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translate done: 20171109 Learn how to use tcpdump command with examples.md
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translating by lujun9972
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Learn how to use tcpdump command with examples
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======
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Tcpdump command is a famous network packet analysing tool that is used to display TCP\IP & other network packets being transmitted over the network attached to the system on which tcpdump has been installed. Tcpdump uses libpcap library to capture the network packets & is available on almost all Linux/Unix flavors.
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Tcpdump command can read the contents from a network interface or from a previously created packet file or we can also write the packets to a file to be used for later. One must use the tcpdump command as root or as a user with sudo privileges.
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In this tutorial, we are going to discuss the uses of tcpdump command along with some examples, but first let's start with installation of tcpdump on various Linux OS.
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**(Recommended Read:[Monitoring network bandwidth with iftop command][1])**
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### Installation
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By default, tcpdump is available on almost all Linux distributions but if that's not the case for you, install it on your system using the following method.
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**CentOS/RHEL**
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Install tcpdump on CentOS & RHEL using the following command ,
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**$ sudo yum install tcpdump**
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**Fedora**
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On Fedora, install tcpdump using the following command,
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**$ dnf install tcpdump**
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**Ubuntu/Debian/Linux Mint**
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On Ubuntu or Debian or Linux Mint, install tcp dumo using the following command,
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**$ apt-get install tcpdump**
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Now that we have install tcpdump on our systems, let's discuss some examples for tcpdump.
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### Examples
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**Get packets from all interfaces**
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To get the network packets from all network interfaces, run the following command,
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**$ tcpdump -i any**
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**Get packets from a single interfaces**
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To get the network packets from a single interface, use
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**$ tcpdump -i eth0**
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**Writing captured packets to file**
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To write all the captured packets to a file, use the '-w' option,
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**$ tcpdump -i eth1 -w packets_file**
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**Reading an old tcpdump file**
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To read an already created, old tcpdump file, use the following command,
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**$ tcpdump -r packets_file**
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**Getting more packets information with readable timestamps**
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To get more information regarding the packets along with readable timestamp, use
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**$ tcpdump -ttttnnvvS**
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**Check packets of whole network**
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To get the packets for whole network, execute the following command from terminal
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**$ tcpdump net 192.168.1.0/24**
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**Check packets based on IP address**
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Get all the packets based on the IP address, whether source or destination or both, using the following command,
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**$ tcpdump host 192.168.1.100**
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To get packets based on source or destination of an IP address, use
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**$ tcpdump src 192.168.1.100**
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**$ tcpdump dst 192.168.1.100**
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**Check packets for a protocol or port number**
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To check all the packets used based on the protocol, run the following command
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**$ tcpdump ssh**
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To get packets for a single port ot for a range of ports, use
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**$ tcpdump port 22**
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**$ tcpdump portrange 22-125**
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We can also use ** 'src'** & **' dst'** options to get packets for ports based on source & destination.
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We can also combine two conditions with AND (and , && ), OR ( or. || ) & EXCEPT (not , ! ). This helps when we have analyze network packets based on the some condtions.
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**Using AND**
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We can use 'and' or symbol '&&' to combine two conditions or mote with tcpdump. An example would be,
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**$ tcpdump src 192.168.1.100 && port 22 -w ssh_packets**
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**Using OR**
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OR will check the command agtcpdump -i eth0 src port not 22ainst one the mentioned conditions in the command, like
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**$ tcpdump src 192.168.1.100 or dst 192.168.1.50 && port 22 -w ssh_packets**
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**$ tcpdump port 443 or 80 -w http_packets**
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**Using EXCEPT**
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EXCEPT will be used when we want not fulfill a condition, like
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**$ tcpdump -i eth0 src port not 22**
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This will monitor all the traffic on eth0 but will not capture port 22.
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This was our tutorial on how to install & use tcpdump command to capture the network packets. Please feel free to send in any queries or suggestions using the comment box below.
