你有没有想过你的 Linux 机器到底安全不安全?Linux 发行版众多，每个发行版都有自己的默认设置，你在上面运行着几十个版本各异的软件包，还有众多的服务在后台运行，而我们几乎不知道或不关心这些。
要想确定安全态势(指你的 Linux 机器上运行的软件、网络和服务的整体安全状态)，你可以运行几个命令，得到一些零碎的相关信息，但你需要解析的数据量是巨大的。
如果能运行一个工具，生成一份关于机器安全状况的报告，那就好得多了。而幸运的是，有一个这样的软件： Lynis 。它是一个非常流行的开源安全审计工具，可以帮助强化基于 Linux 和 Unix 的系统。根据该项目的介绍：
| “它运行在系统本身，可以进行深入的安全扫描。主要目标是测试安全防御措施，并提供进一步强化系统的提示。它还将扫描一般系统信息、易受攻击的软件包和可能的配置问题。Lynis 常被系统管理员和审计人员用来评估其系统的安全防御。” |
安装 Lynis
你的 Linux 软件仓库中可能有 Lynis。如果有的话，你可以用以下方法安装它：
- dnf install lynis
或
- apt install lynis
然而，如果你的仓库中的版本不是最新的，你最好从 GitHub 上安装它。(我使用的是 Red Hat Linux 系统，但你可以在任何 Linux 发行版上运行它)。就像所有的工具一样，先在虚拟机上试一试是有意义的。要从 GitHub 上安装它：
- $ cat /etc/redhat-release
- Red Hat Enterprise Linux Server release 7.8 (Maipo)
- $
- $ uname -r
- 3.10.0-1127.el7.x86_64
- $
- $ git clone https://github.com/CISOfy/lynis.git
- Cloning into 'lynis'...
- remote: Enumerating objects: 30, done.
- remote: Counting objects: 100% (30/30), done.
- remote: Compressing objects: 100% (30/30), done.
- remote: Total 12566 (delta 15), reused 8 (delta 0), pack-reused 12536
- Receiving objects: 100% (12566/12566), 6.36 MiB | 911.00 KiB/s, done.
- Resolving deltas: 100% (9264/9264), done.
- $
一旦你克隆了这个版本库，那么进入该目录，看看里面有什么可用的。主要的工具在一个叫 lynis 的文件里。它实际上是一个 shell 脚本，所以你可以打开它看看它在做什么。事实上，Lynis 主要是用 shell 脚本来实现的：
- $ cd lynis/
- $ ls
- CHANGELOG.md CONTRIBUTING.md db developer.prf FAQ include LICENSE lynis.8 README SECURITY.md
- CODE_OF_CONDUCT.md CONTRIBUTORS.md default.prf extras HAPPY_USERS.md INSTALL lynis plugins README.md
- $
- $ file lynis
- lynis: POSIX shell script, ASCII text executable, with very long lines
- $
运行 Lynis
通过给 Lynis 一个 -h 选项来查看帮助部分，以便有个大概了解：
- $ ./lynis -h
你会看到一个简短的信息屏幕，然后是 Lynis 支持的所有子命令。
接下来，尝试一些测试命令以大致熟悉一下。要查看你正在使用的 Lynis 版本，请运行：
- $ ./lynis show version
- 3.0.0
- $
要查看 Lynis 中所有可用的命令：
- $ ./lynis show commands
- Commands:
- lynis audit
- lynis configure
- lynis generate
- lynis show
- lynis update
- lynis upload-only
- $
审计 Linux 系统
要审计你的系统的安全态势，运行以下命令：
- $ ./lynis audit system
这个命令运行得很快，并会返回一份详细的报告，输出结果可能一开始看起来很吓人，但我将在下面引导你来阅读它。这个命令的输出也会被保存到一个日志文件中，所以你可以随时回过头来检查任何可能感兴趣的东西。
Lynis 将日志保存在这里：
- Files:
- - Test and debug information : /var/log/lynis.log
- - Report data : /var/log/lynis-report.dat
你可以验证是否创建了日志文件。它确实创建了：
- $ ls -l /var/log/lynis.log
- -rw-r-----. 1 root root 341489 Apr 30 05:52 /var/log/lynis.log
- $
- $ ls -l /var/log/lynis-report.dat
- -rw-r-----. 1 root root 638 Apr 30 05:55 /var/log/lynis-report.dat
- $
探索报告
Lynis 提供了相当全面的报告，所以我将介绍一些重要的部分。作为初始化的一部分，Lynis 做的第一件事就是找出机器上运行的操作系统的完整信息。之后是检查是否安装了什么系统工具和插件：
- [+] Initializing program
- ------------------------------------
- - Detecting OS... [ DONE ]
- - Checking profiles... [ DONE ]
- ---------------------------------------------------
- Program version: 3.0.0
- Operating system: Linux
- Operating system name: Red Hat Enterprise Linux Server 7.8 (Maipo)
- Operating system version: 7.8
- Kernel version: 3.10.0
- Hardware platform: x86_64
- Hostname: example
- ---------------------------------------------------
- <<截断>>
- [+] System Tools
- ------------------------------------
- - Scanning available tools...
