socket.getfqdn() does gethostbyaddr() lookup for own hostname

[bluelhf]

Bug report

cpython/Lib/socket.py

Lines 791 to 795 in e1d45b8

	name = name.strip()
	if not name or name in ('0.0.0.0', '::'):
	name = gethostname()
	try:
	hostname, aliases, ipaddrs = gethostbyaddr(name)

When 0.0.0.0, ::, or empty string is passed into socket.get_fqdn(name), instead of returning gethostname(), it performs a host name lookup using the hostname retrieved from gethostname(), and returns the result.

This differs from the documentation, which states

If name was empty, '0.0.0.0' or '::', hostname from gethostname() is returned.

Your environment

• CPython versions tested on: Python 3.13.0a0 (heads/main:01e1a60, Jul 7 2023, 11:22:09)
• Operating system and architecture: Pop!_OS 22.04 on x86_64

Linked PRs

• gh-106505: don't lookup own hostname in getfqdn() #106506

[danderson]

Just encountered this while trying to figure out why http.server hangs for 5s at startup on some of my linux systems, but only when binding to 0.0.0.0. With the following system configuration:

• Linux
• avahi-daemon installed and operating in dual-stack (ipv4 and ipv6) configuration
• nsswitch.conf for host resolution includes mdns_minimal, which attempts to resolve unqualified names through mdns, via avahi
• hostname in /etc/hostname is a single label (e.g. orion), not a fqdn

This configuration can lead to a pathological condition due to an avahi race condition: if it fails to grab its own hostname on mdns, then socket.getfqdn will cause avahi to ask the local LAN for its own hostname. Which will hang the resolution for 5s until that query times out, in the default configuration.

The observable result in my case is that anything on the system which uses python's http.server (django's dev server in my case) and binds to 0.0.0.0 hangs for 5s on startup, because http.server.HTTPServer does a getfqdn of its bind address on construction, which triggers this whole unfortunate chain.

Fortunately I suspect the number of people likely to hit this is relatively low, because the most common linux distros already disable avahi's IPv6 support, which (usually) allows avahi to take ownership of the system's hostname and thus return an instant response to socket.getfqdn.