gethostbyaddr vs getaddrinfo and getnameinfo
 

Why gethostbyaddr treated as obsolete and getaddrinfo and getnameinfo should be used instead, if unde latter one i'm unable to get aliases from /etc/hosts while in case gethostbyaddr i will?

The gethostbyaddr performs netbios lookup. It is very slow when the host can't be found. The getnameinfo only contact DNS server, should be faster.

Thanks, what about aliases in /etc/hosts? Why they become unavailable under DNS?

In general, the file is for domain name and static host name. Some distribution could change file name to another one. It look like no affaction.