How would you find out how much RAM is free on your Linux desktop? That's
a really easy question with a lot of answers—
free, any of
the implementations of
Glances all are valid responses. How
would you find out how much RAM is free on 200 Linux instances, which are
running on a mixture of real and virtual hardware, in dozens of physical
locations spread out around the globe? That's a much bigger problem,
and there is a tool to make the job easier. However, the lack of upkeep on the
standards and lack of development support for the Linux implementation
are resulting in proprietary standards creeping in where there once was
a more open standard.
SNMP (Simple Network Management Protocol) was designed in 1990 to read and write structured data on devices attached to a network, such as how much free RAM there is. Yes, and this is important, the M in SNMP really does stand for "Management", not "Monitoring". Although SNMP is usually used to request operational status information, the SNMP "write" functionality can be used to change the configuration on remote devices. Given the lack of security and authentication in the SNMP protocol, SNMP "write" functionality almost always is disabled on the modern internet, and I will not be discussing it here.
History of SNMP
The original IETF (Internet Engineering Task Force) RFC (Request for Comments) standard for SNMP v1 was published by the IETF in 1990. SNMP v2 was published in 1994–1996 as a series of RFCs and included the first effort to secure SNMP. This effort proved unpopular due to the load it placed on network hardware, which, at the time, had very low performance CPUs. This performance issue exists today and still can cause problems for administrators attempting to secure SNMP. Due to the performance problems, SNMP v2c (SNMP v2 with SNMP v1 communities) became the standard. Concurrently with the release of SNMPv2c, the public began to access the internet, and during the next decade, security would become a serious problem with SNMP since SNMP v2c was entirely unencrypted. SNMPv3 came along in 2003 and added TLS to the previous implementation of SNMP v2c. If all of this seems a bit complicated and unnecessary, it's important to know that many implementations of SNMP still ship with support for SNMP v1, v2c and SNMP v3. This means you're likely to see all of them in the wild.
How Is SNMP Used?
One of the challenges on a modern network is scale, and achieving scale requires managing resources. SNMP provides an agent, which listens for incoming SNMP requests on each host, and a standard communications protocol allowing a central collection system called a Network Management System (NMS) to collect data. NMS is outside the scope of this article, but there are many good open-source NMSes, including Zabbix, OpenNMS, Nagios and Zenoss. The data collected by each NMS is pretty standard, and it includes basic systems information like CPU, memory, network and storage utilization.
SNMP Data Structure
SNMP isn't just an agent, it's also a data structure. Each object in the data structure has an Object IDentifier, or OID. Each OID belongs to an MIB, or Management Information Base. These object identifiers and the hierarchical structure function as a tree. Each sequential number is a branch and has a meaning, and each branch is separated by periods (.), somewhat like an IPv4 address. This means that the meaning of an OID can be decoded very simply.
Andrew Kirch has more than ten years of experience working as a systems/network administrator, with specializations including DevOps, SNMP and NMS.
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