What does 0.0.0.0 gateway mean in routing table?

I am new to being a Linux system admin and I am learning about routing tables.

Currently I have two interfaces in my virtual machine:

vagrant@vagrant-ubuntu-trusty-64:~$ ifconfig
eth0 Link encap:Ethernet HWaddr 08:00:27:2e:8d:5d inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fe2e:8d5d/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:3146 errors:0 dropped:0 overruns:0 frame:0 TX packets:2853 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:218526 (218.5 KB) TX bytes:212044 (212.0 KB)
eth1 Link encap:Ethernet HWaddr 08:00:27:5b:5e:65 inet addr:172.28.128.3 Bcast:172.28.128.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fe5b:5e65/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:14 errors:0 dropped:0 overruns:0 frame:0 TX packets:31 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:5080 (5.0 KB) TX bytes:4622 (4.6 KB)
lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:65536 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

When I execute route -n, I see the following tables:

vagrant@vagrant-ubuntu-trusty-64:~$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.0.2.2 0.0.0.0 UG 0 0 0 eth0
10.0.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
172.28.128.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1

I know that the first entry is the default route. Just wonder what does the 2nd and 3rd entries gateway (0.0.0.0) mean here?

1 Answer

Where the gateway is all zeros, it means there is no gateway.

This is because the networks in question are directly attached to the machine, in that the machine has an IP address on an interface that falls into this network subnet.

Any packets for these networks don't need to be routed, as they are connected, so packets can be sent directly to the destination on the local network.

If the machine has a packet destined for another device on these networks, it will do an ARP request, to find the MAC address that is associated with the IP, and transmit the packet directly to that MAC address.