3.9 How might I increase the security and scalability of my DMZ?A common approach for an attacker is to break into a host that's vulnerable to attack, and exploit trust relationships between the vulnerable host and more interesting targets.
If you are running a number of services that have different levels of security, you might want to consider breaking your DMZ into several ``security zones''. This can be done by having a number of different networks within the DMZ. For example, the access router could feed two ethernets, both protected by ACLs, and therefore in the DMZ.
On one of the ethernets, you might have hosts whose purpose is to service your organization's need for Internet connectivity. These will likely relay mail, news, and host DNS. On the other ethernet could be your web server(s) and other hosts that provide services for the benefit of Internet users.
In many organizations, services for Internet users tend to be less carefully guarded and are more likely to be doing insecure things. (For example, in the case of a web server, unauthenticated and untrusted users might be running CGI or other executable programs. This might be reasonable for your web server, but brings with it a certain set of risks that need to be managed. It is likely these services are too risky for an organization to run them on a bastion host, where a slip-up can result in the complete failure of the security mechanisms.)
By putting hosts with similar levels of risk on networks together in the DMZ, you can help minimize the effect of a breakin at your site. If someone breaks into your web server by exploiting some bug in your web server, they'll not be able to use it as a launching point to break into your private network if the web servers are on a separate LAN from the bastion hosts, and you don't have any trust relationships between the web server and bastion host.
Now, keep in mind that we're running ethernet here. If someone breaks into your web server, and your bastion host is on the same ethernet, an attacker can install a sniffer on your web server, and watch the traffic to and from your bastion host. This might reveal things that can be used to break into the bastion host and gain access to the internal network.
Splitting services up not only by host, but by network, and limiting the level of trust between hosts on those networks, you can greatly reduce the likelihood of a breakin on one host being used to break into the other. Succinctly stated: breaking into the web server in this case won't make it any easier to break into the bastion host.
You can also increase the scalability of your architecture by placing hosts on different networks. The fewer machines that there are to share the available bandwidth, the more bandwidth that each will get.
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