534 lines
15 KiB
Groff
534 lines
15 KiB
Groff
.\" $FreeBSD$
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.\" $OpenBSD: authpf.8,v 1.43 2007/02/24 17:21:04 beck Exp $
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.\"
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.\" Copyright (c) 1998-2007 Bob Beck (beck@openbsd.org>. All rights reserved.
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\"
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.Dd March 28, 2006
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.Dt AUTHPF 8
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.Os
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.Sh NAME
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.Nm authpf
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.Nd authenticating gateway user shell
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.Sh SYNOPSIS
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.Nm authpf
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.Sh DESCRIPTION
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.Nm
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is a user shell for authenticating gateways.
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It is used to change
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.Xr pf 4
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rules when a user authenticates and starts a session with
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.Xr sshd 8
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and to undo these changes when the user's session exits.
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It is designed for changing filter and translation rules for an individual
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source IP address as long as a user maintains an active
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.Xr ssh 1
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session.
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Typical use would be for a gateway that authenticates users before
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allowing them Internet use, or a gateway that allows different users into
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different places.
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.Nm
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logs the successful start and end of a session to
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.Xr syslogd 8 .
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This, combined with properly set up filter rules and secure switches,
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can be used to ensure users are held accountable for their network traffic.
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.Pp
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.Nm
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can add filter and translation rules using the syntax described in
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.Xr pf.conf 5 .
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.Nm
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requires that the
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.Xr pf 4
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system be enabled and a
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.Xr fdescfs 5
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file system be mounted at
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.Pa /dev/fd
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before use.
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.Nm
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can also maintain the list of IP address of connected users
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in the "authpf_users"
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.Pa table .
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.Pp
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.Nm
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is meant to be used with users who can connect via
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.Xr ssh 1
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only.
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On startup,
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.Nm
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retrieves the client's connecting IP address via the
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.Ev SSH_CLIENT
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environment variable and, after performing additional access checks,
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reads a template file to determine what filter and translation rules
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(if any) to add.
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On session exit the same rules that were added at startup are removed.
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.Pp
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Each
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.Nm
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process stores its rules in a separate ruleset inside a
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.Xr pf 4
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.Pa anchor
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shared by all
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.Nm
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processes.
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By default, the
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.Pa anchor
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name "authpf" is used, and the ruleset names equal the username and PID of the
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.Nm
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processes as "username(pid)".
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The following rules need to be added to the main ruleset
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.Pa /etc/pf.conf
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in order to cause evaluation of any
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.Nm
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rules:
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.Bd -literal -offset indent
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nat-anchor "authpf/*"
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rdr-anchor "authpf/*"
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binat-anchor "authpf/*"
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anchor "authpf/*"
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.Ed
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.Pp
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The "/*" at the end of the anchor name is required for
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.Xr pf 4
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to process the rulesets attached to the anchor by
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.Nm authpf .
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.Sh FILTER AND TRANSLATION RULES
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Filter and translation rules for
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.Nm
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use the same format described in
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.Xr pf.conf 5 .
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The only difference is that these rules may (and probably should) use
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the macro
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.Em user_ip ,
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which is assigned the connecting IP address whenever
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.Nm
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is run.
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Additionally, the macro
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.Em user_id
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is assigned the user name.
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.Pp
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Filter and translation rules are stored in a file called
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.Pa authpf.rules .
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This file will first be searched for in
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.Pa /etc/authpf/users/$USER/
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and then in
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.Pa /etc/authpf/ .
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Only one of these files will be used if both are present.
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.Pp
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Per-user rules from the
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.Pa /etc/authpf/users/$USER/
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directory are intended to be used when non-default rules
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are needed on an individual user basis.
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It is important to ensure that a user can not write or change
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these configuration files.
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.Pp
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The
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.Pa authpf.rules
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file must exist in one of the above locations for
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.Nm
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to run.
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.Sh CONFIGURATION
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Options are controlled by the
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.Pa /etc/authpf/authpf.conf
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file.
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If the file is empty, defaults are used for all
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configuration options.
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The file consists of pairs of the form
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.Li name=value ,
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one per line.
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Currently, the allowed values are as follows:
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.Bl -tag -width Ds
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.It anchor=name
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Use the specified
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.Pa anchor
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name instead of "authpf".
