Skip to main content
lethargos
lethargos
New Member
March 1, 2021
Question

Routing through VIP - different handling of source ip

  • March 1, 2021
  • 1 reply
  • 5095 views

Hello,

 

I need help understanding how Fortigate handles two very similar policies.

I've got three vlans with corresponding subinterfaces: 10.20.0.0/16 (project 1), 10.30.0.0/16 (project 2) and 10.40.0.0/16 (project 3).

I've got a VIP mapping 192.168.1.1 (this is actually public, but it's not hugely relevant here) --> 10.30.1.16

When I access the VIP from a server beloning to the same vlan (10.30.1.100), Fortigate changes the source IP from 10.30.1.100 to 10.30.0.1 (so the vlan's gateway).

When I access the VIP from a server belonging to another vlan (10.40.1.12), the receiving server sees the actual ip, so 10.40.1.12.

I can clearly see this in tcpdump and in the packet capture run directly on the Firewall (where the route is for the first scenarion: 10.30.1.100->192.168.1.1->10.30.16 -- and here the ip source changes to 10.30.1.0).

 

The two policies are:

1. interface out - project 1 (subnet 10.30.0.0/16) --> interface in - also project 1 (VIP - 192.168.1.1->10.30.1.16)

2. interfface out - project 3 (subnet 10.40.0.0/16) --> interface in - project 1 (VIP - 192.168.1.1->10.30.1.16)

 

Is this intentional so that it avoid any ip conflict, in case the the packet comes from the server itself (10.30.1.16)?

 

Thank you!

1 reply

lobstercreed
lobstercreed
New Member
March 2, 2021

Are you sure you don't just have NAT enabled on the first policy?  That's the kind of behavior I'd expect if you had NAT enabled on the first policy but not the second one.

lethargos
lethargosAuthor
New Member
March 2, 2021

I've just had a look. No, there's no NAT in the rules themselves that point to the VIP.

 

There is however a general SNAT rule for all three VLANs when the packets go out into the internet. But that shouldn't matter because the destination interface is different (internet-facing one vs the vlan ones) and the packets didn't reach the 10.30.1.16 at all before adding the VIP policies.

lethargos
lethargosAuthor
New Member
March 2, 2021

This was an interesting hint though! I added NAT to the rule, just to see what happens, and interestingly enough, it changed the source ip to the public ip mapping into the VIP, so in my example 192.168.1.1.

So with NAT it's not using the vlan gateway anymore as a source ip.

Terms & ConditionsAccessibility statement