Fix DNS-invalid hostnames with underscores in dotnet new templates by Copilot · Pull Request #64988 · dotnet/aspnetcore

Copilot · 2026-01-09T02:10:27Z

Fix DNS names with underscores in dotnet new templates

Summary:

Fixed issue where dotnet new templates generate DNS-invalid hostnames with underscores when project names contain dots (e.g., my.namespace.web → my_namespace_web), violating RFC 952/1123.

Changes Made:

Technical Implementation:

Templates (EmptyWeb & BlazorWeb):

Created lowerCaseInvariantWithHyphens value form chain that:
- Lowercases the name
- Replaces all invalid DNS chars with hyphens
- Removes leading/trailing hyphens
Added hostName derived symbol that applies the transform and replaces LocalhostTldHostNamePrefix placeholder
Updated launchSettings.json to use the distinct placeholder

Test Infrastructure:

Modified Project.cs to:
- Pass -n parameter when ProjectName is set
- Check if args already contains -n or --name to avoid conflicts
- Added VerifyDnsCompliantHostname method for shared use across test projects
Added overload to ProjectFactoryFixture.CreateProject accepting a project name
Added LocalhostTld constant to ArgConstants

Tests Added (EmptyWeb & BlazorWeb):

Refactored to use ConditionalTheory with InlineData for parameterized testing
Single test method *TemplateLocalhostTld_GeneratesDnsCompliantHostnames covers all scenarios:
- my.namespace.* → my-namespace-* (dots to hyphens)
- .StartWithDot → startwithdot (no leading hyphen)
- EndWithDot. → endwithdot (no trailing hyphen)
- My..Test__Project → my--test--project (multiple invalid chars)
- Project123.Test456 → project123-test456 (numbers preserved per RFC 1123)

Refactoring:

Moved VerifyDnsCompliantHostname from individual test classes to shared Project class
Consolidated 5 separate test methods into 1 parameterized test per template
Improved test maintainability by making it easier to add new test cases
Eliminated code duplication between EmptyWeb and BlazorWeb test classes

Compliance:

✅ RFC 952: Hostnames use letters, digits, and hyphens only
✅ RFC 1123: Allows starting with digits, no underscores
✅ No leading/trailing hyphens in generated hostnames

Original prompt

This section details on the original issue you should resolve

<issue_title>DNS names generated with underscores (_) violate RFC 952/1123 hostname rules during dotnet new</issue_title>
<issue_description>### Is there an existing issue for this?

I have searched the existing issues

Describe the bug

The template/tooling generates DNS names (e.g., hostnames or domain labels) that contain underscores (_). According to DNS hostname specifications (RFC 952 and RFC 1123), underscores are not valid characters in hostnames. This causes failures in environments and libraries that strictly validate DNS names (e.g., Kubernetes, Docker, cloud DNS, and DNS client libraries).

Actual Behavior

Generated names include underscores—e.g., my_service.example.com, MY_SERVICE, or derived labels such as my_service-api. These are rejected by systems enforcing RFC-compliant DNS hostnames.

Impact

Deployment failures in Kubernetes/Docker due to invalid DNS labels.

DNS resolution errors in client libraries.

Operational inconsistencies if environments silently mutate invalid names.

Developer confusion when template output contradicts DNS standards.

References

RFC 952 — DoD Internet Host Table Specification: hostnames use letters, digits, and hyphen; no leading/trailing hyphen.

RFC 1123 — Requirements for Internet Hosts – Application and Support: relaxes RFC 952 to allow starting with a digit; underscores remain invalid for hostnames.

SRV records (underscores in service labels are valid): see service naming like _sip._tcp.example.com.

Additional Notes

If this template also generates SRV/DKIM/DMARC records or service identifiers, underscores are expected only in those service labels—not in hostnames.

Expected Behavior

Generated names should only use:

Letters a–z (case-insensitive)

Digits 0–9

Hyphen -

Additional rules:

Labels must not start or end with a hyphen.

Each label must be ≤ 63 characters.

The full domain must be ≤ 253 characters.

No underscores in hostnames (allowed only in special record labels like SRV/DKIM/DMARC service identifiers, e.g., _sip._tcp).

Steps To Reproduce

Create a new project using localhost tld's and a name including dots (.)
dotnet new web --localhost-tld -n "my.namespace.web"
Exceptions (if any)

No response

.NET Version

10.0.101

Anything else?

No response</issue_description>

<agent_instructions>This is due to these lines in the two effected templates:

aspnetcore/src/ProjectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/BlazorWebCSharp.1/Properties/launchSettings.json

Line 13 in 30d4d50

"applicationUrl": "http://blazorwebcsharp__1.dev.localhost:5500",

aspnetcore/src/ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/Properties/launchSettings.json

Line 9 in 30d4d50

"applicationUrl": "http://company_webapplication1.dev.localhost:5000",

These refer to the template's sourceName value using the "lower case class name" default naming form. This naming form lowercases the value and replaces any characters invalid for use in a C# class name with an underscore.

