From Fedora Project Wiki

The standard C library provides a number of functions related to host and service name resolution and DNS information retrieval. The former are mainly intended to support applications when connecting to services while the latter gives them access to additional DNS

Name resolution for libraries

Problem statement

Current versions of glibc offer the getaddrinfo() function, res_*() functions and _res.options. The former two are used for host and service name resolution and for DNS record retrieval, respectively. While this works well for applications, libraries may need to tweak the configuration without affecting the application. Basically, the configuration needs to be specific to the caller and the caller needs to be able to provide the configuration when performing host/service name resolution and DNS record retrieval.

Example use case

A cryptographic library may want to configure the resolver so that it only receives records secured by DNSSEC. The same library may also want to perform ordinary name resolution where records not secured by DNSSEC are returned as well. None of the configuration should ever affect the running application, therefore three different configuration contexts are needed.

Possible solution

For each libc library function that uses the shared context, provide a new function that would accept an opaque pointer to a specific context object. We also need to provide a set of functions to create and configure the context object.

Existing solutions

For example, netresolve provides a context object and allows for all sorts of name resolution backends including DNS.

Example: Classic getaddrinfo() implemented using netresolve_query_getaddrinfo() using a brand new object for the query. This is not necessarily very practical but it shows the concept.

getaddrinfo(const char *nodename, const char *servname,
        const struct addrinfo *hints, struct addrinfo **res)
    netresolve_t channel;
    netresolve_query_t query;
    int status = EAI_SYSTEM;

    if (!(channel = netresolve_open()))
        return status;

    if ((query = netresolve_query_getaddrinfo(channel, nodename, servname, hints)))
        status = netresolve_query_getaddrinfo_done(query, res);

    return status;

Note: While the getaddrinfo functionality is split into two functions, this code doesn't use the non-blocking functionality and thus can just call the two functions in row.


A global context could still be available and the classic getaddrinfo() function could call the new function with the global context.