Within the configuration of the AD plugin, we are attempting to use the feature 'Use Jenkins Internal Database' as a failback should AD not be available. The Help information for what should be configured is a little unclear though - it appears to suggest setting an AD account, which will become synchronised locally.
We have tested configuring it with an AD account with Jenkins admin permissions, then making AD unavailable. We should then be still able to login with that account, but it fails with the below error in the logs.

Jun 12, 2018 8:02:08 PM WARNING hudson.plugins.active_directory.ActiveDirectorySecurityRealm$DescriptorImpl bind
All attempts to login failed for user CN=Administrator,CN=Users,DC=test,DC=testdomain,DC=co,DC=uk
Jun 12, 2018 8:02:19 PM FINE hudson.plugins.active_directory.ActiveDirectorySecurityRealm
Connecting to ldap://test-ad1.test.testdomain.co.uk:3268/
Jun 12, 2018 8:02:39 PM WARNING hudson.plugins.active_directory.ActiveDirectorySecurityRealm$DescriptorImpl bind
Failed to bind to test-ad1.test.testdomain.co.uk:3268
java.net.UnknownHostException: test-ad1.test.testdomain.co.uk
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at sun.reflect.GeneratedMethodAccessor320.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.sun.jndi.ldap.Connection.createSocket(Connection.java:350)
at com.sun.jndi.ldap.Connection.<init>(Connection.java:203)
Caused: javax.naming.CommunicationException: test-ad1.test.testdomain.co.uk:3268 [Root exception is java.net.UnknownHostException: test-ad1.test.testdomain.co.uk]
at com.sun.jndi.ldap.Connection.<init>(Connection.java:216)
at com.sun.jndi.ldap.LdapClient.<init>(LdapClient.java:137)
at com.sun.jndi.ldap.LdapClient.getInstance(LdapClient.java:1615)
at com.sun.jndi.ldap.LdapCtx.connect(LdapCtx.java:2749)
at com.sun.jndi.ldap.LdapCtx.<init>(LdapCtx.java:319)
at com.sun.jndi.ldap.LdapCtxFactory.getUsingURL(LdapCtxFactory.java:192)
at com.sun.jndi.ldap.LdapCtxFactory.getLdapCtxInstance(LdapCtxFactory.java:151)
at hudson.plugins.active_directory.ActiveDirectorySecurityRealm$DescriptorImpl.bind(ActiveDirectorySecurityRealm.java:643)
at hudson.plugins.active_directory.ActiveDirectorySecurityRealm$DescriptorImpl.bind(ActiveDirectorySecurityRealm.java:628)
at hudson.plugins.active_directory.ActiveDirectorySecurityRealm$DescriptorImpl.bind(ActiveDirectorySecurityRealm.java:575)
at hudson.plugins.active_directory.ActiveDirectoryUnixAuthenticationProvider$1.call(ActiveDirectoryUnixAuthenticationProvider.java:358)
at hudson.plugins.active_directory.ActiveDirectoryUnixAuthenticationProvider$1.call(ActiveDirectoryUnixAuthenticationProvider.java:341)
at com.google.common.cache.LocalCache$LocalManualCache$1.load(LocalCache.java:4767)
at com.google.common.cache.LocalCache$LoadingValueReference.loadFuture(LocalCache.java:3568)
at com.google.common.cache.LocalCache$Segment.loadSync(LocalCache.java:2350)
at com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2313)
at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2228)
at com.google.common.cache.LocalCache.get(LocalCache.java:3965)
at com.google.common.cache.LocalCache$LocalManualCache.get(LocalCache.java:4764)
at hudson.plugins.active_directory.ActiveDirectoryUnixAuthenticationProvider.retrieveUser(ActiveDirectoryUnixAuthenticationProvider.java:341)
at hudson.plugins.active_directory.ActiveDirectoryUnixAuthenticationProvider.retrieveUser(ActiveDirectoryUnixAuthenticationProvider.java:304)
at hudson.plugins.active_directory.ActiveDirectoryUnixAuthenticationProvider.retrieveUser(ActiveDirectoryUnixAuthenticationProvider.java:226)
at org.acegisecurity.providers.dao.AbstractUserDetailsAuthenticationProvider.authenticate(AbstractUserDetailsAuthenticationProvider.java:122)
at org.acegisecurity.providers.ProviderManager.doAuthentication(ProviderManager.java:200)
at org.acegisecurity.AbstractAuthenticationManager.authenticate(AbstractAuthenticationManager.java:47)
at org.acegisecurity.ui.webapp.AuthenticationProcessingFilter.attemptAuthentication(AuthenticationProcessingFilter.java:74)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:252)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:90)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:533)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:524)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:190)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1595)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:188)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1253)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:168)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:473)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1564)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:166)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1155)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.Server.handle(Server.java:530)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:347)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:256)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:102)
at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:247)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.produce(EatWhatYouKill.java:140)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:131)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:382)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

Jun 12, 2018 8:02:39 PM WARNING hudson.plugins.active_directory.ActiveDirectorySecurityRealm$DescriptorImpl bind
All attempts to login failed for user CN=Administrator,CN=Users,DC=test,DC=testdomain,DC=co,DC=uk

We have also tested configuring this field with a local admin user from the Jenkins Internal Database with similar results when AD is made unavailable.

There is one scenario where it does appear to initially work. If the 'Enable cache' feature is turned on, it is possible to login with the AD account for the time period of the 'Cache TTL' setting, however as soon as that period has expired it is no longer possible to login with that account.

Could anyone confirm if they have this feature working or if they are seeing a similar issue during testing of AD unavailability?

We are also wondering if anyone has tested it with the 'Cache TTL' setting turned on and consequently generated a false positive during testing without realising it?