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.It table=name
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Use the specified
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.Pa table
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name instead of "authpf_users".
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.El
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.Sh USER MESSAGES
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On successful invocation,
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.Nm
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displays a message telling the user he or she has been authenticated.
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It will additionally display the contents of the file
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.Pa /etc/authpf/authpf.message
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if the file exists and is readable.
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.Pp
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There exist two methods for providing additional granularity to the control
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offered by
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.Nm
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- it is possible to set the gateway to explicitly allow users who have
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authenticated to
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.Xr ssh 1
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and deny access to only a few troublesome individuals.
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This is done by creating a file with the banned user's login name as the
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filename in
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.Pa /etc/authpf/banned/ .
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The contents of this file will be displayed to a banned user, thus providing
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a method for informing the user that they have been banned, and where they can
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go and how to get there if they want to have their service restored.
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This is the default behaviour.
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.Pp
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It is also possible to configure
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.Nm
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to only allow specific users access.
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This is done by listing their login names, one per line, in
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.Pa /etc/authpf/authpf.allow .
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If "*" is found on a line, then all usernames match.
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If
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.Nm
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is unable to verify the user's permission to use the gateway, it will
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print a brief message and die.
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It should be noted that a ban takes precedence over an allow.
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.Pp
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On failure, messages will be logged to
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.Xr syslogd 8
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for the system administrator.
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The user does not see these, but will be told the system is unavailable due to
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technical difficulties.
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The contents of the file
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.Pa /etc/authpf/authpf.problem
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will also be displayed if the file exists and is readable.
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.Sh CONFIGURATION ISSUES
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.Nm
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maintains the changed filter rules as long as the user maintains an
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active session.
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It is important to remember however, that the existence
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of this session means the user is authenticated.
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Because of this, it is important to configure
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.Xr sshd 8
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to ensure the security of the session, and to ensure that the network
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through which users connect is secure.
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.Xr sshd 8
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should be configured to use the
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.Ar ClientAliveInterval
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and
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.Ar ClientAliveCountMax
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parameters to ensure that a ssh session is terminated quickly if
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it becomes unresponsive, or if arp or address spoofing is used to
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hijack the session.
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Note that TCP keepalives are not sufficient for
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this, since they are not secure.
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Also note that the various SSH tunnelling mechanisms,
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such as
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.Ar AllowTcpForwarding
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and
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.Ar PermitTunnel ,
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should be disabled for
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.Nm
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users to prevent them from circumventing restrictions imposed by the
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packet filter ruleset.
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.Pp
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.Nm
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will remove state table entries that were created during a user's
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session.
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This ensures that there will be no unauthenticated traffic
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allowed to pass after the controlling
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.Xr ssh 1
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session has been closed.
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.Pp
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.Nm
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is designed for gateway machines which typically do not have regular
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(non-administrative) users using the machine.
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An administrator must remember that
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.Nm
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can be used to modify the filter rules through the environment in
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which it is run, and as such could be used to modify the filter rules
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(based on the contents of the configuration files) by regular
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users.
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In the case where a machine has regular users using it, as well
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as users with
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.Nm
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as their shell, the regular users should be prevented from running
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.Nm
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by using the
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.Pa /etc/authpf/authpf.allow
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or
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.Pa /etc/authpf/banned/
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facilities.
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.Pp
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.Nm
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modifies the packet filter and address translation rules, and because
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of this it needs to be configured carefully.
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.Nm
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will not run and will exit silently if the
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.Pa /etc/authpf/authpf.conf
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file does not exist.
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After considering the effect
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.Nm
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may have on the main packet filter rules, the system administrator may
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enable
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.Nm
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by creating an appropriate
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.Pa /etc/authpf/authpf.conf
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file.
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.Sh EXAMPLES
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.Sy Control Files
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\- To illustrate the user-specific access control
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mechanisms, let us consider a typical user named bob.
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Normally, as long as bob can authenticate himself, the
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.Nm
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program will load the appropriate rules.
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Enter the
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.Pa /etc/authpf/banned/
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directory.
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If bob has somehow fallen from grace in the eyes of the
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powers-that-be, they can prohibit him from using the gateway by creating
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the file
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.Pa /etc/authpf/banned/bob
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containing a message about why he has been banned from using the network.
