APT

To limit download speed to 250kBytes/s:

/etc/apt/apt.conf.d/75lowerspeed

Acquire
{
   Queue-mode "access";
   http
   {
      Dl-Limit "250";
   };
};

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 8B48AD6246925553

sudo apt-get install debian-archive-keyring
wget http://ftp.pl.debian.org/debian/pool/main/p/pkg-mozilla-archive-keyring/pkg-mozilla-archive-keyring_1.1_all.deb
sudo dpkg -i pkg-mozilla-archive-keyring_1.1_all.deb
sudo apt-get update

gpg --keyserver pgpkeys.mit.edu --recv-key  8B48AD6246925553      
gpg -a --export 8B48AD6246925553 | sudo apt-key add -

apt-cache policy lxc

lxc:
  Installed: 1:1.0.6-6+deb8u2
  Candidate: 1:1.1.5-1
  Version table:
     1:1.1.5-1 0
        500 http://ftp.pl.debian.org/debian/ stretch/main amd64 Packages
 *** 1:1.0.6-6+deb8u2 0
        100 /var/lib/dpkg/status

apt-get install backupninja=0.9.8.1-1
apt-mark hold backupninja

List version and priorities:

apt-cache policy pulseaudio

apt-get install -t jessie-backports pulseaudio

aptitude -t jessie-backports

Error:

http://httpredir.debian.org squeeze-lts Release: The following signatures were invalid: BADSIG 8B48AD6246925553 Debian Archive Automatic Signing Key (7.0/wheezy) <ftpmaster@debian.org>

cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get update

This will rebuild the cache.

Error:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 13B00F1FD2C19886

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key 13B00F1FD2C19886