4

I have two separate servers running on the same port on a Linux system, one binded to the loopback interface, and the other to the public Ethernet interface. If I connect a client on that system to the public socket, is the system smart enough to keep the traffic internal to itself?

5

No the data never leaves the box when it's addressing an IP address that's assigned to a local interface.

You can convince yourself of this fact by using traceroute to confirm.

Example

Here's my wireless NIC on my Fedora 19 system.

$ ip -4 addr show wlp3s0 | grep inet
    inet 192.168.1.20/24 brd 192.168.1.255 scope global wlp3s0

It's assigned the IP address 192.168.1.20. If we traceroute this IP:

$ traceroute 192.168.1.20
traceroute to 192.168.1.20 (192.168.1.20), 30 hops max, 60 byte packets
 1  greeneggs.bubba.net (192.168.1.20)  0.041 ms  0.013 ms  0.013 ms

Notice we never leave the box. It's a single hop so we're staying local to the system.

  • 1
    Follow-up question: what if I want to send the packet externally? – Gilles Dec 23 '13 at 1:08
  • actually this does not test whether the packet physically appears on the wire. you'll need to run a packet sniffer on the hub or from a different host on the same subnet and see if it shows up. – Michael Martinez Apr 18 '14 at 19:02
  • @MichaelMartinez - yes it does. If the packet egressed from the system we would've seen the IP of the router/switch when we performed the traceroute given how traceroutes work. – slm Apr 18 '14 at 23:52
  • won't a traceroute stop once it gets a reply from the target? in this case it won't record the ip of the router. it stops when it gets a response from eth0. but this still doesn't tell us whether the packet physically appeared on the wire. – Michael Martinez Apr 21 '14 at 17:38
  • @MichaelMartinez - traceroute induces ever node b/w the originator and the target to send a timeout reply packet. – slm Apr 21 '14 at 17:42

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