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Once bob has done suitable penance, his access may be restored by moving or
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removing the file
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.Pa /etc/authpf/banned/bob .
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.Pp
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Now consider a workgroup containing alice, bob, carol and dave.
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They have a
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wireless network which they would like to protect from unauthorized use.
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To accomplish this, they create the file
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.Pa /etc/authpf/authpf.allow
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which lists their login ids, one per line.
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At this point, even if eve could authenticate to
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.Xr sshd 8 ,
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she would not be allowed to use the gateway.
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Adding and removing users from
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the work group is a simple matter of maintaining a list of allowed userids.
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If bob once again manages to annoy the powers-that-be, they can ban him from
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using the gateway by creating the familiar
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.Pa /etc/authpf/banned/bob
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file.
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Though bob is listed in the allow file, he is prevented from using
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this gateway due to the existence of a ban file.
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.Pp
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.Sy Distributed Authentication
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\- It is often desirable to interface with a
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distributed password system rather than forcing the sysadmins to keep a large
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number of local password files in sync.
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The
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.Xr login.conf 5
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mechanism in
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.Ox
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can be used to fork the right shell.
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To make that happen,
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.Xr login.conf 5
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should have entries that look something like this:
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.Bd -literal -offset indent
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shell-default:shell=/bin/csh
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default:\e
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...
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:shell=/usr/sbin/authpf
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daemon:\e
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...
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:shell=/bin/csh:\e
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:tc=default:
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staff:\e
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...
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:shell=/bin/csh:\e
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:tc=default:
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.Ed
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.Pp
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Using a default password file, all users will get
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.Nm
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as their shell except for root who will get
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.Pa /bin/csh .
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.Pp
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.Sy SSH Configuration
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\- As stated earlier,
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.Xr sshd 8
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must be properly configured to detect and defeat network attacks.
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To that end, the following options should be added to
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.Xr sshd_config 5 :
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.Bd -literal -offset indent
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Protocol 2
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ClientAliveInterval 15
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ClientAliveCountMax 3
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.Ed
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.Pp
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This ensures that unresponsive or spoofed sessions are terminated within a
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minute, since a hijacker should not be able to spoof ssh keepalive messages.
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.Pp
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.Sy Banners
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\- Once authenticated, the user is shown the contents of
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.Pa /etc/authpf/authpf.message .
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This message may be a screen-full of the appropriate use policy, the contents
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of
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.Pa /etc/motd
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or something as simple as the following:
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.Bd -literal -offset indent
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This means you will be held accountable by the powers that be
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for traffic originating from your machine, so please play nice.
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.Ed
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.Pp
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To tell the user where to go when the system is broken,
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.Pa /etc/authpf/authpf.problem
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could contain something like this:
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.Bd -literal -offset indent
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Sorry, there appears to be some system problem. To report this
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problem so we can fix it, please phone 1-900-314-1597 or send
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an email to remove@bulkmailerz.net.
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.Ed
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.Pp
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.Sy Packet Filter Rules
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\- In areas where this gateway is used to protect a
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wireless network (a hub with several hundred ports), the default rule set as
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well as the per-user rules should probably allow very few things beyond
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encrypted protocols like
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.Xr ssh 1 ,
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.Xr ssl 8 ,
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or
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.Xr ipsec 4 .
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On a securely switched network, with plug-in jacks for visitors who are
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given authentication accounts, you might want to allow out everything.
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In this context, a secure switch is one that tries to prevent address table
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overflow attacks.
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.Pp
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Example
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.Pa /etc/pf.conf :
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.Bd -literal
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# by default we allow internal clients to talk to us using
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# ssh and use us as a dns server.
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internal_if="fxp1"
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gateway_addr="10.0.1.1"
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nat-anchor "authpf/*"
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rdr-anchor "authpf/*"
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binat-anchor "authpf/*"
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block in on $internal_if from any to any
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pass in quick on $internal_if proto tcp from any to $gateway_addr \e
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port = ssh
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pass in quick on $internal_if proto udp from any to $gateway_addr \e
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port = domain
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anchor "authpf/*"
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.Ed
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.Pp
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.Sy For a switched, wired net
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\- This example
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.Pa /etc/authpf/authpf.rules
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makes no real restrictions; it turns the IP address on and off, logging
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TCP connections.
