WSL generates wrong /etc/hosts file when host system is not part of domain

[jonasPoehler]

Windows Version

Microsoft Windows [Version 10.0.22631.4460]

WSL Version

2.3.26.0

Are you using WSL 1 or WSL 2?

• WSL 2
• WSL 1

Kernel Version

5.15.167.4-1

Distro Version

Ubuntu 24.04

Other Software

No response

Repro Steps

I have a Windows device with the following local hostname (FQDN)

PS C:> [System.Net.Dns]::GetHostByName($env:computerName).HostName
DESKTOP-30N6MHG

Install a Ubuntu 24.04 (maybe even other distros) via wsl --import.

Expected Behavior

The /etc/hosts/ file should contain only one entry for the host, meaning the "regular" hostname:

# This file was automatically generated by WSL. To stop automatic generation of this file, add the following entry to /etc/wsl.conf:
# [network]
# generateHosts = false
127.0.0.1       localhost
127.0.1.1       DESKTOP-30N6MHG
 
# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Actual Behavior

When I install a new WSL-Ubuntu-Distro via wsl --import and then start the system, the /etc/hosts/ (generated by WSL) file looks like this:

# This file was automatically generated by WSL. To stop automatic generation of this file, add the following entry to /etc/wsl.conf:
# [network]
# generateHosts = false
127.0.0.1       localhost
127.0.1.1       DESKTOP-30N6MHG.        DESKTOP-30N6MHG
 
# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

As you can see, the entry with the hostname contains two entries: DESKTOP-30N6MHG. (yes, with a trailing dot) and DESKTOP-30N6MHG. This results in the FQDN inside WSL to be DESKTOP-30N6MHG., which some software (e.g. Postfix) cannot handle. Additionally according to the manpage of hosts, hostnames have to end in an alphanumeric character, which is probably why software cannot handle this behaviour.

Diagnostic Logs

No response

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View similar issues

Please view the issues below to see if they solve your problem, and if the issue describes your problem please consider closing this one and thumbs upping the other issue to help us prioritize it!

Open similar issues:

• HOSTNAME.local: Name or service not known (#10077), similarity score: 0.76
• FQDN hostname no longer works in wsl.conf (#9897), similarity score: 0.73

Closed similar issues:

• /etc/hosts not getting generated with the contents of the windows hosts file (#11719), similarity score: 0.76
• Updating WSL appears to cause generateResolvConf to create only IPv6 nameserver entries, causing DNS resolution to fail. (#12191), similarity score: 0.72
• Extra chars in /etc/hosts (#9642), similarity score: 0.72

Note: You can give me feedback by thumbs upping or thumbs downing this comment.

[focknecht]

Here are the logs:

https://aka.ms/AAtgi7o

[jonasPoehler]

Maybe I need to point to the logs, in order for the bot to remove the label?

Here are the logs:

https://aka.ms/AAtgi7o

[litian1992]

This is affecting cloud-init. Cloud-init uses 'hostname -f' to determine FQDN. The extra dot '.' make it fail setting hostname.

[OneBlue]

Thank you for reporting this.

We have a fix for this issue, it will be included in the next 2.5.X release.

[benhillis]

Fixed with https://github.com/microsoft/WSL/releases/tag/2.5.1