I'm using OpenWrt 18.06.2 with LuCI Web Interface and dnsmasq as dhcp&dns server.
My problem is after altering the local domain suffix and local server specification, my windows hosts are unable to resolve the dns request with the dns suffix (nxdomain response). If I omit the suffix (only a dot appended to hostnames), it works. On android devices I have no problems at all.
DNS requires interoperability support on multiple levels, from the standards body, IANA not suddenly assigning the chosen zone to a registry, your DNS server interpreting and supporting it correctly, the various client operating systems and their name resolving stacks recognizing it as valid domains to be resolved, not to forget the various apps accepting the input - and as the final point also UCI (OpenWrt's config management) and the dnsmasq initscripts expanding the configuration settings into a temporary and valid dnsmasq configuration. The further you diverge from the officially blessed structure, the more likely you'll encounter problems and bugs in the various software stacks involved.
Right now I'm not going to check the various RFCs to check the validity of single character/ single digit top level domains, especially as the rules governing these have been significantly relaxed and redefined in recent years, but considering that neither single ASCII character top level domains nor single digit domains (nor pure number top level domains) are officially assigned (although one would expect them to be rather attractive, paying top dollar) - and the whole concept just feeling wrong to my gut instincts (which might be wrong) made this stick out.
Thank you for your long explanation and your help. I'm experimenting a lot to get a better understanding of how the underlying systems are working (with the risk of breaking everything, but I always do a backup). I reverted the changes I made to the config files and set everything back to default.
Maybe I should consider the RFCs next time before asking. Sorry about that