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PASSWD(5) FreeBSD File Formats Manual PASSWD(5)
NAME
passwd -- format of the password file
DESCRIPTION
The passwd files are files consisting of newline separated records, one
per user, containing ten colon (`:') separated fields. These fields are
as follows:
name User's login name.
password User's encrypted password.
uid User's id.
gid User's login group id.
class User's login class.
change Password change time.
expire Account expiration time.
gecos General information about the user.
home_dir User's home directory.
shell User's login shell.
Lines whose first non-whitespace character is a pound-sign (#) are com-
ments, and are ignored. Blank lines which consist only of spaces, tabs
or newlines are also ignored.
The name field is the login used to access the computer account, and the
uid field is the number associated with it. They should both be unique
across the system (and often across a group of systems) since they con-
trol file access.
While it is possible to have multiple entries with identical login names
and/or identical uids, it is usually a mistake to do so. Routines that
manipulate these files will often return only one of the multiple
entries, and that one by random selection.
The login name must never begin with a hyphen (`-'); also, it is strongly
suggested that neither upper-case characters nor dots (`.') be part of
the name, as this tends to confuse mailers.
The password field is the encrypted form of the password. If the
password field is empty, no password will be required to gain access to
the machine. This is almost invariably a mistake. Because these files
contain the encrypted user passwords, they should not be readable by any-
one without appropriate privileges. Administrative accounts have a pass-
word field containing an asterisk `*' which disallows normal logins.
The group field is the group that the user will be placed in upon login.
Although this system supports multiple groups (see groups(1)) this field
indicates the user's primary group. Secondary group memberships are
selected in /etc/group.
The class field is a key for a user's login class. Login classes are
defined in login.conf(5), which is a termcap(5) style database of user
attributes, accounting, resource and environment settings.
The change field is the number in seconds, GMT, from the epoch, until the
password for the account must be changed. This field may be left empty
or set to 0 to turn off the password aging feature.
The expire field is the number in seconds, GMT, from the epoch, until the
account expires. This field may be left empty or set to 0 to turn off
the account aging feature.
The gecos field normally contains comma (`,') separated subfields as fol-
lows:
user's full name
user's office location
user's work phone number
user's home phone number
This information is used by the finger(1) program, and the first field
used by the system mailer. If an ampersand (`&') character appears
within the fullname field, programs that use this field will substitute
it with a capitalized version of the account's login name.
The user's home directory is the full UNIX path name where the user will
be placed on login.
The shell field is the command interpreter the user prefers. If there is
nothing in the shell field, the Bourne shell (/bin/sh) is assumed. For
security reasons, if the shell is set to a script that disallows access
to the system (the nologin(8) script, for example), care should be taken
not to import any environment variables. With sh(1), this can be done by
specifying the -p flag. Check the specific shell documentation to deter-
mine how this is done with other shells.
YP/NIS INTERACTION
Enabling access to NIS passwd data
The system administrator can configure FreeBSD to use NIS/YP for its
password information by adding special records to the /etc/master.passwd
file. These entries should be added with vipw(8) so that the changes can
be properly merged with the hashed password databases and the /etc/passwd
file ( /etc/passwd should never be edited manually). Alternatively, the
administrator can modify /etc/master.passwd in some other way and then
manually update the password databases with pwd_mkdb(8).
The simplest way to activate NIS is to add an empty record with only a
plus sign (`+') in the name field, such as this:
+:::::::::
The `+' will tell the getpwent(3) routines in FreeBSD's standard C
library to begin using the NIS passwd maps for lookups.
