NAME
authpf —
authenticating gateway user
shell
SYNOPSIS
DESCRIPTION
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 address as long as a user
maintains an active
ssh(1) session.
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.
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 network 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 can also maintain the list of IP address of connected
users in the "authpf_users"
table.
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 "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
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/$USER/ and then in
/etc/authpf/. Only one of these files will be used if both
are present.
Per-user rules from the
/etc/authpf/users/$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.
The
authpf.rules file must exist in one of the above locations
for
authpf to run.
CONFIGURATION
Options are controlled by the
/etc/authpf/authpf.conf file. If
the file is empty, defaults are used for all configuration options. The file
consists of pairs of the form
name=value
, one per
line. Currently, the allowed values are as follows:
-
-
- anchor=name
- Use the specified anchor name instead of
"authpf".
-
-
- table=name
- Use the specified table name instead of
"authpf_users".
USER MESSAGES
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 readable.
There exist two methods for providing additional granularity to the control
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. 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
administrator. 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.
CONFIGURATION ISSUES
authpf maintains the changed filter rules as long as the user
maintains an active session. It is important to remember however, that the
existence of this session means the user is authenticated. Because of this, it
is important to configure
sshd(8)
to ensure the security of the session, 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
tunnelling mechanisms, such as
AllowTcpForwarding and
PermitTunnel, should be disabled for
authpf users to prevent them from circumventing restrictions
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 regular (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
regular 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.
FILES
- /etc/authpf/authpf.conf
-
- /etc/authpf/authpf.allow
-
- /etc/authpf/authpf.rules
-
- /etc/authpf/authpf.message
-
- /etc/authpf/authpf.problem
-
EXAMPLES
Control Files - To illustrate the user-specific access control
mechanisms, let us consider a typical user named bob. Normally, as long as bob
can authenticate himself, the
authpf program will load the
appropriate 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 unauthorized 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) mechanism in
OpenBSD can be used to fork the right shell. To make
that happen,
login.conf(5)
should have entries that look something like this:
shell-default:shell=/bin/csh
default:\
...
:shell=/usr/sbin/authpf
daemon:\
...
:shell=/bin/csh:\
:tc=default:
staff:\
...
:shell=/bin/csh:\
:tc=default:
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
configured 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 messages.
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 remove@bulkmailerz.net.
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) 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 overflow attacks.
Example
/etc/pf.conf:
# by default we allow internal clients to talk to us using
# ssh and use us as a dns server.
internal_if="fxp1"
gateway_addr="10.0.1.1"
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 connections.
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.
internal_if="fxp1"
ipsec_gw="10.2.3.4"
# rdr ftp for proxying by ftp-proxy(8)
rdr on $internal_if proto tcp from $user_ip to any port 21 \
-> 127.0.0.1 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 = 129.128.11.10
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: \
129.128.11.10.60539 > 198.137.240.92.22: 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>
SEE ALSO
pf(4),
pf.conf(5),
ftp-proxy(8)
HISTORY
The
authpf program first appeared in
OpenBSD
3.1.
BUGS
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 address.
authpf is not designed to prevent users from denying service
to other users.