If you’re using Matteo Corti’s check_rbl monitoring plugin, brace yourself. It appears that all your servers are now flagged on this defunct DNSBL:

“Mail servers appear listed on DNSBL spamtrap.drbl.drand.net”

Time to act. Adjust your monitoring list of DNSBLs checked by modifying the configuration file.

If you find that all your servers are appearing on a particular DNSBL while using Matteo Corti’s check_rbl monitoring plugin, adjustments are necessary to the list of checked DNSBLs. This can be done either by modifying the -s parameter or by adjusting the configuration (ini) file.

Here’s how to make the necessary changes:

nagios:~$ grep drand /usr/lib/nagios/plugins/rbl.ini
server=spamtrap.drbl.drand.net

To resolve the issue, delete the entry or comment it out using the # symbol in the configuration file. By doing so, the plugin will cease checking this particular DNSBL.

# getfacl file_name

By executing this command, you obtain the permissions associated with the specified file.

Moreover, if you wish to capture permissions recursively across an entire subdirectory and store the output for future reference, you can utilize the following command:

# getfacl directory > /tmp/files.acl

This command not only retrieves permissions for the directory but also for all its subdirectories and files within it, saving the output in a designated file (/tmp/files.acl in this case).

In cases where it becomes necessary to revert permissions to their previous state, perhaps due to system restoration or accidental modifications, the setfacl command comes into play. Here’s how you can restore permissions using a previously saved ACL file:

# setfacl --restore=/tmp/files.acl

This command applies the permissions specified in the ACL file (/tmp/files.acl) to the respective files and directories, effectively restoring their previous access settings.

By employing these commands judiciously, system administrators can maintain better control over access permissions, ensuring the security and integrity of critical files and directories within the system.

Overview:

The goal is to generate a new CA certificate with an extended expiration date while retaining the same public key. By doing so, previously issued certificates remain valid, ensuring uninterrupted service. The key steps involve generating a new CA certificate, updating the public certificate on both the server and client sides, and distributing the updated configurations.

Steps:

1. Generate New CA Certificate:
   • Using the existing CA key and CSR (Certificate Signing Request), generate a new CA certificate with an extended expiration date.

     openssl ca -keyfile existing_ca_key.pem -in csr.pem -out new_ca_cert.pem -days 3650
     

2. Update Server Configuration:
   • Replace the old public certificate in the OpenVPN server configuration with the new one.

     # Replace 'ca old_ca_cert.pem' with 'ca new_ca_cert.pem' in the server configuration file.
     

3. Update Client Configurations:
   • Similarly, replace the old public certificate in the OpenVPN client configurations with the new one.

     # Replace 'ca old_ca_cert.pem' with 'ca new_ca_cert.pem' in the client configuration file(s).
     

4. Distribute Configurations:
   • Distribute the updated server and client configurations to all relevant systems. Ensure that all clients have the latest configurations with the new CA certificate.
5. Test Connectivity:
   • Thoroughly test connectivity to ensure that the OpenVPN infrastructure is functioning correctly with the new certificates. Verify that clients can connect to the server without any issues.
6. Monitor for Issues:
   • Monitor the OpenVPN infrastructure closely for any anomalies or issues following the certificate renewal. Address any issues promptly to minimize disruptions.
7. Implement Certificate Expiry Monitoring:
   • It’s good practice to implement monitoring for certificate expiration to anticipate future renewals. This ensures that you can act well in advance of the CA certificate’s expiration, minimizing the risk of service disruptions. Consider setting up alerts or automated checks to monitor certificate expiry dates.

Benefits:

• Smooth Transition: By retaining the same public key while extending the CA certificate’s expiration date, service disruptions are minimized.
• Simplified Management: Administrators only need to update the CA certificate, reducing complexity during the transition process.
• Continuous Service: Clients can seamlessly continue using the OpenVPN service without interruptions or manual intervention. There is no need for clients to reissue their certificates with the new CA, avoiding mass client certificate reissuance.

Conclusion:

Renewing an OpenVPN CA certificate doesn’t have to be a daunting task. With a practical approach that preserves the existing public key, you can ensure uninterrupted service while maintaining security. By following these steps, you can smoothly transition to a new CA certificate without disrupting operations. Additionally, implementing certificate expiry monitoring ensures proactive management of future renewals.

Feel free to adjust or expand upon this article as needed for your specific audience or context.

Recognizing the urgency to address this issue, we’re taking steps to phase out MD5 support in OpenVPN. However, we understand that transitioning to stronger cryptographic mechanisms may present challenges for some users.

