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AUTHPF(8)		FreeBSD	System Manager's Manual		     AUTHPF(8)

     authpf, authpf-noip -- authenticating gateway user	shell


     authpf is a user shell for	authenticating gateways.  It is	used to	change
     pf(4) rules when a	user authenticates and starts a	session	with sshd(8)
     and to undo these changes when the	user's session exits.  Typical use
     would be for a gateway that authenticates users before allowing them
     Internet use, or a	gateway	that allows different users into different
     places.  Combined with properly set up filter rules and secure switches,
     authpf can	be used	to ensure users	are held accountable for their network
     traffic.  It is meant to be used with users who can connect via ssh(1)
     only, and requires	the pf(4) subsystem to be enabled.

     authpf-noip is a user shell which allows multiple connections to take
     place from	the same IP address.  It is useful primarily in	cases where
     connections are tunneled via the gateway system, and can be directly
     associated	with the user name.  It	cannot ensure accountability when
     classifying connections by	IP address; in this case the client's IP
     address is	not provided to	the packet filter via the client_ip macro or
     the authpf_users table.  Additionally, states associated with the client
     IP	address	are not	purged when the	session	is ended.

     To	use either authpf or authpf-noip, the user's shell needs to be set to
     /usr/sbin/authpf or /usr/sbin/authpf-noip.

     authpf uses the pf.conf(5)	syntax to change filter	and translation	rules
     for an individual user or client IP address as long as a user maintains
     an	active ssh(1) session, and logs	the successful start and end of	a ses-
     sion to syslogd(8).  authpf retrieves the client's	connecting IP address
     via the SSH_CLIENT	environment variable and, after	performing additional
     access checks, reads a template file to determine what filter and trans-
     lation rules (if any) to add, and maintains the list of IP	addresses of
     connected users in	the authpf_users table.	 On session exit the same
     rules and table entries that were added at	startup	are removed, and all
     states associated with the	client's IP address are	purged.

     Each authpf process stores	its rules in a separate	ruleset	inside a pf(4)
     anchor shared by all authpf processes.  By	default, the anchor name
     "authpf" is used, and the ruleset names equal the username	and PID	of the
     authpf processes as "username(pid)".  The following rules need to be
     added to the main ruleset /etc/pf.conf in order to	cause evaluation of
     any authpf	rules:

	   nat-anchor "authpf/*"
	   rdr-anchor "authpf/*"
	   binat-anchor	"authpf/*"
	   anchor "authpf/*"

     The "/*" at the end of the	anchor name is required	for pf(4) to process
     the rulesets attached to the anchor by authpf.

     Filter and	translation rules for authpf use the same format described in
     pf.conf(5).  The only difference is that these rules may (and probably
     should) use the macro user_ip, which is assigned the connecting IP
     address whenever authpf is	run.  Additionally, the	macro user_id is
     assigned the user name.

     Filter and	translation rules are stored in	a file called authpf.rules.
     This file will first be searched for in /etc/authpf/users/$USER/ and then
     in	/etc/authpf/.  Only one	of these files will be used if both are

     Per-user rules from the /etc/authpf/users/$USER/ directory	are intended
     to	be used	when non-default rules are needed on an	individual user	basis.
     It	is important to	ensure that a user can not write or change these con-
     figuration	files.

     The authpf.rules file must	exist in one of	the above locations for	authpf
     to	run.

     Options are controlled by the /etc/authpf/authpf.conf file.  If the file
     is	empty, defaults	are used for all configuration options.	 The file con-
     sists of pairs of the form	name=value, one	per line.  Currently, the
     allowed values are	as follows:

	     Use the specified anchor name instead of "authpf".

	     Use the specified table name instead of "authpf_users".

     On	successful invocation, authpf displays a message telling the user he
     or	she has	been authenticated.  It	will additionally display the contents
     of	the file /etc/authpf/authpf.message if the file	exists and is read-

     There exist two methods for providing additional granularity to the con-
     trol offered by authpf - it is possible to	set the	gateway	to explicitly
     allow users who have authenticated	to ssh(1) and deny access to only a
     few troublesome individuals.  This	is done	by creating a file with	the
     banned user's login name as the filename in /etc/authpf/banned/.  The
     contents of this file will	be displayed to	a banned user, thus providing
     a method for informing the	user that they have been banned, and where
     they can go and how to get	there if they want to have their service
     restored.	This is	the default behaviour.

     It	is also	possible to configure authpf to	only allow specific users
     access.  This is done by listing their login names, one per line, in
     /etc/authpf/authpf.allow.	A group	of users can also be indicated by
     prepending	"%" to the group name, and all members of a login class	can be
     indicated by prepending "@" to the	login class name.  If "*" is found on
     a line, then all usernames	match.	If authpf is unable to verify the
     user's permission to use the gateway, it will print a brief message and
     die.  It should be	noted that a ban takes precedence over an allow.

