We have configured a load balancer for SMTP on a new VPX Netscaler 14.1 appliance in the classic load balancer way. Client sends a request. Netscaler processes it and sends the request to the Exchange Server. The Exchange server responds to the Netscaler and the Netscaler forwards the response back to the client. Everything works fine.
But now, as you can probably already guess, we don't want to see the SNIP on the Exchange Server, but the client IP address.
Therefore we followed this Docs article (https://docs.netscaler.com/en-us/citrix-adc/current-release/load-balancing/load-balancing-dsrmode.html) and this blog post (https://ingmarverheij.com/citrix-netscaler-dsr-poor-mans-load-balancing-solution/) and configured Direct Server Return (DSR) with the help of MAC based forwarding and USIP.
To summarize:
Service with USIP. Load balancer with Redirection Mode: MAC based, Traffic Settings: Sessionless and Method: SourceIPHash. No SMTP monitoring but simple PING or TCP monitoring. Everything tied together. Then configured the loopback adapter on the Exchange. Done.
But unfortunately the configuration doesn't work and now I can't think of anything else to test. That's why I'm writing to you today and hoping for help from the community.
How is the behavior now?
Ping, TCP, SMTP monitors that we attach to the services or service groups are always positive (green). This can also be seen in the trace and in the nstcpdump.
However, if we now make a Telnet session with port 25 to the load balancer address, we get no response. In a trace from the netscaler and in the nstcpdump we can see that the request arrives at the load balancer address. But what is completely missing in the trace and in the nstcpdump is the packet that should now go from the netscaler to the exchange server. There is simply nothing there.
What am I missing here? Why is the Netscaler not sending out the customized packet? I would be grateful for any tips.
