The Historical Answer
With Windows NT, prior to the advent of Active Directory, there was one Primary Domain Controller (DC) per domain, and every other DC was a Backup.
The Modern Answer
When Active Directory was introduced with Windows 2000, domain controllers became fully multi-master. There is no primary domain controller. However, there are two caveats that may confuse you.
If you are a Linux or OS X power user, then you’re used to having all the necessary tools built into your OS. When you log into a Windows system (What! No command line?) you may feel lost. These tools and shortcuts will help you be productive on Windows systems.
- Alt-x is a magic shortcut key on Windows 8 and Server 2012. It pops up a little menu in the lower right corner of the screen which contains just the items that an admin needs. Try it!
In Part 1, I summarized the basic concepts of SNMP and defined the terms and acronyms used in this post. Now, I will show how to use SNMP to monitor actual devices. As an example, I will monitor an enterprise-grade uninterruptible power supply (UPS) and power distribution unit (PDUs) from Tripp-Lite. These devices have an SNMPWEBCARD installed to support communication over Ethernet.
Command-line tools for SNMP communication should be available for any Linux distribution (or any other UNIX-derived OS). Documentation for the basic SNMP tools is available online. The challenge with SNMP is figuring out what parameters are supported by a particular device. Most devices support a set of standard OIDs that return basic information such as device name, uptime, etc.
SNMP is a protocol for conveying information and controlling devices over a network. SNMP can be used in two ways:
- Active: a device sends a command to set a parameter or request information for another device
- Passive: a device sends an alert (called a trap) to another device, which is configured to receive traps and do something with the information.
The “payload” of an SNMP message is called an Object Identifier, or OID. An OID is an ordered list of non-negative numbers, such as:
The sequence is hierarchical, starting with the highest-level object and progressing to lower-level objects. The above sequence corresponds to:
iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1) system(1) sysUpTime(3) 0
When this command is sent to a device, it will return the uptime of the device.
The translation between the numerical sequence and the human-readable form is stored in a text file called a Management Information Base, or MIB. The format of the MIB is defined in RFC 2578. Some MIB files are standard and contain object IDs that are recognized by almost all devices. Device manufacturers also provide custom MIB files in which they define specialized object IDs for a particular device. Unfortunately, some devices don’t have MIB files, and you will have to query the device to see what objects it supports and decipher what they mean.
In Part 2 of this series, I will use active SNMP to monitor infrastructure.