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authpf - authenticating gateway user shell



      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 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 and a
      fdescfs(5) file system be mounted at /dev/fd before use.  authpf can also
      maintain the list of IP address of connected users in the "authpf_users"
      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 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.  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, 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>


      pf(4), pf.conf(5), fdescfs(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


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