Note that the entry shown above is known as a wildcard entry, because it
matches all users (the `+' without any other information matches every-
body) and allows all NIS password data to be retrieved unaltered. How-
ever, by specifying a username or netgroup next to the `+' in the NIS
entry, the administrator can affect what data are extracted from the NIS
passwd maps and how it is interpreted. Here are a few example records
that illustrate this feature (note that you can have several NIS entries
in a single master.passwd file):
-mitnick:::::::::
+@staff:::::::::
+@permitted-users:::::::::
+dennis:::::::::
+ken:::::::::/bin/csh
+@rejected-users::32767:32767::::::/bin/false
Specific usernames are listed explicitly while netgroups are signified by
a preceding `@'. In the above example, users in the ``staff'' and
``permitted-users'' netgroups will have their password information read
from NIS and used unaltered. In other words, they will be allowed normal
access to the machine. Users ``ken'' and ``dennis'', who have been named
explicitly rather than through a netgroup, will also have their password
data read from NIS, except that user ``ken'' will have his shell remapped
to /bin/csh. This means that value for his shell specified in the NIS
password map will be overridden by the value specified in the special NIS
entry in the local master.passwd file. User ``ken'' may have been
assigned the csh shell because his NIS password entry specified a differ-
ent shell that may not be installed on the client machine for political
or technical reasons. Meanwhile, users in the ``rejected-users'' net-
group are prevented from logging in because their UIDs, GIDs and shells
have been overridden with invalid values.
User ``mitnick'' will be be ignored entirely because his entry is speci-
fied with a `-' instead of a `+'. A minus entry can be used to block out
certain NIS password entries completely; users whose password data has
been excluded in this way are not recognized by the system at all. (Any
overrides specified with minus entries are also ignored since there is no
point in processing override information for a user that the system isn't
going to recognize in the first place.) In general, a minus entry is
used to specifically exclude a user who might otherwise be granted access
because he happens to be a member of an authorized netgroup. For exam-
ple, if ``mitnick'' is a member of the ``permitted-users'' netgroup and
must, for whatever the reason, be permitted to remain in that netgroup
(possibly to retain access to other machines within the domain), the
administrator can still deny him access to a particular system with a
minus entry. Also, it is sometimes easier to explicitly list those users
who are not allowed access rather than generate a possibly complicated
list of users who are allowed access and omit the rest.
Note that the plus and minus entries are evaluated in order from first to
last with the first match taking precedence. This means the system will
only use the first entry that matches a particular user. If, using the
same example, there is a user ``foo'' who is a member of both the
``staff'' netgroup and the ``rejected-users'' netgroup, he will be admit-
ted to the system because the above example lists the entry for ``staff''
before the entry for ``rejected-users''. If the order were reversed,
user ``foo'' would be flagged as a ``rejected-user'' instead and denied
access.
Lastly, any NIS password database records that do not match against at
least one of the users or netgroups specified by the NIS access entries
in the /etc/master.passwd file will be ignored (along with any users
specified using minus entries). In our example shown above, we do not
have a wildcard entry at the end of the list; therefore, the system will
not recognize anyone except ``ken'', ``dennis'', the ``staff'' netgroup,
and the ``permitted-users'' netgroup as authorized users. The
``rejected-users'' netgroup will be recognized but all members will have
their shells remapped and therefore be denied access. All other NIS
password records will be ignored. The administrator may add a wildcard
entry to the end of the list such as:
+:::::::::/sbin/nologin
This entry acts as a catch-all for all users that don't match against any
of the other entries. This technique is sometimes useful when it is
desirable to have the system be able to recognize all users in a particu-
lar NIS domain without necessarily granting them login access. See the
description of the shell field regarding security concerns when using a
shell script as the login shell.
The primary use of this override feature is to permit the administrator
to enforce access restrictions on NIS client systems. Users can be
granted access to one group of machines and denied access to other
machines simply by adding or removing them from a particular netgroup.
Since the netgroup database can also be accessed via NIS, this allows
access restrictions to be administered from a single location, namely the
NIS master server; once a host's access list has been set in
/etc/master.passwd, it need not be modified again unless new netgroups
are created.
NOTES
Shadow passwords through NIS
FreeBSD uses a shadow password scheme: users' encrypted passwords are
stored only in /etc/master.passwd and /etc/spwd.db, which are readable
and writable only by the superuser. This is done to prevent users from
running the encrypted passwords through password-guessing programs and
gaining unauthorized access to other users' accounts. NIS does not sup-
port a standard means of password shadowing, which implies that placing
your password data into the NIS passwd maps totally defeats the security
of FreeBSD's password shadowing system.
