1. Linux网络问题排查
1.1 iptables
iptables is a command-line firewall utility that uses policy chains to allow or block traffic. When a connection tries to establish itself on your system, iptables looks for a rule in its list to match it to. If it doesn’t find one, it resorts to the default action.
You want to be extremely careful when configuring iptables rules, particularly if you’re SSH’d into a server, because one wrong command can permanently lock you out until it’s manually fixed at the physical machine.
ufw 是一个简单的防火墙应用程序，它是 iptables 的前端, 使用ufw命令就是在更新iptables。ufw 的目标是使防火墙配置变得简单，而不是简单化 iptables。
如果linux网络出现了问题，首先要检查iptables是否有规则限制了网络流量。
# 查看iptables规则
sudo iptables -vnL
# 允许所有流量
sudo iptables -P INPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -P OUTPUT ACCEPT
# 禁止所有流量(下面的命令会阻断所有网络流量，包括ssh连接，要慎用)
sudo iptables -P INPUT DROP
sudo iptables -P FORWARD DROP
sudo iptables -P OUTPUT DROP
# 最重要的, 重置规则
sudo iptables -F
1.2 端口与netstat nmap nc telnet ss 命令
1.2.1 端口
Typically, ports identify a specific network service assigned to them. This can be changed by manually configuring the service to use a different port, but in general, the defaults can be used.
The first 1024 ports (port numbers 0 to 1023) are referred to as well-known port numbers and are reserved for the most commonly used services. These include SSH (port 22), HTTP (port 80), HTTPS (port 443).
1.2.2 netstat 等命令
# 列出所有正在监听的进程使用的ip和port
netstat -lntu
# or
ss -lntu
# 列出一些常用命令
telnet {{host}} {{port}}
nc -zv {{host}} {{port}}
nmap -p {{port}} {{host}}
namp {{host}}