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2.2. SNMP Communities

SNMPv1 and SNMPv2 use the notion of communities to establish trust between managers and agents. An agent is configured with three community names: read-only, read-write, and trap. The community names are essentially passwords; there's no real difference between a community string and the password you use to access your account on the computer. The three community strings control different kinds of activities. As its name implies, the read-only community string lets you read data values, but doesn't let you modify the data. For example, it allows you to read the number of packets that have been transferred through the ports on your router, but doesn't let you reset the counters. The read-write community is allowed to read and modify data values; with the read-write community string, you can read the counters, reset their values, and even reset the interfaces or do other things that change the router's configuration. Finally, the trap community string allows you to receive traps (asynchronous notifications) from the agent.

Most vendors ship their equipment with default community strings, typically public for the read-only community and private for the read-write community. It's important to change these defaults before your device goes live on the network. (You may get tired of hearing this because we say it many times, but it's absolutely essential.) When setting up an SNMP agent, you will want to configure its trap destination, which is the address to which it will send any traps it generates. In addition, since SNMP community strings are sent in clear text, you can configure an agent to send an SNMP authentication-failure trap when someone attempts to query your device with an incorrect community string. Among other things, authentication-failure traps can be very useful in determining when an intruder might be trying to gain access to your network.

Because community strings are essentially passwords, you should use the same rules for selecting them as you use for Unix or NT user passwords: no dictionary words, spouse names, etc. An alphanumeric string with mixed upper- and lowercase letters is generally a good idea. As mentioned earlier, the problem with SNMP's authentication is that community strings are sent in plain text, which makes it easy for people to intercept them and use them against you. SNMPv3 addresses this by allowing, among other things, secure authentication and communication between SNMP devices.

There are ways to reduce your risk of attack. IP firewalls or filters minimize the chance that someone can harm any managed device on your network by attacking it through SNMP. You can configure your firewall to allow UDP traffic from only a list of known hosts. For example, you can allow UDP traffic on port 161 (SNMP requests) into your network only if it comes from one of your network-management stations. The same goes for traps; you can configure your router so it allows UDP traffic on port 162 to your NMS only if it originates from one of the hosts you are monitoring. Firewalls aren't 100% effective, but simple precautions such as these do a lot to reduce your risk.

WARNING: It is important to realize that if someone has read-write access to any of your SNMP devices, he can gain control of those devices by using SNMP (for example, he can set router interfaces, switch ports down, or even modify your routing tables). One way to protect your community strings is to use a Virtual Private Network (VPN) to make sure your network traffic is encrypted. Another way is to change your community strings often. Changing community strings isn't difficult for a small network, but for a network that spans city blocks or more and has dozens (or hundreds or thousands) of managed hosts, changing community strings can be a problem. An easy solution is to write a simple Perl script that uses SNMP to change the community strings on your devices.

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