每当我运行 apt-get update 和 upgrade 时，我都会收到各种不同的错误消息。
有很多 503 服务不可用，但我可以 ping IP 地址并获得回复。此外，gnutls_handshake() 失败：pull 函数出错。
以下是我在 apt-get update 结束时遇到的错误
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/main/binary-amd64/Packages 503 Service Unavailable [IP: 91.189.88.152 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial/main/binary-amd64/Packages 503 Service Unavailable [IP: 91.189.88.161 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-updates/main/binary-amd64/Packages 503 Service Unavailable [IP: 91.189.88.161 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-backports/main/binary-amd64/Packages 503 Service Unavailable [IP: 91.189.88.161 80]
E: Failed to fetch http://dl.concerto-signage.org/packages/dists/jessie/main/binary-amd64/Packages 503 Service Unavailable [IP: 173.232.104.112 80]
E: Failed to fetch https://oss-binaries.phusionpassenger.com/apt/passenger/dists/saucy/main/binary-amd64/Packages gnutls_handshake() failed: Error in the pull function.
E: Failed to fetch https://oss-binaries.phusionpassenger.com/apt/passenger/dists/xenial/main/binary-amd64/Packages gnutls_handshake() failed: Error in the pull function.
E: Some index files failed to download. They have been ignored, or old ones used instead.
如您能提供任何帮助我将非常感激。