System Administration Guide, Volume 2

Authentication for Remote Logins (rlogin)

Authentication (establishing who you are) for rlogin operations can be performed either by the remote system or by the network environment.

The main difference between these forms of authentication lies in the type of interaction they require from you and the way they are established. If a remote system tries to authenticate you, you will be prompted for a password, unless you set up the /etc/hosts.equiv or .rhosts file. If the network tries to authenticate you, you won't be asked for a password, since the network already knows who you are. The figure below shows a simplified illustration to describe authentication for remote logins.

Figure 10-2 Authentication for Remote Logins (rlogin)


When the remote system attempts to authenticate you, it relies on information in its local files; specifically if:

Network authentication relies on one of these two methods:

Note -

Network authentication generally supersedes system authentication.

The /etc/hosts.equiv File

The /etc/hosts.equiv file contains a list of trusted hosts for a remote system, one per line. If a user attempts to log in remotely (using rlogin) from one of the hosts listed in this file, and if the remote system can access the user's password entry, the remote system allows the user to log in without a password.

A typical hosts.equiv file has the following structure:

host2 user_a

When a simple entry for a host is made in hosts.equiv, such as the entry above for host1, it means that the host is trusted, and so is any user at that machine.

If the user name is also mentioned, as in the second entry in the example, then the host is trusted only if the specified user is attempting access.

A group name preceded by a plus sign (+) means that all the machines in that netgroup are considered trusted.

A group name preceded by a minus sign (-) means that none of the machines in that netgroup are considered trusted.

Security Risks When Using the /etc/hosts.equiv File

The /etc/hosts.equiv file presents a security risk. If you maintain a /etc/hosts.equiv file on your system, you should include only trusted hosts in your network. The file should not include any host that belongs to a different network, or any machines that are in public areas. (For example, do not include a host that is located in a terminal room.)

This can create a serious security problem. Either replace the /etc/hosts.equiv file with a correctly configured one, or remove the file altogether.

A single line of + in the /etc/hosts.equiv file indicates that every known host is trusted.

The .rhosts File

The .rhosts file is the user equivalent of the /etc/hosts.equiv file. It contains a list of host-user combinations, rather than hosts in general. If a host-user combination is listed in this file, the specified user is granted permission to log in remotely from the specified host without having to supply a password.

Note that a .rhosts file must reside at the top level of a user's home directory. .rhost files located in subdirectories are not consulted.

Users can create .rhosts files in their home directories. Using the .rhosts file is another way to allow trusted access between their own accounts on different systems without using the /etc/hosts.equiv file.

Security Risks When Using the .rhosts File

Unfortunately, the .rhosts file presents a major security problem. While the /etc/hosts.equiv file is under the system administrator's control and can be managed effectively, any user can create a .rhosts file granting access to whomever the user chooses without the system administrator's knowledge.

In a situation in which all of the users' home directories are on a single server and only certain people have superuser access on that server, a good way to prevent a user from using a .rhosts file is to create an empty file as superuser in their home directory. You would then change the permissions in this file to 000 so that it would be difficult to change it, even as superuser. This would effectively prevent a user from risking system security by using a .rhosts file irresponsibly. It would not, however, solve anything if the user is able to change the effective path to his or her home directory.

The only secure way to manage .rhosts files is to completely disallow them. See "How to Search for and Remove .rhosts Files" for detailed instructions. As system administrator, you can check the system often for violations of this policy. One possible exception to this policy is for the root account--you might need to have a .rhosts file to perform network backups and other remote services.