Sadserver Writeup Vol 1

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Work in progress

“Rio de Janeiro”: Do we have another option?

Description: This scenario server is dedicated to Jenkins, a Java application managed by systemd. Jenkins is failing to start. Troubleshoot and find the problem, then apply the solution so Jenkins runs properly.

Root (sudo) Access: True

Test: The service must return the string “Sign in - Jenkins” amongst some other html code. You can check with the command curl -s localhost:8888/login | grep Jenkins | head -n1

Solution

  1. 初始状态检查

    • 发现问题：Jenkins服务处于inactive状态且被禁用
    • 执行命令：systemctl status jenkins
    • 结果：服务dead且disabled 
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      ○ jenkins.service - Jenkins Continuous Integration Server
          Loaded: loaded (/usr/lib/systemd/system/jenkins.service; disabled; preset: enabled)
          Active: inactive (dead)

  2. 服务启动尝试

    • 执行命令：启用并启动Jenkins服务 
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      systemctl enable jenkins
      systemctl start jenkins
    • 结果：启动失败，控制进程退出并报错 
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      Synchronizing state of jenkins.service with SysV service script with /usr/lib/systemd/systemd-sysv-install.
      Executing: /usr/lib/systemd/systemd-sysv-install enable jenkins
      Created symlink '/etc/systemd/system/multi-user.target.wants/jenkins.service' → '/usr/lib/systemd/system/jenkins.service'.
      Job for jenkins.service failed because the control process exited with error code.
      See "systemctl status jenkins.service" and "journalctl -xeu jenkins.service" for details.

  3. 日志分析

    • 执行命令：journalctl -xeu jenkins.service
    • 关键发现：Java版本兼容性问题
    • 错误信息：UnsupportedClassVersionError: class file version 55.0, this version only recognizes up to 52.0
    • 分析：Jenkins需要Java 11（版本55），但系统当前使用Java 8（版本52） 
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      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]: Exception in thread "main" java.lang.UnsupportedClassVersionError: executable/Main has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.lang.ClassLoader.defineClass1(Native Method)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.lang.ClassLoader.defineClass(ClassLoader.java:756)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.net.URLClassLoader.defineClass(URLClassLoader.java:473)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.security.AccessController.doPrivileged(Native Method)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
      Nov 04 17:21:27 i-097edbdc54cac144a jenkins[1336]:         at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:621)
      Nov 04 17:21:27 i-097edbdc54cac144a systemd[1]: jenkins.service: Main process exited, code=exited, status=1/FAILURE

  4. Java环境检查

    • 执行命令：java -version && update-alternatives --config java
    • 发现：
    • 当前使用：Java 8 (Temurin-8)
    • 系统已安装：Java 21 (openjdk-21) 
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      openjdk version "1.8.0_462"
      OpenJDK Runtime Environment (Temurin)(build 1.8.0_462-b08)
      OpenJDK 64-Bit Server VM (Temurin)(build 25.462-b08, mixed mode)
      There are 2 choices for the alternative java (providing /usr/bin/java).

      Selection    Path                                         Priority   Status
      ------------------------------------------------------------
      0            /usr/lib/jvm/java-21-openjdk-amd64/bin/java   2111      auto mode
      1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java   2111      manual mode
      * 2            /usr/lib/jvm/temurin-8-jdk-amd64/bin/java     1081      manual mode

      Press <enter> to keep the current choice[*], or type selection number:

  5. 还是炸了

  • 相同步骤查看日志： 
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    Nov 04 17:21:31 i-097edbdc54cac144a systemd[1]: jenkins.service: Start request repeated too quickly.
    Nov 04 17:21:31 i-097edbdc54cac144a systemd[1]: jenkins.service: Failed with result 'exit-code'.
    Nov 04 17:21:31 i-097edbdc54cac144a systemd[1]: Failed to start jenkins.service - Jenkins Continuous Integration Server.
  • 分析：systemd对频繁失败的服务有保护机制，需重置失败计数器
  • 执行命令：systemctl reset-failed jenkins
  • 再次启动，启动成功： 
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    Nov 04 17:26:30 i-097edbdc54cac144a jenkins[7743]: 2025-11-04 17:26:30.570+0000 [id=34]        INFO        jenkins.InitReactorRunner$1#onAttained: Completed initialization
    Nov 04 17:26:30 i-097edbdc54cac144a jenkins[7743]: 2025-11-04 17:26:30.599+0000 [id=24]        INFO        hudson.lifecycle.Lifecycle#onReady: Jenkins is fully up and running
    Nov 04 17:26:30 i-097edbdc54cac144a systemd[1]: Started jenkins.service - Jenkins Continuous Integration Server.

