Prueba con este script, per primero instala Debian-keyring

#!/bin/bash

HOST=ftp.sld.cu

ROOT=debian

SECTION=main,contrib,non-free

DIST=stretch,stretch-updates,stretch-proposed-updates,sid

ARCH=i386,amd64

METHOD=http

TARGET=/var/repo/debian

PROXY=http://tu.proxy:3128/

debmirror \

--progress \

--verbose \

--proxy=$PROXY \

--host=$HOST \

--root=$ROOT \

--section=$SECTION \

--dist=$DIST \

--arch=$ARCH \

--timeout=3600 \

--method=$METHOD \

--postcleanup \

--i18n \

--nosource \

--diff=none \

--ignore-small-errors \

--md5sums \

--ignore-missing-release \

--ignore-release-gpg \

--rsync-extra=none \

--getcontents \

--no-check-gpg \

${TARGET}

exit 0