How to check which ports are open in Linux

How to check open ports on Linux: 1. Use the nmap tool to detect open ports; 2. Use the netstat tool to detect open ports; 3. Use the lsof tool to detect open ports; 4. Use telnet to detect open ports; 5. Use the netcat tool to detect open ports.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

In network technology, ports include logical ports and physical ports. Physical ports refer to physically existing ports, such as interfaces on ADSL Modems, hubs, switches, and routers used to connect to other network devices, such as RJ-45 ports, SC ports, etc. Logical port refers to the port used to distinguish services in a logical sense, such as the service port in the TCP/IP protocol. The port number ranges from 0 to 65535, such as port 80 for web browsing services and port 21 for FTP services. wait. Due to the large number of physical ports and logical ports, in order to distinguish the ports, each port is numbered, which is the port number

Ports can be divided into 3 categories according to the port number:

1: Well Known Port

Well-known port numbers range from 0 to 1023. They are closely bound to some common services. For example, the FTP service uses port 21, which you can see in /etc/services. this mapping relationship.

2: Registered Ports:

from 1024 to 49151. They are loosely tied to some services. That is to say, there are many services bound to these ports, and these ports are also used for many other purposes.

3: Dynamic and/or Private Ports

Dynamic ports, That is, private port numbers are the number of ports that can be used by any software to communicate with any other software, using the Internet's Transmission Control Protocol, or User Transport Protocol. Dynamic ports are generally from 49152 to 65535

Linux has a limited port range. If I want to reserve certain ports for my program, then I need to control this port range. /proc/sys/net/ipv4/ip_local_port_range defines the local TCP/UDP port range. You can define net.ipv4.ip_local_port_range = 1024 65000

[root@localhost ~]# cat /proc/sys/net/ipv4/ip_local_port_range
32768   61000
[root@localhost ~]#  echo 1024 65535 > /proc/sys/net/ipv4/ip_local_port_range

About ports and services in /etc/sysctl.conf , I once used the analogy of a public toilet. Each toilet in a public toilet is like each port of the system. To provide convenience for people is the so-called service. If you provide these services, then the port (toilet) must be opened. When someone goes to When using the toilet, links are established at these ports. If that toilet is occupied by someone, it means that the port number is occupied by a service. If one day there is no public toilet service here and the public toilet is demolished, naturally there will be no port number. In fact, a more vivid example is like a bank lobby. The port numbers are the counters, and the people who take the numbers to handle business are like various clients linked to the server. They send business contact with the counter through port redirection technology. To give another easy-to-understand example, the port number is like each station on the high-speed rail line. For example, Changsha, Yueyang, etc. each represent a port number. Passengers go to their respective stations through train tickets, which is like each application program sending to the server. Port IP packet.

How to check whether the port is open? In fact, if I don’t sort it out, I don’t know that there are so many methods!

1: nmap tool detects open ports

nmap is a network scanning and host detection tool. Installation of nmap is very simple as shown below for rpm installation.

[root@DB-Server Server]# rpm -ivh nmap-4.11-1.1.x86_64.rpm 
warning: nmap-4.11-1.1.x86_64.rpm: Header V3 DSA signature: NOKEY, key ID 37017186
Preparing...                ########################################### [100%]
   1:nmap                   ########################################### [100%]
[root@DB-Server Server]# rpm -ivh nmap-frontend-4.11-1.1.x86_64.rpm 
warning: nmap-frontend-4.11-1.1.x86_64.rpm: Header V3 DSA signature: NOKEY, key ID 37017186
Preparing...                ########################################### [100%]
   1:nmap-frontend          ########################################### [100%]
[root@DB-Server Server]#

Regarding the use of nmap, you can write a long and close-up article, and I will not expand it here. As shown below, nmap 127.0.0.1 checks the open ports of this machine and scans all ports. Of course, other server ports can also be scanned.

[root@DB-Server Server]# nmap 127.0.0.1
 
Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2016-06-22 15:46 CST
Interesting ports on localhost.localdomain (127.0.0.1):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
25/tcp   open  smtp
111/tcp  open  rpcbind
631/tcp  open  ipp
1011/tcp open  unknown
3306/tcp open  mysql
 
Nmap finished: 1 IP address (1 host up) scanned in 0.089 seconds
You have new mail in /var/spool/mail/root
[root@DB-Server Server]#

2: The netstat tool detects open ports

[root@DB-Server Server]# netstat -anlp | grep 3306
tcp        0      0 :::3306                     :::*                        LISTEN      7358/mysqld         
[root@DB-Server Server]# netstat -anlp | grep 22
tcp        0      0 :::22                       :::*                        LISTEN      4020/sshd           
tcp        0     52 ::ffff:192.168.42.128:22    ::ffff:192.168.42.1:43561   ESTABLISHED 6198/2              
[root@DB-Server Server]#

As shown above, this The tool feels less concise and clear than nmap. Of course, it is not as powerful as nmap.

3: lsof tool detects open ports

[root@DB-Server Server]# service mysql start
Starting MySQL......[  OK  ]
[root@DB-Server Server]# lsof -i:3306
COMMAND  PID  USER   FD   TYPE DEVICE SIZE NODE NAME
mysqld  7860 mysql   15u  IPv6  44714       TCP *:mysql (LISTEN)
[root@DB-Server Server]# service mysql stop
Shutting down MySQL..[  OK  ]
[root@DB-Server Server]# lsof -i:3306
[root@DB-Server Server]#

[root@DB-Server Server]# lsof -i TCP| fgrep LISTEN
cupsd     3153    root    4u  IPv4   9115       TCP localhost.localdomain:ipp (LISTEN)
portmap   3761     rpc    4u  IPv4  10284       TCP *:sunrpc (LISTEN)
rpc.statd 3797 rpcuser    7u  IPv4  10489       TCP *:1011 (LISTEN)
sshd      4020    root    3u  IPv6  12791       TCP *:ssh (LISTEN)
sendmail  4042    root    4u  IPv4  12876       TCP localhost.localdomain:smtp (LISTEN)

4: Use telnet to detect whether the port is open

Even if the server port is in the listening state, but the firewall iptables blocks the port, it is impossible to detect whether the port is open through this method.

5: The netcat tool checks whether the port is open.

[root@DB-Server ~]# nc -vv 192.168.42.128 1521
Connection to 192.168.42.128 1521 port [tcp/ncube-lm] succeeded!
[root@DB-Server ~]# nc -z 192.168.42.128 1521; echo $?
Connection to 192.168.42.128 1521 port [tcp/ncube-lm] succeeded!
0
[root@DB-Server ~]#  nc -vv 192.168.42.128 1433
nc: connect to 192.168.42.128 port 1433 (tcp) failed: No route to host

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