GRE protocol details

Generic Routing Encapsulation (GRE) General routing encapsulation protocol , be based on IP Network layer protocol encapsulates Ethernet message , Can be used in IPSec VPN Multicast routing information messages are transmitted between networks , Or in PPTP Agreement , bearing PPP The data packet . Its position in the data frame is as follows ：

    |-------------------|----------------|----------------------|------------------|
    |  Outer IP Header  |   GRE Header   |    Inner IP Header   |    Payload       |
    |-------------------|----------------|----------------------|------------------|
GRE The head is the smallest 4 Byte length , Followed by optional fields , The flag bit in the first byte determines whether the following field exists , Maximum length is 16 Bytes . The following is a standard GRE Header format ：

    |------------------------------------------------------------------------------|
    |    bit0-3   |      4-12      | 13-15 |                16 - 31                |
    |-------------|----------------|-------|---------------------------------------|
    | C | | K | S |    Reserved0   |  Ver  |            Protocol Type              |
    |--------------------------------------|---------------------------------------|
    |           Checksum(optional)         |         Reserved1(optional)           |         
    |------------------------------------------------------------------------------|
    |                                 Key(optional)                                |
    |------------------------------------------------------------------------------|
    |                           Sequence Number (optional)                         |
    |------------------------------------------------------------------------------|
C, K, and S      : They correspond to each other Checksum、Key and Sequence Number Field , Set up 1 Indicates that there is a corresponding field , Otherwise, there is no such field .
Ver                   : GRE Version number （ by 0）, about PPTP GRE, This field is 1.
Protocol Type  : Encapsulated Ethernet protocol type （ for example IPv4, Here is 0x0800）
Checksum       : If C Position as 1, This field contains information from GRE The checksum of all data at the beginning of the header
Key                  : If K Position as 1, This field contains secret key information
Sequence Number: If S Position as 1, This field contains GRE The serial number of the packet

Linux Configuration command

Two sets of equipment are required （Ubuntu operating system ） Configure and test . A device A Of IP The address is 192.168.1.113, Another device B Of IP The address is 192.168.1.123. First on the device A On the configuration GRE Tunnel , Following commands ：

$ sudo ip tunnel add gre01 mode gre remote 192.168.1.123 local 192.168.1.113 ttl 255
$ sudo ip link set gre01 up
$ sudo ip addr add 10.1.1.1/24 dev gre01