Help Track Down a Flatpak Bug!

I’ve discovered something weird, but I can’t figure it out on my own.

The following Java code works outside of Flatpak, and works inside Flatpak for most distros - But in Flatpak on Arch Linux, it throws an exception:

import java.net.InetAddress;

public final class Main {
  public static void main(String[] args) {
    try {
      InetAddress.getLocalHost();
      System.out.println("Completed without issue!");
      System.exit(0);
    } catch (Exception e) {
      e.printStackTrace();
      System.exit(1);
    }
  }
}

Output outside Flatpak or on most distros:

Completed without issue!

Output in Flatpak on Arch Linux:

java.net.UnknownHostException: archlinux: archlinux: Name or service not known
        at java.base/java.net.InetAddress.getLocalHost(InetAddress.java:1661)
        at Main.main(Main.java:6)
Caused by: java.net.UnknownHostException: archlinux: Name or service not known
        at java.base/java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
        at java.base/java.net.InetAddress$PlatformNameService.lookupAllHostAddr(InetAddress.java:929)
        at java.base/java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1534)
        at java.base/java.net.InetAddress$NameServiceAddresses.get(InetAddress.java:848)
        at java.base/java.net.InetAddress.getAllByName0(InetAddress.java:1524)
        at java.base/java.net.InetAddress.getLocalHost(InetAddress.java:1656)
        ... 1 more

I’ve created a repo with a reproducible example + simple build steps in the README: GitHub - dginovker/FlatpakJavaBug: Example bug in Flatpak Java

• Granting Flatpak with all the permissions via Flatseal doesn’t resolve the issue
• Issue happens with OpenJDK 11 and OpenJDK17 (although I suspect this isn’t actually a Java issue)

Anyone have other suggestions on how to narrow this down?

Seems to work on Asahi Linux with the following /etc/hosts:

# Host addresses
127.0.0.1  localhost
127.0.1.1  asahi-mac
::1        localhost ip6-localhost ip6-loopback
ff02::1    ip6-allnodes
ff02::2    ip6-allrouters

Edit: Matching this hosts file on the computer with issues doesn’t seem to resolve the issue

On my computer with Arch Linux:
FlatpakJavaBug]$ flatpak run io.github.flatpakjavabug
Completed without issue!

does it have --share=network ?

But the app is buggy. It shouldn’t crash on that kind of conditions?

Yup, it has --share=network

It shouldn’t crash

For clarity it’s not crashing, the code is entering the catch branch above and calling System.exit(1), but the problem is it doesn’t do this outside Flatpak

On my computer with Arch Linux:
FlatpakJavaBug]$ flatpak run io.github.flatpakjavabug
Completed without issue!

Thanks for checking . This part is nuts, it’s working haphazadly for different people. Can you post your /etc/hosts? I don’t actually think it is the /etc/hosts causing the issue, but I don’t know enough about Linux to have any other leads

cat /etc/hosts
# Static table lookup for hostnames.
# See hosts(5) for details.
127.0.0.1	localhost
127.0.1.1	bueroal2

Someone said this on Discord, but I’m not sure what the before/after of the hosts file was:

I just tested an install of arch Linux and had the same issue initially but fixed by modifying the hosts file & rebooting

Figured i’d post here instead of Discord

Original, Broken Hosts

broken1307×810 99.6 KB

“New Users Can Only Put One Photo Per Post”

Modified, Working Hosts

image1282×812 108 KB