Here’s a condensed version of the transition plan:

1. Awareness: It’s crucial to recognize if your certificates are using MD5 signatures. You can check this using OpenSSL:

    openssl x509 -in ca.crt -noout -text | grep "Signature Algorithm"
   

   If MD5 is detected, it’s recommended to switch to SHA256 or better.

2. Transitional Period: We’ve decided to implement a transitional period until May 2018, during which MD5-signed certificates will still function. However, users are strongly encouraged to migrate to stronger alternatives.

3. Workarounds: If transitioning immediately isn’t feasible, there are temporary workarounds. One option is to adjust the security level in the OpenVPN configuration file (*.ovpn) on the client side:

    tls-cipher "DEFAULT:@SECLEVEL=0"
   

   Note: Setting SECLEVEL=0 allows any algorithm, including MD5. While this is not recommended for long-term security, it can serve as a temporary measure if needed.

4. Support and Resources: For users facing challenges during the transition, we’re committed to providing support and guidance. Resources such as updated software versions, third-party solutions, and manufacturer support can aid in migrating to secure certificate alternatives.

By transitioning away from MD5-signed certificates, users can mitigate security risks and ensure the integrity of their data. We’re here to support this process and ensure a smooth transition to stronger cryptographic mechanisms.

1. Identify the Tablespace: First, identify the tablespace that needs to be extended. In this case, it’s the “USERS” tablespace.

2. Check Current Size: Before extending, it’s good to check the current size of the datafile associated with the tablespace.

   SELECT file_name, bytes / 1024 / 1024 AS size_mb
   FROM dba_data_files
   WHERE tablespace_name = 'USERS';
   

3. Extend the Tablespace: You can extend the tablespace by adding another datafile or increasing the size of an existing datafile. Here’s how to increase the size of an existing datafile:

   ALTER DATABASE DATAFILE '/path/to/users01.dbf' RESIZE 500M;
   

   This command increases the size of the datafile named ‘users01.dbf’ to 500MB. Adjust the size according to your requirements.

4. Verify Extension: After executing the above command, verify if the datafile size has been extended successfully:

   SELECT file_name, bytes / 1024 / 1024 AS size_mb
   FROM dba_data_files
   WHERE tablespace_name = 'USERS';
   

5. Check Tablespace Usage: Monitor the tablespace usage to ensure that the extension has resolved the space issue:

   SELECT tablespace_name,
          SUM(bytes) / 1024 / 1024 AS total_space_mb,
          SUM(bytes - NVL(free_space, 0)) / 1024 / 1024 AS used_space_mb,
          SUM(NVL(free_space, 0)) / 1024 / 1024 AS free_space_mb
   FROM dba_data_files
   JOIN (SELECT file_id, SUM(bytes) free_space
         FROM dba_free_space
         GROUP BY file_id) ON file_id = file_id
   WHERE tablespace_name = 'USERS'
   GROUP BY tablespace_name;
   

By following these steps, you can extend a tablespace in Oracle Database, addressing the ORA-01658 error and ensuring sufficient space for your database objects. Adjust the commands and sizes according to your specific requirements and environment.

Understanding last:

last is a command-line utility that displays a list of last logged in users. It extracts information from the /var/log/wtmp file, which stores all the login and logout records of users. The typical output of the last command includes the username, terminal, IP address, login time, and logout time (if available). It is an invaluable tool for system administrators to track user activities, audit logins, and diagnose security breaches.

Usage example:

$ last

Introducing lastb:

On the other hand, lastb is a command that is less familiar to many users compared to last. The ‘b’ in lastb stands for “bad”, indicating its primary use case: tracking failed login attempts. While last reads from /var/log/wtmp, lastb reads from /var/log/btmp, which specifically logs failed login attempts.

Usage example:

$ lastb

Differentiating Features:

1. Data Focus:
   • last focuses on successful logins and provides a comprehensive record of user activity.
   • lastb, however, zeroes in on failed login attempts, making it particularly useful for security analysis and intrusion detection.
2. Security Implications:
   • Monitoring lastb can help administrators identify potential security threats by pinpointing repeated failed login attempts, which may signify brute-force attacks or unauthorized access attempts.
   • last, while still crucial for auditing user behavior, may not highlight failed login attempts with the same immediacy as lastb.
3. File Sources:
   • last retrieves data from the /var/log/wtmp file.
   • lastb retrieves data from the /var/log/btmp file.

Best Practices:

• Regularly check both last and lastb logs to maintain a holistic view of user activity on your system.
• Implement automated monitoring systems that parse these logs for anomalies and trigger alerts for suspicious activities.
• Consider integrating these commands into your security protocols to fortify your system against unauthorized access attempts.

