概要

在运维过程中，telnet是一个众所周知比较收悉的测试端口的工具。
但是telnet有一个限制就是，telnet只能测试tcp端口，本文简单介绍如何测试udp端口。

Linux系统

Linux系统下可以使用nc、nmap两种工具来进行测试，两个工具都可通过yum进行安装。
以测试snmp的161端口为例，如下所示：
当udp 161端口开启时：

[root@localhost~]# nc -zvu 192.168.1.123 161
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.1.123:161.
Ncat: UDP packet sent successfully
Ncat: 1 bytes sent, 0 bytes received in 2.02 seconds.
[root@localhost ~]# nmap -sU -p 161 192.168.1.123

Starting Nmap 6.40 ( http://nmap.org ) at 2023-08-30 16:21 CST
Nmap scan report for 192.168.1.123
Host is up (0.0012s latency).
PORT    STATE SERVICE
161/udp open  snmp

Nmap done: 1 IP address (1 host up) scanned in 0.52 seconds

当udp 161端口关闭时。

[root@localhost ~]# nc -zvu 192.168.1.123 161    
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.1.123:161.
Ncat: Connection refused.
[root@localhost ~]# nmap -sU -p 161 192.168.1.123

Starting Nmap 6.40 ( http://nmap.org ) at 2023-08-30 16:22 CST
Nmap scan report for 192.168.1.123
Host is up (0.00058s latency).
PORT    STATE  SERVICE
161/udp closed snmp

Nmap done: 1 IP address (1 host up) scanned in 0.51 seconds

Windows系统

Windows系统可以使用nmap工具，下载链接如下：
https://nmap.org/download.html
以测试snmp的161端口为例，如下所示：
当udp 161端口开启时：

C:\Program Files (x86)\Nmap>nmap.exe -sU -p 161 192.168.1.123
Starting Nmap 7.94 ( https://nmap.org ) at 2023-08-30 17:04 China Standard Time
Nmap scan report for 192.168.1.123
Host is up (0.0011s latency).

PORT    STATE SERVICE
161/udp open  snmp

Nmap done: 1 IP address (1 host up) scanned in 0.44 seconds

当udp 161端口关闭时：

C:\Program Files (x86)\Nmap>nmap.exe -sU -p 161 192.168.1.123
Starting Nmap 7.94 ( https://nmap.org ) at 2023-08-30 17:04 China Standard Time
Nmap scan report for 192.168.1.123
Host is up (0.0010s latency).

PORT    STATE  SERVICE
161/udp closed snmp

Nmap done: 1 IP address (1 host up) scanned in 0.50 seconds