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問題 #119
Which of the following IPv4 networks are reserved by IANA for private address assignment and private routing? (Choose THREE correct answers.)
答案：B,D,E
問題 #120
CORRECT TEXT
Which directory holds the files that configure the xinetd service when using several configuration files instead of an integrated configuration file? (Specify the full path to the directory.)
答案：
解題說明：
/etc/xinetd.d/, /etc/xinetd.d
問題 #121
Task
Persistently configure the OpenSSH daemon to allow keyboard-interactive authentication for user jack.
* Do not modify the user's password.
* Do not modify any other user's ability to log in.
答案：
解題說明：
See the complete solution in Explanation.
Explanation:
Step 1: Connect to the correct Linux server.
The video specifies that this task must be performed on host lfcs00073.
From the candidate terminal, execute:
ssh candidate@lfcs00073
This establishes an SSH connection to the designated Linux server.
All subsequent configuration changes must be performed on this host.
Step 2: Verify that user jack exists.
Execute:
id jack
This displays the user's UID, primary group and supplementary groups.
An example of valid output is:
uid=1001(jack) gid=1001(jack) groups=1001(jack)
The numerical IDs may differ.
You can also verify the account through the system user database:
getent passwd jack
The task requires modifying SSH authentication, not creating or modifying the user account.
Do not execute passwd jack or change the user's existing password.
Step 3: Identify the OpenSSH daemon configuration file.
The primary OpenSSH server configuration file on Ubuntu is:
/etc/ssh/sshd_config
Additionally, OpenSSH may load configuration fragments from:
/etc/ssh/sshd_config.d/
Inspect the primary configuration file:
sudo cat /etc/ssh/sshd_config
Check whether additional configuration files exist:
sudo ls -la /etc/ssh/sshd_config.d/
The effective OpenSSH configuration may depend on directives in both locations.
Step 4: Identify the existing authentication settings.
Execute:
sudo grep -RniE \
'KbdInteractiveAuthentication|ChallengeResponseAuthentication|UsePAM|AuthenticationMethods|Match' \
/etc/ssh/sshd_config /etc/ssh/sshd_config.d/ 2 > /dev/null
This searches for authentication-related directives.
The most important directive is:
KbdInteractiveAuthentication
It controls whether the SSH daemon permits keyboard-interactive authentication.
Possible values:
Configuration
Meaning
KbdInteractiveAuthentication yes
Permits keyboard-interactive authentication
KbdInteractiveAuthentication no
Disables keyboard-interactive authentication
On Ubuntu 22.04, the packaged OpenSSH configuration commonly disables keyboard-interactive authentication globally while enabling PAM.
OpenSSH daemon configuration file
However, the task requires the method to be enabled specifically for jack, without changing other users' login ability.
Therefore, simply changing the global directive from no to yes is not the appropriate solution.
Step 5: Examine the effective configuration before changing anything.
Execute:
sudo sshd -T \
-C user=jack,host=localhost,addr=127.0.0.1
This displays the effective SSH daemon configuration for a connection matching user jack.
To filter the relevant settings:
sudo sshd -T \
-C user=jack,host=localhost,addr=127.0.0.1 \
| grep -E 'kbdinteractiveauthentication|usepam|authenticationmethods'
For example, an initial configuration might show:
kbdinteractiveauthentication no
usepam yes
authenticationmethods any
This is an illustrative output. The actual values must be checked on the server.
Also record the same settings for another existing user, such as candidate:
sudo sshd -T \
-C user=candidate,host=localhost,addr=127.0.0.1 \
| grep -E 'kbdinteractiveauthentication|usepam|authenticationmethods'
These baseline values will be useful when verifying that other users remain unaffected.
Step 6: Back up the existing OpenSSH configuration.
Before modifying the daemon configuration, execute:
sudo cp -a /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
* cp copies the configuration file.
* -a preserves ownership, permissions and other file metadata.
