How do home routers offer DNS services [on hold]

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http://serverfault.com – I have a typical home router/switch DSL modem that provides to my home network with all the common services (DHCP, DNS, web etc). The client machines have in /etc/resolv.conf as nameserver the IP of the router and everything just works. But on the router I cannot see any DNS server running (either TCP/UDP, typically port 53) while I can see that it runs e.g. DHCP on 67 and a web administration interface on 80 and 443. I am using nmap for the port scanning. How does my network get DNS? (HowTos)