The glibc implementation of gethostbyaddr_r will set h_addrtype to the address family that was passed in as an argument. Some programs depend on this (CPython is the one I noticed) for correct operation. The current behavior leaves the field unchanged and likely provides an undefined value to the caller. In the case of CPython, this results in `socket.gethostbyaddr` always raising EAFNOSUPPORT. **Trac**: **Username**: exarkun