Conclusion:

While last may enjoy more recognition among system administrators for its role in tracking user logins, lastb is equally essential, if not more so, for its ability to highlight failed login attempts and potential security threats. By leveraging both commands in tandem, administrators can gain comprehensive insights into user activities and fortify their systems against security breaches. In the realm of cybersecurity, knowledge and utilization of tools like lastb can make all the difference between a secure system and a compromised one.

Understanding DB_RECOVERY_FILE_DEST_SIZE:

DB_RECOVERY_FILE_DEST_SIZE is a crucial parameter in Oracle databases that determines the maximum size limit for the Fast Recovery Area (FRA). The FRA serves as a repository for backup-related files, including RMAN backups, archived redo logs, and flashback logs. When the FRA approaches its defined size limit, Oracle attempts to reclaim disk space to accommodate new backups and logs.

Resolving ORA-19804 by Extending DB_RECOVERY_FILE_DEST_SIZE:

To address the ORA-19804 error and mitigate disk space constraints, extending the DB_RECOVERY_FILE_DEST_SIZE parameter is imperative. This process involves adjusting the parameter value to allocate additional disk space for the FRA, thereby enabling Oracle to reclaim the necessary space during backup operations.

Example Command to Extend DB_RECOVERY_FILE_DEST_SIZE:

To extend the DB_RECOVERY_FILE_DEST_SIZE parameter, execute the following SQL command in Oracle:

ALTER SYSTEM SET DB_RECOVERY_FILE_DEST_SIZE = <new_size> [G|M];

Replace <new_size> with the desired size for the FRA extension. You can specify the size in gigabytes (G) or megabytes (M), depending on your requirements.

Considerations:

• Disk Availability: Ensure that sufficient disk space is available on the filesystem where the FRA resides before extending DB_RECOVERY_FILE_DEST_SIZE.

• Impact of Changes: Assess the potential impact of extending the FRA size on disk usage and system performance.

• Monitoring: Regularly monitor the FRA usage and adjust the DB_RECOVERY_FILE_DEST_SIZE parameter as needed to prevent future disk space constraints.

Conclusion:

Extending the DB_RECOVERY_FILE_DEST_SIZE parameter is a fundamental step in resolving the ORA-19804 error and addressing disk space limitations in Oracle databases. By allocating additional space to the Fast Recovery Area, administrators can ensure uninterrupted backup and recovery operations, safeguarding the integrity and availability of critical data.

1. Using the /proc filesystem: The Linux kernel exposes various system information, including kernel parameters, through the /proc filesystem. The /proc/sys directory contains a hierarchical structure of files and directories that represent different kernel parameters and their values.

To retrieve a kernel parameter, you can navigate to the corresponding file using the cat command. For example, to retrieve the value of the “max_map_count” parameter, execute the following command:

cat /proc/sys/vm/max_map_count

This will display the current value of the parameter.

1. Using the sysctl command: The sysctl command provides a convenient way to retrieve and modify kernel parameters. It reads and writes values directly to the /proc/sys virtual filesystem, simplifying the process for administrators.

To retrieve a kernel parameter using sysctl, use the -n option followed by the parameter’s name. For instance, to retrieve the value of the “net.ipv4.ip_forward” parameter, execute:

sysctl -n net.ipv4.ip_forward

The command will display the current value of the parameter.

1. Listing all kernel parameters: Sometimes, you may want to view a comprehensive list of available kernel parameters. You can achieve this by executing the following command:

sysctl -a

This command will display a list of all kernel parameters and their current values, providing a wealth of information about your system’s configuration.

1. Retrieving kernel parameters from boot-time configuration: Linux systems typically load kernel parameters during the boot process. To retrieve these parameters, you can examine the contents of the /proc/cmdline file. This file contains the command-line arguments passed to the kernel during boot.

To view the contents of the file, use the cat command:

cat /proc/cmdline

This will display the boot-time kernel parameters, such as boot options, root device, and other relevant information.

1. Using the dmesg command: The dmesg command displays the kernel ring buffer, which contains various system messages, including boot-time kernel parameters. By filtering the output, you can retrieve specific kernel parameters.

To retrieve kernel parameters from the dmesg output, use the grep command along with a keyword related to the parameter you are interested in. For example, to find parameters related to the network interface, execute:

dmesg | grep -i network

This command will display the relevant kernel messages containing network-related parameters.

