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>What login does</TITLE
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></A
>10.3. What <B
CLASS="command"
>login</B
> does</H1
><P
>The <B
CLASS="command"
>login</B
> program takes care of
authenticating the user (making sure that the username and
password match), and of setting up an initial environment for
the user by setting permissions for the serial line and starting
the shell. </P
><P
> Part of the initial setup is outputting the contents of
the file <TT
CLASS="filename"
>/etc/motd</TT
> (short for message of the
day) and checking for electronic mail. These can be disabled
by creating a file called <TT
CLASS="filename"
>.hushlogin</TT
> in
the user's home directory. </P
><P
> If the file <TT
CLASS="filename"
>/etc/nologin</TT
>
exists, logins are disabled. That file is typically
created by <B
CLASS="command"
>shutdown</B
> and relatives.
<B
CLASS="command"
>login</B
> checks for this file, and will
refuse to accept a login if it exists. If it does exist,
<B
CLASS="command"
>login</B
> outputs its contents to the terminal
before it quits. </P
><P
> <B
CLASS="command"
>login</B
> logs all failed login attempts in
a system log file (via <B
CLASS="command"
>syslog</B
>). It also logs
all logins by root. Both of these can be useful when tracking
down intruders. </P
><P
> Currently logged in people are listed in
<TT
CLASS="filename"
>/var/run/utmp</TT
>. This file is valid only
until the system is next rebooted or shut down; it is cleared
when the system is booted. It lists each user and the terminal
(or network connection) he is using, along with some other useful
information. The <B
CLASS="command"
>who</B
>, <B
CLASS="command"
>w</B
>,
and other similar commands look in <TT
CLASS="filename"
>utmp</TT
>
to see who are logged in. </P
><P
> All successful logins are recorded into
<TT
CLASS="filename"
>/var/log/wtmp</TT
>. This file will grow without
limit, so it must be cleaned regularly, for example by having
a weekly <B
CLASS="command"
>cron</B
> job to clear it.
<A
NAME="AEN2309"
HREF="#FTN.AEN2309"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
The <B
CLASS="command"
>last</B
> command browses
<TT
CLASS="filename"
>wtmp</TT
>. </P
><P
> Both <TT
CLASS="filename"
>utmp</TT
> and
<TT
CLASS="filename"
>wtmp</TT
> are in a binary format (see the
<TT
CLASS="filename"
>utmp</TT
> manual page); it is unfortunately not
convenient to examine them without special programs. </P
></DIV
><H3
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>Notes</H3
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><SPAN
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>[1]</SPAN
></A
></TD
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><P
>Good Linux distributions do this out
of the box.</P
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