135 lines
4.7 KiB
Groff
135 lines
4.7 KiB
Groff
.TH SKEY.ACCESS 5
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.SH NAME
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skey.access \- S/Key password control table
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.SH DESCRIPTION
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The S/Key password control table (\fI/etc/skey.access\fR) is used by
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\fIlogin\fR-like programs to determine when UNIX passwords may be used
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to access the system.
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.IP \(bu
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When the table does not exist, there are no password restrictions. The
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user may enter the UNIX password or the S/Key one.
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.IP \(bu
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When the table does exist, UNIX passwords are permitted only when
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explicitly specified.
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.IP \(bu
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For the sake of sanity, UNIX passwords are always permitted on the
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systems console.
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.SH "TABLE FORMAT"
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The format of the table is one rule per line. Rules are matched in
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order. 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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.sp
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.in +5
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permit condition condition...
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.br
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deny condition condition...
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.in
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.PP
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where
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.I permit
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and
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.I deny
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may be followed by zero or more conditions. Comments begin with a `#\'
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character, and extend through the end of the line. 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. A rule without
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conditions is always satisfied. For example, the last entry could
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be a line with just the word
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.I deny
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on it.
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.SH CONDITIONS
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.IP "hostname wzv.win.tue.nl"
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True when the login comes from host wzv.win.tue.nl.
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See the WARNINGS section below.
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.IP "internet 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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131.155.210. The general form of a net/mask rule is:
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.sp
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.ti +5
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internet net mask
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.sp
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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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.I address
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and
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.I mask
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equals
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.IR net.
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See the WARNINGS section below.
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.IP "port ttya"
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True when the login terminal is equal to
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.IR /dev/ttya .
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Remember that UNIX passwords are always permitted with logins on the
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system console.
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.IP "user uucp"
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True when the user attempts to log in as
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.IR uucp .
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.IP "group wheel"
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True when the user attempts to log in as a member of the
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.I wheel
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group.
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.SH COMPATIBILITY
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For the sake of backwards compatibility, the
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.I 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 (\fI/etc/skey.access\fR)
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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 UNIX passwords. In particular, this
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means that an invocation of \fIlogin(1)\fR in a pseudo-tty (e.g. from
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within \fIxterm(1)\fR or \fIscreen(1)\fR) 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. Such an invocation of \fIlogin(1)\fR 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. What follows is a list of conceivable attacks to
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force the system to permit UNIX passwords.
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.IP "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. 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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.sp
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Remedies: (1) do not permit UNIX passwords with network logins; (2)
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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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.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. The next obvious attack therefore is:
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.IP "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. 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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.sp
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Remedies: (1) do not permit UNIX passwords with network logins; (2) 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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.PP
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Some applications, such as the UNIX login program, must look up the
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client network address using the client host name. In addition to the
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previous two attacks, this opens up yet another possibility:
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.IP "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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.sp
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Remedies: (1) do not permit UNIX passwords with network logins; (2)
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the skeyaccess() routines ignore network addresses that appear to
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belong to someone else.
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.SH DIAGNOSTICS
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Syntax errors are reported to the syslogd. When an error is found
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the rule is skipped.
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.SH FILES
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/etc/skey.access, password control table
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.SH AUTHOR
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.nf
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Wietse Venema
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Eindhoven University of Technology
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The Netherlands
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