21.1. NAT resource

21.1. NAT resource
	Chapter 21. NAT - Network Address Translation

Abilis implements NAT (Network Address Translation) with PAT (Port Address Translation) and IPACL (IP Access Control List) services.

By properly specifying:

source IP address or network;
destination IP address or network;
source UDP/TCP port;
destination UDP/TCP port;
Source/Destination IP resource;
TOS (Type of Service) for packets priority management by services.

A high level of security is granted.

	Note
	Abilis isn't a Firewall! It implements basic firewall function, but evolved features must be commissioned to a “real” firewall. Abilis and firewall can coexist with no problems.

21.1.1. NAT overview

ABILIS has 4 NAT zones: INside, OUTside, VPN, DMZ.

The relationships between the IP router and the NAT functional unit can be represented schematically as follows:

The NAT table contains rules of descending priority PR:0, PR:1, ... (the first rule, PR:0, is the one which is checked first):

[15:20:49] ABILIS_CPX:d nat

UPNP maps not present

Configured maps
-------------------------------------------------------------------------------
PR: [DESCR:]
    INAT:         ADD: SNET:              DNET:              ANET:
    ONAT:              SPO:               DPO:               APO:          PAT:
    SIP:  DIP:         PROT:              TOUT:
-------------------------------------------------------------------------------
0   IN            SRC  192.168.000.000/24 *                  OUT-IP
    OUT                *                  *                  AUTO          YES
-------------------------------------------------------------------------------
1   VPN           DST  *                  010.000.011.000/24 192.168.000.000/24
    IN                 *                  *                  *             YES
-------------------------------------------------------------------------------
2   OUT           SRC  212.002.038.222/32 088.081.001.165/32 192.168.000.178/32
    IN                 *                  8001               http(80)      YES
    3     1            *                  SYS
-------------------------------------------------------------------------------

	Important
	These rules only apply to packets which enter the router through a port which belongs to a different zone, with respect to the IP port used to go out. In other words, in order to have packets processed by NAT, the values of `INAT` and `ONAT` must be different.

The rules contain both the packets selection criteria and the kind of desired processing:

Packet-selection
When the IP packet does not match the specifications of a rule, the rule is skipped; the next rule will be immediately checked.
When a rule matches the IP packet's specs, the address/port processing specified in the rule itself is executed.
The rule's parameters used to select the IP packets are:
1. The zone through which packets enter into the IP router. In our example IP-1, IP-2 and IP-3 have their NAT parameter set to INSIDE. If the purpose is to select packets incoming through the port IP-1, then the selection criterion is INAT:INSIDE.
2. The zone where packets shall leave the IP router. If packets have to be sent-out of the router through a port marked with NAT:OUTSIDE, then the right criterion is ONAT:OUTSIDE.
3. The IP-packet's source network, by means of the SNET parameter (“source network”). For example SNET:192.168.0.0/24.
4. The IP-packet's destination address, by means of the DNET parameter (“destination network”). For example, DNET:* if every possible destination-network is concerned.
5. The filtering specified with the INAT parameter can be further restricted by specifying the SIP, source IP port (SIP:1 for IP-1). The rule will be applied only if the packet's source port is actually the one specified with SIP.
6. The filtering specified with the ONAT parameter can be further restricted by specifying the DIP, destination IP port (DIP:5 for IP-5).
7. When the protocol is TCP or UDP, the filtering can by further restricted by specifying the SPO, source port (for example SPO:25).
8. When the protocol is TCP or UDP, the filtering can by further restricted by specifying the DPO, destination port (for example DPO:25).
9. When the Port Address Translation is enabled, the selection criterion can also define PROT the protocol in use: ICMP, TCP, UDP, * (all).
Packet processing
Once the IP-packet fulfils all the selection criteria specified in a rule, it is processed according to the specs written in the rule itself.
Please mind: the NAT table is scanned twice:
- The first scan is reserved for the rules which request the processing of the destination address (ADD:DST).
- The second scan is reserved to the rules which request the processing of the source address (ADD:SRC).
In the first scan, this is the applied processing:
1. The selected DESTINATION address is changed as specified in the “alias network”, described by the ANET parameter. For instance: ANET:88.81.111.22/32.
2. When the protocol is TCP or UDP and PAT (port address translation) is active (PAT:YES), it is also possible to define the change of the DESTINATION port. The target port is defined in the APO (alias port) parameter.
In the second scan, this is the applied processing:
1. The selected SOURCE address is changed as specified in the “alias network”, described by the ANET parameter. For instance: ANET:OUT-IP makes all the packets use the current public IP-address in use for the output port.
2. When the protocol is TCP or UDP and PAT (port address translation) is active (PAT:YES), it is also possible to define the change of the SOURCE port. The target port is defined in the APO (alias port) parameter.