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--------------------------------------------------------------------------------
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via: http://linuxtechlab.com/learn-use-tcpdump-command-examples/
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作者：[Shusain ][a]
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译者：[lujun9972](https://github.com/lujun9972)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:http://linuxtechlab.com/author/shsuain/
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[1]:http://linuxtechlab.com/monitoring-network-bandwidth-iftop-command/
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@ -0,0 +1,165 @@
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通过案例学习 TCPDUMP 命令
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======
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Tcpdump 是一个很常用的网络包分析工具，可以用来显示通过网络传输到本系统的 TCP\IP 以及其他网络的数据包。Tcpdump 使用 libpcap 库来抓取网络报，这个库在几乎存在于所有的 Linux/Unix 中。
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Tcpdump 可以从网卡或之前创建的数据包文件中读取内容，也可以将包写入文件中以供后续使用。必须是 root 用户或者使用 sudo 特权来运行 tcpdump。
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在本文中，我们将会通过一些案例来演示如何使用 tcpdump 命令，但首先让我们来看看在各种 Linux 操作系统中是如何安装 tcpdump 的。
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**(推荐阅读：[使用 iftop 命令监控网络带宽 ][1])**
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### 安装
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tcpdump 默认在几乎所有的 Linux 发行版中都可用，但若你的 Linux 上没有的话，使用下面方法进行安装。
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#### CentOS/RHEL
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使用下面命令在 CentOS 和 RHEL 上安装 tcpdump，
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```
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$ sudo yum install tcpdump*
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```
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#### Fedora
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使用下面命令在 Fedora 上安装 tcpdump，
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```
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$ dnf install tcpdump
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```
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#### Ubuntu/Debian/Linux Mint
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在 Ubuntu/Debain/Linux Mint 上使用下面命令安装 tcpdump
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```
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$ apt-get install tcpdump
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```
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安装好 tcpdump 后，现在来看一些例子。
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### 案例演示
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#### 从所有网卡中捕获数据包
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运行下面命令来从所有网卡中捕获数据包，run the following command，
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```
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$ tcpdump -i any
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```
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#### 从指定网卡中捕获数据包
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要从指定网卡中捕获数据包，运行
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```
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$ tcpdump -i eth0
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```
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#### 将捕获的包写入文件
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使用 ‘-w’ 选项将所有捕获的包写入文件，
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```
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$ tcpdump -i eth1 -w packets_file
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```
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#### 读取之前产生的 tcpdump 文件
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使用下面命令从之前创建的 tcpdump 文件中读取内容
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```
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$ tcpdump -r packets_file
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```
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#### 获取更多的包信息并且以可读的形式显示时间戳
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要获取更多的包信息同时以可读的形式显示时间戳，使用
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```
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$ tcpdump -ttttnnvvS
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```
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#### 查看整个网络的数据包
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要获取整个网络的数据包，在终端执行下面命令
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```
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$ tcpdump net 192.168.1.0/24
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```
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#### 根据 IP 地址查看报文
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要获取指定 IP 的数据包，不管是作为源地址还是目的地址，使用下面命令，
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```
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$ tcpdump host 192.168.1.100
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```
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要指定 IP 地址是源地址或是目的地址则使用
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```
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$ tcpdump src 192.168.1.100
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$ tcpdump dst 192.168.1.100
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```
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#### 查看某个协议或端口号的数据包
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要查看某个协议的数据包，运行下面命令
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```
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$ tcpdump ssh
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```
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要捕获某个端口或一个范围的数据包，使用
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```
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$ tcpdump port 22
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$ tcpdump portrange 22-125
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```
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我们也可以与 'src' 和 'dst' 选项连用来捕获指定源端口或指定目的端口的报文
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我们还可以使用 AND (与，&& )，OR ( 或。|| ) & EXCEPT (非，！) 来将两个条件组合起来。当我们需要基于某些条件来分析网络报文是非常有用。
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#### 使用 AND
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可以使用 'and' 或者符号 '&&' 来将两个或多个条件组合起来。比如，
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```
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$ tcpdump src 192.168.1.100 && port 22 -w ssh_packets
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```
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#### 使用 OR
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OR 会检查是否匹配命令所列条件中的其中一条，像这样
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```
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$ tcpdump src 192.168.1.100 or dst 192.168.1.50 && port 22 -w ssh_packets
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$ tcpdump port 443 or 80 -w http_packets
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```
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#### 使用 EXCEPT
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当我们想表达不匹配某项条件时可以使用 EXCEPT，像这样
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```
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$ tcpdump -i eth0 src port not 22
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```
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这会捕获 eth0 上除了 22 号端口的所有通讯。
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我们的教程至此就结束了，在本教程中我们讲解了如何安装并使用 tcpdump 来捕获网络数据包。如有任何疑问或建议，欢迎留言。
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--------------------------------------------------------------------------------
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via: http://linuxtechlab.com/learn-use-tcpdump-command-examples/
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作者：[Shusain ][a]
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译者：[lujun9972](https://github.com/lujun9972)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:http://linuxtechlab.com/author/shsuain/
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[1]:http://linuxtechlab.com/monitoring-network-bandwidth-iftop-command/
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