com.mojang.authlib.exceptions.AuthenticationUnavailableException: Cannot contact authentication server

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Check if ip adress is correct, if the port is correct, and if the server is up and running and waiting for connections. Also, check if there's no firewall blocking you and if your internet connection is working.

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via GitHub by rawlingsj

host port isnt set

com.mojang.authlib.exceptions.AuthenticationUnavailableException: Cannot contact authentication server
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:579)
at sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:635)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:527)
at sun.net.www.protocol.https.HttpsClient.(HttpsClient.java:264)
at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:367)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:191)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:977)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:177)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1093)
at com.mojang.authlib.HttpAuthenticationService.performPostRequest(HttpAuthenticationService.java:73)
at com.mojang.authlib.yggdrasil.YggdrasilAuthenticationService.makeRequest(YggdrasilAuthenticationService.java:54)
at com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication.logInWithToken(YggdrasilUserAuthentication.java:132)
at com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication.logIn(YggdrasilUserAuthentication.java:55)
at net.minecraft.launcher.Launcher.ensureLoggedIn(Launcher.java:215)
at net.minecraft.launcher.Launcher$2.run(Launcher.java:169)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

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