Table of Contents

About

This article is about a connection within the TCP protocol.

A TCP server may serve several clients concurrently with the same local socket address (ie local IP address and port number) by creating a child process for each client for each connection.

A TCP connection is the name for the full request and response process in which:

  • a client connects to the server,
  • the server generates a response,
  • and the server closes the connection.

Id

Every connection is uniquely identified by an identifier called a socket.

Connection Reset

The Connection reset error occurs when the server isn’t sending back any data.

Browser

Browsers may deal with closed connections by retrying, because the problem might be temporary.

Standard Stream

After having created a connection, an application sends a standard input (on a io level) to the host via the connection, and receive a standard output.

For instance, lets request the home page of this website via the below HTTP get request 

GET / HTTP/1.1
Host: gerardnico.com

we can send it as standard input with echo to a connection created with netcat

For Https, you need to use another utility such as openssl because netcast does not support it. 

echo -e "GET / HTTP/1.1\nHost: gerardnico.com\n" | nc gerardnico.com 80

and you should get a redirection response telling you that you need to go to https://gerardnico.com/ 

HTTP/1.1 301 Moved Permanently
Date: Mon, 13 Apr 2020 12:26:26 GMT
Transfer-Encoding: chunked
Connection: keep-alive
Cache-Control: max-age=3600
Expires: Mon, 13 Apr 2020 13:26:26 GMT
Location: https://gerardnico.com/
Server: cloudflare
CF-RAY: 58352ceb1ba9bf78-AMS

Process

TCP connections process (how a TCP socket gets started)

Duration: anywhere from 30-100ms to establish a connection and if you had SSL, there is more round trip.

Properties

State

During the lifetime of a TCP connection the local end-point undergoes a series of state changes:

Management

List

Powershell

Get-NetTCPConnection

# All listening port
Get-NetTCPConnection -State Listen
# One port
Get-NetTCPConnection -State Listen –LocalPort 1313

Netstat

On Unix-like and Microsoft Windows based operating systems the netstat command line tool may be used to list all currently established connection (ie sockets).

  • Command line - Example: Count all tcp connection for a the web server nginx
netstat \
  -all \
  --programs \
  -n \ # only numeric value (no host name, no port name)
  --tcp \ # only tcp 
  |  { IFS= read a; echo $a; grep nginx; } # capture and print the first line

Active Internet connections (servers and established)
tcp        0      0 0.0.0.0:443             0.0.0.0:*               LISTEN      15549/nginx: worker
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      15549/nginx: worker
tcp        0      0 164.132.99.202:443      162.158.183.132:28246   ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      143.176.206.82:7414     ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      108.162.246.202:22880   ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      143.176.206.82:7410     ESTABLISHED 15550/nginx: worker
tcp        0      0 127.0.0.1:56812         127.0.0.1:19999         ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      162.158.183.172:54598   ESTABLISHED 15550/nginx: worker
tcp        0      0 127.0.0.1:57484         127.0.0.1:19999         ESTABLISHED 15549/nginx: worker
tcp        0      0 127.0.0.1:57482         127.0.0.1:19999         ESTABLISHED 15549/nginx: worker
tcp        0      0 127.0.0.1:80            127.0.0.1:44084         ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      173.245.54.88:41618     ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      143.176.206.82:6910     ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      172.69.63.26:25284      ESTABLISHED 15549/nginx: worker
tcp        0      0 164.132.99.202:443      143.176.206.82:7377     ESTABLISHED 15550/nginx: worker
tcp6       0      0 :::443                  :::*                    LISTEN      15549/nginx: worker
  • UI

Netstat Tcp Listening Socket

Tcpview

Network - TCPview

Process Explorer

Windows Process Explorer

Tcp Connection State Process Explorer

Get file

See Network File

Create

  • On a utility level, the utility netcat create a TCP connection.
  • On a library level, check the network package of your language.

Count

Count all TCP connection with netstat

Example for the nginx programs 

netstat \
  -all \
  --programs \
  --tcp \ # only tcp 
  | grep nginx
  | wc

Test

You can test if a connection is possible with the netcat utility and it's z options that report connection status only.

  • In case of a connection
nc -z localhost 80 && echo tcp connection is possible
  • In case of no possible connection
nc -z localhost 80 || echo no tcp connection possible

Documentation / Reference