     On	failure, messages will be logged to syslogd(8) for the system adminis-
     trator.  The user does not	see these, but will be told the	system is
     unavailable due to	technical difficulties.	 The contents of the file
     /etc/authpf/authpf.problem	will also be displayed if the file exists and
     is	readable.

     authpf maintains the changed filter rules as long as the user maintains
     an	active session.	 It is important to remember however, that the exis-
     tence of this session means the user is authenticated.  Because of	this,
     it	is important to	configure sshd(8) to ensure the	security of the	ses-
     sion, and to ensure that the network through which	users connect is
     secure.  sshd(8) should be	configured to use the ClientAliveInterval and
     ClientAliveCountMax parameters to ensure that a ssh session is terminated
     quickly if	it becomes unresponsive, or if arp or address spoofing is used
     to	hijack the session.  Note that TCP keepalives are not sufficient for
     this, since they are not secure.  Also note that the various SSH tun-
     nelling mechanisms, such as AllowTcpForwarding and	PermitTunnel, should
     be	disabled for authpf users to prevent them from circumventing restric-
     tions imposed by the packet filter	ruleset.

     authpf will remove	state table entries that were created during a user's
     session.  This ensures that there will be no unauthenticated traffic
     allowed to	pass after the controlling ssh(1) session has been closed.

     authpf is designed	for gateway machines which typically do	not have regu-
     lar (non-administrative) users using the machine.	An administrator must
     remember that authpf can be used to modify	the filter rules through the
     environment in which it is	run, and as such could be used to modify the
     filter rules (based on the	contents of the	configuration files) by	regu-
     lar users.	 In the	case where a machine has regular users using it, as
     well as users with	authpf as their	shell, the regular users should	be
     prevented from running authpf by using the	/etc/authpf/authpf.allow or
     /etc/authpf/banned/ facilities.

     authpf modifies the packet	filter and address translation rules, and
     because of	this it	needs to be configured carefully.  authpf will not run
     and will exit silently if the /etc/authpf/authpf.conf file	does not
     exist.  After considering the effect authpf may have on the main packet
     filter rules, the system administrator may	enable authpf by creating an
     appropriate /etc/authpf/authpf.conf file.

     Control Files - To	illustrate the user-specific access control mecha-
     nisms, let	us consider a typical user named bob.  Normally, as long as
     bob can authenticate himself, the authpf program will load	the appropri-
     ate rules.	 Enter the /etc/authpf/banned/ directory.  If bob has somehow
     fallen from grace in the eyes of the powers-that-be, they can prohibit
     him from using the	gateway	by creating the	file /etc/authpf/banned/bob
     containing	a message about	why he has been	banned from using the network.
     Once bob has done suitable	penance, his access may	be restored by moving
     or	removing the file /etc/authpf/banned/bob.

     Now consider a workgroup containing alice,	bob, carol and dave.  They
     have a wireless network which they	would like to protect from unautho-
     rized use.	 To accomplish this, they create the file
     /etc/authpf/authpf.allow which lists their	login ids, group prepended
     with "%", or login	class prepended	with "@", one per line.	 At this
     point, even if eve	could authenticate to sshd(8), she would not be
     allowed to	use the	gateway.  Adding and removing users from the work
     group is a	simple matter of maintaining a list of allowed userids.	 If
     bob once again manages to annoy the powers-that-be, they can ban him from
     using the gateway by creating the familiar	/etc/authpf/banned/bob file.
     Though bob	is listed in the allow file, he	is prevented from using	this
     gateway due to the	existence of a ban file.

     Distributed Authentication	- It is	often desirable	to interface with a
     distributed password system rather	than forcing the sysadmins to keep a
     large number of local password files in sync.  The	login.conf(5) mecha-
     nism in OpenBSD can be used to fork the right shell.  To make that	hap-
     pen, login.conf(5)	should have entries that look something	like this:





     Using a default password file, all	users will get authpf as their shell
     except for	root who will get /bin/csh.

     SSH Configuration - As stated earlier, sshd(8) must be properly config-
     ured to detect and	defeat network attacks.	 To that end, the following
     options should be added to	sshd_config(5):

	   Protocol 2
	   ClientAliveInterval 15
	   ClientAliveCountMax 3

     This ensures that unresponsive or spoofed sessions	are terminated within
     a minute, since a hijacker	should not be able to spoof ssh	keepalive mes-

     Banners - Once authenticated, the user is shown the contents of
     /etc/authpf/authpf.message.  This message may be a	screen-full of the
     appropriate use policy, the contents of /etc/motd or something as simple
     as	the following:

	   This	means you will be held accountable by the powers that be
	   for traffic originating from	your machine, so please	play nice.

