I've got a couple of services forwarded through my primary gateway (192.168.0.1) to my linux server (192.168.0.2). Everything works great. Awesome. I have a secondary ISP with a secondary gateway (192.168.0.3) on the same subnet (255.255.255.0). If I specify the secondary as my gateway, everything works fine, which is very nice to have when the primary ISP goes down. I've forwarded some ports (notably 22 for ssh) on the secondary gateway, just like I've done on the primary gateway. That's so I have a back door into the network if the primary ISP goes down. Makes sense, right? However, the server totally ignores all connections (on any port) which came through the secondary gateway. It's probably not routing; outbound connections work just fine if I specify the secondary as the gateway. It's probably not firewalling; the iptables list is completely empty. It's probably not the router configuration; a windows machine on the same network (192.168.0.10) accepts forwarded connections just fine. Can anybody suggest anything easy to check before I start trying to learn how to use packet sniffers?