feat(ldap): FallbackServers option + boot-time validation

ZB.MOM.WW.Auth/src/ZB.MOM.WW.Auth.Abstractions/Ldap/LdapContracts.cs

@@ -19,6 +19,17 @@ public sealed record LdapOptions
     public string GroupAttribute { get; init; } = "memberOf";
     public int ConnectionTimeoutMs { get; init; } = 10_000;
 
+    /// <summary>
+    /// Ordered fallback LDAP endpoints tried when the primary (and any earlier fallback)
+    /// fails with a system-side error (connect/TLS, service-account bind, or search).
+    /// Each entry is <c>"host"</c> (adopts <see cref="Port"/>) or <c>"host:port"</c>.
+    /// Empty (the default) disables failover entirely — single-endpoint behavior is
+    /// unchanged. The TLS validation policy (<see cref="ServerCertificateValidationCallback"/>
+    /// / <see cref="AllowInsecure"/>) applies to every endpoint. Endpoint preference is
+    /// sticky: the service keeps using the last endpoint that answered until it fails.
+    /// </summary>
+    public IReadOnlyList<string> FallbackServers { get; init; } = Array.Empty<string>();
+
     /// <summary>
     /// Optional hook to harden (or, in dev, relax) TLS server-certificate validation for the
     /// <see cref="LdapTransport.Ldaps"/> / <see cref="LdapTransport.StartTls"/> transports. When

ZB.MOM.WW.Auth/src/ZB.MOM.WW.Auth.Ldap/LdapOptionsValidator.cs

@@ -10,7 +10,7 @@ namespace ZB.MOM.WW.Auth.Ldap;
 /// </summary>
 /// <remarks>
 /// Validation is skipped entirely when <see cref="LdapOptions.Enabled"/> is <c>false</c>
-/// (a disabled provider's connection fields are inert). When enabled, four conditions
+/// (a disabled provider's connection fields are inert). When enabled, five conditions
 /// are enforced:
 /// <list type="bullet">
 /// <item>plaintext transport (<see cref="LdapTransport.None"/>) is rejected unless
@@ -19,7 +19,10 @@ namespace ZB.MOM.WW.Auth.Ldap;
 /// <item><see cref="LdapOptions.SearchBase"/> must be specified (the DN root every
 /// search runs against);</item>
 /// <item><see cref="LdapOptions.ServiceAccountDn"/> must be specified — an empty value
-/// would bind anonymously, defeating the search-then-bind authentication flow.</item>
+/// would bind anonymously, defeating the search-then-bind authentication flow;</item>
+/// <item>every <see cref="LdapOptions.FallbackServers"/> entry must parse as
+/// <c>"host"</c> or <c>"host:port"</c> — a typo'd backup DC must not lie dormant
+/// until the primary outage it exists to survive.</item>
 /// </list>
 /// </remarks>
 public sealed class LdapOptionsValidator : IValidateOptions<LdapOptions>
@@ -68,6 +71,21 @@ public sealed class LdapOptionsValidator : IValidateOptions<LdapOptions>
                 "(e.g. \"cn=svc,dc=example,dc=com\").");
         }
 
+        // Fallback endpoints are walked by LdapAuthService on system-side failures. A
+        // malformed entry would be silently skipped there (the walk degrades to the
+        // parseable subset rather than throwing), so it must be caught at boot instead —
+        // otherwise an operator's typo'd backup DC would only surface as a failed
+        // failover during the primary outage it was meant to survive.
+        foreach (var entry in options.FallbackServers)
+        {
+            if (!Internal.LdapEndpointParser.TryParse(entry, options.Port, out _, out var error))
+            {
+                return ValidateOptionsResult.Fail(
+                    $"{nameof(LdapOptions.FallbackServers)} contains an invalid entry — {error}. " +
+                    "Each entry must be \"host\" or \"host:port\" (port 1–65535).");
+            }
+        }
+
         return ValidateOptionsResult.Success;
     }
 }

ZB.MOM.WW.Auth/tests/ZB.MOM.WW.Auth.Ldap.Tests/LdapOptionsValidatorTests.cs

@@ -88,4 +88,32 @@ public class LdapOptionsValidatorTests
                 ServiceAccountDn = "",
             })
             .Failed);
+
+    [Fact]
+    public void Validator_Succeeds_WithValidFallbackServers() =>
+        Assert.False(new LdapOptionsValidator()
+            .Validate(null, Opts() with { FallbackServers = new[] { "dc2", "dc3:3894" } })
+            .Failed);
+
+    [Theory]
+    [InlineData("")]
+    [InlineData(":636")]
+    [InlineData("dc2:0")]
+    [InlineData("dc2:notaport")]
+    public void Validator_Fails_OnMalformedFallbackServer(string entry)
+    {
+        var result = new LdapOptionsValidator()
+            .Validate(null, Opts() with { FallbackServers = new[] { entry } });
+
+        Assert.True(result.Failed);
+        Assert.Contains(nameof(LdapOptions.FallbackServers), result.FailureMessage);
+    }
+
+    [Fact]
+    public void Validator_Skips_FallbackServers_WhenDisabled() =>
+        // Same inert-when-disabled rule as every other connection field: a disabled
+        // provider never opens a connection, so its fallback list is never walked.
+        Assert.False(new LdapOptionsValidator()
+            .Validate(null, Opts() with { Enabled = false, FallbackServers = new[] { ":::" } })
+            .Failed);
 }