Important

Frequent mistakes:

Missing to define the right NAT zone for each used IP resource. By default, the NAT parameter is inactive (NAT:NO).
Missing to activate the NAT resource. By default the NAT resource is not active (ACT:NO). Set it active with: S P NAT ACT:YES.
Leaving the NAT table empty. By default Abilis does not produce any NAT entry.
Missing to specify PAT:YES in the rule used to share the only public IP address available, makes your ABILIS no more reachable by the remote support. Therefore, in the mentioned cases don't miss to specify PAT:YES, as in the following example:
A NAT PR:1 INAT:IN ONAT:OUT ADD:SRC SNET:192.168.0.0/24 DNET:* ANET:OUT-IP PAT:YES

Tip

Interesting chapters:

Section 21.4, “Examples of NAT configuration”;

Section 71.12, “How to configure the Network Address Translation (NAT)”;

Section 71.13, “How to share a single Internet connection among multiple computers”;

Section 71.14, “How to access IP hosts and services which are behind a NAT manager (port forwarding)”.

21.1.2. Frequently used expression

Inside. The set of networks, there's subject of translation, usually “private” networks.
Outside. All other networks, usually “public” addresses located on the Internet.
Inside local IP address. The IP address which was assigned to a host on the inside network.
Inside global IP address. The IP address of an inside host as it appears to the outside networks. If, as usual, the outside network is the Internet, the address must be one of the “public” addresses which the ISPs have assigned to the user's router for those connections. In the table of NAT Aliases this term called as ANET:.
Processed IP packet. It means in this packet was changed a source or destination address in some cases a source or destination port was changed too.
Ignored IP packet. It means that this packet wasn't changed.
Static Address Translation. The user can establish a one-to-one mapping between the inside local and global addresses, which happens when the number (netmask) of inside local and global addresses are identical.
Dynamic Source Address Translation. The user can establish a dynamic mapping between the inside and global addresses, which happens when the number (netmask) of inside local and global addresses are different.
Port Address Translation (PAT). The user can save addresses in the global address pool by allowing source ports in TCP connections or UDP conversations to be translated. Different local addresses will be mapped to the same global address, with port translation providing the necessary uniqueness for TCP/UDP and other tricks providing uniqueness for ICMP.
Extended filtering in PAT mode. The purpose of this feature is to allow a selective activation of the PAT translation based on the destination TCP/UDP port and on the IP protocol, with the result that network managers can empower their control of the network by:
- Granting access only to some service, e.g. WEB and FTP.
- Blocking access only to specific services, e.g. RealAudio / RealVideo servers.
- Precisely distinguishing inbound connections from outbound ones.
- Allowing internal users to access ANY service on the Internet while outside users may access only a restricted set.
Destination port mapping. This behavior is very useful in many situations, the most frequents are:
- The user has just one public IP address and on the internal LAN there are more computers with several IP addresses running different services that must be reached from outside, e.g. FTP, HTTP, SMTP etc.
- The user has just one public IP address and on the internal LAN there are more computers with several IP addresses running the same service with different contents, e.g. a commercial web, a technical web, a restricted access web.

21.1.3. Activating the NAT resource

Add the resource to the Abilis system with the command:

[11:01:39] ABILIS_CPX:a res:nat

RES:NAT ALREADY EXISTS

The NAT resource may already exist in the system, but may not yet be active: set it active with the command:

[11:01:48] ABILIS_CPX:s act res:nat

COMMAND EXECUTED

	Caution
	After adding or setting the NAT active, you must restart the Abilis to make the resource running (use the command warm start to reboot the Abilis).

