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VPN(8)                  OpenBSD System Manager's Manual                 VPN(8)

NAME
     vpn - configuring the system for virtual private networks

DESCRIPTION
     A virtual private network is used to securely connect two or more subnets
     over the internet.  For each subnet there is a security gateway which is
     linked via a cryptographically secured tunnel to the security gateway of
     the other subnet.  ipsec(4) is used to provide the necessary network-lay-
     er cryptographic services.  This document describes the configuration
     process for setting up a VPN.

     Briefly, creating a VPN consists of the following steps:

     1.   Choose a key exchange method: manual keyed, or automated via
          isakmpd(8).
     2.   For manual keying, create the Security Associations (SA), one for
          each endpoint.
     3.   For manual keying, create the appropriate IPsec flows.
     4.   For automated keying, create a configuration file for the keying
          daemon.
     5.   Configure your firewall rules appropriately.

   Choosing a key exchange method
     There are currently two key exchange methods available:

     o   manual (symmetric shared secret)
     o   isakmpd(8)

   Enabling Packet Forwarding
     For security gateways, proper operation often requires packet forwarding
     to be enabled using sysctl(8):

           # sysctl net.inet.ip.forwarding=1
           # sysctl net.inet6.ip6.forwarding=1

     Packet forwarding defaults to 'off'.

     For more permanent operation, the appropriate option(s) should be enabled
     in your sysctl.conf(5).

   Generating Manual Keys [manual keying]
     The shared secret symmetric keys used to create a VPN can be any hexadec-
     imal value, so long as both sides of the connection use the same values.
     Since the security of the VPN is based on these keys being unguessable,
     it is very important that the keys be chosen using a strong random
     source.  One practical method of generating them is by using the
     random(4) device.  To produce 160 bits (20 bytes) of randomness, for ex-
     ample, do:

           # openssl rand 20 | hexdump -e '20/1 "%02x"'

     or:

           # openssl rand 20 | perl -pe 's/./unpack("H2",$&)/ges'

     Different cipher types may require different sized keys.

           Cipher    Key Length
           DES       56 bits
           3DES      168 bits
           AES       Variable (128 bits recommended)
           BLF       Variable (160 bits recommended)
           CAST      Variable (128 bits maximum and recommended)
           SKIPJACK  80 bits

     Use of DES or SKIPJACK as an encryption algorithm is not recommended (ex-
     cept for backwards compatibility) due to their short key length.  Fur-
     thermore, recent attacks on SKIPJACK have shown severe weaknesses in its
     structure.

     Note that DES requires 8 bytes to form a 56-bit key and 3DES requires 24
     bytes to form its 168-bit key.  This is because the most significant bit
     of each byte is ignored by both algorithms.

   Creating Security Associations [manual keying]
     Before the IPsec flows can be defined, two Security Associations (SAs)
     must be defined on each end of the VPN, e.g.:

           # ipsecadm new esp -spi $SPI_AB -src $GATEWAY_A \
               -dst $GATEWAY_B -forcetunnel -enc 3des -auth sha1 \
               -keyfile $ENCRYPTION_KEY_FILE \
               -authkeyfile $AUTHENTICATION_KEY_FILE
           # ipsecadm new esp -spi $SPI_BA -src $GATEWAY_B \
               -dst $GATEWAY_A -forcetunnel -enc 3des -auth sha1 \
               -keyfile $ENCRYPTION_KEY_FILE \
               -authkeyfile $AUTHENTICATION_KEY_FILE

     Note that the -key and -authkey options may be used to specify the keys
     directly in the ipsecadm(8) command line.  However, another user could
     view the keys by using the ps(1) command at the appropriate time (or use
     a program for doing so).