- - Checking system binaries...
- [+] Plugins (phase 1)
- ------------------------------------
- Note: plugins have more extensive tests and may take several minutes to complete
- - Plugin: pam
- [..]
- - Plugin: systemd
- [................]
接下来，该报告被分为不同的部分，每个部分都以 [+] 符号开头。下面可以看到部分章节。(哇，要审核的地方有这么多，Lynis 是最合适的工具!)
- [+] Boot and services
- [+] Kernel
- [+] Memory and Processes
- [+] Users, Groups and Authentication
- [+] Shells
- [+] File systems
- [+] USB Devices
- [+] Storage
- [+] NFS
- [+] Name services
- [+] Ports and packages
- [+] Networking
- [+] Printers and Spools
- [+] Software: e-mail and messaging
- [+] Software: firewalls
- [+] Software: webserver
- [+] SSH Support
- [+] SNMP Support
- [+] Databases
- [+] LDAP Services
- [+] PHP
- [+] Squid Support
- [+] Logging and files
- [+] Insecure services
- [+] Banners and identification
- [+] Scheduled tasks
- [+] Accounting
- [+] Time and Synchronization
- [+] Cryptography
- [+] Virtualization
- [+] Containers
- [+] Security frameworks
- [+] Software: file integrity
- [+] Software: System tooling
- [+] Software: Malware
- [+] File Permissions
- [+] Home directories
- [+] Kernel Hardening
- [+] Hardening
- [+] Custom tests
Lynis 使用颜色编码使报告更容易解读。
- 绿色。一切正常
- 黄色。跳过、未找到，可能有个建议
- 红色。你可能需要仔细看看这个
在我的案例中，大部分的红色标记都是在 “Kernel Hardening” 部分找到的。内核有各种可调整的设置，它们定义了内核的功能，其中一些可调整的设置可能有其安全场景。发行版可能因为各种原因没有默认设置这些，但是你应该检查每一项，看看你是否需要根据你的安全态势来改变它的值：
- [+] Kernel Hardening
- ------------------------------------
- - Comparing sysctl key pairs with scan profile
- - fs.protected_hardlinks (exp: 1) [ OK ]
- - fs.protected_symlinks (exp: 1) [ OK ]
- - fs.suid_dumpable (exp: 0) [ OK ]
- - kernel.core_uses_pid (exp: 1) [ OK ]
- - kernel.ctrl-alt-del (exp: 0) [ OK ]
- - kernel.dmesg_restrict (exp: 1) [ DIFFERENT ]
- - kernel.kptr_restrict (exp: 2) [ DIFFERENT ]
- - kernel.randomize_va_space (exp: 2) [ OK ]
- - kernel.sysrq (exp: 0) [ DIFFERENT ]
- - kernel.yama.ptrace_scope (exp: 1 2 3) [ DIFFERENT ]
- - net.ipv4.conf.all.accept_redirects (exp: 0) [ DIFFERENT ]
- - net.ipv4.conf.all.accept_source_route (exp: 0) [ OK ]
- - net.ipv4.conf.all.bootp_relay (exp: 0) [ OK ]
- - net.ipv4.conf.all.forwarding (exp: 0) [ OK ]
- - net.ipv4.conf.all.log_martians (exp: 1) [ DIFFERENT ]
- - net.ipv4.conf.all.mc_forwarding (exp: 0) [ OK ]
- - net.ipv4.conf.all.proxy_arp (exp: 0) [ OK ]
- - net.ipv4.conf.all.rp_filter (exp: 1) [ OK ]
- - net.ipv4.conf.all.send_redirects (exp: 0) [ DIFFERENT ]
- - net.ipv4.conf.default.accept_redirects (exp: 0) [ DIFFERENT ]
- - net.ipv4.conf.default.accept_source_route (exp: 0) [ OK ]
- - net.ipv4.conf.default.log_martians (exp: 1) [ DIFFERENT ]
- - net.ipv4.icmp_echo_ignore_broadcasts (exp: 1) [ OK ]
- - net.ipv4.icmp_ignore_bogus_error_responses (exp: 1) [ OK ]
- - net.ipv4.tcp_syncookies (exp: 1) [ OK ]
- - net.ipv4.tcp_timestamps (exp: 0 1) [ OK ]
- - net.ipv6.conf.all.accept_redirects (exp: 0) [ DIFFERENT ]
- - net.ipv6.conf.all.accept_source_route (exp: 0) [ OK ]
- - net.ipv6.conf.default.accept_redirects (exp: 0) [ DIFFERENT ]
- - net.ipv6.conf.default.accept_source_route (exp: 0) [ OK ]
看看 SSH 这个例子，因为它是一个需要保证安全的关键领域。这里没有什么红色的东西，但是 Lynis 对我的环境给出了很多强化 SSH 服务的建议：
- [+] SSH Support
- ------------------------------------
- - Checking running SSH daemon [ FOUND ]
- - Searching SSH configuration [ FOUND ]
- - OpenSSH option: AllowTcpForwarding [ SUGGESTION ]
