我在 Ubuntu 16.04 中使用来自 Debian 多媒体的未签名存储库：
deb http://www.deb-multimedia.org jessie main
要安装deb-multimedia-keyring，我正在运行：
apt-get update && apt-get install deb-multimedia-keyring -y
这给出了一个错误：
W: GPG error: http://www.deb-multimedia.org jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5C808C2B65558117
E: The repository 'http://www.deb-multimedia.org jessie InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
我在尝试更新我的包裹列表时遇到的错误。
W: GPG error: http://download.opensuse.org ./ Release: The following signatures were invalid: KEYEXPIRED 1436387333
W: Failed to fetch http://ppa.launchpad.net/samrog131/ppa/ubuntu/dists/trusty/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
我正在我的 Ubuntu-12.04 系统上安装 mongo DB，但是当我在终端中给出以下命令时
apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10
它给了我以下错误
gpg: requesting key 7F0CEB10 from hkp server keyserver.ubuntu.com
gpg: keyserver timed out
gpg: keyserver receive failed: keyserver error
有没有其他安装mongo db的方法或者可以做些什么来避免上述情况？