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.Bd -literal
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external_if = "xl0"
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internal_if = "fxp0"
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pass in log quick on $internal_if proto tcp from $user_ip to any
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pass in quick on $internal_if from $user_ip to any
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.Ed
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.Pp
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.Sy For a wireless or shared net
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\- This example
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.Pa /etc/authpf/authpf.rules
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could be used for an insecure network (such as a public wireless network) where
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we might need to be a bit more restrictive.
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.Bd -literal
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internal_if="fxp1"
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ipsec_gw="10.2.3.4"
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# rdr ftp for proxying by ftp-proxy(8)
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rdr on $internal_if proto tcp from $user_ip to any port 21 \e
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-> 127.0.0.1 port 8021
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# allow out ftp, ssh, www and https only, and allow user to negotiate
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# ipsec with the ipsec server.
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pass in log quick on $internal_if proto tcp from $user_ip to any \e
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port { 21, 22, 80, 443 }
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pass in quick on $internal_if proto tcp from $user_ip to any \e
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port { 21, 22, 80, 443 }
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pass in quick proto udp from $user_ip to $ipsec_gw port = isakmp
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pass in quick proto esp from $user_ip to $ipsec_gw
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.Ed
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.Pp
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.Sy Dealing with NAT
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\- The following
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.Pa /etc/authpf/authpf.rules
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shows how to deal with NAT, using tags:
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.Bd -literal
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ext_if = "fxp1"
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ext_addr = 129.128.11.10
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int_if = "fxp0"
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# nat and tag connections...
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nat on $ext_if from $user_ip to any tag $user_ip -> $ext_addr
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pass in quick on $int_if from $user_ip to any
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pass out log quick on $ext_if tagged $user_ip
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.Ed
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.Pp
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With the above rules added by
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.Nm ,
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outbound connections corresponding to each users NAT'ed connections
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will be logged as in the example below, where the user may be identified
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from the ruleset name.
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.Bd -literal
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# tcpdump -n -e -ttt -i pflog0
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Oct 31 19:42:30.296553 rule 0.bbeck(20267).1/0(match): pass out on fxp1: \e
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129.128.11.10.60539 > 198.137.240.92.22: S 2131494121:2131494121(0) win \e
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16384 <mss 1460,nop,nop,sackOK> (DF)
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.Ed
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.Pp
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.Sy Using the authpf_users table
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\- Simple
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.Nm
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settings can be implemented without an anchor by just using the "authpf_users"
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.Pa table .
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For example, the following
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.Xr pf.conf 5
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lines will give SMTP and IMAP access to logged in users:
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.Bd -literal
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table <authpf_users> persist
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pass in on $ext_if proto tcp from <authpf_users> \e
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to port { smtp imap }
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.Ed
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.Pp
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It is also possible to use the "authpf_users"
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.Pa table
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in combination with anchors.
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For example,
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.Xr pf 4
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processing can be sped up by looking up the anchor
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only for packets coming from logged in users:
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.Bd -literal
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table <authpf_users> persist
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anchor "authpf/*" from <authpf_users>
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rdr-anchor "authpf/*" from <authpf_users>
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.Ed
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.Sh FILES
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.Bl -tag -width "/etc/authpf/authpf.conf" -compact
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.It Pa /etc/authpf/authpf.conf
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.It Pa /etc/authpf/authpf.allow
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.It Pa /etc/authpf/authpf.rules
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.It Pa /etc/authpf/authpf.message
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.It Pa /etc/authpf/authpf.problem
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.El
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.Sh SEE ALSO
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.Xr pf 4 ,
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.Xr pf.conf 5 ,
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.Xr fdescfs 5 ,
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.Xr securelevel 7 ,
|
|
.Xr ftp-proxy 8
|
|
.Sh HISTORY
|
|
The
|
|
.Nm
|
|
program first appeared in
|
|
.Ox 3.1 .
|
|
.Sh BUGS
|
|
Configuration issues are tricky.
|
|
The authenticating
|
|
.Xr ssh 1
|
|
connection may be secured, but if the network is not secured the user may
|
|
expose insecure protocols to attackers on the same network, or enable other
|
|
attackers on the network to pretend to be the user by spoofing their IP
|
|
address.
|
|
.Pp
|
|
.Nm
|
|
is not designed to prevent users from denying service to other users.
|