The /etc/hosts.equiv and .rhosts files provide the “remote authentication” database for rlogin(1), rsh(1), rcp(1), and rcmd(3SOCKET). The files specify remote hosts and users that are considered “trusted”. Trusted users are allowed to access the local system without supplying a password. The library routine ruserok() (see rcmd(3SOCKET)) performs the authentication procedure for programs by using the /etc/hosts.equiv and .rhosts files. The /etc/hosts.equiv file applies to the entire system, while individual users can maintain their own .rhosts files in their home directories.
These files bypass the standard password-based user authentication mechanism. To maintain system security, care must be taken in creating and maintaining these files.
The remote authentication procedure determines whether a user from a remote host should be allowed to access the local system with the identity of a local user. This procedure first checks the /etc/hosts.equiv file and then checks the .rhosts file in the home directory of the local user who is requesting access. Entries in these files can be of two forms. Positive entries allow access, while negative entries deny access. The authentication succeeds when a matching positive entry is found. The procedure fails when the first matching negative entry is found, or if no matching entries are found in either file. The order of entries is important. If the files contain both positive and negative entries, the entry that appears first will prevail. The rsh (1) and rcp(1) programs fail if the remote authentication procedure fails. The rlogin program falls back to the standard password-based login procedure if the remote authentication fails.
Both files are formatted as a list of one-line entries. Each entry has the form:
Hostnames must be the official name of the host, not one of its nicknames.
Negative entries are differentiated from positive entries by a `−' character preceding either the hostname or username field.
If the form:
is used, then users from the named host are trusted. That is, they may access the system with the same user name as they have on the remote system. This form may be used in both the /etc/hosts.equiv and .rhosts files.
If the line is in the form:
then the named user from the named host can access the system. This form may be used in individual .rhosts files to allow remote users to access the system as a different local user. If this form is used in the /etc/hosts.equiv file, the named remote user will be allowed to access the system as any local user.
netgroup(4) can be used in either the hostname or username fields to match a number of hosts or users in one entry. The form:
allows access from all hosts in the named netgroup. When used in the username field, netgroups allow a group of remote users to access the system as a particular local user. The form:
allows all of the users in the named netgroup from the named host to access the system as the local user. The form:
allows the users in netgroup2 from the hosts in netgroup1 to access the system as the local user.
The special character `+' can be used in place of either hostname or username to match any host or user. For example, the entry
will allow a user from any remote host to access the system with the same username. The entry
will allow the named user from any remote host to access the system. The entry
will allow any user from the named host to access the system as the local user.
Negative entries are preceded by a `−' sign. The form:
will disallow all access from the named host. The form:
means that access is explicitly disallowed from all hosts in the named netgroup. The form:
disallows access by the named user only from the named host, while the form:
will disallow access by all of the users in the named netgroup from all hosts.
To help maintain system security, the /etc/hosts.equiv file is not checked when access is being attempted for root. If the user attempting access is not the root, /etc/hosts.equiv is searched for lines of the form described above.
Checks are made for lines in this file in the following order:
The user is granted access if a positive match occurrs. Negative entries apply only to /etc/hosts.equiv and may be overridden by subsequent .rhosts entries.
If no positive match occurred, the .rhosts file is then searched if the user attempting access maintains such a file. This file is searched whether or not the user attempting access is the root. As a security feature, the .rhosts file must be owned by the user who is attempting access.
Checks are made for lines in .rhosts in the following order:
system trusted hosts and users
user's trusted hosts and users
The PAM configuration of pam_rhosts_auth(5) can cause the hosts.equiv support to be disabled.
Positive entries in /etc/hosts.equiv that include a username field (either an individual named user, a netgroup, or `+' sign) should be used with extreme caution. Because /etc/hosts.equiv applies system-wide, these entries allow one, or a group of, remote users to access the system as any local user. This can be a security hole. For example, because of the search sequence, an /etc/hosts.equiv file consisting of the entries
will not deny access to “hostxxx”.