- - OpenSSH option: ClientAliveCountMax [ SUGGESTION ]
- - OpenSSH option: ClientAliveInterval [ OK ]
- - OpenSSH option: Compression [ SUGGESTION ]
- - OpenSSH option: FingerprintHash [ OK ]
- - OpenSSH option: GatewayPorts [ OK ]
- - OpenSSH option: IgnoreRhosts [ OK ]
- - OpenSSH option: LoginGraceTime [ OK ]
- - OpenSSH option: LogLevel [ SUGGESTION ]
- - OpenSSH option: MaxAuthTries [ SUGGESTION ]
- - OpenSSH option: MaxSessions [ SUGGESTION ]
- - OpenSSH option: PermitRootLogin [ SUGGESTION ]
- - OpenSSH option: PermitUserEnvironment [ OK ]
- - OpenSSH option: PermitTunnel [ OK ]
- - OpenSSH option: Port [ SUGGESTION ]
- - OpenSSH option: PrintLastLog [ OK ]
- - OpenSSH option: StrictModes [ OK ]
- - OpenSSH option: TCPKeepAlive [ SUGGESTION ]
- - OpenSSH option: UseDNS [ SUGGESTION ]
- - OpenSSH option: X11Forwarding [ SUGGESTION ]
- - OpenSSH option: AllowAgentForwarding [ SUGGESTION ]
- - OpenSSH option: UsePrivilegeSeparation [ OK ]
- - OpenSSH option: AllowUsers [ NOT FOUND ]
- - OpenSSH option: AllowGroups [ NOT FOUND ]
我的系统上没有运行虚拟机或容器，所以这些显示的结果是空的：
- [+] Virtualization
- ------------------------------------
- [+] Containers
- ------------------------------------
Lynis 会检查一些从安全角度看很重要的文件的文件权限：
- [+] File Permissions
- ------------------------------------
- - Starting file permissions check
- File: /boot/grub2/grub.cfg [ SUGGESTION ]
- File: /etc/cron.deny [ OK ]
- File: /etc/crontab [ SUGGESTION ]
- File: /etc/group [ OK ]
- File: /etc/group- [ OK ]
- File: /etc/hosts.allow [ OK ]
- File: /etc/hosts.deny [ OK ]
- File: /etc/issue [ OK ]
- File: /etc/issue.net [ OK ]
- File: /etc/motd [ OK ]
- File: /etc/passwd [ OK ]
- File: /etc/passwd- [ OK ]
- File: /etc/ssh/sshd_config [ OK ]
- Directory: /root/.ssh [ SUGGESTION ]
- Directory: /etc/cron.d [ SUGGESTION ]
- Directory: /etc/cron.daily [ SUGGESTION ]
- Directory: /etc/cron.hourly [ SUGGESTION ]
- Directory: /etc/cron.weekly [ SUGGESTION ]
- Directory: /etc/cron.monthly [ SUGGESTION ]
在报告的底部，Lynis 根据报告的发现提出了建议。每项建议后面都有一个 “TEST-ID”(为了下一部分方便，请将其保存起来)。
- Suggestions (47):
- ----------------------------
- * If not required, consider explicit disabling of core dump in /etc/security/limits.conf file [KRNL-5820]
- https://cisofy.com/lynis/controls/KRNL-5820/
- * Check PAM configuration, add rounds if applicable and expire passwords to encrypt with new values [AUTH-9229]
- https://cisofy.com/lynis/controls/AUTH-9229/
Lynis 提供了一个选项来查找关于每个建议的更多信息，你可以使用 show details 命令和 TEST-ID 号来访问：
- ./lynis show details TEST-ID
这将显示该测试的其他信息。例如，我检查了 SSH-7408 的详细信息：
- $ ./lynis show details SSH-7408
- 2020-04-30 05:52:23 Performing test ID SSH-7408 (Check SSH specific defined options)
- 2020-04-30 05:52:23 Test: Checking specific defined options in /tmp/lynis.k8JwazmKc6
- 2020-04-30 05:52:23 Result: added additional options for OpenSSH < 7.5
- 2020-04-30 05:52:23 Test: Checking AllowTcpForwarding in /tmp/lynis.k8JwazmKc6
- 2020-04-30 05:52:23 Result: Option AllowTcpForwarding found
- 2020-04-30 05:52:23 Result: Option AllowTcpForwarding value is YES
- 2020-04-30 05:52:23 Result: OpenSSH option AllowTcpForwarding is in a weak configuration state and should be fixed
- 2020-04-30 05:52:23 Suggestion: Consider hardening SSH configuration [test:SSH-7408] [details:AllowTcpForwarding (set YES to NO)] [solution:-]
试试吧
如果你想更多地了解你的 Linux 机器的安全性，请试试 Lynis。如果你想了解 Lynis 是如何工作的，可以研究一下它的 shell 脚本，看看它是如何收集这些信息的。
