I added a rule to accept connections from 192.168.1.135/24, since my router is configured to hand out /24 addresses. Then, iptables -L -v showed that connections from 192.168.1.0/24 are accepted. When I change the rule to accept connections from .135/32 - or from .135 without specifying the subnet -, it not only works as intended, but it also resolves the hostname correctly. Why? unsolicited “why do you still use iptables” advice not welcome :D

The routing and firewalling is a bit different in terms of why certain CIDR masks are used. For the router, the /24 prefix is usually defined for itself on the LAN interface to denote the address space it may send route information to, and what addresses are controlled by the device. Almost certainly, (unless using a lower CIDR range and actually handing out /24 blocks to subsequent routers), you are granting /32 IPv4 addresses to your device from your router. For your system firewall, 192.168.1.135/24 is identical to 192.168.1.0/24 as they are the same address space. You’re simply allowing from a subnet of hosts to accept from. Given the /24 mask is 255.255.255.0, it does not matter what the last number of the IPv4 address is, but the lowest possible number to match the mask is standard form. Without knowing what rule(s) specifically are being applied, I couldn’t tell you if your firewall rules are something that would affect hostname resolution of other hosts from your system or not.