Fetched 28.1 MB in 11s (2344 kB/s)
W: GPG error: http://archive.canonical.com xenial Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://mirrors.aliyun.com xenial Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://mirrors.aliyun.com xenial-security Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://mirrors.aliyun.com xenial-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://mirrors.aliyun.com xenial-backports Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://mirrors.aliyun.com xenial-proposed Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://mirrors.aliyun.com xenial Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
问题处理
apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5
来源：oschina
链接：https://my.oschina.net/u/4288933/blog/3746411