     To	tell the user where to go when the system is broken,
     /etc/authpf/authpf.problem	could contain something	like this:

	   Sorry, there	appears	to be some system problem. To report this
	   problem so we can fix it, please phone 1-900-314-1597 or send
	   an email to

     Packet Filter Rules - In areas where this gateway is used to protect a
     wireless network (a hub with several hundred ports), the default rule set
     as	well as	the per-user rules should probably allow very few things
     beyond encrypted protocols	like ssh(1), ssl(8), or	ipsec(4).  On a
     securely switched network,	with plug-in jacks for visitors	who are	given
     authentication accounts, you might	want to	allow out everything.  In this
     context, a	secure switch is one that tries	to prevent address table over-
     flow attacks.

     Example /etc/pf.conf:

     # by default we allow internal clients to talk to us using
     # ssh and use us as a dns server.
     nat-anchor	"authpf/*"
     rdr-anchor	"authpf/*"
     binat-anchor "authpf/*"
     block in on $internal_if from any to any
     pass in quick on $internal_if proto tcp from any to $gateway_addr \
	   port	= ssh
     pass in quick on $internal_if proto udp from any to $gateway_addr \
	   port	= domain
     anchor "authpf/*"

     For a switched, wired net - This example /etc/authpf/authpf.rules makes
     no	real restrictions; it turns the	IP address on and off, logging TCP

     external_if = "xl0"
     internal_if = "fxp0"

     pass in log quick on $internal_if proto tcp from $user_ip to any
     pass in quick on $internal_if from	$user_ip to any

     For a wireless or shared net - This example /etc/authpf/authpf.rules
     could be used for an insecure network (such as a public wireless network)
     where we might need to be a bit more restrictive.


     # rdr ftp for proxying by ftp-proxy(8)
     rdr on $internal_if proto tcp from	$user_ip to any	port 21	\
	   ->	port 8021

     # allow out ftp, ssh, www and https only, and allow user to negotiate
     # ipsec with the ipsec server.
     pass in log quick on $internal_if proto tcp from $user_ip to any \
	   port	{ 21, 22, 80, 443 }
     pass in quick on $internal_if proto tcp from $user_ip to any \
	   port	{ 21, 22, 80, 443 }
     pass in quick proto udp from $user_ip to $ipsec_gw	port = isakmp
     pass in quick proto esp from $user_ip to $ipsec_gw

     Dealing with NAT -	The following /etc/authpf/authpf.rules shows how to
     deal with NAT, using tags:

     ext_if = "fxp1"
     ext_addr =
     int_if = "fxp0"
     # nat and tag connections...
     nat on $ext_if from $user_ip to any tag $user_ip -> $ext_addr
     pass in quick on $int_if from $user_ip to any
     pass out log quick	on $ext_if tagged $user_ip

     With the above rules added	by authpf, outbound connections	corresponding
     to	each users NAT'ed connections will be logged as	in the example below,
     where the user may	be identified from the ruleset name.

     # tcpdump -n -e -ttt -i pflog0
     Oct 31 19:42:30.296553 rule 0.bbeck(20267).1/0(match): pass out on	fxp1: \ > S	2131494121:2131494121(0) win \
     16384 <mss	1460,nop,nop,sackOK> (DF)

     Using the authpf_users table - Simple authpf settings can be implemented
     without an	anchor by just using the "authpf_users"	table.	For example,
     the following pf.conf(5) lines will give SMTP and IMAP access to logged
     in	users:

     table <authpf_users> persist
     pass in on	$ext_if	proto tcp from <authpf_users> \
	     to	port { smtp imap }

     It	is also	possible to use	the "authpf_users" table in combination	with
     anchors.  For example, pf(4) processing can be sped up by looking up the
     anchor only for packets coming from logged	in users:

     table <authpf_users> persist
     anchor "authpf/*" from <authpf_users>
     rdr-anchor	"authpf/*" from	<authpf_users>

     Tunneled users - normally authpf allows only one session per client IP
     address.  However in some cases, such as when connections are tunneled
     via ssh(1)	or ipsec(4), the connections can be authorized based on	the
     userid of the user	instead	of the client IP address.  In this case	it is
     appropriate to use	authpf-noip to allow multiple users behind a NAT gate-
     way to connect.  In the /etc/authpf/authpf.rules example below, the
     remote user could tunnel a	remote desktop session to their	workstation:


     pass out on $internal_if from (self) to $workstation_ip port 3389 \
	    user $user_id


     pf(4), pf.conf(5),	securelevel(7),	ftp-proxy(8)

     The authpf	program	first appeared in OpenBSD 3.1.

     Configuration issues are tricky.  The authenticating 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

     authpf is not designed to prevent users from denying service to other

FreeBSD	11.1			January	6 2009			  FreeBSD 11.1


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