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
1421877	dotnet/runtime	System.Net.NameResolution.Tests.GetHostAddressesTest.DnsGetHostAddresses_LocalhostWithTrailingDot_ReturnsLoopback	#128067
1418093	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.DnsGetHostEntry_LocalhostWithTrailingDot_ReturnsLoopback	#126520
1416588	dotnet/runtime	System.Net.NameResolution.Tests.GetHostAddressesTest.DnsGetHostAddresses_LocalhostWithTrailingDot_ReturnsLoopback	#128057
1412514	dotnet/runtime	System.Net.NameResolution.Tests.GetHostAddressesTest.DnsGetHostAddresses_LocalhostWithTrailingDot_ReturnsLoopback	#127927
1407254	dotnet/runtime	System.Net.NameResolution.Tests.GetHostAddressesTest.DnsGetHostAddresses_LocalhostWithTrailingDot_ReturnsLoopback
1405444	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.DnsGetHostEntry_LocalhostWithTrailingDot_ReturnsLoopback	#127655
1405410	dotnet/runtime	System.Net.NameResolution.Tests.GetHostAddressesTest.DnsGetHostAddresses_LocalhostWithTrailingDot_ReturnsLoopback	#126367
1405052	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.DnsGetHostEntry_LocalhostWithTrailingDot_ReturnsLoopback	#127328
1404771	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.DnsGetHostEntry_LocalhostWithTrailingDot_ReturnsLoopback	#125795
1402028	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.DnsGetHostEntry_LocalhostWithTrailingDot_ReturnsLoopback	#127562
1400249	dotnet/runtime	System.Net.NameResolution.Tests.GetHostEntryTest.DnsGetHostEntry_LocalhostWithTrailingDot_ReturnsLoopback	#127464

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	0	11

[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.

[kg]

Summary:

DNS resolution fails with SocketException on Helix machines where hostname is not in /etc/hosts.

Failed in (3):

• runtime-coreclr libraries-jitstress 20260409.1

• runtime-coreclr libraries-jitstressregs 20260408.1

• runtime-coreclr libraries-pgo 20260408.1

Console Log: Console Log

Source: runtime-coreclr libraries-jitstress / net11.0-linux-Release-x64-no_tiered_compilation-AzureLinux.3.Amd64.Open / System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_HostString_Ok

Failed tests:

runtime-coreclr libraries-jitstress

- net11.0-linux-Release-x64-disabler2r-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitminopts-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress1-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress1_tiered-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress2-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress2_tiered-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-no_tiered_compilation-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-tailcallstress-AzureLinux.3.Amd64.Open

runtime-coreclr libraries-jitstressregs

- net11.0-linux-Release-x64-jitstressregs0x10-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs0x1000-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs0x2000-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs0x80-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs1-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs2-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs3-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs4-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs8-AzureLinux.3.Amd64.Open

runtime-coreclr libraries-pgo

- net11.0-linux-Release-x64-defaultpgo-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_methodprofiling-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_random_gdv-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_random_gdv_edge-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_random_gdv_methodprofiling_only-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitosr_stress-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitosr_stress_random-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitrlcse-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-syntheticpgo-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-syntheticpgo_blend-AzureLinux.3.Amd64.Open

  - System.Net.NameResolution.Tests.GetHostByNameTest.DnsObsoleteBeginEndGetHostByName_EmptyString_ReturnsHostName

  - System.Net.NameResolution.Tests.GetHostByNameTest.DnsObsoleteGetHostByName_EmptyString_ReturnsHostName

  - System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntryAsync_HostString_Ok

  - System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_HostString_Ok

  - System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses

Error Message:

System.Net.Sockets.SocketException : Name or service not known

Stack Trace:

   at System.Net.Dns.GetHostEntryOrAddressesCore(String hostName, Boolean justAddresses, AddressFamily addressFamily, Nullable`1 activityOrDefault) in /_/src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs:line 522
   at System.Net.Dns.GetHostEntry(String hostNameOrAddress, AddressFamily family) in /_/src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs:line 87
   at System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_HostString_Ok(String hostName) in /_/src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs:line 53

Analysis:

These are DNS resolution failures where Dns.GetHostEntry("") fails with "Name or service not known" on Helix Linux machines. The test machines hostnames are not resolvable via DNS or /etc/hosts. This is a Helix infrastructure issue, not JIT/stress related. Tracked by #126641.

• Generated by ci-pipeline-monitor/scripts/generate_report.py *

[JulieLeeMSFT]

Summary:

DNS resolution failures in Helix test machines. Tests fail because hostname cannot be resolved via DNS. Known infrastructure issue affecting jitstress, jitstressregs, and pgo pipelines.

