tcpdump prints out a description of the contents of packets on a network interface that match the Boolean expression (see pcap-filter(7) for the expression syntax); the description is preceded by a time stamp, printed, by default, as hours, minutes, seconds, and fractions of a second since midnight. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -r flag, which causes it to read from a saved packet file rather than to read packets from a network interface. It can also be run with the -V flag, which causes it to read a list of saved packet files. In all cases, only packets that match expression will be processed by tcpdump.
tcpdump打印出网络接口上与布尔表达式匹配的数据包内容的描述（表达式语法见pcap-filter（7））;描述之前有一个时间戳，默认情况下打印为小时、分钟、秒和自午夜以来的几分之一秒。它也可以使用-w标志运行，这会使它将数据包数据保存到文件中以供以后分析，和/或使用-r标志运行，这会使它从保存的数据包文件中读取，而不是从网络接口读取数据包。它也可以使用-V标志运行，这会导致它读取保存的数据包文件列表。在所有情况下，只有匹配expression的数据包才会被tcpdump处理。

tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically control-C) or a SIGTERM signal (typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.
如果不使用-c标志运行，tcpdump将继续捕获数据包，直到被SIGINT信号中断（例如，通过键入中断字符（通常为control-C）或SIGTERM信号生成（通常由kill（1）命令生成）;如果使用-c标志运行，它将捕获数据包，直到它被SIGINT或SIGTERM信号中断，或者已经处理了指定数量的数据包。

When tcpdump finishes capturing packets, it will report counts of:
当tcpdump完成捕获数据包时，它将报告以下计数：

packets ``captured'' (this is the number of packets that tcpdump has received and processed);
packets `` captured ''（这是tcpdump接收和处理的数据包数量）;

packets ``received by filter'' (the meaning of this depends on the OS on which you're running tcpdump, and possibly on the way the OS was configured - if a filter was specified on the command line, on some OSes it counts packets regardless of whether they were matched by the filter expression and, even if they were matched by the filter expression, regardless of whether tcpdump has read and processed them yet, on other OSes it counts only packets that were matched by the filter expression regardless of whether tcpdump has read and processed them yet, and on other OSes it counts only packets that were matched by the filter expression and were processed by tcpdump);
数据包“由筛选器接收”（这一点的含义取决于运行tcpdump的操作系统，可能还取决于操作系统的配置方式-如果在命令行上指定了过滤器，则在某些操作系统上，它会对数据包进行计数，而不管它们是否与过滤器表达式匹配，即使它们与过滤器表达式匹配，也不管tcpdump是否已经读取并处理它们，在其他操作系统上，它只计数与过滤器表达式匹配的数据包，而不管tcpdump是否已经读取并处理它们，并且在其他操作系统上，它只计数与过滤器表达式匹配并由tcpdump处理的数据包）;

packets ``dropped by kernel'' (this is the number of packets that were dropped, due to a lack of buffer space, by the packet capture mechanism in the OS on which tcpdump is running, if the OS reports that information to applications; if not, it will be reported as 0).
packets ``dropped by kernel ''（这是由于缺少缓冲区空间而由运行tcpdump的操作系统中的数据包捕获机制丢弃的数据包数量，如果操作系统将该信息报告给应用程序;否则，它将报告为0）。

On platforms that support the SIGINFO signal, such as most BSDs (including macOS) and Digital/Tru64 UNIX, it will report those counts when it receives a SIGINFO signal (generated, for example, by typing your ``status'' character, typically control-T, although on some platforms, such as macOS, the ``status'' character is not set by default, so you must set it with stty(1) in order to use it) and will continue capturing packets. On platforms that do not support the SIGINFO signal, the same can be achieved by using the SIGUSR1 signal.
在支持SIGINFO信号的平台上，例如大多数BSD（包括macOS）和Digital/Tru 64 UNIX时，它将在收到SIGINFO信号时报告这些计数（例如，通过键入您的“状态”字符（通常为control-T）生成，尽管在某些平台（如macOS）上，默认情况下不会设置“状态”字符，所以你必须设置它与stty（1）为了使用它），并将继续捕获数据包.在不支持SIGINFO信号的平台上，可以通过使用SIGUSR 1信号来实现相同的功能。

Using the SIGUSR2 signal along with the -w flag will forcibly flush the packet buffer into the output file.
使用SIGUSR 2信号沿着-w标志将强制将数据包缓冲区刷新到输出文件中。

