By analyzing the log ngrok speculated that principle and mode
https://bob.kim/articles/2018/05/09/1525819680566.html
grok is a proxy server to provide network proxy function, for example:
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Following conditions:
1 A, B are two applications, including network operation;
2 ngrok server running a server in the public network;
A 3 ngrok client running on the same system;
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If B wants to interact with A, the following process:
1 B sends data to ngrok server;
2 ngrok server forwards the data to the ngrok client;
3 ngrok client forwards the data to A;
4 A data processing, the need to return to the data B, the process returns to the ngrok client;
5 ngrok client forwards the data to the ngrok server;
6 ngrok server forwards the data to the B;
Finally, A, B by forwarding it ngrok complete interaction data;
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We can think, ngrok A proxy objects outside the service provided, when B and A to interact with objects, only the object A proxy ngrok interact with it;
Well, all this interaction, there is a certain "protocol" communications;
The following explains the interactive process, involving the "Agreement":
Hereinafter referred to: client == ngrok client, server == ngrok server;
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(1) CMD = Auth
authentication --- Certification
client authentication to register with the server;
client starts, CMD = Auth send commands to the server; tell the server, my client needs to connect to the server, "certification"; for example:
{"Type":"Auth","Payload":{"Version":"2","MmVersion":"1.7","User":"","Password": "","OS":"darwin","Arch":"amd64","ClientId":""}}
//========================================================================
(2) CMD = AuthResp
authentication response --- authentication response
server response to client commands CMD = Auth; for example:
{"Type":"AuthResp","Payload":{"Version":"2","MmVersion":"1.7","ClientId":"c53d9a89c0ff87c37f14be30ff1f0ecb","Error":""}}
//========================================================================
(3) CMD = ReqTunnel
request tunnel --- request tunnel
client request to establish a tunnel to the Server; example:
{"Type":"ReqTunnel","Payload":{"Protocol":"http","ReqId":"NWLRB","Hostname": "","Subdomain":"wkf168","HttpAuth":"","RemotePort":0,"authtoken":""}
client sends their information to the server, requesting server establish a tunnel;
Protocol --- protocol request
an ID of the information ReqId --- Client;
Subdomain subdomain ---
//========================================================================
(4) CMD = ReqProxy
request proxy --- Request Broker
server to a client, client requests a "service agent"; for example:
{"Type":"ReqProxy","Payload":{}}
At this point, client creates a socket fd connected to server, server to accept to put the socket fd as a "tunnel";
//========================================================================
(5) CMD = RegProxy
register proxy --- registered agent
When the client receives server's "Request Broker" command, you can send a "registered agent" command to the server, for example:
{"Type":"RegProxy","Payload":{"ClientId":"c53d9a89c0ff87c37f14be30ff1f0ecb"}}
Which, Client ID is an authentication ID of the current client, it's time to carry AuthResp server command, information returned;
At this point, client sends this ID to sever, carry RegProxy command, pledged to register as an agent;
//========================================================================
(6) CMD = NewTunnel
Create a new tunnel --- tunnel;
After the server receives a client's registered agent command to the client registers a proxy identity;
At the same time, the client returns the information in the proxy authentication server, for example:
{"Type":"NewTunnel","Payload":{"ReqId":"NWLRB","Url":"http://wkf168.ngrok.bob.kim","Protocol":"http","Error":""}}
At this point, client has completed the creation of "agency status" in the server end, returned:
RegId --- is a client ID information carried when sending ReqTunnel command;
the Url --- is a server for the URL of the current distribution of the client agent;
Then, when other applications access Url, put the requested data is forwarded to the current client;
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In this case, after the above several steps to complete the establishment of a "tunnel" of;
So, when there are other applications to access Url, there is the following command interaction:
//========================================================================
(7) CMD = StartProxy
start proxy --- started agent
E.g:
{"Type":"StartProxy","Payload":{"Url":"http://wkf168.ngrok.bob.kim","ClientAddr":"183.14.133.76:49127"}}
When there is an application to access the Url, ngrok server receives the data; then, to the client to send StartProxy order requiring the client agent service started;
After then, in ReqProxy command, server sends the command to the client, client to create a socket fd is connected to the server;
So, we define the socket fd is fdA;
At this point, fdA it as a "tunnel" interfaces;
In this case, client is from this tunnel interface fdA received "StartProxy" command;
Then, client creates a new socket fd, connected to the local IP + PORT; we define the socket fd is fdB;
Finally, fdA relationship with fdB form of a map, are:
1 fdA received data, forwarded to the FDB;
2 fdB received data, forwarded to the FDA;
//========================================================================
(8) CMD = Ping --- used as a heartbeat packet
(9) CMD = Pong --- is a reply to the Ping command
//========================================================================
Like RTSP protocols, they are:
Between 1 Client and Server, as the channel has a socket fd "protocol interaction" used;
The command CMD is a transceiver mentioned above;
2 Client and Server re-transmission of audio and video data, the need to establish a new socket fd to interactive data;
Similarly, in ngrok in, too, when ngrok client begin to create a socket fd connected ngrok server, interactive communication command;
When ngrok client received when CMD = ReqProxy command creates a new socket fd, connected to ngrok server, as a "data exchange" tunnel interfaces;
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So, ngrok client and ngrok server during data forwarding tunnel when the following process:
1 client begins to establish a connection to the socket fd server, a command CMD interaction channel; socket fd is defined in the FDA;
2 server transmits to the client through the CMD command fdA = ReqProxy; requesting client proxy activity;
Then, client on the new socket fd is connected to a server, the definition of the socket fdB;
After receiving the connection fdB 3 server, and transmitting to the interface command CMD = StartProxy, the interface required to start the fdB agent activity;
Ultimately, this fdB to interact with the server as a client for forwarding data "tunnel" interfaces;
A more detailed analysis of the source code level reference: https://blog.csdn.net/lyb3290/article/details/80239890
Annex:
[ngrok client windows version 64 _bob.rar]
If you have questions or peers welcome to add the group to discuss: 
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