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deb http://httpredir.debian.org/debian stretch main contrib non-free
deb-src http://httpredir.debian.org/debian stretch main contrib non-free
deb http://httpredir.debian.org/debian stretch-updates main contrib non-free
deb-src http://httpredir.debian.org/debian stretch-updates main contrib non-free
deb http://security.debian.org/ stretch main
deb-src http://security.debian.org/ stretch main
deb http://security.debian.org/ stretch/updates main
deb-src http://security.debian.org/ stretch/updates main
...
W: Failed to fetch http://security.debian.org/dists/stretch/main/source/Sources 404 Not Found [IP: 128.61.240.73 80]
W: Failed to fetch http://security.debian.org/dists/stretch/main/binary-amd64/Packages 404 Not Found [IP: 128.61.240.73 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.