What is a Martian Packet? The IANA defines a Martian packet as one which arrives on an interface where the interface does not use that network. For Linux, it's any packet that arrives on an interface which is not configured for that subnet in any way. Any martian packet notice should be investigated. Martian packets: Are frequently used in hacking intrusion. May be a symptom of a misconfigured server elsewhere on the network. May indicate a network infrastructure issue. If … [Read more...] about How to interpret Linux martian source messages
What is the kernel parameter net.ipv4.conf.all.log_martians used for
Question: What is the kernel parameter net.ipv4.conf.all.log_martians for? What are "martian source" messages? A "martian" is a packet received in a network interface, where the packet's source and/or destination IP address does not make sense. Some examples of martian packets are: Source IP is already an address on the system receiving the packet Source IP is the broadcast address 255.255.255.255 Source IP is the local subnet broadcast address Source IP is a multicast … [Read more...] about What is the kernel parameter net.ipv4.conf.all.log_martians used for
How to create LUKS-Encrypted image and mount it at boot in CentOS/RHEL 7
Question: How to create a LUKS-Encrypted image (file) and mount it automatically during boot? 1. Create a directory: # mkdir /var/loopfs 2. Preallocate a space to a file: # fallocate -l 1G /var/loopfs/Stage 3. Set up a loop device: # losetup /dev/loop0 /var/loopfs/Stage 4. Initialize a LUKS partition and set the initial passphrase: # cryptsetup --verbose --verify-passphrase luksFormat /dev/loop0 WARNING! ======== This will overwrite data on /dev/loop0 irrevocably. Are you … [Read more...] about How to create LUKS-Encrypted image and mount it at boot in CentOS/RHEL 7
How to set quota with xfs_quota for groups
Question: Need to set quota on xfs filesystem. Additionally, want various groups with different quota restriction and each user assigned to a group should have individually the same amount of capacity how it was defined as group quota. For example: Group A with 10 GB has some users. (Each user in group A should have 10 GB capacity, same for other groups as well) Group B with 15 GB has some other users. Group C with 5 GB compromises of rest of all user. As group quota can be used only … [Read more...] about How to set quota with xfs_quota for groups
Troubleshooting Oracle RAC Node Evictions (Reboots) [ 11.2 and above ]
This post provides a reference for troubleshooting Clusterware node evictions in versions 11.2 and above. NODE EVICTION OVERVIEW The Oracle Clusterware is designed to perform a node eviction by removing one or more nodes from the cluster if some critical problem is detected. A critical problem could be a node not responding via a network heartbeat, a node not responding via a disk heartbeat, a hung or severely degraded machine, or a hung ocssd.bin process. The purpose of this node eviction … [Read more...] about Troubleshooting Oracle RAC Node Evictions (Reboots) [ 11.2 and above ]
Oracle Interview Questions : Grid Infrastructure Single Client Access Name (SCAN)
SCAN Concepts Single client access name (SCAN) is the virtual hostname to provide for all clients connecting to the cluster (as opposed to the VIP hostnames in 10g and 11gR1). SCAN is a domain name registered to at least one and up to three IP addresses, either in the domain name service (DNS) or the Grid Naming Service (GNS). By default, the name used as the SCAN is also the name of the cluster and must be globally unique throughout your enterprise. The default value for the SCAN is based … [Read more...] about Oracle Interview Questions : Grid Infrastructure Single Client Access Name (SCAN)
How to copy directories recursively using rsync while excluding specific files
Question: How to copy folders recursively while excluding specific folders/files whhen using rsync? In general, we use 'cp' command to copy files, but unfortunately, 'cp' command doesn't have the "exclude" feature, so we need to write some scripts to exclude the unwanted files, or we can use another tool 'rsync'. From the man page of rsync command: $ man rsync ... -r, --recursive recurse into directories --exclude=PATTERN exclude files matching PATTERN The … [Read more...] about How to copy directories recursively using rsync while excluding specific files
Comparing Solaris and Red Hat Enterprise Linux kernel network parameters
Question: What are the relations between Red Hat Enterprise Linux and Solaris network kernel parameters? This table shows the mapping the kernel parameters between Solaris and RHEL 6.x: SolarisRHEL tcp_keepalive_intervalnet.ipv4.tcp_keepalive_time : How often TCP sends out keepalive messages when keepalive is enabled.(Default: 2hours.) net.ipv4.tcp_keepalive_probes : How many keepalive probes TCP sends out, until it decides that the connection is broken.( Default value: 9.) … [Read more...] about Comparing Solaris and Red Hat Enterprise Linux kernel network parameters
How to customize rngd service startup in CentOS/RHEL 7
Question: The rngd service fails at startup on a virtual server as an entropy device is not available. How can I customize rngd service startup? With CentOS/RHEL 7 and the use of systemd for tools like rngd which do not have a config file which enables changing its default settings there is an option to override it, following these steps: 1. Copy the /usr/lib/systemd/system/rngd.service to /etc/systemd/system: # cp /usr/lib/systemd/system/rngd.service /etc/systemd/system 2. Edit the … [Read more...] about How to customize rngd service startup in CentOS/RHEL 7
How to Modify an Existing ASM Spfile in a RAC Environment
This post provides an example on how to modify an existing Automatic Storage Management (ASM) spfile in a two-node Real Application Clusters (RAC) Environment. These steps can be applied to a multiple node Real Application Clusters (RAC) Environment. Applies To: Oracle Database - Enterprise Edition - Version 10.2.0.1 to 11.1.0.7 [Release 10.2 to 11.1] If a spfile is used in Clustered ASM environments, then the spfile must be on a shared Clusterware device, i.e. shared spfile. Note: … [Read more...] about How to Modify an Existing ASM Spfile in a RAC Environment