c-ares

c-ares
Name	c-ares
Developer	T. J. Fontaine; community contributors
Released	2001
Platform	Cross-platform
License	MIT License

c-ares

c-ares is a C library providing asynchronous DNS request and name resolution capabilities for networked applications. It supplies non-blocking DNS query routines and utilities to perform host name lookups without stalling event loops used in projects such as curl, libuv, NGINX, and Node.js integrations. Designed for portability and embeddability, the library emphasizes minimal dependencies, small footprint, and interoperability with event-driven frameworks like libevent, GLib, and Boost.Asio.

Overview

c-ares implements asynchronous resolution of DNS records and related operations, enabling applications to issue parallel lookups for A, AAAA, SRV, MX, CNAME and other resource records. It sits alongside standard resolver implementations like the GNU C Library resolver and system-specific resolvers in Microsoft Windows and BSD variants, offering an alternative when synchronous, thread-based, or blocking behavior is unsuitable. Many networking stacks and middleware — for example HAProxy, Exim, and Postfix integrations — use or can integrate with c-ares to avoid per-request thread creation while maintaining compatibility with POSIX sockets and native socket APIs in macOS and Linux.

Features and Architecture

c-ares exposes a small set of core features: non-blocking DNS queries, asynchronous host name resolution for IPv4 and IPv6, reverse DNS (PTR) lookups, TXT queries, and support for handling multiple parallel queries with per-query callbacks. The architecture comprises a state machine for managing query transactions, retransmission timers modeled after DNS behavior, and a socket/event integration layer that allows consumer code to drive the library from an existing event loop such as libevent, epoll, kqueue, or select-based loops used in FreeBSD and OpenBSD. It supports EDNS0 options, configurable timeouts, and custom query retries, while leaving TLS-over-DNS and DNSSEC to complementary libraries such as LDNS or BIND components. c-ares is designed to be used without a dedicated threadpool, contrasting with designs in Apache HTTP Server module threading, and aligns with single-threaded reactor patterns employed in Nginx and Node.js.

API and Usage

The API centers on functions to initiate asynchronous queries and to process events: initialization, channel creation, query submission for specific record types, and callbacks invoked when responses arrive or errors occur. Integration typically requires registering file descriptor readiness and timer expiration notifications with an event dispatcher like libuv, GLib's main loop, or the platform-native I/O mechanism in I/O Completion Ports. Example usage patterns include resolving hostnames for HTTP clients such as curl and performing cluster service discovery used by projects like Consul or Etcd that rely on DNS SRV or TXT records. Error handling maps to standard DNS rcode values and internal status codes, enabling interoperable behavior with server-side resolvers like Unbound or authoritative servers such as BIND.

Implementations and Language Bindings

Although implemented in C, c-ares has numerous language bindings and integrations enabling use from higher-level runtimes. Bindings exist for Python (via extension modules), Ruby gems, Go wrappers, Perl XS modules, and integration into Node.js through native addons. Projects like curl and libcurl include optional hooks to use c-ares for name resolution; OpenSSL-based clients can combine c-ares DNS resolution with TLS handshakes. Bindings and adapters often expose idiomatic async interfaces compatible with asyncio in Python, evented callbacks in Node.js, and futures/promises patterns used in Boost.Asio or Rust ecosystems, facilitating adoption in web servers, SMTP clients, and proxy software such as Squid.

History and Development

Development of c-ares began in the early 2000s to address blocking resolver limitations in portable networking codebases. The project was initially championed by contributors seeking a lightweight, permissively licensed alternative to heavier resolver stacks, drawing inspiration from resolver behaviors in BIND and resolver strategies used in Mozilla code. Over time, maintainers and contributors from organizations and projects including curl maintainers, distribution vendors, and independent developers expanded support for IPv6, EDNS0, and integration hooks for event loops. The codebase evolved through public repositories and mailing lists, with periodic releases that added features, addressed portability issues across Windows, Solaris, and embedded Linux platforms, and responded to DNS protocol developments tracked by the IETF.

Security and Performance Considerations

Security concerns with DNS resolution — such as spoofing, cache poisoning, and amplification — necessitate careful configuration when using c-ares alongside authoritative servers or recursive resolvers like Unbound or BIND. c-ares does not provide native DNSSEC validation; applications requiring authenticated responses should combine c-ares with DNSSEC-aware validators or use secure transports like DNS over TLS as implemented in projects such as stubby or getdns. Performance tuning involves adjusting retransmission intervals, parallel query limits, and socket reuse to match server capacity and latency characteristics observed in deployments like high-throughput proxies or load balancers such as HAProxy and Envoy. Profiling with tools like Valgrind and perf on Linux and static analysis with scanners used by organizations like Google or Microsoft helps identify memory safety and concurrency issues in integration code.

Category:Software libraries