July 25, 2017

Using arp

arp translates ip address to mac addresses.

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The arp command let’s you see which one of your interfaces has access to the default gateway, and therefore can access the internet.

the following shows which interface has access to the the default gateway:

$ arp
Address                  HWtype  HWaddress           Flags Mask            Iface
172.28.128.2             ether   08:00:27:88:f1:e5   C                     enp0s10
172.28.128.1             ether   0a:00:27:00:00:04   C                     enp0s10
gateway                  ether   52:54:00:12:35:02   C                     enp0s3
192.168.56.100           ether   08:00:27:8a:11:d7   C                     enp0s8
192.168.56.100           ether   08:00:27:8a:11:d7   C                     enp0s9

This command shows the default gateway’s ip address:

$ arp -n
Address                  HWtype  HWaddress           Flags Mask            Iface
172.28.128.2             ether   08:00:27:88:f1:e5   C                     enp0s10
172.28.128.1             ether   0a:00:27:00:00:04   C                     enp0s10
10.0.2.2                 ether   52:54:00:12:35:02   C                     enp0s3
192.168.56.100           ether   08:00:27:8a:11:d7   C                     enp0s8
192.168.56.100           ether   08:00:27:8a:11:d7   C                     enp0s9

This matches up with the information provided in the routing tables:

$ 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    100    0        0 enp0s3
10.0.2.0        0.0.0.0         255.255.255.0   U     100    0        0 enp0s3
172.28.128.0    0.0.0.0         255.255.255.0   U     100    0        0 enp0s10
192.168.56.0    0.0.0.0         255.255.255.0   U     100    0        0 enp0s9
192.168.56.0    0.0.0.0         255.255.255.0   U     101    0        0 enp0s8
192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0

$ netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         10.0.2.2        0.0.0.0         UG        0 0          0 enp0s3
10.0.2.0        0.0.0.0         255.255.255.0   U         0 0          0 enp0s3
172.28.128.0    0.0.0.0         255.255.255.0   U         0 0          0 enp0s10
192.168.56.0    0.0.0.0         255.255.255.0   U         0 0          0 enp0s9
192.168.56.0    0.0.0.0         255.255.255.0   U         0 0          0 enp0s8
192.168.122.0   0.0.0.0         255.255.255.0   U         0 0          0 virbr0

or:

[root@localhost ~]# ip route list
default via 10.0.2.2 dev enp0s3  proto static  metric 100
10.0.2.0/24 dev enp0s3  proto kernel  scope link  src 10.0.2.15  metric 100
172.28.128.0/24 dev enp0s10  proto kernel  scope link  src 172.28.128.4  metric 100
192.168.56.0/24 dev enp0s9  proto kernel  scope link  src 192.168.56.104  metric 100
192.168.56.0/24 dev enp0s8  proto kernel  scope link  src 192.168.56.103  metric 101
192.168.122.0/24 dev virbr0  proto kernel  scope link  src 192.168.122.1

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