SKEY.ACCESS(5)

NAME
skey.access - S/Key password control table

DESCRIPTION
The S/Key password control table (/etc/skey.access) is used by login-like programs to determine when UNIX passwords may be used to access the system.

•   When the table does not exist, there are no password restrictions. The user may enter the UNIX password or the S/Key one.

•   When the table does exist, UNIX passwords are permitted only when explicitly specified.

•   For the sake of sanity, UNIX passwords are always permitted on the systems console.

TABLE FORMAT
The format of the table is one rule per line. Rules are matched in order. The search terminates when the first matching rule is found, or when the end of the table is reached.

Rules have the form:

permit condition condition...
deny condition condition...

where permit and deny may be followed by zero or more conditions. Comments begin with a `#' character, and extend through the end of the line. Empty lines or lines with only comments are ignored.

A rule is matched when all conditions are satisfied. A rule without conditions is always satisfied. For example, the last entry could be a line with just the word deny on it.

CONDITIONS

hostname wzv.win.tue.nl
    True when the login comes from host wzv.win.tue.nl. See the WARNINGS section below.

internet 131.155.210.0 255.255.255.0
    True when the remote host has an internet address in network 131.155.210. The general form of a net/mask rule is:

    internet net mask

    The expression is true when the host has an internet address for which the bitwise and of address and mask equals net. See the WARNINGS section below.

port ttya
    True when the login terminal is equal to /dev/ttya. Remember that UNIX passwords are always permitted with logins on the system console.

user uucp
    True when the user attempts to log in as uucp.

group wheel
    True when the user attempts to log in as a member of the wheel group.

COMPATIBILITY
For the sake of backwards compatibility, the internet keyword may be omitted from net/mask patterns.

WARNINGS
Several rule types depend on host name or address information obtained through the network. What follows is a list of conceivable attacks to force the system to permit UNIX passwords.

Host address spoofing (source routing)
    An intruder configures a local interface to an address in a trusted network and connects to the victim using that source address. Given the wrong client address, the victim draws the wrong conclusion from rules based on host addresses or from rules based on host names derived from addresses.

    Remedies: (1) do not permit UNIX passwords with network logins; (2) use network software that discards source routing information (e.g. a tcp wrapper).

Almost every network server must look up the client host name using the client network address. The next obvious attack therefore is:

Host name spoofing (bad PTR record)
    An intruder manipulates the name server system so that the client network address resolves to the name of a trusted host. Given the wrong host name, the victim draws the wrong conclusion from rules based on host names, or from rules based on addresses derived from host names.

    Remedies: (1) do not permit UNIX passwords with network logins; (2) use network software that verifies that the hostname resolves to the client network address (e.g. a tcp wrapper).

Some applications, such as the UNIX login program, must look up the client network address using the client host name. In addition to the previous two attacks, this opens up yet another possibility:

Host address spoofing (extra A record)
    An intruder manipulates the name server system so that the client host name (also) resolves to a trusted address.

    Remedies: (1) do not permit UNIX passwords with network logins; (2) the skeyaccess() routines ignore network addresses that appear to belong to someone else.

DIAGNOSTICS
Syntax errors are reported to the syslogd. When an error is found the rule is skipped.

FILES
/etc/skey.access, password control table

AUTHOR
Wietse Venema
Eindhoven University of Technology
The Netherlands