Recently, I focus on OpenStack things where an interesting network question comes out: If a host machine has multiple network interfaces, is it possible to ping all of them from another host?

To be clearer, we assume that Host A has a unique network interface 192.168.11.0/24, Host B has 2 interfaces: 192.168.125.0/24 and 192.168.126.0/24. Meanwhile, the 3 interfaces are connected by a router, as the graph illustrated.

2hosts

If we look at the routing table of Host B, we’ll find something like

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[root@eta-bmc2 ~]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.128.0   0.0.0.0         255.255.255.0   U     0      0        0 eth3
192.168.125.0   0.0.0.0         255.255.255.0   U     0      0        0 eth0
192.168.126.0   0.0.0.0         255.255.255.0   U     0      0        0 eth2
192.168.127.0   0.0.0.0         255.255.255.0   U     0      0        0 eth1
0.0.0.0         192.168.125.1   0.0.0.0         UG    0      0        0 eth0

If we ping from Host B to Host A, Host B looks up its routing table and find no specific rools for destination 192.168.11.0/24, so it will go via gateway 192.168.125.1.

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[root@eta-bmc2 ~]# tracepath 192.168.11.20
 1:  192.168.125.1 (192.168.125.1)                          1.437ms
 2:  testagent (192.168.11.20)                              3.174ms reached
     Resume: pmtu 65535 hops 2 back 63

And if we ping from Host A to Host B of interface 192.168.125.0/24, it goes well. However, if we ping to Host B’s another interface, it will fail.

The reason is obvious, Host A send a packet to 192.168.126.92 and expect a response from that destination. Unfortunately, Host B receives the packet but can only response through 192.168.125.1, which will never match for Host A.

So in this case, the answer is NO. But if Host A’s network interface is in 192.168.126.0/24, it can ping the 2 interfaces of Host B, or if Host A also has multiple network interfaces, with proper routing table setting, it can ping both 2 interfaces of Host B as well.