* /etc/ssh/sshd_config.bak is the backup copy.
If a configuration error occurs, the original file can be restored.
Keep the existing SSH session open while making these changes.
Step 7: Edit the OpenSSH daemon configuration.
Open the configuration file:
sudo nano /etc/ssh/sshd_config
Alternatively, use Vim:
sudo vim /etc/ssh/sshd_config
Review any existing Match directives before adding the required configuration.
Step 8: Configure keyboard-interactive authentication specifically for jack.
Provided no earlier matching block takes precedence, append the following configuration at the end of /etc/ssh
/sshd_config:
Match User jack
KbdInteractiveAuthentication yes
This is the central configuration required for the task.
Match User jack
The Match directive begins a conditional configuration section.
The User jack condition means that the directives inside this section apply to connections authenticating as jack.
KbdInteractiveAuthentication yes
This enables keyboard-interactive authentication for connections satisfying the preceding Match condition.
Other users do not match User jack, so they retain their existing global or user-specific settings.
OpenSSH's official documentation permits KbdInteractiveAuthentication inside a Match section.
OpenSSH daemon configuration file
Important: If the existing configuration already contains a matching Match User jack or Match All block setting this option, edit the appropriate existing rule instead. OpenSSH generally uses the first occurrence of a directive among satisfied Match blocks. Simply appending a later block may not override an earlier matching rule.
Step 9: Save the configuration file.
If using Nano:
* Press Ctrl + O to save.
* Press Enter to confirm.
* Press Ctrl + X to exit.
If using Vim, press Esc, type :wq, and press Enter.
The modified file should contain the user-specific rule:
Match User jack
KbdInteractiveAuthentication yes
Do not add a global AllowUsers jack rule. That could prevent other users from logging in and would violate the task.
Step 10: Verify the PAM authentication setting.
Execute:
sudo sshd -T \
-C user=jack,host=localhost,addr=127.0.0.1 \
| grep usepam
For the normal Ubuntu PAM-based authentication setup, the expected configuration is:
usepam yes
UsePAM yes allows OpenSSH to use PAM for keyboard-interactive authentication.
On Ubuntu, this directive normally appears in the global configuration.
Do not place UsePAM inside the Match User jack block. It is not a valid Match-specific directive in the relevant OpenSSH configuration.
If PAM is globally disabled, investigate the existing authentication design before changing it. Enabling PAM globally could affect other users, contrary to the task's constraints.
Step 11: Validate the SSH configuration syntax.
Before reloading the daemon, execute:
sudo sshd -t
* sshd is the OpenSSH server daemon.
* -t validates the configuration and checks relevant server key requirements.
A successful validation normally produces no output and returns exit code 0.
Check the result:
echo $?
Expected:
0
If errors are displayed, correct them before proceeding.
Do not restart or reload SSH with an invalid configuration.
Step 12: Verify that keyboard-interactive authentication is enabled for jack.
Execute:
sudo sshd -T \
-C user=jack,host=localhost,addr=127.0.0.1 \
| grep kbdinteractiveauthentication
Expected output:
kbdinteractiveauthentication yes
This confirms the effective server configuration for the specified user and connection context.
Unlike simply inspecting the configuration file, sshd -T -C evaluates applicable Match rules.
OpenSSH daemon
Step 13: Confirm that other users' authentication settings have not changed.
Execute:
sudo sshd -T \
-C user=candidate,host=localhost,addr=127.0.0.1 \
| grep -E 'kbdinteractiveauthentication|usepam|authenticationmethods'
Compare the output with the baseline recorded in Step 5.
If the original configuration had keyboard-interactive authentication disabled for candidate, it should remain disabled.
For example:
kbdinteractiveauthentication no
usepam yes
authenticationmethods any
The important requirement is that other users retain their original effective authentication settings, not that every server necessarily produces these example values.
