LOGIN(1) Linux Programmer's Manual LOGIN(1)
NAME
login - sign on
SYNOPSIS
login [ name ]
login -p
login -h hostname
login -f name
DESCRIPTION
login is used when signing onto a system. It can also be used to switch from one user to
another at any time (most modern shells have support for this feature built into them,
however).
If an argument is not given, login prompts for the username.
If the user is not root, and if /etc/nologin exists, the contents of this file are printed
to the screen, and the login is terminated. This is typically used to prevent logins when
the system is being taken down.
If special access restrictions are specified for the user in /etc/usertty, these must be
met, or the log in attempt will be denied and a syslog message will be generated. See the
section on "Special Access Restrictions".
If the user is root, then the login must be occurring on a tty listed in /etc/securetty.
Failures will be logged with the syslog facility.
After these conditions have been checked, the password will be requested and checked (if a
password is required for this username). Ten attempts are allowed before login dies, but
after the first three, the response starts to get very slow. Login failures are reported
via the syslog facility. This facility is also used to report any successful root logins.
If the file .hushlogin exists, then a "quiet" login is performed (this disables the check-
ing of mail and the printing of the last login time and message of the day). Otherwise,
if /var/log/lastlog exists, the last login time is printed (and the current login is
recorded).
Random administrative things, such as setting the UID and GID of the tty are performed.
The TERM environment variable is preserved, if it exists (other environment variables are
preserved if the -p option is used). Then the HOME, PATH, SHELL, TERM, MAIL, and LOGNAME
environment variables are set. PATH defaults to /usr/local/bin:/bin:/usr/bin for normal
users, and to /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin for root.
Last, if this is not a "quiet" login, the message of the day is printed and the file with
the user's name in /var/spool/mail will be checked, and a message printed if it has non-
zero length.
The user's shell is then started. If no shell is specified for the user in /etc/passwd,
then /bin/sh is used. If there is no directory specified in /etc/passwd, then / is used
(the home directory is checked for the .hushlogin file described above).
OPTIONS
-p Used by getty(8) to tell login not to destroy the environment
-f Used to skip a second login authentication. This specifically does not work for
root, and does not appear to work well under Linux.
-h Used by other servers (i.e., telnetd(8)) to pass the name of the remote host to
login so that it may be placed in utmp and wtmp. Only the superuser may use this
option.
SPECIAL ACCESS RESTRICTIONS
The file /etc/securetty lists the names of the ttys where root is allowed to log in. One
name of a tty device without the /dev/ prefix must be specified on each line. If the file
does not exist, root is allowed to log in on any tty.
On most modern Linux systems PAM (Pluggable Authentication Modules) is used. On systems
that do not use PAM, the file /etc/usertty specifies additional access restrictions for
specific users. If this file does not exist, no additional access restrictions are
imposed. The file consists of a sequence of sections. There are three possible section
types: CLASSES, GROUPS and USERS. A CLASSES section defines classes of ttys and hostname
patterns, A GROUPS section defines allowed ttys and hosts on a per group basis, and a
USERS section defines allowed ttys and hosts on a per user basis.
Each line in this file in may be no longer than 255 characters. Comments start with #
character and extend to the end of the line.
The CLASSES Section
A CLASSES section begins with the word CLASSES at the start of a line in all upper case.
Each following line until the start of a new section or the end of the file consists of a
sequence of words separated by tabs or spaces. Each line defines a class of ttys and host
patterns.
The word at the beginning of a line becomes defined as a collective name for the ttys and
host patterns specified at the rest of the line. This collective name can be used in any
subsequent GROUPS or USERS section. No such class name must occur as part of the defini-
tion of a class in order to avoid problems with recursive classes.
An example CLASSES section:
CLASSES
myclass1 tty1 tty2
myclass2 tty3 @.foo.com
This defines the classes myclass1 and myclass2 as the corresponding right hand sides.
The GROUPS Section
A GROUPS section defines allowed ttys and hosts on a per Unix group basis. If a user is a
member of a Unix group according to /etc/passwd and /etc/group and such a group is men-
tioned in a GROUPS section in /etc/usertty then the user is granted access if the group
is.
A GROUPS section starts with the word GROUPS in all upper case at the start of a line, and
each following line is a sequence of words separated by spaces or tabs. The first word on
a line is the name of the group and the rest of the words on the line specifies the ttys
and hosts where members of that group are allowed access. These specifications may involve
the use of classes defined in previous CLASSES sections.
