HOSTS.EQUIV(4) File Formats and Configurations HOSTS.EQUIV(4)


NAME


hosts.equiv, rhosts - trusted remote hosts and users

DESCRIPTION


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:

hostname [username]


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.

Positive Entries


If the form:

hostname


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:

hostname username


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:

+@netgroup


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:

hostname +@netgroup


allows all of the users in the named netgroup from the named host to
access the system as the local user. The form:

+@netgroup1 +@netgroup2


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

+ username


will allow the named user from any remote host to access the system. The
entry

hostname +


will allow any user from the named host to access the system as the local
user.

Negative Entries


Negative entries are preceded by a `-' sign. The form:

-hostname


will disallow all access from the named host. The form:

-@netgroup


means that access is explicitly disallowed from all hosts in the named
netgroup. The form:

hostname -username


disallows access by the named user only from the named host, while the
form:

+ -@netgroup


will disallow access by all of the users in the named netgroup from all
hosts.

Search Sequence


To help maintain system security, the /etc/hosts.equiv file is not
checked when access is being attempted for super-user. If the user
attempting access is not the super-user, /etc/hosts.equiv is searched for
lines of the form described above. Checks are made for lines in this file
in the following order:

1. +

2. +@netgroup

3. -@netgroup

4. -hostname

5. hostname


The user is granted access if a positive match occurs. 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 super-user. 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:

1. +

2. +@netgroup

3. -@netgroup

4. -hostname

5. hostname

FILES


/etc/hosts.equiv
system trusted hosts and users


~/.rhosts
user's trusted hosts and users


SEE ALSO


rcp(1), rlogin(1), rsh(1), rcmd(3SOCKET), hosts(4), netgroup(4),
passwd(4)

WARNINGS


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

+
-hostxxx


will not deny access to "hostxxx".


November 26, 2017 HOSTS.EQUIV(4)