Also verify any existing AuthenticationMethods policy. If it requires a method sequence that excludes standalone keyboard-interactive authentication, simply enabling KbdInteractiveAuthentication may not permit a keyboard-interactive-only login. Any necessary adjustment must remain specific to jack.
Step 14: Reload the OpenSSH daemon.
After verifying the configuration, execute:
sudo systemctl reload ssh
On Ubuntu, the systemd service is normally named ssh.
Reloading causes the daemon to apply its updated configuration without unnecessarily terminating established SSH sessions.
Step 15: Verify the SSH service is operational.
Execute:
sudo systemctl status ssh --no-pager
Expected active status:
Active: active (running)
You can also execute:
systemctl is-active ssh
Expected:
active
Step 16: Test keyboard-interactive authentication for jack.
From a separate SSH session, execute:
ssh -o PreferredAuthentications=keyboard-interactive \
-o PubkeyAuthentication=no \
-o PasswordAuthentication=no \
jack@localhost
* PreferredAuthentications=keyboard-interactive selects keyboard-interactive authentication.
* PubkeyAuthentication=no prevents the client from authenticating using a public key.
* PasswordAuthentication=no disables the separate SSH password authentication method, without disabling keyboard-interactive prompts.
* jack@localhost attempts to connect as user jack.
If PAM supports the account and the user supplies the correct existing credentials, the server should permit keyboard-interactive authentication.
If the server uses a multi-factor AuthenticationMethods policy, adapt the test to that policy and confirm keyboard-interactive authentication is offered at the appropriate stage.
Do not modify jack's password to perform the test.
Step 17: Perform final configuration checks.
Execute:
sudo sshd -t
Verify the setting for jack:
sudo sshd -T \
-C user=jack,host=localhost,addr=127.0.0.1 \
| grep kbdinteractiveauthentication
Expected:
kbdinteractiveauthentication yes
Verify another user's setting:
sudo sshd -T \
-C user=candidate,host=localhost,addr=127.0.0.1 \
| grep kbdinteractiveauthentication
The value must remain consistent with that user's original configuration.
Finally:
systemctl is-active ssh
Expected:
active
Final Solution
1. Connect to the correct host.
ssh candidate@lfcs00073
2. Back up the SSH configuration.
sudo cp -a /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
3. Edit the configuration file.
sudo nano /etc/ssh/sshd_config
4. Add the required configuration at the appropriate location, after checking existing Match rules.
Match User jack
KbdInteractiveAuthentication yes
5. Validate the SSH configuration.
sudo sshd -t
6. Verify the effective setting for jack.
sudo sshd -T \
-C user=jack,host=localhost,addr=127.0.0.1 \
| grep kbdinteractiveauthentication
Expected:
kbdinteractiveauthentication yes
7. Reload the SSH daemon.
sudo systemctl reload ssh
8. Confirm that SSH remains active.
systemctl is-active ssh
Expected:
active
Technical Verification and References
The solution has been checked against the official OpenSSH documentation for Ubuntu.
* Match User applies specified configuration directives to connections matching a particular username.
* KbdInteractiveAuthentication controls whether keyboard-interactive authentication is permitted.
* UsePAM enables PAM integration used for keyboard-interactive authentication on typical Ubuntu systems.
* sshd -t validates server configuration syntax.
* sshd -T -C evaluates effective configuration settings for a particular connection, including applicable Match sections.
Official references:
* Ubuntu OpenSSH sshd_config(5)
* Ubuntu OpenSSH sshd(8)
* OpenSSH Configuration Reference
Expected result: Keyboard-interactive authentication is persistently permitted for user jack, without changing the user's password or altering other users' existing login settings. Successful end-to-end authentication must also be confirmed with a valid login test.
問題 #122
Which of the following fields can be found in the /etc/group file? (Choose THREE correct answers.)
答案：A,C,D
問題 #123
What is the default nice level when a process is started using the nice command?
答案：A
問題 #124
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