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AUTHPF(8) System Manager's Manual AUTHPF(8) NAME authpf, authpf-noip -- authenticating gateway user shell SYNOPSIS authpf authpf-noip 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. 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 and an fdescfs(4) file system mounted at /dev/fd 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 connec- tions are tunneled via the gateway system, and can be directly associated with the user name. It cannot ensure accountability when classifying con- nections by IP address; in this case the client's IP address is not pro- vided to the packet filter via the client_ip macro or the authpf_users ta- ble. 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 session 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 translation 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 "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/*" 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 configura- tion 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 val- ues 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 trou- blesome 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 ac- cess. 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 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. 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 im- portant 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 al- lowed 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 remem- ber that authpf can be used to modify the filter rules through the environ- ment 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 run- ning 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 exist. Af- ter 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. 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 au- thenticate 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 mes- sage 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, 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 al- lowed 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 dis- tributed 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, lo- gin.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 ex- cept 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 mes- sages. 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 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), ssl(8), or ipsec(4). On a securely switched network, with plug-in jacks for visitors who are given authentica- tion 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 connec- tions. 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 an- chors. For example, pf(4) processing can be sped up by looking up the an- chor 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 ad- dress. 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 appropri- ate to use authpf-noip to allow multiple users behind a NAT gateway to con- nect. In the /etc/authpf/authpf.rules example below, the remote user could tunnel a remote desktop session to their workstation: internal_if="bge0" workstation_ip="10.2.3.4" pass out on $internal_if from (self) to $workstation_ip port 3389 \ user $user_id FILES /etc/authpf/authpf.conf /etc/authpf/authpf.allow /etc/authpf/authpf.rules /etc/authpf/authpf.message /etc/authpf/authpf.problem SEE ALSO fdescfs(4), pf(4), pf.conf(5), securelevel(7), 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. FreeBSD ports 15.quarterly January 29 2014 AUTHPF(8)
NAME | SYNOPSIS | DESCRIPTION | FILTER AND TRANSLATION RULES | CONFIGURATION | USER MESSAGES | CONFIGURATION ISSUES | EXAMPLES | FILES | SEE ALSO | HISTORY | BUGS
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