====== Default Gateway ======
===== Find current setting =====
==== IPv4 ====
Find Default Gateway in Linux using following options:
<code>
$ ip route show
---------------
default via 192.168.1.1 dev eth1  proto static 
192.168.1.0/24 dev eth1  proto kernel  scope link  src 192.168.1.100  metric 1
</code>
Alternative option using route:
<code>
$ route -n
----------
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth1
192.168.1.0     0.0.0.0         255.255.255.0   U     1      0        0 eth1
</code>
The above output shows my default gateway is 192.168.1.1. UG stands for the network link is Up and G stands for Gateway. 
==== IPv6 ====
Find Default Gateway in Linux using following options:
<code>
$ ip -6 route show
2001:1c00:2e16:720a::806 dev enp1s0 proto kernel metric 100 pref medium
2001:1c00:2e16:720a::/64 dev enp1s0 proto ra metric 100 pref medium
fdaa:66:67:a::806 dev enp1s0 proto kernel metric 100 pref medium
fdaa:66:67:a::/64 dev enp1s0 proto ra metric 100 pref medium
fe80::/64 dev enp1s0 proto kernel metric 1024 pref medium
default via fe80::ee08:6bff:fe84:2043 dev enp1s0 proto ra metric 100 pref medium
</code>
===== Setting Default Gateway =====
===== IPv4 =====
In case of DHCP the gateway is communicated via the DHCP options. Here is the list of the most common DHCP options exchanged with clients:
  * DHCP option 1: subnet mask to be applied on the interface asking for an IP address
  * DHCP option 3: default router or last resort gateway for this interface
  * DHCP option 6: which DNS (Domain Name Server) 
In case of manual configuration the gateway is specified in the /etc/network/interfaces file:
<code>
cat /etc/network/interfaces 
---------------------------
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1
</code>
===== IPv6 =====
IPv6 works very differently from IPv4 when it comes to gateways. With IPv4, you need to specify a gateway manually or via a DHCP option. With IPv6, the IP stack locates gateways automatically by using ICMP-v6 Neighbor Discovery packets. Specifically, a client will listen for Router Advertisements, which may be sent gratuitously by a router or as a response to a Router Solicitation package.
In addition to that a default route can be set with:
<code>
# ip -6 route add default via xx:xx:xx::xx dev eth0
</code>