Reading packets from a network interface may require that you have special privileges; see the pcap(3PCAP) man page for details. Reading a saved packet file doesn't require special privileges.
从网络接口阅读数据包可能需要您具有特殊权限;有关详细信息，请参见pcap（3 PCAP）手册页。阅读保存的数据包文件不需要特殊权限。

-A

Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
以ASCII格式打印每个数据包（减去其链路级报头）。方便捕捉网页。

-b

Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.
用ASDOT符号而不是ASPLAIN符号打印BGP数据包中的AS编号。

-B buffer_size

--buffer-size=buffer_size

Set the operating system capture buffer size to buffer_size, in units of KiB (1024 bytes).
将操作系统捕获缓冲区大小设置为buffer_size，单位为KiB（1024字节）。

-c count

Exit after receiving count packets.
收到计数数据包后退出。

--count

Print only on stdout the packet count when reading capture file(s) instead of parsing/printing the packets. If a filter is specified on the command line, tcpdump counts only packets that were matched by the filter expression.
当阅读捕获文件时，仅在标准输出上打印数据包计数，而不是解析/打印数据包。如果在命令行中指定了筛选器，则tcpdump仅计算与筛选器表达式匹配的数据包。

-C file_size

Before writing a raw packet to a savefile, check whether the file is currently larger than file_size and, if so, close the current savefile and open a new one. Savefiles after the first savefile will have the name specified with the -w flag, with a number after it, starting at 1 and continuing upward. The default unit of file_size is millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
在将原始数据包写入保存文件之前，请检查该文件当前是否大于file_size，如果是，请关闭当前保存文件并打开一个新文件。第一个存储文件之后的存储文件将具有使用-w标志指定的名称，后面有一个数字，从1开始，一直向上。file_size的默认单位是百万字节（1，000，000字节，而不是1，048，576字节）。

By adding a suffix of k/K, m/M or g/G to the value, the unit can be changed to 1,024 (KiB), 1,048,576 (MiB), or 1,073,741,824 (GiB) respectively.
通过在值上添加后缀k/K、m/M或g/G，单位可以分别更改为1，024（KiB）、1，048，576（MiB）或1，073，741，824（GiB）。

-d

Dump the compiled packet-matching code in a human readable form to standard output and stop.
将编译后的包匹配代码以人类可读的形式转储到标准输出并停止。

Please mind that although code compilation is always DLT-specific, typically it is impossible (and unnecessary) to specify which DLT to use for the dump because tcpdump uses either the DLT of the input pcap file specified with -r, or the default DLT of the network interface specified with -i, or the particular DLT of the network interface specified with -y and -i respectively. In these cases the dump shows the same exact code that would filter the input file or the network interface without -d.
请注意，尽管代码编译总是特定于DLT的，但通常不可能（也没有必要）指定要用于转储的DLT，因为tcpdump使用-r指定的输入pcap文件的DLT，或-i指定的网络接口的默认DLT，或分别使用-y和-i指定的网络接口的特定DLT。在这些情况下，dump显示的代码与过滤输入文件或网络接口时不使用-d的代码完全相同。

However, when neither -r nor -i is specified, specifying -d prevents tcpdump from guessing a suitable network interface (see -i). In this case the DLT defaults to EN10MB and can be set to another valid value manually with -y.
但是，当既没有指定-r也没有指定-i时，指定-d可以防止tcpdump猜测合适的网络接口（参见-i）。在这种情况下，DLT默认为EN 10 MB，可以使用-y手动设置为另一个有效值。

-dd

Dump packet-matching code as a C program fragment.
将包匹配代码转储为C程序片段。

-ddd

Dump packet-matching code as decimal numbers (preceded with a count).
将数据包匹配代码转储为十进制数（前面有一个计数）。

-D

--list-interfaces

Print the list of the network interfaces available on the system and on which tcpdump can capture packets. For each network interface, a number and an interface name, possibly followed by a text description of the interface, are printed. The interface name or the number can be supplied to the -i flag to specify an interface on which to capture.
打印系统上可用的网络接口列表，tcpdump可以在这些接口上捕获数据包。对于每个网络接口，打印一个编号和接口名称，后面可能还有接口的文本描述。接口名称或编号可以提供给-i标志，以指定要捕获的接口。

This can be useful on systems that don't have a command to list them (e.g., Windows systems, or UNIX systems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems, where the interface name is a somewhat complex string.
这在没有命令列出它们的系统上可能很有用（例如，Windows系统，或缺少ifconfig -a的UNIX系统）;该数字在Windows 2000及更高版本的系统上可能很有用，其中接口名称是一个有点复杂的字符串。

