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Network protocol of yyds dry goods inventory: datagram socket for detailed explanation of socket protocol

2022-06-11 11:38:00 【Procedural stuff】

brief introduction

In the last article we explained Socket Classification and the most commonly used Stream Socket,Stream Socket It's usually based on TCP Agreed , So we are often in web They can be seen in the service . Of course TCP The agreement has a twin brother called UDP, Then based on UDP To do the transmission protocol socket The agreement is called Datagram Socket, Today, let's take a closer look at Datagram Socket.

What is? Datagram Socket

And connected Stream Socket Different ,Datagram Socket It's disconnected . Connected Stream Socket Indicates that the socket It's stable , So we can Stream socket Stable data transmission in , Of course, this stability means that packets will not be lost , But it is not always possible to ensure that the packet is not tampered with .

Datagram Socket This connectionless is usually used in scenarios that allow partial loss of data , Like voice 、 Video and so on , The advantage of no connection is that you don't need TCP The complex steps of establishing a connection , So it's relatively simpler .

Datagram Socket The usual use is UDP Protocol as the underlying data transmission protocol .

about UDP Come on , because UDP The protocol itself does not guarantee the order of data and the handling of data exceptions , These need to be implemented in the application itself .

common UDP Applications include DNS(Domain Name System) service ,NTP(Network Time Protocol) Services, etc. .

stay JDK Of java.net The package provides a pair of Datagram Socket Encapsulation , The state of three connections is defined ：

class DatagramSocket implements java.io.Closeable {
    ...
    static final int ST_NOT_CONNECTED = 0;
    static final int ST_CONNECTED = 1;
    static final int ST_CONNECTED_NO_IMPL = 2;
    ...
}


     
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Indicates that no connection has been established , Established connection and established connection , But it hasn't been realized yet level.

in addition , stay DatagramSocket It also contains the address and port of a connection ：

Use socat To create UDP service

Be careful , Before using subsequent commands , Need to be in unix Execute the installation command in the environment ：yum install iproute2 netcat-openbsd socat

And before Stream Socket equally , We can also use socat command , To build a UDP The server , We need to use socat The following parameters ：

We need to monitor udp4 and udp6 The data of , So here we use udp4-listen and udp6-listen Two parameters .

The following port number can be customized , Here we still use the same 8888 port , The corresponding command is as follows ：

The command above , We are 8888 Listening on the port UDP4 and UDP6 Connection information , among fork Parameter indicates that the program continues to run after receiving the package , If not fork, Then the program will automatically exit .

socat There was going to be one in the back bi-address, Here we use /dev/null, It means discarding all income Information .

UDP6-LISTEN There is a special parameter called ipv6only, Indicates that the received packet should not be sent to IPv4-mapped IPv6 addresses.

What is? IPv4-mapped IPv6 addresses？ Simply put, it means to IPv4 Mapped to IPv6 Address in .

Execute the above command , We will get the following output ：

Because it is executed in the background , So we returned to the process ID.

Use ss Command to monitor Datagram Sockets

ss The command can be used to check socket The state of , Here we need to use ss These parameters ：

   -4, --ipv4          display only IP version 4 sockets
   -u, --udp           display only UDP sockets
   -l, --listening     display listening sockets
   -n, --numeric       don't resolve service names

     
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Because we only monitor ipv4 and ipv6 The data of , So here we use -4 and -6 These two parameters .

In addition, because you only need to monitor udp sockets, So we need to use -u Parameters .

Because it's monitoring , So use -l Parameters , Finally, we hope to see specific figures , Instead of being resolved to a service name , So here we use -n Parameters .

Let's use the following command to see the result ：

You can get the following results ：

State       Recv-Q Send-Q                      Local Address:Port                                     Peer Address:Port              
UNCONN      0      0                                       *:8888                                                *:*  

     
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The above command only monitors Ipv4, Let's see Ipv6：

You can get the following results ：

State       Recv-Q Send-Q                      Local Address:Port                                     Peer Address:Port              
UNCONN      0      0                                      :::8888                                               :::*  

     
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and Ipv4 It's very similar , It means that we are Ipv6 The supervisor heard the port 8888.

Use nc Establish and UDP Socket The connection of

We've set up a monitor UDP Connected servers , Next we try to use nc Command to connect .

nc yes Ncat For short , Is a very small and efficient network tool . Let's look at the parameters that will be used in this example ：

  -4                         Use IPv4 only
  -6                         Use IPv6 only
  -u, --udp                  Use UDP instead of default TCP
  -v, --verbose              Set verbosity level (can be used several times)
  -z                         Zero-I/O mode, report connection status only

     
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Because you need to connect to Ipv4 and Ipv6, So we need to -4 and -6 Parameters .

By default nc It uses TCP agreement , If you want to use udp You need to use -u This parameter .

In addition, we need to output detailed information , So we need to -v Parameters , Finally, we directly establish the connection , Don't send any data , So here we use -z Parameters , Let's execute it and see the effect ：

Look at the output below ：

Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 127.0.0.1:8888.
Ncat: UDP packet sent successfully
Ncat: 1 bytes sent, 0 bytes received in 2.02 seconds.

     
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Express UDP Successful connection .

alike , We can use the following command to connect to UDP socket：

among ::1 It represents the local ipv6 Address .

You can get the following results ：

Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:8888.
Ncat: UDP packet sent successfully
Ncat: 1 bytes sent, 0 bytes received in 2.02 seconds.

     
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Express UDP Successful connection .

summary

This article explains datagram socket Basic concepts of , And use some unix The basic commands to build udp Servers and clients , So that you can understand .

This article has been included in http://www.flydean.com/16-datagram-socket/
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