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Support SNAT on internal Azure load Balancer

Currently it seems Azure Internal Load Balancer does not support Source NAT.
this mean that if 2 different services hosted on 2 different VM and the VM are on the same vnet the traffic is not load balanced if the ILB route the traffic to the same VM that start the request.
Service A (exposed on port x) and B (exposed on port y) are hosted on VM 1 and VM2 on the same vnet.
Service A has VIP z and Service B has VIP m.
if service A is recalled via VIP z from VM 1 and ILB route the traffic to service A on VM1 a connection timeout is returned.
all works if the ILB route traffic to service A on VM2.
the same scenario works in other clouds (public and private) if on the ILB the source NAT can be enabled

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  • Andy Johnson commented  ·   ·  Flag as inappropriate

    Yes, this is a really common problem, I am surprised it's not addressed in ILB. You don't even need multiple services to trigger this bug: a single VIP, serving two VM's. If your VM's have to make a self-referencing call (to the ILB VIP), the Source NAT rules don't treat it any differently!

    This means if VM1(client) makes call to ILB VIP, and ILB VIP load balances back to VM1(server), the *initial* SYN packet makes it's way to ILB VIP, no problem. When the ILB VIP forwards the packet to VM1(server), the Source NAT steps in, and changes the source IP to become VM1 (just, likely with a different port, for translation purposes). VM1(server) replies to that packet, but since the reply destination is ITS OWN IP, it doesn't leave the network interface.

    I am pretty sure most mature load balancers address this by performing NAT translation of any requests coming from the real servers, with a destination of any of the hosted VIPs, to an IP address OTHER than their own.

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