nmap 工具 (在同名软件包中) 可以在未登录的前提下，快速地探测一台线上机器上运行着哪些互联网服务。只需在连接到同一网络的另一台计算机上调用以下命令：
$nmap mirwizStarting Nmap 7.80 ( https://nmap.org ) at 2021-04-29 14:41 CEST Nmap scan report for mirwiz (192.168.1.104) Host is up (0.00062s latency). Not shown: 992 closed ports PORT STATE SERVICE 22/tcp open ssh 25/tcp open smtp 80/tcp open http 111/tcp open rpcbind 139/tcp open netbios-ssn 445/tcp open microsoft-ds 5666/tcp open nrpe 9999/tcp open abyss Nmap done: 1 IP address (1 host up) scanned in 0.06 seconds
ps auxw displays a list of all processes with their user identity. By checking this information against the output of the who or w commands, which give a list of logged in users, it is possible to identify rogue or undeclared servers or programs running in the background. Looking at crontabs (tables listing automatic actions scheduled by users) will often provide interesting information on functions fulfilled by the server (a complete explanation of cron is available in 第 9.7 节 “使用 cron 和 atd运行计划任务”).
/etc/ 文件夹内，有时放在 /usr/local/ 的子文件夹里。比如源码安装的程序，配置档就会放在前述的子文件夹内，有时候也被放在 /opt/ 。
/etc/debian_version，通常包括 Debian 系统的版本编号 (它是 base-files 软件包的一部分)。若以 代码/sid 方式呈现，表示此系统从该代码版本取得的最新版软件包 (测试版或不稳定版)。
apt-show-versions 程序 (来自 Debian 同名软件包) 检查已安装软件包的列表并标识可用的版本。aptitude 程序可以做到同样的工作，但是不是特别系统化。
/etc/apt/sources.list 文件 (以及 /etc/apt/sources.list.d/ 文件夹) 可看到已安装 Debian 软件包的来源。若出现不明来源，管理员可能选择重新安装电脑系统确保其软件与 Debian 兼容。
sources.list file is often a good indicator: the majority of administrators keep, at least in comments, the list of APT sources that were previously used. But you should not forget that sources used in the past might have been deleted, and that some random packages grabbed on the Internet might have been manually installed (with the help of the dpkg command). In this case, the machine is misleading in its appearance of being a “standard” Debian system. This is why you should pay attention to any indication that will give away the presence of external packages (appearance of deb files in unusual directories, package version numbers with a special suffix indicating that it originated from outside the Debian project, such as ubuntu or lmde, etc.).
/usr/local/ 文件夹的内容，其目的是收录编译后与手动安装后的程序。列出以此方式安装的软件清单，指出一个问题，为什么不使用 Debian 对应的软件包。
表 3.1. 操作系统与架构对照表
| 操作系统 | 架构 |
|---|---|
| DEC Unix (OSF/1) | alpha, mipsel |
| HP Unix | ia64, hppa |
| IBM AIX | powerpc |
| Irix | mips |
| OS X | amd64, powerpc, i386 |
| z/OS, MVS | s390x, s390 |
| Solaris, SunOS | sparc, i386, m68k |
| Ultrix | mips |
| VMS | alpha |
| Windows 95/98/ME | i386 |
| Windows NT/2000 | i386, alpha, ia64, mipsel |
| Windows XP / Windows Server 2008 | i386, amd64, ia64 |
| Windows RT | armel, armhf, arm64 |
| Windows Vista / Windows 7-8-10 | i386, amd64 |
