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For the sake of simplicity, I’ll start by denying all access to a server over the SSH service and then explicitly allow certain users access.
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On many of the popular Linux distributions, two files live inside the /etc , I’ll focus on the use of IP addresses and names for configuring who is allowed to connect to ports. Ignoring the use of a somewhat ancient network service, namely identd
#Software wrapper for security software#
The story goes that the talented Venema needed to keep track of attacks on workstations at a University and wrote a piece of software capable of limiting port access by rules. TCP Wrappers have been around since the early days of the Internet and were written by a programmer named Wietse Venema, a Dutchman who is most famous for writing the truly excellent mail server software, Postfix. It also handles hostname lookups with ease, whether they’re entered in a local database or a public DNS database. To log, check, restrict, and avoid spoofed network connections efficiently and in real time. You can achieve this easily using a fantastic piece of software called TCP Wrappers. It’s highly unlikely that you would want to restrict all HTTP traffic to your server by IP address (e.g., unless the web server was on an Intranet), but you might only want email from specific mail servers locked down by IP address, and to my mind, almost every Internet-facing SSH server should be locked down by IP address. The open ports example I used was that of HTTP, SMTP, and SSH. I mentioned ACLs, and in this case, I’m talking about limiting access to services installed on your server, such as SSH, to specific IP addresses or restricting access with other rulesets. Another side effect is that fewer packages might mean less services that can accidentally open up a network port (which would otherwise add to your risk such a scenario might only be mitigated against by running only a comprehensive firewalling policy like a efficacious IPtables default-deny
#Software wrapper for security code#
Amongst other benefits keeping packages at a minimum means not only that your backups are leaner (and therefore easier to store because they’ll use less storage space and in addition because they’re smaller they should also be quicker to restore in an emergency) but also that the code on your server is less vulnerable to attack. This is important because you can lock all the network-facing ports down but make your applications prone by adding screeds of unneeded code to your server. By only having, say, three ports open on your server, such as HTTP, SMTP and SSH, you’re significantly limiting the number of attack vectors on your system.Īside from network ports posing a threat there’s a loosely followed rule of thumb that for every thousand lines of code you add to a server you potentially add another security hole. I’m referring to keeping the number of packages (and more specifically network services) to a minimum. With a little planning and some consideration it is possible to connect Linux boxes to the Internet safely without anything but some Access Control Lists (ACLs) combined with an eye for minimalism. A word to the wise, therefore, is that if you fail to implement correctly the approach I run through in this article, iptables is the perfect hero to come to your rescue and make that tiny mistake less disastrous to your servers’ security. My brief addendum to the last two sentences has to be that running Netfilter – or, to most peoples minds, the tool that controls Netfilter, iptables – on a Linux server brings a great number of benefits, such as automatically dropping illegitimately formed traffic that might pose a threat to your applications or catching traffic to a port you forgot to close. If you’re wondering what I mean by “successfully,” I mean without the servers being compromised. This piqued my interest because all my customers talked about when it came to Internet security was how much their proprietary firewall had cost them or which bundled features with their firewall guaranteed greater security for their servers.Īdmittedly, it goes against the grain – and more than just a little – to totally dismiss firewalls, but you might be surprised to hear that I’ve successfully run several sets of production servers for many years with the absence of a firewall entirely. Many years ago I remember somebody mentioning that rather than running a firewall, they were just using TCP Wrappers.