[arielb1]

> nping --tcp --flags SA --source-ip 192.168.12.1 --dest-ip 10.8.0.8 -- rate 3 -c 3 -e ap0 --dest-mac 08:00:27:9c:53:12

Why is Linux accepting packets coming from one interface into an IP address belonging to a different interface? It feels like it is "forwarding" the packets internally, but `ip_forward` is turned off.

Is there any case where this behavior is legitimately useful?

[ajross]

IP addresses don't "belong" to interfaces in the general case. It's just a hard problem. In fact there are lots of multi-homed use cases where you want to internally route packets across interfaces without an affirmative mapping of what address is supposed to be used where.

For the specific case of point to point VPNs, there's a rule that makes sense. But that's not part of the network stack per se and there's no way to enforce it generically.

[loeg]

Do network stacks drop 127.0/8 packets from external interfaces today? Superficially (I'm not an experienced TCP/IP or routing stack developer, although I do work in the kernel) it seems like the same treatment could be used for VPN-registered interface addresses. You just need an API to specify "I'm a VPN interface" when the device is created or the IP assigned, no?

[arielb1]

Is there a place where I can read about these cases?

[loeg]

From the referenced systemd commit,

> such as keeping connections via one default route alive after another one appears (e.g. plugging an Ethernet cable when connected via Wi-Fi).

[arielb1]

How is this supposed to work? How will the packet destined to the WiFi IP address get to the Ethernet interface?

[vetrom]

I use this behavior in production systems where I have 'well-known' RFC1918 addresses I use for service bootstrapping/configuration. In the network engineering world, extra loopback interfaces are also used for similar reasons.