Chose default GW based on SrcAddr after SRC NAT

RouterOS Forwarding Protocols
1

We have a WISP network fronted by a CCR1036 that has two upstream connections - a 1 Gbps Ethernet link that was until recently our only upstream, and a 10 Gbps link that will be our only link once a transition is complete. We serve a mix of business customers who get public IP addresses and residential customers on one-to-many NAT using 100.64.0.0/10 addresses. The NAT function is performed on the CCR1036. For the transition, we need to migrate customers from addresses in two existing /25 address blocks (delivered over the 1 Gbps link) to addresses in a new /24 address block (delivered over the 10 Gbps link).

At first I thought we could use mangle to mark packets based on their source address and then add a second default route for that routing mark. Then I noticed that SRC-NAT happens near the end of the packet flow, long after mangle. Presumably I can work around this by creating a new set of NAT rules (mapping to addresses in the new block) and a mangle rule to mark 100.64.0.0/10 packets for the new route.

But before I go down that path, I thought I’d ask if there are other, potentially better approaches. ??

2

You can put mangle rules at different points in the packet flow, so it should be possible to solve it using
some well placed mark and src-nat rules.

3

Thanks Pe1chl, but as I read the packet flow diagrams, SRC-NAT is done in PostRouting and after Mangle Postrouting. There doesn’t appear to be any way to do SRC-NAT and then do routing. Am I missing something?

4

You can do marking in prerouting and then the routing based on those routing marks. The NAT will be done after that.

5

But that means marking based on pre-NAT source address.
It’s the public IP address range that determines which default gateway to use, so marking based on pre-NAT source addresses was what I was trying to avoid.