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[Andreas <2121898@xxxxxxxxxxxxxxxxxx>]

** Information type changed from Public to Public Security

** Description changed:

  sudo on Ubuntu (tested on 24.04) is compiled with the --with-fqdn flag,
  which causes it to perform FQDN resolution on the machine's own hostname
  before executing any command (even when there is no Defaults fqdn line
  in /etc/sudoers).
  
  This can lead to noticeable hangs (30–60 seconds or more) if hostname
  resolution fails. For example:
  
- If the system hostname is changed via hostnamectl or GNOME Settings, but
- /etc/hosts is not updated accordingly (a common and silent
- misconfiguration).
- 
- If /etc/nsswitch.conf falls through to DNS and DNS is blocked (e.g. by a
- VPN kill-switch).
- 
- If systemd-resolved has no cached answer and cannot reach upstream
- nameservers.
+ - If the system hostname is changed via hostnamectl or GNOME Settings, but /etc/hosts is not updated accordingly (a common and silent misconfiguration).
+ - If /etc/nsswitch.conf falls through to DNS and DNS is blocked (e.g. by a VPN kill-switch).
+ - If systemd-resolved has no cached answer and cannot reach upstream nameservers.
  
  In this scenario, sudo hangs until name resolution times out, then
  eventually proceeds. This behavior is surprising, and introduces an
  unnecessary point of failure in a critical tool that is expected to work
  even when the network is down.
  
- Notably, upstream sudo does not enable FQDN resolution by default — this
- is a Debian/Ubuntu-specific build option (--with-fqdn). Other
- distributions like Fedora and Arch do not compile sudo this way, and do
- not exhibit this behavior unless Defaults fqdn is explicitly configured.
- 
+ Notably, upstream sudo does not enable FQDN resolution by default. This
+ behavior comes from a Debian and Ubuntu-specific build option (--with-
+ fqdn). Other distributions, such as Fedora and Arch, do not compile sudo
+ with this option and therefore do not exhibit this behavior unless
+ Defaults fqdn is explicitly set in the sudoers file.
  
  ---
  System Information:
  - Ubuntu version: 24.04.2 LTS
  - sudo version: 1.9.15p5-3ubuntu5.24.04.1
  
  ---
  
  For a detailed write-up and reproduction scenario see:
  https://anagogistis.com/posts/vpn-sudo-hang/

** Description changed:

  sudo on Ubuntu (tested on 24.04) is compiled with the --with-fqdn flag,
  which causes it to perform FQDN resolution on the machine's own hostname
  before executing any command (even when there is no Defaults fqdn line
  in /etc/sudoers).
  
- This can lead to noticeable hangs (30–60 seconds or more) if hostname
- resolution fails. For example:
- 
+ This can lead to noticeable hangs (30–60 seconds or more) if hostname resolution fails. For example:
  - If the system hostname is changed via hostnamectl or GNOME Settings, but /etc/hosts is not updated accordingly (a common and silent misconfiguration).
  - If /etc/nsswitch.conf falls through to DNS and DNS is blocked (e.g. by a VPN kill-switch).
  - If systemd-resolved has no cached answer and cannot reach upstream nameservers.
  
  In this scenario, sudo hangs until name resolution times out, then
- eventually proceeds. This behavior is surprising, and introduces an
- unnecessary point of failure in a critical tool that is expected to work
- even when the network is down.
+ eventually proceeds.
  
  Notably, upstream sudo does not enable FQDN resolution by default. This
  behavior comes from a Debian and Ubuntu-specific build option (--with-
  fqdn). Other distributions, such as Fedora and Arch, do not compile sudo
  with this option and therefore do not exhibit this behavior unless
  Defaults fqdn is explicitly set in the sudoers file.
  
  ---
  System Information:
  - Ubuntu version: 24.04.2 LTS
  - sudo version: 1.9.15p5-3ubuntu5.24.04.1
  
  ---
  
  For a detailed write-up and reproduction scenario see:
  https://anagogistis.com/posts/vpn-sudo-hang/

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https://bugs.launchpad.net/bugs/2121898

Title:
  sudo hangs when hostname resolution fails due to FQDN lookup being
  enabled by default (--with-fqdn)

Status in sudo package in Ubuntu:
  New

Bug description:
  sudo on Ubuntu (tested on 24.04) is compiled with the --with-fqdn
  flag, which causes it to perform FQDN resolution on the machine's own
  hostname before executing any command (even when there is no Defaults
  fqdn line in /etc/sudoers).

  This can lead to noticeable hangs (30–60 seconds or more) if hostname resolution fails. For example:
  - If the system hostname is changed via hostnamectl or GNOME Settings, but /etc/hosts is not updated accordingly (a common and silent misconfiguration).
  - If /etc/nsswitch.conf falls through to DNS and DNS is blocked (e.g. by a VPN kill-switch).
  - If systemd-resolved has no cached answer and cannot reach upstream nameservers.

  In this scenario, sudo hangs until name resolution times out, then
  eventually proceeds.

  Notably, upstream sudo does not enable FQDN resolution by default.
  This behavior comes from a Debian and Ubuntu-specific build option
  (--with-fqdn). Other distributions, such as Fedora and Arch, do not
  compile sudo with this option and therefore do not exhibit this
  behavior unless Defaults fqdn is explicitly set in the sudoers file.

  ---
  System Information:
  - Ubuntu version: 24.04.2 LTS
  - sudo version: 1.9.15p5-3ubuntu5.24.04.1

  ---

  For a detailed write-up and reproduction scenario see:
  https://anagogistis.com/posts/vpn-sudo-hang/

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