4. ipsec-tools on ubuntu14.04

4. ipsec-tools on ubuntu14.04
X.509 Certificates

Racoon supports the usage of X.509 certificates for the authentication process. These certificates may be checked against a certificate authority (CA). The configuration is similar to the PSK configuration and differs only on the authentication part:

path certificate "/etc/certs";

remote 192.168.2.100 {
        exchange_mode main;
        certificate_type x509 "my_certificate.pem" "my_private_key.pem";
    verify_cert on;
        my_identifier asn1dn;
    peers_identifier asn1dn;
        proposal {
                encryption_algorithm 3des;
                hash_algorithm md5;
                authentication_method rsasig;
                dh_group modp1024;
        }
}

sainfo address 172.16.1.0/24 any address 172.16.2.0/24 any {
        pfs_group modp768;
        encryption_algorithm 3des;
        authentication_algorithm hmac_md5;
        compression_algorithm deflate;
}
    

The certificate and the private key are stored in the certificate path /etc/certs. This path is set using the option path certificate in the configuration file. The certificates and the certificate revocation lists are stored in PEM format as generated with openssl. For the generation of certificates see the chapter on X.509 certificates. If the certificate of the peer is to be checked against a certificate authority (verify_cert on; is the default), then the certificate of the CA has to be also stored in this directory. For OpenSSL to find the certificate it has to be renamed or linked using the hashed name:

ln -s CAfile.pem `openssl x509 -noout -hash < CAfile.pem`.0
    

If the certificate additionally is to be checked against a certificate revocation file (CRL) the CRL must be stored in the same directory using a similar linked hashed name:

ln -s CRLfile.pem `openssl x509 -noout -hash < CAfile.pem`.r0
    

When storing the certificates and the private key it is important to note that racoon cannot decrypt a private key. Therefore the private key must be stored in its decrypted cleartext form. If you created a crypted private key, you have to decrypt it:

# openssl rsa -in my_private_key.pem -out my_private_key.pem
read RSA key
Enter PEM pass phrase: password
writing RSA key
    

Roadwarrior

Roadwarriors are clients using unknown dynamic IP addresses to connect to a VPN gateway. In combination with racoon this poses two problems:

    The IP address is not known and cannot be specified in the racoon configuration file or in the /etc/psk.txt file. A different way to determine the identity of the client must be found. When using pre-shared keys this requires the aggressive mode! The best solution is the usage of X.509 certificates though.

    No security policy can be created for racoon to act on, since the destination IP address is not known. racoon must create the security policy and the security association when the connection is initiated.

To achieve this the configuration file /etc/racoon.conf needs several modifications:

path certificate "/etc/certs";

remote anonymous {
        exchange_mode main;
        generate_policy on;
        passive on;
        certificate_type x509 "my_certificate.pem" "my_private_key.pem";
        my_identifier asn1dn;
        peers_identifier asn1dn;
        proposal {
                encryption_algorithm 3des;
                hash_algorithm md5;
                authentication_method rsasig;
                dh_group modp1024;
        }
}


sainfo anonymous {
        pfs_group modp1024;
        encryption_algorithm 3des;
        authentication_algorithm hmac_md5;
        compression_algorithm deflate;
}
    

The option generate_policy on instructs racoon to create the appropriate policy when a new connection is initiated. The option passive on tells racoon to remain passive and wait for new connection to be started from the outside. racoon may not start a connection.

The most important change though is the definition of anonymous in the remote and sainfo line. This instructs racoon to accept the connection from anywhere.
NAT-Traversal

The Linux kernel 2.6 is capable of using NAT traversal in tunnel mode. Transport mode is not supported yet. This can be used by Racoon starting with version 0.3.3 of the ipsec-tools.

To configure Racoon for NAT traversal several options have been added to the configuration file. These are natt_keepalive, isakmp_natt, nat_traversal.

The most important option is nat_traversal. This can be set to on, off or force. When set to on this peer will use NAT traversal as soon as a NAT device is detected on the path. Off will disable this behavior. When using force NAT traversal will be used regardless wether a NAT device is found or not.

Since many NAT devices forget the entries in their internal tables quite fast when no traffic is seen, racoon offers to send keepalive packets across the wire. These are send every 20 seconds by default. You can change this value using natt_keepalive. Setting this to 0 seconds will disable this feature.

If you want to use NAT traversal you have to specify the IP address and the port to use in the listen section of the racoon configuration file. This is done using isakmp_natt.

For clarity a typical configuration file is shown, where the peer 192.168.2.100 is hidden by a NAT gateway with the IP address 192.168.1.1:

path pre_shared_key "/etc/psk.txt";

timer  {
       natt_keepalive 10sec;
       }

listen {
       isakmp 192.168.1.100 [500];
       isakmp_natt 192.168.1.100 [4500];
       }

remote 192.168.1.1 {
        exchange_mode main;
        nat_traversal on;
        proposal {
                encryption_algorithm 3des;
                hash_algorithm md5;
                authentication_method pre_shared_key;
                dh_group modp1024;
        }
}

sainfo address 172.16.1.0/24 any address 172.16.2.0/24 any {
        pfs_group modp768;
        encryption_algorithm 3des;
        authentication_algorithm hmac_md5;
        compression_algorithm deflate;
}
    

