diff --git a/build/tools/before_install.sh b/build/tools/before_install.sh
index 0ad23f4d18..e207adcc7c 100755
--- a/build/tools/before_install.sh
+++ b/build/tools/before_install.sh
@@ -11,6 +11,13 @@ SUDO=sudo
 case $(uname -s) in
     Linux)
         if [ -f /etc/apt/sources.list ]; then
+            # Show information about the repositories and priorities used.
+            echo 'APT sources used:'
+            $SUDO grep --no-messages '^[^#]' /etc/apt/sources.list /etc/apt/sources.list.d/* || true
+            echo 'APT preferences:'
+            $SUDO grep --no-messages '^[^#]' /etc/apt/preferences /etc/apt/preferences.d/* || true
+            echo '--- End of APT files dump ---'
+
             run_apt() {
                 echo "Running apt-get $@"