There are several names for a null route, such as a “bit bucket”, a “black hole”, or just a null0 route. They all refer to the same basic mechanism that points traffic to a virtual interface on a router. That in turn is used for managing unwanted traffic to prevent loops or entering routes into the RIB (routing information base) of a router.
Continue reading What is a null route and why do I need one?
By default, the PAM configuration files in Linux allow for null or empty passwords due to the nullok feature. From the manpage,
# man pam_unix
nullok
The default action of this module is to not permit the user access to a service if their official password is blank. The nullok argument overrides this default.
Continue reading Disable null passwords 
We have all used snmp for many years to help monitor our systems and networks but most admins have been reluctant to migrate to v3 due to the perceived increase in complexity. This post will show you how to quickly and easily enable snmpv3 on your linux system to take advantage of the additional security features to support authentication and privacy.
# yum install net-snmp net-snmp-utilsContinue reading Configuring snmpv3 in Linux
After setting up your Chrony NTP Server and Client, we are now ready to configure authentication using randomly generated symmetric keys. This is an important option beyond the allow/deny rules within your /etc/chrony.conf file to maintain the integrity of the service.
Continue reading RHEL 8 and Chrony – Part 3
In Part 1, we discussed setting up Chrony from a client perspective. This post will show how to configure the server side and investigate some of the options available within the /etc/chrony.conf configuration file.
Install the Chrony package
Continue reading RHEL 8 and Chrony – Part 2
The Network Time Protocol or NTP is essential for synchronizing system clocks across your environment. Having a reliable and accurate time service is not only important for many different applications but for logging and auditing as well. In RHEL 8, Chrony is used for implementing NTP. In Part 1, we will review setting this service up as a client and look at the basic functionality of the chronyc command to interact with the chrony daemon, chronyd.
Continue reading RHEL 8 and Chrony – Part 1
Sometimes, due to some new specific server requirements, you will find it necessary to increase your swap space. Even if your swap partition is setup as a Logical Volume, your requirements may exceed what is available. This is where creating a new swap file is the best option. In this example, we are going to add a new 12GB swap file.
Verify the total amount of used and free physical and swap memory with the free command and the -h human-readable flag
# free -ht
total used free shared buff/cache available
Mem: 755G 321G 3.0G 62G 430G 670G
Swap: 4G 1G 3G
Total: 771G 322G 6G
Display the swap usage summary by device using swapon. Same as cat /proc/swaps
Continue reading Add a Swap File to RHEL/CentOS
Sometimes you may want to quickly block a scanning/probing IP address without having to deal directly with the Access Control Policy. This can be done in the FMC within the Events view. Go to Analysis -> Connections -> Events
Depending on volume of traffic, you may have to click ‘Edit Search‘ and look by Initiator or Responder IP. Once found, right-click on the IP address and select ‘Blacklist IP Now‘ and confirm,
Continue reading Blacklist an IP in Firepower
The Network Logician 