Support Forum
The Forums are a place to find answers on a range of Fortinet products from peers and product experts.
SMGK74
New Contributor II

How do the VIP address are source natted and the purpose to bind a VIP to an Interface

Here to clarify the way a VIP address can be source natted on an external interface.

 

A Vip address could need to exit WAN1 using the same its IP address or using a dedicated ip pool, but the way to do it is not so obvious.

In some configuration the behavior of the VIP in SNAT, sometimes, could seems to work badly, for example the address could be source natted every time with its vip address also for internal traffic or not natted with its vip address but with the ip pools used by the policy.

 

Following all the different cases and the results shown

 

Initial configuration:

WAN1 Interface ip           : 172.16.100.200/24

DMZ  Interface Ip             : 192.168.3.1/24

LAN  Interface Ip              : 192.168.2.1/24

 

VIP with extip 172.16.100.221 and mappedip "192.168.2.21"

       config firewall vip

               edit "vip_dc1"

                       set extip 172.16.100.221

                       set mappedip "192.168.2.21"

                       set extintf "any"

                next

       end

#policy from WAN1 to DMZ with VIP

    edit 1

        set srcintf "WAN1"

        set dstintf "DMZ"

        set srcaddr "all"

        set dstaddr "vip_dc1"

        set action accept

        set schedule "always"

        set service "ALL"

    next

                   

#policy from DMZ to WAN1

     edit 2

        set srcintf "DMZ"

        set dstintf "WAN1"

        set srcaddr "DC1"

        set dstaddr "all"

        set action accept

        set schedule "always"

        set service "ALL"

        set nat enable

    next

#policy from DMZ to LAN

    edit 3

        set srcintf "DMZ"

        set dstintf "LAN"

        set srcaddr "DC1"

        set dstaddr "192.168.3.0/24"

        set action accept

        set schedule "always"

        set service "ALL_ICMP"

        set nat enable

    next

 

 

To test how the vip address is source natted, we'll do a ping from the 192.168.2.21 to an external Ip (8.8.8.8) and to an client on LAN network : 192.168.3.100.

Then we get the result by the cli command "get sys session list"

 

Case 1 : Starting Point  : Vip Address with option "interface any", outgoing policy with NAT interface

 

Result of "get sys session list":

                               PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT

                               icmp    59     192.168.2.21:1   172.16.100.221:1 192.168.3.100:8  -              

                               icmp    59     192.168.2.21:1   172.16.100.221:1 8.8.8.8:8        -

 

As you can observe the vip is Source Natted with its vip external address both on the external interface and internal.               

 

 

Case 2 : Vip Address with option "interface any", outgoing policy with NAT Ip pool

Created two ip pools :

    edit "ip_pool_192.168.3.200"

        set startip 192.168.3.200

        set endip 192.168.3.200

    next

      edit "ip_pool_dc1_172.16.100.199"

        set startip 172.16.100.199

        set endip 172.16.100.199

    next

 

Applied to both the policies with nat:

    edit 2

        set srcintf "DMZ"

        set dstintf "WAN1"

        set srcaddr "DC1"

        set dstaddr "all"

        set action accept

        set schedule "always"

        set service "ALL"

        set ippool enable

        set poolname "ip_pool_dc1_172.16.100.199"

        set nat enable

    next 

    edit 3

        set name "DC1-INSIDE"

        set srcintf "DMZ"

        set dstintf "LAN"

        set srcaddr "DC1"

        set dstaddr "192.168.3.0/24"

        set action accept

        set schedule "always"

        set service "ALL_ICMP"

        set ippool enable

        set poolname "ip_pool_192.168.3.200"

        set nat enable

    next

               

Result of "get sys session list":

                PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT

                icmp    59     192.168.2.21:1   192.168.3.200:60417 192.168.3.100:8  -              

                icmp    59     192.168.2.21:1   172.16.100.199:60417 8.8.8.8:8        -              

 

Good, my server going out using the ippool assigned to the policies, right behavior...

