Linux: Block Port With IPtables

由Internet和其他网络协议识别端口号，使计算机能够与其他人进行交互。每个Linux服务器都有一个端口号（参见/ etc / services文件）

Block Incoming Port

The syntax is as follows to block incoming port using IPtables:

/sbin/iptables -A INPUT -p tcp --destination-port {PORT-NUMBER-HERE} -j DROP  

### interface section use eth1 ###

/sbin/iptables -A INPUT -i eth1 -p tcp --destination-port {PORT-NUMBER-HERE} -j DROP  

### only drop port for given IP or Subnet ##

/sbin/iptables -A INPUT -i eth0 -p tcp --destination-port {PORT-NUMBER-HERE} -s {IP-ADDRESS-HERE} -j DROP /sbin/iptables -A INPUT -i eth0 -p tcp --destination-port {PORT-NUMBER-HERE} -s {IP/SUBNET-HERE} -j DROP

To block port 80 (HTTP server), enter (or add to your iptables shell script):
# /sbin/iptables -A INPUT -p tcp --destination-port 80 -j DROP
# /sbin/service iptables save

Block Incomming Port 80 except for IP Address 1.2.3.4

#/sbin/iptables -A INPUT -p tcp -i eth1 -s ! 1.2.3.4 --dport 80 -j DROP

Block Outgoing Port

The syntax is as follows:

/sbin/iptables -A OUTPUT -p tcp --dport {PORT-NUMBER-HERE} -j DROP  

### interface section use eth1 ###

/sbin/iptables -A OUTPUT -o eth1 -p tcp --dport {PORT-NUMBER-HERE} -j DROP  

### only drop port for given IP or Subnet ##

/sbin/iptables -A OUTPUT -o eth0 -p tcp --destination-port {PORT-NUMBER-HERE} -s {IP-ADDRESS-HERE} -j DROP /sbin/iptables -A OUTPUT -o eth0 -p tcp --destination-port {PORT-NUMBER-HERE} -s {IP/SUBNET-HERE} -j DROP

To block outgoing port # 25, enter:
# /sbin/iptables -A OUTPUT -p tcp --dport 25 -j DROP
# /sbin/service iptables save
You can block port # 1234 for IP address 192.168.1.2 only:
# /sbin/iptables -A OUTPUT -p tcp -d 192.168.1.2 --dport 1234 -j DROP
# /sbin/service iptables save

How Do I Log Dropped Port Details?

Use the following syntax:

 # Logging #

### If you would like to log dropped packets to syslog, first log it ###

/sbin/iptables -A INPUT -m limit --limit 5/min -j LOG --log-prefix "PORT 80 DROP: " --log-level 7  

### now drop it ###

/sbin/iptables -A INPUT -p tcp --destination-port 80 -j DROP

How Do I Block Cracker (IP: 123.1.2.3) Access To UDP Port # 161?

/sbin/iptables -A INPUT -s 123.1.2.3 -i eth1 -p udp -m state --state NEW -m udp --dport 161 -j DROP  

# drop students 192.168.1.0/24 subnet to port 80

/sbin/iptables -A INPUT -s 192.168.1.0/24 -i eth1 -p tcp -m state --state NEW -m tcp --dport 80 -j DROP

How do I view blocked ports rules?

Use the iptables command:
# /sbin/iptables -L -n -v
# /sbin/iptables -L -n -v | grep port
# /sbin/iptables -L -n -v | grep -i DROP
# /sbin/iptables -L OUTPUT -n -v
# /sbin/iptables -L INPUT -n -v

---

check whether port is open or block

iptables -nL | grep <port number>

netstat :

netstat -plnt | grep ':25'

ss :

 ss -lntu | grep ':25'

nmap :

nmap -sT -O localhost | grep 25

lsof:

lsof -i:25

Refer to: https://www.cyberciti.biz/faq/iptables-block-port/

Refer to: https://unix.stackexchange.com/questions/306195/how-to-check-whether-port-25-is-open-or-blocked