# Fix hostname validation in SCMU

# Fix hostname validation in SCMU

## Summary

This PR completes the fix for issue #5190 by adding proper hostname validation to ServiceControl.Config. While PR #5238 partially addressed the issue by allowing dashes in hostnames, it didn't implement comprehensive hostname validation, leaving users able to enter invalid hostnames that would cause configuration issues.

## Background

**Original Issue**: #5190 reported that ServiceControl.Config was not properly validating hostname fields, allowing users to enter invalid values that would cause runtime failures.

**Previous Fix**: PR #5238 addressed part of the problem by allowing dashes in hostnames, but the validation was still incomplete and didn't prevent other invalid hostname formats.

**This Fix**: Implements comprehensive RFC-compliant hostname validation using .NET's built-in `Uri.CheckHostName()` method.

## Validation Behavior

The new validation accepts:
- ✅ Valid DNS hostnames (server01, myhost.domain.com)
- ✅ localhost
- ✅ Hostnames with dashes (server-01, my-host.domain.com) - *previously fixed by PR #5238*

Author: Christian

src/ServiceControl.Config/UI/InstanceAdd/ServiceControlAddViewModelValidator.cs

@@ -52,6 +52,7 @@ public ServiceControlAddViewModelValidator()
 
             RuleFor(viewModel => viewModel.ErrorHostName)
                 .NotEmpty()
+                .ValidHostname()
                 .When(viewModel => viewModel.InstallErrorInstance);
 
             RuleFor(x => x.ErrorPortNumber)
@@ -155,6 +156,7 @@ public ServiceControlAddViewModelValidator()
 
             RuleFor(viewModel => viewModel.AuditHostName)
                 .NotEmpty()
+                .ValidHostname()
                 .When(viewModel => viewModel.InstallAuditInstance);
 
             RuleFor(x => x.AuditPortNumber)

src/ServiceControl.Config/UI/InstanceEdit/ServiceControlAuditEditViewModelValidator.cs

@@ -14,6 +14,7 @@ public ServiceControlAuditEditViewModelValidator()
 
             RuleFor(x => x.HostName)
                 .NotEmpty()
+                .ValidHostname()
                 .When(x => x.SubmitAttempted);
 
             RuleFor(x => x.PortNumber)

src/ServiceControl.Config/UI/InstanceEdit/ServiceControlEditViewModelValidator.cs

@@ -14,6 +14,7 @@ public ServiceControlEditViewModelValidator()
 
             RuleFor(x => x.HostName)
                 .NotEmpty()
+                .ValidHostname()
                 .When(x => x.SubmitAttempted);
 
             RuleFor(x => x.PortNumber)

src/ServiceControl.Config/UI/SharedInstanceEditor/SharedMonitoringEditorViewModelValidator.cs

@@ -9,6 +9,7 @@ protected SharedMonitoringEditorViewModelValidator()
         {
             RuleFor(x => x.HostName)
                 .NotEmpty()
+                .ValidHostname()
                 .When(x => x.SubmitAttempted);
 
             RuleFor(x => x.LogPath)

src/ServiceControl.Config/UI/Upgrades/AddNewAuditInstanceViewModelValidator.cs

@@ -60,6 +60,11 @@ public AddNewAuditInstanceViewModelValidator()
                 .NotEmpty()
                 .NotEqual(x => x.ServiceControlAudit.AuditQueueName).WithMessage(string.Format(Validation.Validations.MSG_UNIQUEQUEUENAME, "Audit"))
                 .When(x => x.SubmitAttempted && (x.ServiceControlAudit.AuditForwarding?.Value ?? false));
+
+            RuleFor(x => x.ServiceControlAudit.HostName)
+                .NotEmpty()
+                .ValidHostname()
+                .When(x => x.SubmitAttempted);
         }
     }
 }

src/ServiceControl.Config/Validation/Validations.cs

@@ -202,6 +202,14 @@ public static IRuleBuilderOptions<T, string> MustBeUniqueWindowsServiceName<T>(t
             });
         }
 
+        public static IRuleBuilderOptions<T, string> ValidHostname<T>(this IRuleBuilder<T, string> ruleBuilder)
+        {
+            return ruleBuilder.Must((t, hostname) =>
+                    !string.IsNullOrWhiteSpace(hostname) &&
+                    Uri.CheckHostName(hostname) == UriHostNameType.Dns)
+                .WithMessage(MSG_INVALID_HOSTNAME);
+        }
+
         public const string MSG_EMAIL_NOT_VALID = "Not Valid.";
 
         public const string MSG_THIS_TRANSPORT_REQUIRES_A_CONNECTION_STRING = "This transport requires a connection string.";
@@ -234,6 +242,8 @@ public static IRuleBuilderOptions<T, string> MustBeUniqueWindowsServiceName<T>(t
 
         public const string MSG_ILLEGAL_PATH_CHAR = "Paths cannot contain characters {0}";
 
+        public const string MSG_INVALID_HOSTNAME = "Hostname is not valid.";
+
         static char[] ILLEGAL_PATH_CHARS =
         {
             '*',