HOST }}/undionly.kpxe
+# Fallback: serve x86_64 when PXE client is detected but architecture
+# could not be determined. Legacy BIOS does not support IPv6 PXE.
+dhcp-option=tag:pxe6,tag:!amd64,tag:!arm64,option6:bootfile-url,tftp://{{ env.IRONIC_URL_HOST }}/snponly-x86_64.efi
 {%- if env.IPXE_TLS_SETUP != "true" %}
 dhcp-option=tag:ipxe6,option6:bootfile-url,{{ env.IRONIC_HTTP_URL }}/boot.ipxe
 {% endif %}