根本原因

Java版本不兼容：Jenkins需要Java 11或更高版本，但系统默认使用Java 8。

经验总结

  • Java版本兼容性是Jenkins部署中的常见问题
  • 使用update-alternatives可以灵活管理多个Java版本
  • systemd服务的失败计数器需要重置，以防止频繁失败导致的启动阻塞：systemctl reset-failed <service-name>

“Nuuk”: More SSH Troubles

Level: Easy

Description: SSH seems broken in this server. The user admin has an id_ed25519 SSH key pair in their ~/.ssh directory with the public key in ~/.ssh/authorized_keys but ssh 127.0.0.1 won’t work.

Test: You can ssh locally, i.e. ssh admin@127.0.0.1 works.

Solution

  1. 分析SSH连接详细日志

    • 命令: ssh -vvv admin@127.0.0.1
    • 关键发现:
      • 日志显示Permission denied错误当尝试读取~/.ssh/known_hosts和私钥文件（如id_ed25519）。
      • 具体错误信息：
        • load_hostkeys: fopen /home/admin/.ssh/known_hosts: Permission denied
        • no such identity: /home/admin/.ssh/id_ed25519: Permission denied
    • 结论: 用户admin对~/.ssh目录或文件缺乏读取权限，导致SSH认证失败。 
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      debug1: Trying private key: /home/admin/.ssh/id_rsa
      debug3: no such identity: /home/admin/.ssh/id_rsa: Permission denied
      debug1: Trying private key: /home/admin/.ssh/id_ecdsa
      debug3: no such identity: /home/admin/.ssh/id_ecdsa: Permission denied
      debug1: Trying private key: /home/admin/.ssh/id_ecdsa_sk
      debug3: no such identity: /home/admin/.ssh/id_ecdsa_sk: Permission denied
      debug1: Trying private key: /home/admin/.ssh/id_ed25519
      debug3: no such identity: /home/admin/.ssh/id_ed25519: Permission denied
      debug1: Trying private key: /home/admin/.ssh/id_ed25519_sk
      debug3: no such identity: /home/admin/.ssh/id_ed25519_sk: Permission denied
      debug1: Trying private key: /home/admin/.ssh/id_xmss
      debug3: no such identity: /home/admin/.ssh/id_xmss: Permission denied

  2. 检查目录权限和所有权

    • 命令: whoami && ls -la /home/admin/ | grep .ssh
    • 结果:
      • 当前用户为admin。
      • .ssh目录权限为d---------，所有者为admin。

  3. 修复.ssh目录权限

    • 命令: chmod 700 /home/admin/.ssh
    • 结果: 无输出，表示权限修改成功。
    • 验证: 再次运行ls -la /home/admin/.ssh/确认权限已改为drwx------。

  4. 检查.ssh内部文件权限

    • 命令: ls -la /home/admin/.ssh/
    • 结果: 
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      drwx------ 2 admin admin 4096 Oct 21 17:27 .
      drwx------ 5 admin admin 4096 Sep  7 16:32 ..
      -rw------- 1 admin admin  177 Nov  4 17:55 authorized_keys
      -rw------- 1 admin admin  387 Oct 21 17:27 id_ed25519
      -rw------- 1 admin admin   82 Oct 21 17:27 id_ed25519.pub
    • 分析: 文件权限正确（私钥和authorized_keys应为600）。

根本原因

/home/admin/.ssh目录的权限设置不正确（初始为d---------，可能由于权限位异常），导致用户admin无法读取目录内的私钥和已知主机文件，从而SSH公钥认证失败。修复目录权限为700后，SSH客户端能正常访问所需文件。

经验总结

  • SSH权限要求: SSH对文件和目录权限有严格限制：
    • ~/.ssh目录必须为700（drwx------）。
    • 私钥文件（如id_ed25519）必须为600（-rw-------）。
    • authorized_keys文件通常为600或644（但600更安全）。
  • 诊断技巧: 使用ssh -vvv可快速定位认证问题，如权限错误、密钥路径问题等。
  • 运维最佳实践: 在部署SSH密钥时，应始终验证权限设置，避免因权限过松（安全风险）或过紧（连接失败）导致问题。