If you configured everything correctly NAT will be detected automatically:

2004-12-22 07:34:53: INFO: @(#)ipsec-tools 0.4 (http://ipsec-tools.sourceforge.net)
2004-12-22 07:34:53: INFO: @(#)This product linked OpenSSL 0.9.7a Feb 19 2003 (http://www.openssl.org/)
2004-12-22 07:34:53: INFO: 192.168.1.100[4500] used as isakmp port (fd=6)
2004-12-22 07:34:53: INFO: 192.168.1.100[4500] used for NAT-T
2004-12-22 07:34:53: INFO: 192.168.1.100[500] used as isakmp port (fd=7)
2004-12-22 07:35:09: INFO: respond new phase 1 negotiation: 192.168.1.100[500]<=>192.168.1.1[500]
2004-12-22 07:35:09: INFO: begin Identity Protection mode.
2004-12-22 07:35:09: INFO: received Vendor ID: draft-ietf-ipsec-nat-t-ike-02
2004-12-22 07:35:09: INFO: received Vendor ID: RFC XXXX
2004-12-22 07:35:09: INFO: Selected NAT-T version: RFC XXXX
2004-12-22 07:35:09: INFO: Hashing 192.168.1.100[500] with algo #1
2004-12-22 07:35:09: INFO: NAT-D payload #0 verified
2004-12-22 07:35:09: INFO: Hashing 192.168.1.1[500] with algo #1
2004-12-22 07:35:09: INFO: NAT-D payload #1 doesn't match
2004-12-22 07:35:09: INFO: NAT detected: PEER
2004-12-22 07:35:10: INFO: Hashing 192.168.1.1[500] with algo #1
2004-12-22 07:35:10: INFO: Hashing 192.168.1.100[500] with algo #1
2004-12-22 07:35:10: INFO: Adding remote and local NAT-D payloads.
2004-12-22 07:35:10: INFO: NAT-T: ports changed to: 192.168.1.1[4500]<->192.168.1.100[4500]
2004-12-22 07:35:10: INFO: KA list add: 192.168.1.100[4500]->192.168.1.1[4500]
2004-12-22 07:35:10: INFO: ISAKMP-SA established 192.168.1.100[4500]-192.168.1.1[4500] spi:0613dc09c4ccc828:9cc9dfc9acc82eb5
2004-12-22 07:35:11: INFO: respond new phase 2 negotiation: 192.168.1.100[0]<=>192.168.1.1[0]
2004-12-22 07:35:11: INFO: Adjusting my encmode UDP-Tunnel->Tunnel
2004-12-22 07:35:11: INFO: Adjusting peer's encmode UDP-Tunnel(3)->Tunnel(1)
2004-12-22 07:35:11: INFO: IPsec-SA established: ESP/Tunnel 192.168.1.1->192.168.1.100 spi=95762109(0x5b536bd)
2004-12-22 07:35:11: INFO: IPsec-SA established: ESP/Tunnel 192.168.1.100->192.168.1.1 spi=222871470(0xd48bfae)
      

When looking at the packets on the wire you will see UDP traffic traversing back and forth:

[root@bibo root]# tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tap1, link-type EN10MB (Ethernet), capture size 96 bytes
13:37:41.920621 IP 192.168.1.1.isakmp > 192.168.1.100.isakmp: isakmp: phase 1 I iden
t
13:37:41.941296 IP 192.168.1.100.isakmp > 192.168.1.1.isakmp: isakmp: phase 1 R iden
t
13:37:42.051826 IP 192.168.1.1.isakmp > 192.168.1.100.isakmp: isakmp: phase 1 I iden
t
13:37:42.157134 IP 192.168.1.100.isakmp > 192.168.1.1.isakmp: isakmp: phase 1 R iden
t
13:37:42.353942 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 72
13:37:42.361530 IP 192.168.1.100.4500 > 192.168.1.1.4500: UDP, length 72
13:37:42.373799 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 88
13:37:43.374630 IP 192.168.1.100.4500 > 192.168.1.1.4500: UDP, length 1
13:37:43.384476 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 256
13:37:43.431219 IP 192.168.1.100.4500 > 192.168.1.1.4500: UDP, length 256
13:37:43.436680 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 56
13:37:44.492976 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 1
13:37:45.390137 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 116
13:37:45.390612 IP 192.168.1.100.4500 > 192.168.1.1.4500: UDP, length 116
13:37:46.395603 IP 192.168.1.1.4500 > 192.168.1.100.4500: UDP, length 116
13:37:46.396009 IP 192.168.1.100.4500 > 192.168.1.1.4500: UDP, length 116
      

If you are using your Racoon not in a roadwarrior setup but with fixed addresses as above you need to modify your Security Policies too. These need to reflect the natted addresses! The correct Policies for the above scenario are:

#!/usr/sbin/setkey -f
#
# Flush SAD and SPD
flush;
spdflush;

# Create policies for racoon
spdadd 172.16.1.0/24 172.16.2.0/24 any -P out ipsec
           esp/tunnel/192.168.1.100-192.168.1.1/require;

spdadd 172.16.2.0/24 172.16.1.0/24 any -P in ipsec
           esp/tunnel/192.168.1.1-192.168.1.100/require;
    

These policies are automatically setup if you use generate_policy on; in your Racoon configuration.