FreeBSD provides a few special features to help get around this problem.
It is possible to implement password shadowing between FreeBSD NIS
clients and FreeBSD NIS servers. The getpwent(3) routines will search
for a master.passwd.byname and master.passwd.byuid maps which should con-
tain the same data found in the /etc/master.passwd file. If the maps
exist, FreeBSD will attempt to use them for user authentication instead
of the standard passwd.byname and passwd.byuid maps. FreeBSD's ypserv(8)
will also check client requests to make sure they originate on a privi-
leged port. Since only the superuser is allowed to bind to a privileged
port, the server can tell if the requesting user is the superuser; all
requests from non-privileged users to access the master.passwd maps will
be refused. Since all user authentication programs run with superuser
privilege, they should have the required access to users' encrypted pass-
word data while normal users will only be allowed access to the standard
passwd maps which contain no password information.
Note that this feature cannot be used in an environment with non-FreeBSD
systems. Note also that a truly determined user with unrestricted access
to your network could still compromise the master.passwd maps.
UID and GID remapping with NIS overrides
Unlike SunOS and other operating systems that use Sun's NIS code, FreeBSD
allows the user to override all of the fields in a user's NIS passwd
entry. For example, consider the following /etc/master.passwd entry:
+@foo-users:???:666:666:0:0:0:Bogus user:/home/bogus:/bin/bogus
This entry will cause all users in the `foo-users' netgroup to have all
of their password information overridden, including UIDs, GIDs and pass-
words. The result is that all `foo-users' will be locked out of the sys-
tem, since their passwords will be remapped to invalid values.
This is important to remember because most people are accustomed to using
an NIS wildcard entry that looks like this:
+:*:0:0:::
This often leads to new FreeBSD administrators choosing NIS entries for
their master.passwd files that look like this:
+:*:0:0::::::
Or worse, this
+::0:0::::::
DO _NOT_ PUT ENTRIES LIKE THIS IN YOUR master.passwd FILE!! The first
tells FreeBSD to remap all passwords to `*' (which will prevent anybody
from logging in) and to remap all UIDs and GIDs to 0 (which will make
everybody appear to be the superuser). The second case just maps all UIDs
and GIDs to 0, which means that all users will appear to be root!
Compatibility of NIS override evaluation
When Sun originally added NIS support to their getpwent(3) routines, they
took into account the fact that the SunOS password /etc/passwd file is in
plain ASCII format. The SunOS documentation claims that adding a `+'
entry to the password file causes the contents of the NIS password data-
base to be ``inserted'' at the position in the file where the `+' entry
appears. If, for example, the administrator places a `+::::::' entry in
the middle of /etc/passwd, then the entire contents of the NIS password
map would appear as though it had been copied into the middle of the
password file. If the administrator places `+::::::' entries at both the
middle and the end of /etc/passwd, then the NIS password map would appear
twice: once in the middle of the file and once at the end. (By using
override entries instead of simple wildcards, other combinations could be
achieved.)
By contrast, FreeBSD does not have a single ASCII password file: it has a
hashed password database. This database does not have an easily-defined
beginning, middle or end, which makes it very hard to design a scheme
that is 100% compatible with SunOS. For example, the getpwnam() and
getpwuid() functions in FreeBSD are designed to do direct queries to the
hash database rather than a linear search. This approach is faster on
systems where the password database is large. However, when using direct
database queries, the system does not know or care about the order of the
original password file, and therefore it cannot easily apply the same
override logic used by SunOS.
Instead, FreeBSD groups all the NIS override entries together and con-
structs a filter out of them. Each NIS password entry is compared
against the override filter exactly once and treated accordingly: if the
filter allows the entry through unaltered, it's treated unaltered; if the
filter calls for remapping of fields, then fields are remapped; if the
filter calls for explicit exclusion (i.e., the entry matches a `-' over-
ride), the entry is ignored; if the entry doesn't match against any of
the filter specifications, it's discarded.
