GRE (Generic Routing Encapsulation)

GRE (Generic Routing Encapsulation) is a lightweight tunneling protocol, identified as IP protocol number 47, that encapsulates a wide range of network-layer packets inside IP packets. It creates a virtual point-to-point link between two routers.

Why it matters

GRE can carry things plain IPsec cannot, such as multicast and routing protocol traffic like OSPF, because the tunnel looks like an ordinary interface. That makes it a common building block for site-to-site links and cloud connectivity. It adds only a small header, typically 24 bytes with the outer IPv4 header, but offers no confidentiality or authentication, so it is often combined with IPsec (GRE over IPsec) when crossing the internet.

Example

On Linux:

ip tunnel add gre1 mode gre remote 198.51.100.2 local 203.0.113.5 ttl 64
ip addr add 10.255.0.1/30 dev gre1
ip link set gre1 up

The peer is configured symmetrically. Traffic routed to gre1 is wrapped and sent to 198.51.100.2. Reduce the interface MTU to about 1476 to avoid fragmentation.

Common mistakes

Sending sensitive data through GRE alone.
Blocking IP protocol 47 on a firewall; it is not TCP or UDP, so port rules do not apply.
Overlooking the extra header size when setting MTU.

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