Name Resolution on Linux

With glibc, name resolution goes getaddrinfo() → nsswitch.conf (for Name Service Switch, which is not Network Security Services) → /etc/hosts or DNS. If DNS: → libresolv → host.conf → /etc/hosts or real DNS.

With musl, name resolution goes getaddrinfo() → /etc/hosts or DNS. name_from_hosts() reads /etc/hosts directly, and name_from_dns_search() calls name_from_dns() for each search domain with resolver configuration read from /etc/resolv.conf.

Both glibc and musl defaults to returning all valid addresses for a host that appears in the /etc/hosts file, instead of only the first. musl’s behavior is not configurable and has a fixed limit of at most 48 results. glibc’s behavior can be configured with the multi keyword in the /etc/host.conf file, and can be overridden by the RESOLV_MULTI environment variable.

gethostbyname()

gethostbyname() is obsolete, does not support IPv6 and only returns one IP address even if there are multiple entries.

Applications

Some applications do name resolution on their own, some always pick the first address returned from getaddrinfo(). The behavior is very application-specific and you should always test it before making conclusion.