   Creating IPsec Flows [manual keying]
     Both IPsec gateways need to configure ipsec(4) routes with the
     ipsecadm(8) tool:

     On the security gateway of subnet A:

           # ipsecadm flow -out -require -proto esp \
               -src $GATEWAY_A -dst $GATEWAY_B \
               -addr $NETWORK_A $NETWORK_B
           # ipsecadm flow -in -require -proto esp \
               -src $GATEWAY_A -dst $GATEWAY_B \
               -addr $NETWORK_B $NETWORK_A

     and on the security gateway of subnet B:

           # ipsecadm flow -out -require -proto esp \
               -src $GATEWAY_B -dst $GATEWAY_A \
               -addr $NETWORK_B $NETWORK_A
           # ipsecadm flow -in -require -proto esp \
               -src $GATEWAY_B -dst $GATEWAY_A \
               -addr $NETWORK_A $NETWORK_B

   Configure and run the keying daemon [automated keying]
     Unless manual keying is used, both security gateways need to start the
     isakmpd(8) key management daemon.  To make sure the daemon is properly
     configured to provide the required security services (typically, encryp-
     tion and authentication) start the daemon with debugging or verbose out-
     put.

     isakmpd(8) implements security policy using the KeyNote trust management
     system.

   Configuring Firewall Rules
     pf(4) needs to be configured such that all packets from the outside are
     blocked by default.  Only successfully IPsec-processed packets (from the
     enc(4) interface), or key management packets (for isakmpd(8), UDP packets
     with source and destination ports of 500) should be allowed to pass.

     The pf.conf(5) rules for a tunnel which uses encryption (the ESP IPsec
     protocol) and isakmpd(8) on security gateway A might look like this:

     GATEWAY_A = "192.168.1.254/32"
     GATEWAY_B = "192.168.2.1/32"
     NETWORK_A = "10.0.50.0/24"
     NETWORK_B = "10.0.99.0/24"

     # default deny
     # ne0 is the only interface going to the outside.
     block in log on { enc0, ne0 } all
     block out log on { enc0, ne0 } all

     # Passing in encrypted traffic from security gateways
     pass in proto esp from $GATEWAY_B to $GATEWAY_A
     pass out proto esp from $GATEWAY_A to $GATEWAY_B

     # Need to allow ipencap traffic on enc0.
     pass in on enc0 proto ipencap all

     # Passing in traffic from the designated subnets.
     pass in on enc0 from $NETWORK_B to $NETWORK_A
     pass out on enc0 from $NETWORK_A to $NETWORK_B

     # Passing in isakmpd(8) traffic from the security gateways
     pass in on ne0 proto udp from $GATEWAY_B port = 500 to $GATEWAY_A port = 500
     pass out on ne0 proto udp from $GATEWAY_A port = 500 to $GATEWAY_B port = 500

     If there are no other pf.conf(5) rules, the "quick" clause can be added
     to the last four rules.  NAT rules can also be used on the enc(4) inter-
     face.  Note that it is strongly encouraged that instead of detailed PF
     rules, the SPD (IPsec flow database) be utilized to specify security pol-
     icy, if only to avoid filtering conflicts.

FILES
     /usr/share/ipsec/rc.vpn    Sample VPN configuration file
     /etc/isakmpd/isakmpd.conf  isakmpd(8) configuration file
     /etc/pf.conf               Firewall configuration file

EXAMPLES
   Manual keying
     To create a manual keyed VPN between two class C networks using 3DES en-
     cryption and the following IP addresses:

            GATEWAY_A  = 192.168.1.254
            NETWORK_A = 10.0.50.0/24
            GATEWAY_B  = 192.168.2.1
            NETWORK_B = 10.0.99.0/24

     1.   Choose the shared secrets using a suitably random method.  The 3DES
          encryption key needs 192 bits (3x64), or 24 bytes.  The SHA-1 au-
          thentication key for needs 160 bits, or 20 bytes.