 

 

Case 3 : Vip Address with option "interface <WAN1>", outgoing policy with NAT interface

 

                Ok now start from begging but  first let's bind the vip address to the WAN1 interface:

 

                config firewall vip

                               edit "vip_dc1"

                                               set extip 172.16.100.221

                                               set mappedip "192.168.2.21"

                                               set extintf "WAN1"

                               next

                end

Result of "get sys session list":

                               PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT

                               icmp    59     192.168.2.21:1   192.168.3.1:60417 192.168.3.100:8  -              

                               icmp    59     192.168.2.21:1   172.16.100.221:1 8.8.8.8:8        -     

 

As you can see only the traffic versus WAN1 is source natted with its vip address but not the traffic natted on other interfaces.

So, to bind the vip address to interface plays main role in choosing the the source nat

 

Case 4 : Vip Address with option "interface <WAN1>", outgoing policy with NAT Ippool

Now let's use again the ipools asin the case 2

                              

                Result of "get sys session list":              

                               PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT

                               icmp    59     192.168.2.21:1   192.168.3.200:60417 192.168.3.100:8  -              

                               icmp    59     192.168.2.21:1   172.16.100.199:60417 8.8.8.8:8        -              

 

                Ok same result as case 2, ipools wins over the "interface bind"

 

Case 5 : Vip Address with option "nat-source-vip enable" and "interface any", outgoing policy with NAT interface

 

Let's now to introduce the vip option "nat-source-vip enable".

This option will ever force VIP to EVER use its external ip for source nat when going out natted on a policy

 

Let's start again from begging but with the option "nat-source-vip" enabled:

 

config firewall vip

    edit "vip_dc1"

        set extip 172.16.100.221

        set mappedip "192.168.2.21"

        set extintf "any"

        set nat-source-vip enable

    next

end

 

Result of "get sys session list":

                PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT

                icmp    59     192.168.2.21:1   172.16.100.221:1 192.168.3.100:8  -              

                icmp    59     192.168.2.21:1   172.16.100.221:1 8.8.8.8:8        -              

 

As can you see the source nat is the extip on  both session

 

Case 6 : Vip Address with option "nat-source-vip enable" and  "interface any", outgoing policy with NAT Ippool

let's now add the ippools on the policy nat...

 

Result of "get sys session list":

                PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT      

                icmp    59     192.168.2.21:1   172.16.100.221:1 192.168.3.100:8  -              

                icmp    59     192.168.2.21:1   172.16.100.221:1 8.8.8.8:8        -              

 

Same result...

 

Case 7 : Vip Address with option "nat-source-vip enable" and  "interface <WAN1>", outgoing policy with NAT interface

Let's try to bind the vip to the outgoing interface:

 

config firewall vip

    edit "vip_dc1"

        set extip 172.16.100.221

        set mappedip "192.168.2.21"

        set extintf "WAN1"

        set nat-source-vip enable

    next

end

 

Result of "get sys session list":

                PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT      

                icmp    59     192.168.2.21:1   172.16.100.221:1 192.168.3.100:8  -              

                icmp    60     192.168.2.21:1   172.16.100.221:1 8.8.8.8:8        -

 

Again, the same result!

 

Case 8 : Vip Address with option "nat-source-vip enable" and  "interface <WAN1>", outgoing policy with NAT Ippool

Let's a last try using also the ippools on the nat policies:

 

Result of "get sys session list":

                PROTO   EXPIRE SOURCE           SOURCE-NAT       DESTINATION      DESTINATION-NAT      

                icmp    59     192.168.2.21:1   172.16.100.221:1 192.168.3.100:8  -              

                icmp    59     192.168.2.21:1   172.16.100.221:1 8.8.8.8:8        -              

Clearly the same result, as told before this option ever wins!

So, keep it in mind when you going to use it, because could lead to misconfigurations!

 

#fortigate #vip #SNAT

Sergio Marchi
Sergio Marchi
1 REPLY 1
jintrah_FTNT
Staff
Staff
Announcements

Select Forum Responses to become Knowledge Articles!

Select the “Nominate to Knowledge Base” button to recommend a forum post to become a knowledge article.

Labels
Top Kudoed Authors