An example GROUPS section.
GROUPS
sys tty1 @.bar.edu
stud myclass1 tty4
This example specifies that members of group sys may log in on tty1 and from hosts in the
bar.edu domain. Users in group stud may log in from hosts/ttys specified in the class
myclass1 or from tty4.
The USERS Section
A USERS section starts with the word USERS in all upper case at the start of a line, and
each following line is a sequence of words separated by spaces or tabs. The first word on
a line is a username and that user is allowed to log in on the ttys and from the hosts
mentioned on the rest of the line. These specifications may involve classes defined in
previous CLASSES sections. If no section header is specified at the top of the file, the
first section defaults to be a USERS section.
An example USERS section:
USERS
zacho tty1 @130.225.16.0/255.255.255.0
blue tty3 myclass2
This lets the user zacho login only on tty1 and from hosts with IP addreses in the range
130.225.16.0 - 130.225.16.255, and user blue is allowed to log in from tty3 and whatever
is specified in the class myclass2.
There may be a line in a USERS section starting with a username of *. This is a default
rule and it will be applied to any user not matching any other line.
If both a USERS line and GROUPS line match a user then the user is allowed access from the
union of all the ttys/hosts mentioned in these specifications.
Origins
The tty and host pattern specifications used in the specification of classes, group and
user access are called origins. An origin string may have one of these formats:
o The name of a tty device without the /dev/ prefix, for example tty1 or ttyS0.
o The string @localhost, meaning that the user is allowed to telnet/rlogin from the
local host to the same host. This also allows the user to for example run the com-
mand: xterm -e /bin/login.
o A domain name suffix such as @.some.dom, meaning that the user may rlogin/telnet
from any host whose domain name has the suffix .some.dom.
o A range of IPv4 addresses, written @x.x.x.x/y.y.y.y where x.x.x.x is the IP address
in the usual dotted quad decimal notation, and y.y.y.y is a bitmask in the same
notation specifying which bits in the address to compare with the IP address of the
remote host. For example @130.225.16.0/255.255.254.0 means that the user may
rlogin/telnet from any host whose IP address is in the range 130.225.16.0 -
130.225.17.255.
Any of the above origins may be prefixed by a time specification according to the syntax:
timespec ::= '[' [':' ]* ']'
day ::= 'mon' | 'tue' | 'wed' | 'thu' | 'fri' | 'sat' | 'sun'
hour ::= '0' | '1' | ... | '23'
hourspec ::= | '-'
day-or-hour ::= |
For example, the origin [mon:tue:wed:thu:fri:8-17]tty3 means that log in is allowed on
mondays through fridays between 8:00 and 17:59 (5:59 pm) on tty3. This also shows that an
hour range a-b includes all moments between a:00 and b:59. A single hour specification
(such as 10) means the time span between 10:00 and 10:59.
Not specifying any time prefix for a tty or host means log in from that origin is allowed
any time. If you give a time prefix be sure to specify both a set of days and one or more
hours or hour ranges. A time specification may not include any white space.
If no default rule is given then users not matching any line /etc/usertty are allowed to
log in from anywhere as is standard behavior.
FILES
/var/run/utmp
/var/log/wtmp
/var/log/lastlog
/var/spool/mail/*
/etc/motd
/etc/passwd
/etc/nologin
/etc/usertty
.hushlogin
SEE ALSO
init(8), getty(8), mail(1), passwd(1), passwd(5), environ(7), shutdown(8)
BUGS
The undocumented BSD -r option is not supported. This may be required by some rlogind(8)
programs.
A recursive login, as used to be possible in the good old days, no longer works; for most
purposes su(1) is a satisfactory substitute. Indeed, for security reasons, login does a
vhangup() system call to remove any possible listening processes on the tty. This is to
avoid password sniffing. If one uses the command "login", then the surrounding shell gets
killed by vhangup() because it's no longer the true owner of the tty. This can be avoided
by using "exec login" in a top-level shell or xterm.
AUTHOR
Derived from BSD login 5.40 (5/9/89) by Michael Glad () for HP-UX
Ported to Linux 0.12: Peter Orbaek ()
Util-linux 1.6 4 November 1996 LOGIN(1)
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