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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  TCP/IP Network Management Framework and Protocols (SNMP and RMON)                 9  TCP/IP Simple Network Management Protocol (SNMP) Protocol                      9  SNMP Protocol Operations

SNMP Protocol Information Notification Using Trap(v2) and InformRequest Messages 1 2 3 SNMP Protocol Messaging and Message Formats

SNMPv1 Security

Unfortunately, the security incorporated into SNMPv1 was extremely limited; it really took the form of only one policy and one simple technology:

• “Weak Objects”: SNMP was created with the mindset that the MIB objects used in the protocol would be relatively weak. This means that the objects are designed so that any problems in working with them result in minimal damage. The policy of the designers of SNMP was that MIB objects that are normally read should not contain critical information, and objects that are written should not control critical functions.
  
  So, a read-only MIB object containing a description of a machine is fine, but one containing the administrative password is not. Similarly, a read-write MIB object that controls when the computer next reboots is acceptable, but one that tells the object to reformat its hard disk is (definitely) not!
  
  
• Community Strings: All the devices in an SNMP network managed by a particular set of network management stations are considered to be in a “community”. Each SNMPv1 message sent between members of the community is identified by a community string that appears in a field in the message header. This string is like a simple password; any messages received with the wrong string will be rejected by the recipient.

These security features are better than nothing, but not much. The use of weak objects is comparable to a policy that says not to leave your car in front of the convenience store with the doors unlocked and the key in the ignition—it is basically saying “don't ask for trouble”. This is wise, but it’s not a complete security solution. The community strings protect against obvious tampering in the form of unauthorized messages. However, the strings are sent in plain open text and can easily be discovered and then used to compromise the “community”. So this is like locking your doors when parking your car—it protects against the casual thief but not a pro.

Of course, for some people, not leaving their car running and locking the doors when they park are enough security, and SNMPv1's security was also sufficient for some users of SNMP. But in newer, larger internetworks, especially ones spanning large distances or using public carriers, SNMPv1 wasn't up to the task. This is why all that fun stuff occurred with SNMP version 2.

SNMP Protocol Information Notification Using Trap(v2) and InformRequest Messages 1 2 3 SNMP Protocol Messaging and Message Formats