Host Name Server (Assuming you refer to DNS)
Purpose: Host Name Server, or DNS (Domain Name System), translates domain names into IP addresses, facilitating the resolution of human-readable domain names to machine-readable IP addresses.
Vulnerabilities: DNS Spoofing, Cache Poisoning, DDoS attacks (such as DNS Amplification attacks), Zone Transfer attacks, DNS Tunneling.
Tools: Nmap, DNSrecon, dnswalk, dnsenum, fierce, Metasploit.