NetworkInterface.GetAllNetworkInterfaces throws DirectoryNotFoundException on Linux when /etc does not exist #135568

Description

On Linux, NetworkInterface.GetAllNetworkInterfaces() throws System.IO.DirectoryNotFoundException when /etc does not exist. When /etc exists but /etc/resolv.conf does not, the same call returns the interfaces with no DNS server addresses.

/etc/resolv.conf is read with File.ReadAllText, and the catch covers only FileNotFoundException and UnauthorizedAccessException:

LinuxNetworkInterface.cs:55-64

                try
                {
                    string resolverConfig = File.ReadAllText(NetworkFiles.EtcResolvConfFile);
                    DnsSuffix = StringParsingHelpers.ParseDnsSuffixFromResolvConfFile(resolverConfig);
                    DnsAddresses = new InternalIPAddressCollection(StringParsingHelpers.ParseDnsAddressesFromResolvConfFile(resolverConfig));
                }
                catch (Exception e) when (e is FileNotFoundException || e is UnauthorizedAccessException)
                {
                    DnsAddresses = new InternalIPAddressCollection();
                }

When the parent directory is missing, ENOENT is mapped to DirectoryNotFoundException, which that catch does not cover:

Interop.IOErrors.cs:120-138

            case Error.ENOENT:
                // For Windows compatibility, throw DirectoryNotFoundException instead of FileNotFoundException
                // when the parent folder does not exist.
                if (!isDirError && (path is null || ParentDirectoryExists(path)))
                {
                    return !string.IsNullOrEmpty(path) ?
                        new FileNotFoundException(SR.Format(SR.IO_FileNotFound_FileName, path), path) :
                        new FileNotFoundException(SR.IO_FileNotFound);
                }
                goto case Error.ENOTDIR;

            case Error.ENOTDIR:
                return !string.IsNullOrEmpty(path) ?
#if NET11_0_OR_GREATER
                    new DirectoryNotFoundException(SR.Format(SR.IO_PathNotFound_Path, path), path) :
#else
                    new DirectoryNotFoundException(SR.Format(SR.IO_PathNotFound_Path, path)) :
#endif
                    new DirectoryNotFoundException(SR.IO_PathNotFound_NoPathName);

The two /etc/resolv.conf readers in UnixIPInterfaceProperties, which the BSD and Apple implementations call from the IPInterfaceProperties constructor, catch only FileNotFoundException (UnixIPInterfaceProperties.cs:87-110). Those platforms were not run.

Reproduction Steps

Run as root on Linux. run.sh runs the app twice, each time in its own mount namespace: once with an empty tmpfs mounted over /etc (so /etc exists but /etc/resolv.conf does not), and once chrooted into a tmpfs root that has no /etc. The chroot root contains only /bin, /lib, /lib64, /usr, /proc, /sys, /dev, /tmp, /home and /root, so in the second case the real paths (run.sh applies readlink -f) of the app and the dotnet host must be under one of those.

Program.cs:

using System;
using System.IO;
using System.Net.NetworkInformation;

Console.WriteLine($"/etc exists: {Directory.Exists("/etc")}, /etc/resolv.conf exists: {File.Exists("/etc/resolv.conf")}");

foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
{
    IPInterfaceProperties props = nic.GetIPProperties();
    Console.WriteLine($"{nic.Name}: IsDnsEnabled={props.IsDnsEnabled}, DnsAddresses={props.DnsAddresses.Count}");
}

repro.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net11.0</TargetFramework>
    <ImplicitUsings>disable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <!-- Lets the app run on a locally built runtime with a higher major version. -->
    <RollForward>LatestMajor</RollForward>
  </PropertyGroup>
</Project>

run.sh:

#!/bin/sh
# Usage (as root): ./run.sh <path to dotnet host> <path to repro.dll>
# Runs the repro with /etc present but no /etc/resolv.conf, then with no /etc at all.
# Each case runs in its own mount namespace (unshare --mount), so the host's /etc is untouched.
set -u
DOTNET=$(readlink -f "$1")
APP=$(readlink -f "$2")
export DOTNET

echo "== /etc present, /etc/resolv.conf missing (empty tmpfs mounted over /etc)"
unshare --mount sh -c 'mount -t tmpfs tmpfs /etc && "$DOTNET" "$0"' "$APP"
echo "exit code: $?"

echo "== /etc missing (chroot into a tmpfs root that has no /etc)"
unshare --mount sh -c '
    ROOT=$(mktemp -d)
    mount -t tmpfs tmpfs "$ROOT"
    for d in bin lib lib64 usr proc sys dev tmp home root; do
        if [ -L "/$d" ]; then
            cp -P "/$d" "$ROOT/$d"
        elif [ -d "/$d" ]; then
            mkdir "$ROOT/$d" && mount --bind "/$d" "$ROOT/$d"
        fi
    done
    exec chroot "$ROOT" "$DOTNET" "$0"' "$APP"
echo "exit code: $?"

dotnet build -c Release -o out
./run.sh "$(command -v dotnet)" out/repro.dll

Expected behavior

With no /etc, the same result as with no /etc/resolv.conf: the interfaces are returned, DnsAddresses is empty and IsDnsEnabled is false.

Actual behavior

Output on a runtime built from 9bfbe1d (source paths and the file-open frames between Interop.CheckIo and LinuxNetworkInterfaceSystemProperties..ctor trimmed):

== /etc present, /etc/resolv.conf missing (empty tmpfs mounted over /etc)
/etc exists: True, /etc/resolv.conf exists: False
lo: IsDnsEnabled=False, DnsAddresses=0
eth0: IsDnsEnabled=False, DnsAddresses=0
exit code: 0
== /etc missing (chroot into a tmpfs root that has no /etc)
/etc exists: False, /etc/resolv.conf exists: False
Unhandled exception. System.IO.DirectoryNotFoundException: Could not find a part of the path '/etc/resolv.conf'.
Directory path: '/etc/resolv.conf'
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
   at Interop.CheckIo(Error error, String path, Boolean isDirError)
   ...
   at System.Net.NetworkInformation.LinuxNetworkInterface.LinuxNetworkInterfaceSystemProperties..ctor()
   at System.Net.NetworkInformation.LinuxNetworkInterface.GetLinuxNetworkInterfaces()
   at System.Net.NetworkInformation.NetworkInterfacePal.GetAllNetworkInterfaces()
   at System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces()
   at Program.<Main>$(String[] args)
Aborted (core dumped)
exit code: 134

Regression?

Not checked against earlier releases.

Known Workarounds

An /etc directory, even an empty one, avoids the exception (the first case in the output above).

Configuration

Runtime built from dotnet/runtime 9bfbe1d (./build.sh clr+libs -rc Release), app built with SDK 11.0.100-rc.1.26420.103. Ubuntu 22.04.5 LTS on WSL2, x64, Linux 6.18.40.1-microsoft-standard-WSL2. Only seen when /etc is missing.

Other information

Found while porting .NET to another operating system, where /etc did not exist in the test environment.

A fix: read /etc/resolv.conf through one internal helper that returns null for both FileNotFoundException and DirectoryNotFoundException, used by all three readers. No public API change.

A missing /etc/resolv.conf used to make DnsAddresses throw PlatformNotSupportedException on Linux (#109582); #111471 fixed that by setting an empty DnsAddresses in the Linux reader's FileNotFoundException catch. A missing /etc throws DirectoryNotFoundException instead, which that catch does not cover.

I have a fix with a test ready and will open a PR for it shortly. Could this be assigned to me?