Demons Seeking Dæmons—A Practical Approach to Hardening Your OpenSSH Configuration

A few simple configuration tweaks could save you sleepless nights over whether or not someone might crack your SSH server.

Chances are, if you are the owner or administrator of a Linux machine, you access it remotely from time to time, if not constantly. Be it a workstation at home, a co-located server or a hobby machine, if you are accessing Linux remotely, there is a good chance that you are using the OpenSSH server on the remote machine with some type of SSH client locally. (If you are not, you probably should be.) Although it is true that the OpenSSH server and clients do a tremendous job at encrypting the traffic that passes between systems, it also is true that any dæmon listening for connections is a door handle waiting to be turned by the black hats, the evildoers or the crackers. The less-desirable folks in these situations are, as the title states, demons seeking dæmons, a person or group of people that usually are up to no good seeking listening dæmons that, depending on the configuration of the machine, may or may not be secure. Hackers can be defined as people who have relatively benign motives for breaking into a system. Crackers often are thought of as hackers with malicious intent. Neither the former nor latter is welcome on my machines. In this article, I expand on the basic OpenSSH configuration and cover ways to improve the security of the SSH dæmon to offer more protection to your machines. If you have a machine that suffers a compromise, your machine may be helping to spread your unpleasantness to others as well. The Internet is one of the largest insecure data routes on the face of our planet.

OpenSSH provides the means for secure communication over insecure channels. The file sshd_config is the ruling party in the game of secure shell configuration. The sshd_config file consists of multiple options that can be changed to help improve the security of the listening dæmon. Although it may seem that because it is a remote access tool it should be secure right out of the box, this is hardly the case. For the most part, a default installation of the OpenSSH server will provide you with a relatively secure default configuration but one that can be improved substantially by the machine's administrator.

When planning for remote access initially, it is suggested that you consider three major things for the access of your machines:

  1. Who will be allowed access to the machines?

  2. How will this access be provided?

  3. From where will this access be allowed?

We are going to assume that we will be using the OpenSSH server to provide remote access. This leaves us with the questions of who will be allowed remote access and from where they will be allowed. For some, this may be simple; perhaps the machine supports only a single user and remote access is carried out from a single domain. For others, this can be quite a challenge when multiple users that travel frequently are the machine's users.

Who Is on the Invite List for This Devilish Party?

The first and foremost person on a Linux machine is the root user. This is universally known, and it also should be universally known that if you need to have remote root access to a machine, there are many better ways to access as root than simply using SSH and logging in as root. If you think about the fundamentals of a brute-force attack attempt, it is obvious that the most relevant account that will be attacked is the root account. One need not guess whether the account is present; it is there. The sshd_config file lets us specify that root is not allowed to log in remotely at all through the PermitRootLogin directive.

I fully believe that the saying “an ounce of prevention equals a pound of cure” is highly accurate when dealing with remote-use accounts. Two options allowed in the sshd_config file, UsersAllow and UsersDeny, are more than an ounce of prevention, and although it may be one extra step when adding an account, modifying the UsersAllow for each account added provides that pound of cure that you may (thankfully) never need to seek. To expand on the UsersAllow directive, you cannot specify only particular users, but you also can specify particular users at specific hosts. So, if in advance you know exactly who will need to log in and from where, the minimal time overhead associated with adding these directives to the sshd_config file provides the peace of mind for you to know you are allowing remote access for only specific accounts from specific machines. Valid users can be retrieved from the /etc/passwd file in standalone machines or from the corresponding files in NIS or LDAP environments. Listing 1 is an example of parsing the previous month's security files to verify which accounts have logged in successfully using SSH.

______________________

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

ssh work

Steve Jim1's picture

The new work about ssh server is to be access very carefully to get fixed.

ssh -p <alternate port>

Keith Daniels's picture
Doing ssh -p will allow a second dæmon to run and provide you with a secondary secure connection Shouldn't that be: sshd -p ?

"I have always wished that my computer would be as easy to use as my telephone.
My wish has come true. I no longer know how to use my telephone."
-- Bjarne Stroustrup

Denyhosts

Anonymous's picture

"DenyHosts is a python program that automatically blocks ssh attacks by adding entries to /etc/hosts.deny. DenyHosts will also inform Linux administrators about offending hosts, attacked users and suspicious logins." Very nifty when your machine is getting hammered with brute force attacks. You can find it here.

White Paper
Linux Management with Red Hat Satellite: Measuring Business Impact and ROI

Linux has become a key foundation for supporting today's rapidly growing IT environments. Linux is being used to deploy business applications and databases, trading on its reputation as a low-cost operating environment. For many IT organizations, Linux is a mainstay for deploying Web servers and has evolved from handling basic file, print, and utility workloads to running mission-critical applications and databases, physically, virtually, and in the cloud. As Linux grows in importance in terms of value to the business, managing Linux environments to high standards of service quality — availability, security, and performance — becomes an essential requirement for business success.

Learn More

Sponsored by Red Hat

White Paper
Private PaaS for the Agile Enterprise

If you already use virtualized infrastructure, you are well on your way to leveraging the power of the cloud. Virtualization offers the promise of limitless resources, but how do you manage that scalability when your DevOps team doesn’t scale? In today’s hypercompetitive markets, fast results can make a difference between leading the pack vs. obsolescence. Organizations need more benefits from cloud computing than just raw resources. They need agility, flexibility, convenience, ROI, and control.

Stackato private Platform-as-a-Service technology from ActiveState extends your private cloud infrastructure by creating a private PaaS to provide on-demand availability, flexibility, control, and ultimately, faster time-to-market for your enterprise.

Learn More

Sponsored by ActiveState