The -D flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_findalldevs(3PCAP) function.
如果tcpdump是用缺少pcap_findalldevs（3 PCAP）函数的libpcap旧版本构建的，则不支持-D标志。

-e

Print the link-level header on each dump line. This can be used, for example, to print MAC layer addresses for protocols such as Ethernet and IEEE 802.11.
在每个转储行上打印链接级标题。例如，这可以用于打印以太网和IEEE 802.11等协议的MAC层地址。

-E

Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain Security Parameter Index value spi. This combination may be repeated with comma or newline separation.
使用spi@ipaddr algo：secret对寻址到addr并包含安全参数索引值spi的ESPs数据包进行解密。这种组合可以用逗号或换行符分隔。

Note that setting the secret for IPv4 ESP packets is supported at this time.
请注意，此时支持为IPv4 ESP数据包设置密码。

Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default is des-cbc. The ability to decrypt packets is only present if tcpdump was compiled with cryptography enabled.
算法可以是des-cbc、3des-cbc、blowfish-cbc、rc 3-cbc、cast 128-cbc或无。默认值为des-cbc。只有在编译tcpdump时启用了加密，才能解密数据包。

secret is the ASCII text for ESP secret key. If preceded by 0x, then a hex value will be read.
secret是ESP密钥的ASCII文本。如果前面是0x，则将读取十六进制值。

The option assumes RFC 2406 ESP, not RFC 1827 ESP. The option is only for debugging purposes, and the use of this option with a true `secret' key is discouraged. By presenting IPsec secret key onto command line you make it visible to others, via ps(1) and other occasions.
该选项假定为RFC 2406 ESP，而不是RFC 1827 ESP。该选项仅用于调试目的，并且不鼓励将该选项与真正的“secret”密钥一起使用。通过在命令行中提供私钥，您可以通过ps（1）和其他场合使其他人看到它。

In addition to the above syntax, the syntax file name may be used to have tcpdump read the provided file in. The file is opened upon receiving the first ESP packet, so any special permissions that tcpdump may have been given should already have been given up.
除了上面的语法，语法文件名可以用来让tcpdump读入提供的文件。该文件在接收到第一个ESP数据包时被打开，因此tcpdump可能被赋予的任何特殊权限都应该已经被放弃。

-f

Print `foreign' IPv4 addresses numerically rather than symbolically (this option is intended to get around serious brain damage in Sun's NIS server --- usually it hangs forever translating non-local internet numbers).
用数字而不是符号打印“外国”IPv4地址（此选项旨在避免Sun的NIS服务器中的严重脑损伤-通常它会永远挂起翻译非本地互联网号码）。

The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of the interface on that capture is being done. If that address or netmask are not available, either because the interface on that capture is being done has no address or netmask or because it is the "any" pseudo-interface, which is available in Linux and in recent versions of macOS and Solaris, and which can capture on more than one interface, this option will not work correctly.
使用正在捕获的接口的IPv4地址和网络掩码完成对“外来”IPv4地址的测试。如果该地址或网络掩码不可用，或者是因为正在执行捕获的接口没有地址或网络掩码，或者是因为它是“any”伪接口（在Linux以及最新版本的macOS和Solaris中可用，并且可以在多个接口上捕获），则此选项将无法正常工作。

-F file

Use file as input for the filter expression. An additional expression given on the command line is ignored.
使用文件作为筛选器表达式的输入。忽略命令行上给出的其他表达式。

-G rotate_seconds

If specified, rotates the dump file specified with the -w option every rotate_seconds seconds. Savefiles will have the name specified by -w which should include a time format as defined by strftime(3). If no time format is specified, each new file will overwrite the previous. Whenever a generated filename is not unique, tcpdump will overwrite the preexisting data; providing a time specification that is coarser than the capture period is therefore not advised.
如果指定，则每隔rotate_seconds秒旋转一次使用-w选项指定的转储文件。保存文件将具有由-w指定的名称，该名称应包含由strftime（3）定义的时间格式。如果未指定时间格式，则每个新文件将覆盖前一个文件。只要生成的文件名不是唯一的，tcpdump就会覆盖先前存在的数据;因此不建议提供比捕获周期更粗略的时间规范。

If used in conjunction with the -C option, filenames will take the form of `file<count>'.
如果与-C选项一起使用，文件名将采用“file”的形式<count>。