Failed in (3):

• runtime-coreclr libraries-jitstress 20260410.1

• runtime-coreclr libraries-jitstressregs 20260408.1

• runtime-coreclr libraries-pgo 20260408.1

Console Log: Console Log

Source: runtime-coreclr libraries-jitstress / net11.0-linux-Release-x64-no_tiered_compilation-AzureLinux.3.Amd64.Open / System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_HostString_Ok

Failed tests:

runtime-coreclr libraries-jitstress

- net11.0-linux-Release-x64-disabler2r-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitminopts-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress1-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress1_tiered-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress2-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstress2_tiered-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-no_tiered_compilation-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-tailcallstress-AzureLinux.3.Amd64.Open

runtime-coreclr libraries-jitstressregs

- net11.0-linux-Release-x64-jitstressregs0x10-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs0x1000-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs0x2000-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs0x80-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs1-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs2-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs3-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs4-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitstressregs8-AzureLinux.3.Amd64.Open

runtime-coreclr libraries-pgo

- net11.0-linux-Release-x64-defaultpgo-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_methodprofiling-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_random_gdv-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_random_gdv_edge-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-fullpgo_random_gdv_methodprofiling_only-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitosr_stress-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitosr_stress_random-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-jitrlcse-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-syntheticpgo-AzureLinux.3.Amd64.Open

- net11.0-linux-Release-x64-syntheticpgo_blend-AzureLinux.3.Amd64.Open

  - System.Net.NameResolution.Tests.GetHostByNameTest.DnsObsoleteBeginEndGetHostByName_EmptyString_ReturnsHostName

  - System.Net.NameResolution.Tests.GetHostByNameTest.DnsObsoleteGetHostByName_EmptyString_ReturnsHostName

  - System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntryAsync_HostString_Ok

  - System.Net.NameResolution.Tests.GetHostEntryTest.Dns_GetHostEntry_HostString_Ok

  - System.Net.NameResolution.Tests.GetHostEntryTest.GetHostEntry_DisableIPv6_ExcludesIPv6Addresses

Error Message:

System.Net.Sockets.SocketException : Name or service not known

Stack Trace:

at System.Net.Dns.GetHostEntryOrAddressesCore(String hostName, Boolean justAddresses, AddressFamily addressFamily, Nullable`1 activityOrDefault) in /_/src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs:line 522
at System.Net.Dns.GetHostEntry(String hostNameOrAddress, AddressFamily family) in /_/src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs:line 87

Analysis:

Infrastructure issue with DNS resolution on Helix machines. Not a product bug. Known from prior runs.

Generated by ci-pipeline-monitor/scripts/generate_report.py

[danmoseley]

Note

This comment was drafted with Copilot assistance.

Some additional findings from investigating dotnet-helix-machines:

Tests run directly on the VM, not in Docker. The Helix logs show hostname a00BVKN (no D prefix — Docker containers prepend D to the VM hostname via dockerhelper.py). The VM's /etc/hosts only has localhost entries — the hostname is completely absent. Multiple machines are affected (a00BOMC, a00C1G0, a00C13G, a00BVKN), so this is fleet-wide.

An artifact to fix exactly this already exists — linux-add-localhost-to-hosts (created in PR 13243 for "self-resolution issues"). It adds 127.0.0.1\t$(hostname) to /etc/hosts. However, it is not applied to any AzureLinux 3 queue in definitions/shared/linux.yaml.

Timeline correlates with AutoScalerACA deployment:

• March 18: AutoScalerSystem: AutoScalerACA added to AzureLinux.3.Amd64.Open.Rt (PR 59010)
• April 1: Production rollout deploys this change
• April 2: AutoScalerACA also added to AzureLinux.3.Amd64.Open and .Open.Svc (PR 59711)
• April 3: Last green jitstress build (existing VMs still had working DNS)
• April 4–7: No jitstress runs (gap)
• April 8: First DNS failures appear

The likely mechanism: old VMs (provisioned under AutoScalerSF) had hostname DNS resolution working through Azure infrastructure. When AutoScalerACA took over and cycled the VMSS (scale down/up over the weekend gap), newly provisioned VMs lost that resolution — possibly due to differences in VNET config, custom extension ordering, or Azure DNS behavior for the new VMSS.

wfurt has an in-progress fix on branch wfurt/systemd (commit 41a553fa) — adds a linux-enable-systemd-resolver artifact that enables systemd-resolved and points /etc/resolv.conf at its stub resolver. This is a more comprehensive DNS fix than just patching /etc/hosts.

A simpler alternative would be to add the existing linux-add-localhost-to-hosts artifact to the AzureLinux 3 artifact list — that would fix hostname self-resolution specifically without changing the broader DNS setup.