Ping/traceroute/etc appears to work, but I don’t see the return packets inside my router. What’s up with that?
If you’re printing debug information or logging packets, on some topologies you may see what seems like an impossible series of events: • You run ping, traceroute, an ftp download, or some other command aimed at a server on your topology. • Your router gets a packet from the firewall destined for one of the servers. You route it to the server appropriately after doing your ARP processing and other appropriate bits. • You see the ping or download complete, but never see the return path (echo response from the server, etc.) inside your router. What’s happening here is that your topology is slightly malfunctioning- your router is working correctly, but at the application server packets are being routed back out through the firewall directly rather than passing through your topology. This happens because interactions between the servers and the topologies that they are multiplexed to are governed by special routing rules called policy routes; policy routes enable a routing decision to be m
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