authpf(8) - Manual pages

NAME

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 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 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

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 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.

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".

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.

authpf will remove statetable 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 ssl(8). 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 \
      keep state
pass in quick on $internal_if from $user_ip to any

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 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: \
129.128.11.10.60539 > 198.137.240.92.22: S 2131494121:2131494121(0) win \
16384 <mss 1460,nop,nop,sackOK> (DF)

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.

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