Conclusion: Retrieving Linux kernel parameters is essential for midlevel administrators to understand their system’s configuration and troubleshoot issues effectively. By utilizing the /proc filesystem, sysctl command, examining boot-time configuration, and leveraging the dmesg command, administrators can retrieve valuable information about their system’s kernel parameters. Understanding these parameters enables administrators to optimize performance, diagnose problems, and make informed decisions to maintain a stable and efficient Linux environment.

1. Using the /proc filesystem: The Linux kernel exposes various system information, including kernel parameters, through the /proc filesystem. The /proc/sys directory contains a hierarchical structure of files and directories that represent different kernel parameters and their values.

To retrieve a kernel parameter, you can navigate to the corresponding file using the cat command. For example, to retrieve the value of the “max_map_count” parameter, execute the following command:

cat /proc/sys/vm/max_map_count

This will display the current value of the parameter.

1. Using the sysctl command: The sysctl command provides a convenient way to retrieve and modify kernel parameters. It reads and writes values directly to the /proc/sys virtual filesystem, simplifying the process for administrators.

To retrieve a kernel parameter using sysctl, use the -n option followed by the parameter’s name. For instance, to retrieve the value of the “net.ipv4.ip_forward” parameter, execute:

sysctl -n net.ipv4.ip_forward

The command will display the current value of the parameter.

1. Listing all kernel parameters: Sometimes, you may want to view a comprehensive list of available kernel parameters. You can achieve this by executing the following command:

sysctl -a

This command will display a list of all kernel parameters and their current values, providing a wealth of information about your system’s configuration.

1. Retrieving kernel parameters from boot-time configuration: Linux systems typically load kernel parameters during the boot process. To retrieve these parameters, you can examine the contents of the /proc/cmdline file. This file contains the command-line arguments passed to the kernel during boot.

To view the contents of the file, use the cat command:

cat /proc/cmdline

This will display the boot-time kernel parameters, such as boot options, root device, and other relevant information.

1. Using the dmesg command: The dmesg command displays the kernel ring buffer, which contains various system messages, including boot-time kernel parameters. By filtering the output, you can retrieve specific kernel parameters.

To retrieve kernel parameters from the dmesg output, use the grep command along with a keyword related to the parameter you are interested in. For example, to find parameters related to the network interface, execute:

dmesg | grep -i network

This command will display the relevant kernel messages containing network-related parameters.

Conclusion: Retrieving Linux kernel parameters is essential for midlevel administrators to understand their system’s configuration and troubleshoot issues effectively. By utilizing the /proc filesystem, sysctl command, examining boot-time configuration, and leveraging the dmesg command, administrators can retrieve valuable information about their system’s kernel parameters. Understanding these parameters enables administrators to optimize performance, diagnose problems, and make informed decisions to maintain a stable and efficient Linux environment.

In this article, we will delve into the fascinating world of Linux command history. We will explore how you can leverage this powerful tool to streamline your workflow, improve productivity, and gain a deeper understanding of your command line interactions. Whether you’re a Linux newcomer or an experienced user looking to enhance your efficiency, this guide will provide you with the knowledge and techniques to make the most of command history.

Running commands from history:

To display command history, type ‘history’ and press Enter. It will show numbered commands from the history. To run a specific command from history, type ‘!nr’, where ‘nr’ is the command number in history. For example, if you want to run command number 42, type ‘!42’ and press Enter. Environment variables and history formatting:

To add date and time to command history, you can set the environment variable ‘HISTTIMEFORMAT’. For example, in the terminal, type:

export HISTTIMEFORMAT="%F %T "

The above format ‘%F %T’ represents date and time in ISO 8601 format. After applying this change, new commands will include date and time information in the history. Formatting history:

Command history can be formatted using options of the ‘history’ command. Here are a few examples: ‘history -c’: Clear command history. ‘history -d nr’: Delete the command with the specified number ‘nr’ from history. ‘history -a’: Save the current state of history to the file ‘~/.bash_history’. You can find more options by typing ‘man history’ in the terminal. Synchronizing history between sessions:

Command history is typically saved in the ‘~/.bash_history’ file for most shells. To synchronize history between sessions, follow these steps: After executing important commands, type ‘history -a’ to save the current state of history to the ‘~/.bash_history’ file. To load history from the ‘~/.bash_history’ file in a new session, type ‘history -r’. Examples:

Displaying command history:

history

Running a command from history:

!42

Setting the HISTTIMEFORMAT environment variable:

export HISTTIMEFORMAT="%F %T "

Formatting history: Clearing command history:

history -c

Deleting command number 15 from history:

history -d 15

Saving the current state of history to the file ‘~/.bash_history’:

history -a

Please note that the examples assume the usage of the Bash shell.