          # openssl rand 24 | hexdump -e '24/1 "%02x"' > enc_key
          # openssl rand 20 | hexdump -e '20/1 "%02x"' > auth_key

     2.   Create the Security Associations (on both endpoints):

          # /sbin/ipsecadm new esp -src 192.168.2.1 -dst 192.168.1.254 \
             -forcetunnel -spi 1000 -enc 3des -auth sha1 \
             -keyfile enc_key -authkeyfile auth_key

          # /sbin/ipsecadm new esp -src 192.168.1.254 -dst 192.168.2.1 \
             -forcetunnel -spi 1001 -enc 3des -auth sha1 \
             -keyfile enc_key -authkeyfile auth_key

     3.   Create the IPsec flows on machine A (the first is for outbound
          flows, the latter is the ingress filter for the incoming security
          association):

          # ipsecadm flow -out -require -proto esp \
              -src 192.168.1.254 -dst 192.168.2.1 \
              -addr 10.0.50.0/24 10.0.99.0/24
          # ipsecadm flow -in -require -proto esp \
              -src 192.168.1.254 -dst 192.168.2.1 \
              -addr 10.0.99.0/24 10.0.50.0/24

     4.   Create the matching IPsec flows on machine B:

          # ipsecadm flow -out -require -proto esp \
              -src 192.168.1.254 -dst 192.168.2.1 \
              -addr 10.0.50.0/24 10.0.99.0/24
          # ipsecadm flow -in -require -proto esp \
              -src 192.168.1.254 -dst 192.168.2.1 \
              -addr 10.0.99.0/24 10.0.50.0/24

     5.   Configure the firewall rules on machine A using the previously de-
          fined ruleset:

          GATEWAY_A = "192.168.1.254/32"
          GATEWAY_B = "192.168.2.1/32"
          NETWORK_A = "10.0.50.0/24"
          NETWORK_B = "10.0.99.0/24"

          (rest of ruleset)

     6.   Configure the firewall rules on machine B, modifying the definitions
          as appropriate:

          GATEWAY_B = "192.168.1.254/32"
          GATEWAY_A = "192.168.2.1/32"
          NETWORK_B = "10.0.50.0/24"
          NETWORK_A = "10.0.99.0/24"

          (rest of ruleset)

   Automated keying
     To create a VPN between the same two C class networks as the example
     above, using isakmpd(8):

     1.   Create /etc/isakmpd/isakmpd.conf for machine A:


          # Incoming phase 1 negotiations are multiplexed on the source IP
          # address. Phase 1 is used to set up a protected channel just
          # between the two gateway machines. This channel is then used for
          # the phase 2 negotiation traffic (i.e. encrypted & authenticated).

          [Phase 1]
          192.168.2.1=            peer-machineB

          # 'Phase 2' defines which connections the daemon should establish.
          # These connections contain the actual "IPsec VPN" information.

          [Phase 2]
          Connections=            VPN-A-B

          # ISAKMP phase 1 peers (from [Phase 1])

          [peer-machineB]
          Phase=                  1
          Transport=              udp
          Address=                192.168.2.1
          Configuration=          Default-main-mode
          Authentication=         yoursharedsecret

          # IPSEC phase 2 connections (from [Phase 2])

          [VPN-A-B]
          Phase=                  2
          ISAKMP-peer=            peer-machineB
          Configuration=          Default-quick-mode
          Local-ID=               machineA-internal-network
          Remote-ID=              machineB-internal-network

          # ID sections (as used in [VPN-A-B])

          [machineA-internal-network]
          ID-type=                IPV4_ADDR_SUBNET
          Network=                10.0.50.0
          Netmask=                255.255.255.0

          [machineB-internal-network]
          ID-type=                IPV4_ADDR_SUBNET
          Network=                10.0.99.0
          Netmask=                255.255.255.0

          # Main and Quick Mode descriptions (as used by peers and connections)

          [Default-main-mode]
          DOI=                    IPSEC
          EXCHANGE_TYPE=          ID_PROT
          Transforms=             3DES-SHA,BLF-SHA

          [Default-quick-mode]
          DOI=                    IPSEC
          EXCHANGE_TYPE=          QUICK_MODE
          Suites=                 QM-ESP-3DES-SHA-SUITE

     2.   Create /etc/isakmpd/isakmpd.conf for machine B:


          # Incoming phase 1 negotiations are multiplexed on the source IP
          # address. Phase 1 is used to set up a protected channel just
          # between the two gateway machines. This channel is then used for
          # the phase 2 negotiation traffic (i.e. encrypted & authenticated).

