System.Net.NameResolution.Tests DNS failures: Name or service not known

[kotlarmilos]

Build Information

Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1368622
Build error leg or test failing: System.Net.NameResolution.Tests.GetHostByNameTest.DnsObsoleteBeginEndGetHostByName_EmptyString_ReturnsHostName
Pull request: #125647

Error Message

Fill the error message using step by step known issues guidance.

{
  "ErrorMessage": "",
  "ErrorPattern": "System\\.Net\\.Sockets\\.SocketException : Name or service not known|System\\.Exception : Failed for empty hostname",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1368622
Error message validated: [System\.Net\.Sockets\.SocketException : Name or service not known|System\.Exception : Failed for empty hostname]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 4/8/2026 12:37:43 PM UTC

Report

Build	Definition	Test	Pull Request
1370807	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126587
1370894	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses
1370423	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126322
1370594	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126621
1370561	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126124
1370564	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126522
1367903	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126348
1367834	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126543
1370568	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125613
1366930	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126371
1369043	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126112
1368922	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126509
1367818	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126521
1370450	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125835
1370412	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125559
1370311	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126562
1370306	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126648
1370295	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#108158
1370287	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126445
1370270	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126587
1370333	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126152
1370261	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126588
1370239	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#122649
1370220	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126645
1370255	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125955
1370161	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126047
1370118	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126622
1368340	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126579
1370070	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126124
1370025	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126629
1369180	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125900
1369946	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125347
1369794	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125559
1369801	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126199
1369758	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#126633
1368420	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#124266
1368622	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses	#125647

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
37	37	37

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[MihuBot]

I'm a bot. Here are possible related and/or duplicate issues (I may be wrong):

• Tests failures: System.Net.NameResolution.Tests / DnsObsolete* & Dns_GetHostEntry* #1488
• System.Net.Dns does not always resolve "" and the system's own hostname on Unix #36849

[wfurt]

It seems like the machines cannot resolve own name.

[mangod9]

diag analysis:

Analysis

What's happening

The test calls Dns.GetHostEntry("") which:

 1. Gets the local hostname via Dns.GetHostName() → returns a00BOMC (the
Helix machine name)
 2. Tries to resolve a00BOMC via DNS → "Name or service not known"

The test's diagnostic output reveals the root cause:

 Dns.GetHostName() == a00BOMC
 /etc/hosts ==
 ::1         localhost localhost.localdomain ipv6-localhost ipv6-loopback

The machine's /etc/hosts file only has the localhost entry. The hostname
a00BOMC is not in /etc/hosts and is not resolvable via DNS.

Why it's happening now

 - All 10 hits in the last 24 hours — this started today (April 8)
 - All on the AzureLinux.3.Amd64.Open queue
 - All different machines (a00BPIG, a00BPYE, a00BQ20, a00BQ0P, a00BOMC) — not
 a single bad machine
 - All on the coreclr_checked test configuration with
net11.0-linux-Release-x64

This pattern indicates a fleet-wide infrastructure change on the AzureLinux 3
Helix machines — either:

 1. New machines were provisioned without /etc/hosts entries for their own
hostname
 2. A configuration management change removed the hostname entry from
/etc/hosts
 3. A DNS server change caused the machine names to stop resolving

Fix options

Infrastructure fix (preferred): Ensure /etc/hosts on AzureLinux 3 Helix
machines includes a line like:

 127.0.0.1   a00BOMC a00BOMC.localdomain

Test resilience fix: The test at GetHostEntryTest.cs:53 could skip or mark as
inconclusive when the local hostname can't be resolved, since this is an
environment issue:

 [InlineData("")]
 public async Task Dns_GetHostEntry_HostString_Ok(string hostName)
 {
     // Skip if local hostname can't be resolved (Helix infra issue)
     if (hostName == "" && !CanResolveLocalHostname())
         return;
     // ...
 }

This should be reported to @dotnet/dnceng or the Helix infrastructure team as
a fleet configuration issue on the AzureLinux.3.Amd64.Open queue.