Unbound is a production-grade, modular, recursive DNS resolver with extensive DNSSEC validation, caching, and policy enforcement. It functions as a system daemon that iteratively resolves DNS queries from root servers or forwards to upstream resolvers, caches results in multi-layered caches (message, RRset, infrastructure), and validates responses according to DNSSEC. The daemon supports multiple transport protocols: UDP, TCP, TLS (DoT), HTTPS (DoH), and QUIC (DoQ), as well as experimental DNSCrypt encryption. It includes a modular processing pipeline where the iterator, validator, response-IP modification (respip), DNS64 synthesis, and other modules are composable at startup. An embedded library (libunbound) provides a public C and Python API for synchronous and asynchronous DNS lookups, enabling integration into other applications.
ircd-hybrid is a high-performance, modular IRC daemon written in C. It implements the full IRC protocol (RFC 1459, 2812, 2813) with modern IRCv3 capabilities (account-notify, extended-join, etc.), server-to-server linking (TS6), TLS support (GnuTLS, OpenSSL, wolfSSL), and a dynamic module system via GNU libltdl. The architecture is event-driven with selectable I/O backends (epoll, kqueue, poll) and a self-contained I/O abstraction library (libio) that provides networking, DNS resolution, event timers, data structures (patricia trie, doubly-linked lists, dynamic buffers), and TLS abstraction. Configuration is parsed by a Bison/Flex-generated parser supporting a rich set of server options. The codebase is well-structured with a clear separation between core daemon, library, and loadable modules, making it extensible and maintainable. Originally forked from the hybrid IRCD project in 1997, it has evolved to support modern IRC standards while maintaining backwards compatibility.
Keepalived is a production-grade Linux daemon for high-availability and load balancing. It implements VRRP v2/v3 for router failover, integrates with the Linux Virtual Server (IPVS) kernel module for Layer 4 load balancing, and includes a suite of health checkers (HTTP, SSL, TCP, UDP, SMTP, DNS, PING, MISC) plus BFD for fast failure detection. The architecture is multi-process: a parent process manages dedicated children for VRRP, checkers, and BFD, coordinating reloads and termination via signals and eventfd. It deeply interacts with the Linux kernel through netlink, nftables/iptables, raw sockets, and proc connector, and exposes operational interfaces via SNMP, D-Bus, JSON state dumps, SMTP alerts, and notification scripts. The codebase is written in C with careful attention to Linux-specific features, including network namespaces, VRFs, VMAC/macvlan, and multicast group handling. It is a mature, widely deployed critical infrastructure component rather than a research artifact or library.