[11:01:39] ABILIS_CPX:s p nat act:yes

COMMAND EXECUTED

[11:01:39] ABILIS_CPX:d p nat

RES:Nat - Not Saved (SAVE CONF), Not Refreshed (INIT) -------------------------
Run    DESCR:Network_Address_Translator
       LOG:DS             ACT:YES         dimtable:1000   
       DNS-ALG:NO         SNMP-ALG:NO     PPTP-ALG:NO         IKE-AWARE:NO 
       NAT-LOOPBACK:NO 
       - Not PAT mode parameters ----------------------------------------------
       TOUT:3600   
       - PAT mode parameters --------------------------------------------------
       FRAG:5             ICMP:5          DNS:5    SNMP:5     SNTP:5    
       UDP-OPENING:5      UDP-CONN:180    
       TCP-OPENING:5      TCP-CONN:360    TCP-CLOSING:15      TCP-CLOSED:1    
       TCP-RST:YES        PPTP-CONN:360    
       INSIDE-MIL:NOLIMIT OUTSIDE-MIL:NOLIMIT VPN-MIL:NOLIMIT DMZ-MIL:NOLIMIT
       MIL-TIME:2

21.1.4. NAT resource parameters

Use the following command to display resource parameters; the command d p nat ? displays the meaning of all parameters.

[11:01:39] ABILIS_CPX:d p nat

Run    DESCR:Network_Address_Translator
       LOG:DS             ACT:YES         dimtable:1000   
       DNS-ALG:NO         SNMP-ALG:NO     PPTP-ALG:NO         IKE-AWARE:NO 
       NAT-LOOPBACK:NO 
       - Not PAT mode parameters ----------------------------------------------
       TOUT:3600   
       - PAT mode parameters --------------------------------------------------
       FRAG:5             ICMP:5          DNS:5    SNMP:5     SNTP:5    
       UDP-OPENING:5      UDP-CONN:180    
       TCP-OPENING:5      TCP-CONN:360    TCP-CLOSING:15      TCP-CLOSED:1    
       TCP-RST:YES        PPTP-CONN:360    
       INSIDE-MIL:NOLIMIT OUTSIDE-MIL:NOLIMIT VPN-MIL:NOLIMIT DMZ-MIL:NOLIMIT
       MIL-TIME:2

Meaning of the most important parameters:

LOG

Logging functionalities activation/deactivation.

ACT

NAT runtime activation/deactivation.

dimtable

It specifies how many translations can be created at the same time.

DNS-ALG

Enable/disable DNS Application Level Gateway [NO, YES].

SNMP-ALG

Enable/disable SNMP Application Level Gateway [NO, YES].

PPTP-ALG

Enable/disable PPTP Application Level Gateway [NO, YES].

IKE-AWARE

Enable/disable IKE awareness. Minimise remap of UDP ports 500 and 4500 [NO, YES].

NAT-LOOPBACK

Enable/disable the packets loopback for destination translations [NO, YES]. NAT loopback permits the access of a service via the public IP address from inside the local network.

	Tip
	Interesting chapter: Section 71.15, “How to configure the NAT loopback”.

TOUT

This value sets the timeout of static and dynamic translations, i.e. those created without the PAT mode. If the translation isn't used for the specified time (i.e. no packets need it), it times out and it has removed by the translation table.

FRAG

Time-out for linkable fragment identifier [1..240 sec].

ICMP

Time-out for ICMP link type [1..240 sec].

DNS

Time-out for DNS link type [1..240 sec].

SNMP

Time-out for SNMP link type [1..240 sec].

SNTMP

Time-out for SNTP link type [1..240 sec].

UDP-OPENING

Time-out for UDP links in Opening state [1..240 sec].

UDP-CONN

Time-out for UDP links in Connected state [60..65535 sec].

TCP-OPENING

Time-out for TCP links in Opening state [1..240 sec].

TCP-CONN

Time-out for TCP links in Connected state [60..65535 sec].

TCP-CLOSING

Time-out for TCP links in Closed state [1..240 sec].

TCP-RST

Enable sending of RESET for expired links [NO, YES].

PPTP-CONN

Time-out for PPTP links in Connected state [60..65535 sec].

INSIDE-MIL

Maximal number of links that same IP address on INSIDE interface can open at the same time [NOLIMIT, 1..10000].

OUTSIDE-MIL

Maximal number of links that same IP address on OUTSIDE interface can open at the same time [NOLIMIT, 1..10000].

VPN-MIL

Maximal number of links that same IP address on VPN interface can open at the same time [NOLIMIT, 1..10000].

DMZ-MIL

Maximal number of links that same IP address on DMZ interface can open at the same time [NOLIMIT, 1..10000].

MIL-TIME

Resync time used for maximal initiator links procedure [2..30 sec].

The following command allows the administrator to change the configuration of the resource:

s p nat parameter:value...

	Caution
	To activate the changes made on the upper case parameters, execute the initialization command init nat; while to set act the changes made on the lowercase parameters a save conf and an Abilis restart are required (i.e. With warm start command).


Chapter 21. NAT - Network Address Translation		21.2. NAT Aliases table