下載 package source code

Step 1. 確認sources.list檔案是否有deb-src
$ cat /etc/apt/sources.list|grep deb-src
deb-src http://opensource.nchc.org.tw/debian/ jessie main
deb-src http://security.debian.org/ jessie/updates main
deb-src http://opensource.nchc.org.tw/debian/ jessie-updates main

Step 2. 下載 package source code
$ apt-get source libc6
# ls -l
total 13136
drwxr-xr-x 73 root root 4096 Apr 8 05:50 glibc-2.19
-rw-r--r-- 1 root root 1043764 Feb 28 10:24 glibc_2.19-18+deb8u4.debian.tar.xz
-rw-r--r-- 1 root root 8238 Feb 28 10:24 glibc_2.19-18+deb8u4.dsc
-rw-r--r-- 1 root root 12387008 Jun 18 2014 glibc_2.19.orig.tar.xz

Ref:https://www.debian.org/doc/manuals/apt-howto/ch-sourcehandling.en.html