root@debian:/# apt update
Hit:1 http://security.debian.org/debian-security stretch/updates InRelease
Ign:2 http://deb.debian.org/debian stretch InRelease
Hit:3 http://deb.debian.org/debian stretch-updates InRelease
Hit:4 http://deb.debian.org/debian stretch Release
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
W: http://security.debian.org/debian-security/dists/stretch/updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
W: http://deb.debian.org/debian/dists/stretch-updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
W: http://deb.debian.org/debian/dists/stretch/Release.gpg: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.

----
/etc/apt/sources.list:

root@debian:/# cat /etc/apt/sources.list
#

# deb cdrom:[Debian GNU/Linux 9.4.0 _Stretch_ - Official amd64 NETINST 20180310-11:21]/ stretch main

# deb cdrom:[Debian GNU/Linux 9.4.0 _Stretch_ - Official amd64 NETINST 20180310-11:21]/ stretch main

deb http://deb.debian.org/debian/ stretch main non-free contrib
deb-src http://deb.debian.org/debian/ stretch main non-free contrib

deb http://security.debian.org/debian-security stretch/updates main non-free contrib
deb-src http://security.debian.org/debian-security stretch/updates main non-free contrib

# stretch-updates, previously known as 'volatile'
deb http://deb.debian.org/debian/ stretch-updates main non-free contrib
deb-src http://deb.debian.org/debian/ stretch-updates main non-free contrib