several top-level domains. Knot Resolver Knot Resolver is an open source modern resolver implementation designed for scalability, resiliency, and flexibility. Its core architecture is tiny and efficient, and most of the rich features are implemented as optional modules, which limits attack surface and improves performance. Many re­solver fea­tures are avail­able out-of-the-box as modules while keep­ing core tiny and ef­ficient. Modular archi­tec­ture pro­vides a state-machine like API for ex­ten­sions, such as C and Lua modules. MaraDNS MaraDNS is a free software DNS server by Sam Trenholme that claims a good security history and ease of use. In order to change any DNS records, MaraDNS needs to be restarted. Like djbdns dnscache, the MaraDNS 2.0 stand-alone recursive resolver ("Deadwood") does not use threads. Microsoft DNS Windows DNS Server component of Microsoft DNS. The same software can be configured to support authoritative, recursive and hybrid mode. The software is integrated with Active Directory which makes it the default DNS software for many enterprise networks that are based on Active Directory. It also allows creating zones by the standard DNS zone file. The software comes packaged as a role in Windows Server. The server software is shipped with a command line application dnscmd, a DNS management GUI wizard, and a DNS PowerShell package. In Windows Server 2012, the Windows DNS added support for DNSSEC, with full-fledged online signing, with Dynamic DNS and NSEC3 support, along with RSASHA and ECDSA signing algorithms. It provides an inbuilt key storage provider and support for any third party CNG compliant key storage provider. User interface and PowerShell support for managing DNS and DNSSEC were improved as well. In the Windows Server 2016, the DNS Server supports DNS policies using which the admins can have more control over the name resolution process. NSD NSD is a free software authoritative server provided by NLNet Labs. NSD is a test-bed server for DNSSEC; new DNSSEC protocol features are often prototyped using the NSD code base. NSD hosts several top-level domains, and operates three of the root nameservers. pdnsd Pdnsd is a caching DNS proxy server that stores cached DNS records on disk for long term retention. Pdnsd is designed to be highly adaptable to situations where net connectivity is slow, unreliable, unavailable, or highly dynamic, with limited capability of acting as an authoritative nameserver. It is licensed under the GPL. Posadis Posadis is a free software DNS server, written in C++,