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

     authpf -- 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.  It is designed
     for changing filter and translation rules for an individual source	IP ad-
     dress as long as a	user maintains an active ssh(1)	session.  Typical use
     would be for a gateway that authenticates users before allowing them In-
     ternet use, or a gateway that allows different users into different
     places.  authpf logs the successful start and end of a session to
     syslogd(8).  This,	combined with properly set up filter rules and secure
     switches, can be used to ensure users are held accountable	for their net-
     work traffic.

     authpf can	add filter and translation rules using the syntax described in
     pf.conf(5).  authpf requires that the pf(4) system	be enabled before use.

     authpf is meant to	be used	with users who can connect via ssh(1) only.
     On	startup, 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	translation
     rules (if any) to add.  On	session	exit the same rules that were added at
     startup are removed.

     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 "au-
     thpf" 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

     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	ad-
     dress whenever authpf is run.  Additionally, the macro user_id is as-
     signed the	user name.

     Filter and	nat rules will first be	searched for in
     /etc/authpf/users/$USER/ and then in /etc/authpf/.	 Per-user rules	from
     the /etc/authpf/users/$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 configuration files.

     Filter and	translation rules are loaded from the file
     /etc/authpf/users/$USER/authpf.rules.  If this file does not exist	the
     file /etc/authpf/authpf.rules is used.  The authpf.rules file must	exist
     in	one of the above locations for authpf to run.

     Translation rules are also	loaded from this file.	The use	of translation
     rules in an authpf.rules file is optional.

     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 al-
     lowed values are as follows:

	     Use the specified anchor name instead of "authpf".

     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 re-
     stored.  This is the default behaviour.

     It	is also	possible to configure authpf to	only allow specific users ac-
     cess.  This is done by listing their login	names, one per line, in
     /etc/authpf/authpf.allow.	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 un-
     available 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 se-
     cure.  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.

     authpf will remove	statetable entries that	were created during a user's
     session.  This ensures that there will be no unauthenticated traffic al-
     lowed 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 be-
     cause 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 ex-
     ist.  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, 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 ap-
     propriate 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 be-
     yond encrypted protocols like ssh(1), ssl(8), or ipsec(4).	 On a securely
     switched network, with plug-in jacks for visitors who are given authenti-
     cation accounts, you might	want to	allow out everything.  In this con-
     text, a secure switch is one that tries to	prevent	address	table overflow

     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 \
	   keep	state
     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 8081

     # 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 } flags S/SA
     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 \
	   keep	state
     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 keep	state

     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)


     pf(4), pf.conf(5),	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 in-
     secure protocols to attackers on the same network,	or enable other	at-
     tackers on	the network to pretend to be the user by spoofing their	IP ad-

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

BSD			       January 10, 2002				   BSD


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