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7.2. Security Concerns

Chapter 2, "A Closer Look at SNMP" discussed the security issues with SNMPv1 and SNMPv2. The biggest problem, of course, is that the read-only and read-write community strings are sent as clear-text strings; the agent or the NMS performs no encryption. Therefore, the community strings are available to anyone with access to a packet sniffer. That certainly means almost anyone on your network with a PC and the ability to download widely available software. Does that make you uncomfortable? It should.

Obviously, you need to take the same precautions with the community strings that you would with your superuser or administrator passwords. Choose community strings that are hard to guess. Mixed-case alphanumeric strings are good choices for community strings; don't use dictionary words. Although someone with the read-only community string can't do as much damage as someone with the read-write string, you might as well take the same precautions for both. Don't forget to change your community strings -- most devices ship with preconfigured community strings that are extremely easy to guess.

That doesn't solve the problems with packet sniffers. When you're configuring an agent, it's a good idea to limit the devices that can make SNMP requests (assuming that your agent allows you to make this restriction). That way, even if someone gets the community strings, he'll have to spoof the IP address of one of your management stations to do any damage.

Of course, many people know how to spoof IP addresses these days, and it's not a really good idea to assume that you can trust your employees. A better solution to the problem is to prevent the SNMP packets from being visible on your external network connections and parts of your network where you don't want them to appear. This requires configuring your routers and firewalls with access lists that block SNMP packets from the outside world (which may include parts of your own network). If you don't trust the users of your network, you may want to set up a separate administrative network to be used for SNMP queries and other management operations. This is expensive and inflexible -- it's hard to imagine extending such a network beyond your core routers and servers -- but it may be what your situation requires.

If you want to use SNMP to monitor your network from home, be extremely careful. You do not want your community strings traveling over the public Internet in an unencrypted form. If you plan to use SNMP tools directly from home, make sure to install VPN software, or some form of tunneling, to keep your SNMP traffic private. A better approach to home monitoring is to use a web interface; by using SSL, you can prevent others from seeing your usage graphs. (No network-management products that we're aware of support SSL out of the box; but they do allow you to integrate with external servers, such as Apache, which do support SSL).

SNMPv3 (discussed in Appendix F, "SNMPv3") fixes most of the security problems; in particular, it makes sure that the community strings are always encrypted. Unfortunately, there are very few implementations of SNMPv3 out there. It's clear what direction you want to head in, but you can't get there yet.

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