检查SMTP服务器是否在Linux上运行

codingman included in Linux

2016-06-04 990 words 2 minutes

Contents

1. 简介

SMTP 服务器处理电子邮件的发送和接收。因此，其正常运行时间至关重要。

在本教程中，我们将研究可用于检查 SMTP 服务器是否已启动并正在运行的各种方法。

2. 使用Telnet

Telnet 是一种用于与远程主机交互的应用程序协议。我们可以使用telnet 命令来检查 SMTP 连接。在我们的例子中，我们将在本地运行一个 SMTP 服务器。因此，我们的域名是localhost。

让我们使用telnet检查 SMTP 服务器是否在端口 2525 上运行：

[user@localhost ~]# telnet localhost 2525
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 a42502410059 smtp4dev ready

连接在终止之前会打开一小段时间：

[user@localhost ~]# telnet localhost 2525
...
Connection closed by foreign host.

我们可以在连接打开时键入示例电子邮件，看看 SMTP 服务器是否会接收并转发它：

[user@localhost ~]# telnet localhost 2525
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 5cf184409a80 smtp4dev ready
helo user.sample.com
mail from:<sender@sample.com>
rcpt to:<receiver@sample.com>
data
From: test@sample.com
Subject: This is a test
Hi there :)
How are you doing?
.
quit
250 Nice to meet you
250 New message started
250 Recipient accepted
354 End message with period
250 Mail accepted
Connection closed by foreign host.

我们看到我们的本地 SMTP 服务器收到了这封电子邮件：

Session started. Client address 10.88.0.1.
Message received. Client address 10.88.0.1. From sender@sample.com. To receiver@sample.com.
Processing received message
Processing received message DONE

相反，当 SMTP 服务器无法访问时，我们应该会看到“连接被拒绝”消息：

[user@localhost ~]# telnet localhost 2525
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
telnet: connect to address 127.0.0.1: Connection refused

3. 使用 NetCat

nc 或ncat ( NetCat ) 命令是一个网络实用程序，用于跨网络读取和写入数据。我们可以使用这个工具来测试我们的 SMTP 服务器的连接：

[user@localhost ~]# nc -v localhost 2525
Ncat: Version 7.70 ( https://nmap.org/ncat )
Ncat: Connection to ::1 failed: Connection refused.
Ncat: Trying next address...
Ncat: Connected to 127.0.0.1:2525.
220 b9b103518a80 smtp4dev ready

nc命令**可以使用 TCP 或 UDP 两种协议中的任何一种来读取或写入数据。*TCP 协议是默认协议设置。不过，如果我们想使用 UDP 协议，我们可以简单地添加-u*选项：

[user@localhost ~]# nc -vu localhost 2525
Ncat: Version 7.70 ( https://nmap.org/ncat )
Ncat: Connected to ::1:2525.

此外，如果 SMTP 服务器不可访问，我们应该会看到“连接被拒绝”消息：

[user@localhost ~]# nc -v localhost 2525
Ncat: Version 7.70 ( https://nmap.org/ncat )
Ncat: Connection to ::1 failed: Connection refused.
Ncat: Trying next address...
Ncat: Connection refused.

4. 使用nmap

nmap 命令有助于网络探索。虽然该工具的目的是扫描大型网络，但它也可以扫描单个主机。需要注意的是，该工具存在法律问题——特别是关于端口扫描。因此，建议我们仅在本地使用该工具：

[user@localhost ~]# nmap localhost -p 2525 
Starting Nmap 7.70 ( https://nmap.org ) at 2021-11-11 20:38 SAST
Nmap scan report for localhost (127.0.0.1)
Host is up.
Other addresses for localhost (not scanned): ::1
PORT     STATE    SERVICE
2525/tcp filtered ms-v-worlds
Nmap done: 1 IP address (1 host up) scanned in 2.08 seconds

** STATE列的目的是描述一个端口**。该列的值是open、filtered和closed。filtered 状态意味着防火墙已到位。此外，当我们的 SMTP 服务器不可访问时，我们可以期望看到closed状态列的值：

[user@localhost ~]# nmap localhost -p 2525 
Starting Nmap 7.70 ( https://nmap.org ) at 2021-11-11 20:42 SAST
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00010s latency).
Other addresses for localhost (not scanned): ::1
PORT     STATE  SERVICE
2525/tcp closed ms-v-worlds
Nmap done: 1 IP address (1 host up) scanned in 0.27 seconds