gevent.resolver.ares – c-ares based hostname resolver#

c-ares based hostname resolver.

class Resolver(hub=None, use_environ=True, **kwargs)[source]#

Bases: AbstractResolver

Implementation of the resolver API using the c-ares library.

This implementation uses the c-ares library to handle name resolution. c-ares is natively asynchronous at the socket level and so integrates well into gevent’s event loop.

In comparison to gevent.resolver_thread.Resolver (which delegates to the native system resolver), the implementation is much more complex. In addition, there have been reports of it not properly honoring certain system configurations (for example, the order in which IPv4 and IPv6 results are returned may not match the threaded resolver). However, because it does not use threads, it may scale better for applications that make many lookups.

There are some known differences from the system resolver.

  • gethostbyname_ex and gethostbyaddr may return different for the aliaslist tuple member. (Sometimes the same, sometimes in a different order, sometimes a different alias altogether.)

  • gethostbyname_ex may return the ipaddrlist in a different order.

  • getaddrinfo does not return SOCK_RAW results.

  • getaddrinfo may return results in a different order.

  • Handling of .local (mDNS) names may be different, even if they are listed in the hosts file.

  • c-ares will not resolve broadcasthost, even if listed in the hosts file prior to 2020-04-30.

  • This implementation may raise gaierror(4) where the system implementation would raise herror(1) or vice versa, with different error numbers. However, after 2020-04-30, this should be much reduced.

  • The results for localhost may be different. In particular, some system resolvers will return more results from getaddrinfo than c-ares does, such as SOCK_DGRAM results, and c-ares may report more ips on a multi-homed host.

  • The system implementation may return some names fully qualified, where this implementation returns only the host name. This appears to be the case only with entries found in /etc/hosts.

  • c-ares supports a limited set of flags for getnameinfo and getaddrinfo; unknown flags are ignored. System-specific flags such as AI_V4MAPPED_CFG are not supported.

  • getaddrinfo may return canonical names even without the AI_CANONNAME being set.

  • getaddrinfo does not appear to support IPv6 symbolic scope IDs.


This module is considered extremely experimental on PyPy, and due to its implementation in cython, it may be slower. It may also lead to interpreter crashes.

Changed in version 1.5.0: This version of gevent typically embeds c-ares 1.15.0 or newer. In that version of c-ares, domains ending in .onion are never resolved or even sent to the DNS server.

Changed in version 20.5.0: getaddrinfo is now implemented using the native c-ares function from c-ares 1.16 or newer.

Changed in version 20.5.0: Now herror and gaierror are raised more consistently with the standard library resolver, and have more consistent errno values.

Handling of localhost and broadcast names is now more consistent.

Changed in version 22.10.1: Now has a __del__ method that warns if the object is destroyed without being properly closed.


Release resources held by this object.

Subclasses that define resources should override.

New in version 22.10.1.