Again, note that the NIS `+' and `-' entries themselves are handled in
the order in which they were specified in the /etc/master.passwd file,
since doing otherwise would lead to unpredictable behavior.
The end result is that FreeBSD's provides a very close approximation of
SunOS's behavior while maintaining the database paradigm, though the
getpwent(3) functions do behave somewhat differently from their SunOS
counterparts. The primary differences are:
Each NIS password map record can be mapped into the password
local password space only once.
The placement of the NIS `+' and `-' entries does not necessar-
ily affect where NIS password records will be mapped into the
password space.
In 99% of all FreeBSD configurations, NIS client behavior will be indis-
tinguishable from that of SunOS or other similar systems. Even so, users
should be aware of these architectural differences.
Using groups instead of netgroups for NIS overrides
FreeBSD offers the capability to do override matching based on user
groups rather than netgroups. If, for example, an NIS entry is specified
as:
+@operator:::::::::
the system will first try to match users against a netgroup called
`operator'. If an `operator' netgroup doesn't exist, the system will try
to match users against the normal `operator' group instead.
Changes in behavior from older versions of FreeBSD
There have been several bug fixes and improvements in FreeBSD's NIS/YP
handling, some of which have caused changes in behavior. While the
behavior changes are generally positive, it is important that users and
system administrators be aware of them:
1. In versions prior to 2.0.5, reverse lookups (i.e. using
getpwuid()) would not have overrides applied, which is to say
that it was possible for getpwuid() to return a login name
that getpwnam() would not recognize. This has been fixed:
overrides specified in /etc/master.passwd now apply to all
getpwent(3) functions.
2. Prior to FreeBSD 2.0.5, netgroup overrides did not work at
all, largely because FreeBSD did not have support for reading
netgroups through NIS. Again, this has been fixed, and net-
groups can be specified just as in SunOS and similar NIS-capa-
ble systems.
3. FreeBSD now has NIS server capabilities and supports the use
of master.passwd NIS maps in addition to the standard Sixth
Edition format passwd maps. This means that you can specify
change, expiration and class information through NIS, provided
you use a FreeBSD system as the NIS server.
FILES
/etc/passwd ASCII password file, with passwords removed
/etc/pwd.db db(3) -format password database, with passwords
removed
/etc/master.passwd ASCII password file, with passwords intact
/etc/spwd.db db(3) -format password database, with passwords
intact
SEE ALSO
chpass(1), login(1), passwd(1), getpwent(3), login_getclass(3),
login.conf(5), adduser(8), pw(8), pwd_mkdb(8), vipw(8), yp(8)
BUGS
User information should (and eventually will) be stored elsewhere.
The YP/NIS password database makes encrypted passwords visible to ordi-
nary users, thus making password cracking easier unless you use shadow
passwords with the master.passwd maps and FreeBSD's ypserv(8) server.
Unless you're using FreeBSD's ypserv(8), which supports the use of
master.passwd type maps, the YP/NIS password database will be in old-
style (Sixth Edition) format, which means that site-wide values for user
login class, password expiration date, and other fields present in the
current format will not be available when a FreeBSD system is used as a
client with a standard NIS server.
COMPATIBILITY
The password file format has changed since 4.3BSD. The following awk
script can be used to convert your old-style password file into a new
style password file. The additional fields ``class'', ``change'' and
``expire'' are added, but are turned off by default. These fields can
then be set using vipw(8) or pw(8).
BEGIN { FS = ":"}
{ print $1 ":" $2 ":" $3 ":" $4 "::0:0:" $5 ":" $6 ":" $7 }
HISTORY
A passwd file format appeared in Version 6 AT&T UNIX. The YP/NIS func-
tionality is modeled after SunOS and first appeared in FreeBSD 1.1 The
override capability is new in FreeBSD 2.0. The override capability was
updated to properly support netgroups in FreeBSD 2.0.5. Support for com-
ments first appeared in FreeBSD 3.0.
FreeBSD 4.9 September 29, 1994 FreeBSD 4.9
Федотов А.М.
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