need sources.list example for lan with approx-server

[rlharris]

Would someone kindly point me to (or email me) an example sources.list 
for machines running Debian-9 (Stretch) in a LAN with an approx server?

Following the installation of Stretch on a machine in the LAN, I used 
Synaptic to add packages, but pressing the RELOAD button of Synaptic 
produces the following error message:


http://192.168.1.40:9999/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.http://deb.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.http://192.168.1.40:9999/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.Failed to fetch 
http://192.168.1.40:9999/debian/dists/stretch-updates/main/source/Sources.xz
Hash Sum mismatch
<snip>


The approx server is running a recent installation of Stretch, and was 
used as a mirror for installation of the machine which now is generating 
the error message.  I then attempted to edit the sources.list created by 
the installer on the complaining machine, and in the process I overwrote 
the version left by the installer.