socket.getfqdn() returns unqualifed hostname if the resolver is configured to trim local domains

Bug report

socket.getfqdn() is returning an unqualified address for my host. I tracked this down to the use of the trim keyword in /etc/host.conf the removal of which would cause other unrelated problems.

Looking at the source code for the system hostname command, they appear to call getaddrinfo with the service parameter set to NULL and pull out the ai_canonname field from the result. See: https://sources.debian.org/src/hostname/3.23/hostname.c lines 332-340 and 386 The python getaddrinfo doesn't especially help with this but perhaps this provides clues as to a more robust implementation.

Looking at Lib/socket.py, gethostname() actually is returning a fully qualified hostname and the gethostbyaddr() step is turning it back into an unqualified address. If this is hard to do portably, perhaps it could check for a dot in the values.

Your environment

Tested on a variety of RHEL based systems - Oracle Linux and CentOS Stream versions 8 and 9 both with distributed Python(s) and a build from source of the latest Python. Can also verify the issue by breaking down and tracing the source code in cpython/Lib/socket.py

/etc/host.conf on these systems includes the trim directive to remove the local domain from returned results.

從 Lib/socket.py 開始，追蹤 getfqdn() 經由 gethostname() 和 gethostbyaddr() 的呼叫流程；將其行為與 hostname 命令對 getaddrinfo 和 ai_canonname 的使用進行比較。在 /etc/host.conf 中設定 trim 的情況下重現該案例，並在 getfqdn() 無需移除該指示詞即可保留完整限定主機名稱時，視為結果完成。