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225 lines
5.0 KiB
Groff
225 lines
5.0 KiB
Groff
.\" $FreeBSD$
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.\"
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.Dd January 12, 2001
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.Dt SKEY.ACCESS 5
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.Os
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.Sh NAME
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.Nm skey.access
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.Nd "S/Key password control table"
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.Sh DESCRIPTION
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The S/Key password control table
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.Pq Pa /etc/skey.access
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is used by
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.Nm login Ns \-like
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programs to determine when
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.Ux
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passwords may be used
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to access the system.
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.Bl -bullet
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.It
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When the table does not exist, there are no password restrictions.
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The user may enter the
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.Ux
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password or the S/Key one.
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.It
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When the table does exist,
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.Ux
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passwords are permitted only when
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explicitly specified.
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.It
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For the sake of sanity,
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.Ux
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passwords are always permitted on the
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systems console.
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.El
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.Sh TABLE FORMAT
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The format of the table is one rule per line.
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Rules are matched in order.
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The search terminates when the first matching rule is found, or
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when the end of the table is reached.
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.Pp
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Rules have the form:
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.Pp
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.Bl -item -offset indent -compact
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.It
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.Ic permit
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.Ar condition condition ...
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.It
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.Ic deny
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.Ar condition condition ...
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.El
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.Pp
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where
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.Ic permit
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and
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.Ic deny
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may be followed by zero or more
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.Ar conditions .
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Comments begin with a
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.Ql #
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character, and extend through the end of the line.
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Empty lines or
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lines with only comments are ignored.
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.Pp
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A rule is matched when all conditions are satisfied.
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A rule without
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conditions is always satisfied.
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For example, the last entry could
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be a line with just the word
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.Ic deny
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on it.
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.Sh CONDITIONS
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.Bl -tag -width indent
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.It Ic hostname Ar wzv.win.tue.nl
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True when the login comes from host
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.Ar wzv.win.tue.nl .
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See the
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.Sx WARNINGS
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section below.
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.It Ic internet Ar 131.155.210.0 255.255.255.0
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True when the remote host has an internet address in network
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.Ar 131.155.210 .
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The general form of a net/mask rule is:
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.Pp
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.D1 Ic internet Ar net mask
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.Pp
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The expression is true when the host has an internet address for which
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the bitwise and of
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.Ar address
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and
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.Ar mask
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equals
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.Ar net .
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See the
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.Sx WARNINGS
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section below.
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.It Ic port Ar ttya
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True when the login terminal is equal to
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.Pa /dev/ttya .
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Remember that
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.Ux
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passwords are always permitted with logins on the
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system console.
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.It Ic user Ar uucp
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True when the user attempts to log in as
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.Ar uucp .
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.It Ic group Ar wheel
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True when the user attempts to log in as a member of the
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.Ar wheel
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group.
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.El
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.Sh COMPATIBILITY
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For the sake of backwards compatibility, the
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.Ic internet
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keyword may be omitted from net/mask patterns.
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.Sh WARNINGS
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When the S/Key control table
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.Pq Pa /etc/skey.access
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exists, users without S/Key passwords will be able to login only
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where its rules allow the use of
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.Ux
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passwords.
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In particular, this
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means that an invocation of
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.Xr login 1
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in a pseudo-tty (e.g. from
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within
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.Xr xterm 1
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or
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.Xr screen 1
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will be treated as a login
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that is neither from the console nor from the network, mandating the use
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of an S/Key password.
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Such an invocation of
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.Xr login 1
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will necessarily
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fail for those users who do not have an S/Key password.
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.Pp
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Several rule types depend on host name or address information obtained
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through the network.
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What follows is a list of conceivable attacks to force the system to permit
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.Ux
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passwords.
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.Ss "Host address spoofing (source routing)"
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An intruder configures a local interface to an address in a trusted
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network and connects to the victim using that source address.
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Given
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the wrong client address, the victim draws the wrong conclusion from
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rules based on host addresses or from rules based on host names derived
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from addresses.
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.Pp
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Remedies:
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.Bl -enum
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.It
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do not permit
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.Ux
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passwords with network logins;
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.It
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use network software that discards source routing information (e.g.\&
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a tcp wrapper).
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.El
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.Pp
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Almost every network server must look up the client host name using the
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client network address.
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The next obvious attack therefore is:
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.Ss "Host name spoofing (bad PTR record)"
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An intruder manipulates the name server system so that the client
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network address resolves to the name of a trusted host.
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Given the
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wrong host name, the victim draws the wrong conclusion from rules based
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on host names, or from rules based on addresses derived from host
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names.
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.Pp
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Remedies:
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.Bl -enum
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.It
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do not permit
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.Ux
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passwords with network logins;
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.It
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use
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network software that verifies that the hostname resolves to the client
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network address (e.g. a tcp wrapper).
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.El
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.Pp
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Some applications, such as the
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.Ux
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.Xr login 1
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program, must look up the
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client network address using the client host name.
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In addition to the
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previous two attacks, this opens up yet another possibility:
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.Ss "Host address spoofing (extra A record)"
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An intruder manipulates the name server system so that the client host
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name (also) resolves to a trusted address.
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.Pp
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Remedies:
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.Bl -enum
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.It
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do not permit
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.Ux
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passwords with network logins;
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.It
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the
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.Fn skeyaccess
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routines ignore network addresses that appear to
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belong to someone else.
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.El
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.Sh DIAGNOSTICS
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Syntax errors are reported to the
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.Xr syslogd 8 .
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When an error is found
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the rule is skipped.
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.Sh FILES
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.Bl -tag -width /etc/skey.access
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.It Pa /etc/skey.access
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password control table
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.El
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.Sh SEE ALSO
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.Xr login 1 ,
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.Xr syslogd 8
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.Sh AUTHORS
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.An Wietse Venema ,
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Eindhoven University of Technology,
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The Netherlands.
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