To fix this the effected templates will need to define custom value forms that are then used by a declared derived symbol that transforms the sourceName into a value safe for use in a host name, then the launchSettings.json files can be updated to use the value declared in the symbol's replaces value.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes DNS names generated with underscores (_) violate RFC 952/1123 hostname rules during dotnet new #64978

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

Co-authored-by: DamianEdwards <[email protected]>

...rojectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/.template.config/template.json

…acement Co-authored-by: DamianEdwards <[email protected]>

Copilot

Pull request overview

This PR fixes DNS-invalid hostname generation in ASP.NET Core templates by replacing underscores with hyphens to comply with RFC 952/1123 specifications when project names contain dots or other invalid DNS characters.

Key Changes:

Added custom value form chain (lowerCaseInvariantWithHyphens) that transforms project names into DNS-compliant hostnames
Created hostName derived symbol that applies the transformation and replaces the LocalhostTldHostNamePrefix placeholder
Updated launchSettings.json files to use the new placeholder instead of hardcoded lowercase class names

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
`src/ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/.template.config/template.json`	Added custom value forms and hostName symbol to transform project names into DNS-compliant hostnames
`src/ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/Properties/launchSettings.json`	Replaced hardcoded lowercase hostname with LocalhostTldHostNamePrefix placeholder for both HTTP and HTTPS profiles
`src/ProjectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/.template.config/template.json`	Added custom value forms and hostName symbol to transform project names into DNS-compliant hostnames
`src/ProjectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/BlazorWebCSharp.1/Properties/launchSettings.json`	Replaced hardcoded lowercase hostname with LocalhostTldHostNamePrefix placeholder for both HTTP and HTTPS profiles

...ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/.template.config/template.json

...rojectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/.template.config/template.json

Co-authored-by: DamianEdwards <[email protected]>

src/ProjectTemplates/test/Templates.Tests/EmptyWebTemplateTest.cs

Co-authored-by: DamianEdwards <[email protected]>

...ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/Properties/launchSettings.json

src/ProjectTemplates/test/Templates.Blazor.Tests/BlazorWebTemplateTest.cs

src/ProjectTemplates/Shared/Project.cs

Co-authored-by: DamianEdwards <[email protected]>

…64988) * Initial plan * Add DNS-safe hostname forms and symbols to templates Co-authored-by: DamianEdwards <[email protected]> * Add trimming of leading/trailing hyphens for DNS-safe hostnames Co-authored-by: DamianEdwards <[email protected]> * Use distinct placeholder LocalhostTldHostNamePrefix for hostname replacement Co-authored-by: DamianEdwards <[email protected]> * Add tests for DNS-compliant hostname generation with --localhost-tld Co-authored-by: DamianEdwards <[email protected]> * Add tests for DNS-compliant hostname generation in Blazor templates Co-authored-by: DamianEdwards <[email protected]> * Move VerifyDnsCompliantHostname to shared Project class Co-authored-by: DamianEdwards <[email protected]> * Fix -n parameter conflict and add tests with numbers in project names Co-authored-by: DamianEdwards <[email protected]> * Refactor DNS hostname tests to use Theory with InlineData Co-authored-by: DamianEdwards <[email protected]> * Fix razorcomponent item test --------- Co-authored-by: copilot-swe-agent[bot] <[email protected]> Co-authored-by: DamianEdwards <[email protected]> Co-authored-by: Damian Edwards <[email protected]>

…stnames in launch profiles (#65048) * Fix DNS-invalid hostnames with underscores in dotnet new templates (#64988) * Initial plan * Add DNS-safe hostname forms and symbols to templates Co-authored-by: DamianEdwards <[email protected]> * Add trimming of leading/trailing hyphens for DNS-safe hostnames Co-authored-by: DamianEdwards <[email protected]> * Use distinct placeholder LocalhostTldHostNamePrefix for hostname replacement Co-authored-by: DamianEdwards <[email protected]> * Add tests for DNS-compliant hostname generation with --localhost-tld Co-authored-by: DamianEdwards <[email protected]> * Add tests for DNS-compliant hostname generation in Blazor templates Co-authored-by: DamianEdwards <[email protected]> * Move VerifyDnsCompliantHostname to shared Project class Co-authored-by: DamianEdwards <[email protected]> * Fix -n parameter conflict and add tests with numbers in project names Co-authored-by: DamianEdwards <[email protected]> * Refactor DNS hostname tests to use Theory with InlineData Co-authored-by: DamianEdwards <[email protected]> * Fix razorcomponent item test --------- Co-authored-by: copilot-swe-agent[bot] <[email protected]> Co-authored-by: DamianEdwards <[email protected]> Co-authored-by: Damian Edwards <[email protected]> * Add replaceRepeatedHyphens step to template processing and update tests for DNS-compliant hostnames (#65027) Related to #64978 --------- Co-authored-by: Copilot <[email protected]> Co-authored-by: DamianEdwards <[email protected]>

Initial plan

ee22359

Copilot AI assigned Copilot and DamianEdwards Jan 9, 2026

Copilot started work on behalf of DamianEdwards January 9, 2026 02:10 View session

github-actions bot added the needs-area-label Used by the dotnet-issue-labeler to label those issues which couldn't be triaged automatically label Jan 9, 2026