          [Phase 1]
          192.168.1.254=          peer-machineA

          # 'Phase 2' defines which connections the daemon should establish.
          # These connections contain the actual "IPsec VPN" information.

          [Phase 2]
          Connections=            VPN-B-A

          # ISAKMP phase 1 peers (from [Phase 1])

          [peer-machineA]
          Phase=                  1
          Transport=              udp
          Address=                192.168.1.254
          Configuration=          Default-main-mode
          Authentication=         yoursharedsecret

          # IPSEC phase 2 connections (from [Phase 2])

          [VPN-B-A]
          Phase=                  2
          ISAKMP-peer=            peer-machineA
          Configuration=          Default-quick-mode
          Local-ID=               machineB-internal-network
          Remote-ID=              machineA-internal-network

          # ID sections (as used in [VPN-A-B])

          [machineA-internal-network]
          ID-type=                IPV4_ADDR_SUBNET
          Network=                10.0.50.0
          Netmask=                255.255.255.0

          [machineB-internal-network]
          ID-type=                IPV4_ADDR_SUBNET
          Network=                10.0.99.0
          Netmask=                255.255.255.0

          # Main and Quick Mode descriptions (as used by peers and connections)

          [Default-main-mode]
          DOI=                    IPSEC
          EXCHANGE_TYPE=          ID_PROT
          Transforms=             3DES-SHA,BLF-SHA

          [Default-quick-mode]
          DOI=                    IPSEC
          EXCHANGE_TYPE=          QUICK_MODE
          Suites=                 QM-ESP-3DES-SHA-SUITE

     3.   Read through the configuration one more time.  The only real differ-
          ences between the two files in this example are the IP-addresses,
          and ordering of Local- and Remote-ID for the VPN itself.  Note that
          the shared secret (the Authentication tag) must match between ma-
          chineA and machineB.

          Due to the shared secret information in the configuration file, it
          must be installed without any permissions for "group" or "other".

                # chmod og-rwx /etc/isakmpd/isakmpd.conf

     4.   Create a simple /etc/isakmpd/isakmpd.policy file for machineA:

                Keynote-version: 2
                Authorizer: "POLICY"
                Conditions: app_domain == "IPsec policy" &&
                            esp_present == "yes" &&
                            esp_enc_alg != "null" -> "true";

     5.   Create a simple /etc/isakmpd/isakmpd.policy file for machineB:

                Keynote-version: 2
                Authorizer: "POLICY"
                Conditions: app_domain == "IPsec policy" &&
                            esp_present == "yes" &&
                            esp_enc_alg != "null" -> "true";

     6.   Configure the firewall rules on machines A and B:

          Use the same ruleset as the manual keying example, but add permis-
          sion for the isakmpd(8) control traffic, on UDP port 500.

          For machineA, add:

          # Permit ISAKMPD control traffic between A and B
          pass in proto udp from 192.168.2.1/32 to 193.127.1.254/32 port = 500
          pass out proto udp from 192.168.1.254/32 to 193.127.2.1/32 port = 500

          For machineB, add:

          # Permit ISAKMPD control traffic between A and B
          pass in proto udp from 192.168.1.254/32 to 193.127.2.1/32 port = 500
          pass out proto udp from 192.168.2.1/32 to 193.127.1.254/32 port = 500

     7.   Start isakmpd(8)

          On both machines, run:

                # /sbin/isakmpd

          To run with verbose debugging enabled, instead start with:

                # /sbin/isakmpd -d -DA=99

SEE ALSO
     openssl(1), enc(4), ipsec(4), keynote(4), options(4), isakmpd.conf(5),
     isakmpd.policy(5), pf.conf(5), ipsecadm(8), isakmpd(8), pfctl(8),
     sysctl(8)

BUGS
     In situations where the gateway IPs are outside the tunnelled network
     range, such as when tunnelling private (RFC 1918) networks over the In-
     ternet, isakmpd(8) or manual keying must be used.

OpenBSD 3.6                    February 9, 1999                              7