Copilot AI and others added 2 commits January 9, 2026 02:17

Add DNS-safe hostname forms and symbols to templates

5b1e1e3

Co-authored-by: DamianEdwards <[email protected]>

Add trimming of leading/trailing hyphens for DNS-safe hostnames

f48b724

Co-authored-by: DamianEdwards <[email protected]>

DamianEdwards reviewed Jan 9, 2026

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...rojectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/.template.config/template.json Outdated Show resolved Hide resolved

Copilot AI changed the title ~~[WIP] Fix DNS names generated with underscores in dotnet new~~ Fix DNS-invalid hostnames with underscores in dotnet new templates Jan 9, 2026

Copilot AI requested a review from DamianEdwards January 9, 2026 02:33

Copilot finished work on behalf of DamianEdwards January 9, 2026 02:33

Copilot started work on behalf of DamianEdwards January 9, 2026 02:33 View session

Use distinct placeholder LocalhostTldHostNamePrefix for hostname repl…

80a5e82

…acement Co-authored-by: DamianEdwards <[email protected]>

Copilot finished work on behalf of DamianEdwards January 9, 2026 02:39

DamianEdwards marked this pull request as ready for review January 9, 2026 06:11

Copilot AI review requested due to automatic review settings January 9, 2026 06:11

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DamianEdwards requested review from BrennanConroy and halter73 January 9, 2026 06:12

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...ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/.template.config/template.json Show resolved Hide resolved

...rojectTemplates/Web.ProjectTemplates/content/BlazorWeb-CSharp/.template.config/template.json Show resolved Hide resolved

Copilot started work on behalf of DamianEdwards January 9, 2026 16:20 View session

DamianEdwards self-requested a review January 9, 2026 16:23

Add tests for DNS-compliant hostname generation with --localhost-tld

4cc41eb

Co-authored-by: DamianEdwards <[email protected]>

Copilot finished work on behalf of DamianEdwards January 9, 2026 16:31

Copilot started work on behalf of DamianEdwards January 9, 2026 16:32 View session

Add tests for DNS-compliant hostname generation in Blazor templates

cac285e

Co-authored-by: DamianEdwards <[email protected]>

Copilot finished work on behalf of DamianEdwards January 9, 2026 16:39

DamianEdwards reviewed Jan 9, 2026

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src/ProjectTemplates/test/Templates.Tests/EmptyWebTemplateTest.cs Outdated Show resolved Hide resolved

Copilot started work on behalf of DamianEdwards January 9, 2026 18:37 View session

Move VerifyDnsCompliantHostname to shared Project class

4461e9d

Co-authored-by: DamianEdwards <[email protected]>

Copilot AI requested a review from DamianEdwards January 9, 2026 18:46

Copilot finished work on behalf of DamianEdwards January 9, 2026 18:46

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...ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/Properties/launchSettings.json Show resolved Hide resolved

src/ProjectTemplates/test/Templates.Blazor.Tests/BlazorWebTemplateTest.cs Show resolved Hide resolved

DamianEdwards reviewed Jan 9, 2026

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src/ProjectTemplates/Shared/Project.cs Show resolved Hide resolved

Copilot started work on behalf of DamianEdwards January 9, 2026 22:49 View session

Fix -n parameter conflict and add tests with numbers in project names

ec21b91

Co-authored-by: DamianEdwards <[email protected]>

Copilot AI requested a review from DamianEdwards January 9, 2026 22:58

Copilot finished work on behalf of DamianEdwards January 9, 2026 22:58

Copilot started work on behalf of DamianEdwards January 9, 2026 22:58 View session

DamianEdwards requested a review from BrennanConroy January 9, 2026 23:04

Refactor DNS hostname tests to use Theory with InlineData

d2c69f8

Co-authored-by: DamianEdwards <[email protected]>

Copilot finished work on behalf of DamianEdwards January 9, 2026 23:06

Copilot AI mentioned this pull request Jan 9, 2026

Fix DNS-invalid hostnames with underscores in templates using --localhost-tld dotnet/aspire#13825

Merged

10 tasks

Fix razorcomponent item test

71c2e9c

BrennanConroy approved these changes Jan 10, 2026

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build-analysis bot mentioned this pull request Jan 10, 2026

Known Build Error: error : Cannot create a file when that file already exists. #61178

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[main] Source code updates from dotnet/aspnetcore dotnet/dotnet#4189

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[dkurepa/test] Source code updates from maestro-auth-test/aspnetcore maestro-auth-test/dotnet#236

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[dkurepa/test] Source code updates from maestro-auth-test/aspnetcore maestro-auth-test/dotnet#237

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Fix DNS-invalid hostnames with underscores in dotnet new templates#64988

Fix DNS-invalid hostnames with underscores in dotnet new templates#64988
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Fix DNS names with underscores in dotnet new templates

Summary:

Changes Made:

Technical Implementation:

Compliance:

Describe the bug

Actual Behavior

Impact

References

Additional Notes

Expected Behavior

Steps To Reproduce

Exceptions (if any)

.NET Version

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Copilot AI commented Jan 9, 2026 •

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