-h

--help

Print the tcpdump and libpcap version strings, print a usage message, and exit.
打印tcpdump和libpcap版本字符串，打印用法消息，然后退出。

--version

Print the tcpdump and libpcap version strings and exit.
打印tcpdump和libpcap版本字符串并退出。

-H

Attempt to detect 802.11s draft mesh headers.
尝试检测802.11s草案网格标题。

-i interface

--interface=interface

Listen, report the list of link-layer types, report the list of time stamp types, or report the results of compiling a filter expression on interface. If unspecified and if the -d flag is not given, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback), which may turn out to be, for example, ``eth0''.
监听、报告链路层类型列表、报告时间戳类型列表或报告接口上的过滤器表达式的编译结果。如果未指定，并且没有给出-d标志，tcpdump将搜索系统接口列表中编号最低的已配置up接口（不包括upi），例如，它可能是“eth0”。

On Linux systems with 2.2 or later kernels and on recent versions of macOS and Solaris, an interface argument of ``any'' can be used to capture packets from all interfaces. Note that captures on the ``any'' pseudo-interface will not be done in promiscuous mode.
在具有2.2或更高版本内核的Linux系统以及最新版本的macOS和Solaris上，接口参数"any“可用于捕获来自所有接口的数据包。请注意，在“any”伪接口上的捕获不会在混杂模式下完成。

If the -D flag is supported, an interface number as printed by that flag can be used as the interface argument, if no interface on the system has that number as a name.
如果支持-D标志，则该标志打印的接口编号可以用作接口参数，前提是系统上没有接口将该编号作为名称。

-I

--monitor-mode

Put the interface in "monitor mode"; this is supported only on IEEE 802.11 Wi-Fi interfaces, and supported only on some operating systems.
将接口置于“监视模式”;这仅在IEEE 802.11 Wi-Fi接口上受支持，并且仅在某些操作系统上受支持。

Note that in monitor mode the adapter might disassociate from the network with which it's associated, so that you will not be able to use any wireless networks with that adapter. This could prevent accessing files on a network server, or resolving host names or network addresses, if you are capturing in monitor mode and are not connected to another network with another adapter.
请注意，在监视模式下，适配器可能会与它所关联的网络解除关联，因此您将无法使用该适配器的任何无线网络。这可能会阻止访问网络服务器上的文件，或解析主机名或网络地址，如果您在监视器模式下捕获，并且没有使用其他适配器连接到其他网络。

This flag will affect the output of the -L flag. If -I isn't specified, only those link-layer types available when not in monitor mode will be shown; if -I is specified, only those link-layer types available when in monitor mode will be shown.
此标志将影响-L标志的输出。如果未指定-I，则仅显示未处于监视模式时可用的链路层类型;如果指定-I，则仅显示处于监视模式时可用的链路层类型。

--immediate-mode

Capture in "immediate mode". In this mode, packets are delivered to tcpdump as soon as they arrive, rather than being buffered for efficiency. This is the default when printing packets rather than saving packets to a ``savefile'' if the packets are being printed to a terminal rather than to a file or pipe.
在“即时模式”下捕获。在这种模式下，数据包一到达就被传递到tcpdump，而不是为了提高效率而进行缓冲。如果数据包被打印到终端而不是文件或管道，则这是打印数据包而不是将数据包保存到“保存文件”时的默认设置。

-j tstamp_type

--time-stamp-type=tstamp_type

Set the time stamp type for the capture to tstamp_type. The names to use for the time stamp types are given in pcap-tstamp(7); not all the types listed there will necessarily be valid for any given interface.
将捕获的时间戳类型设置为tstamp_type。pcap-tstamp（7）中给出了用于时间戳类型的名称;不是所有列出的类型都必须对任何给定的接口有效。

-J

--list-time-stamp-types

List the supported time stamp types for the interface and exit. If the time stamp type cannot be set for the interface, no time stamp types are listed.
列出接口支持的时间戳类型并退出。如果无法为接口设置时间戳类型，则不会列出任何时间戳类型。

--time-stamp-precision=tstamp_precision

When capturing, set the time stamp precision for the capture to tstamp_precision. Note that availability of high precision time stamps (nanoseconds) and their actual accuracy is platform and hardware dependent. Also note that when writing captures made with nanosecond accuracy to a savefile, the time stamps are written with nanosecond resolution, and the file is written with a different magic number, to indicate that the time stamps are in seconds and nanoseconds; not all programs that read pcap savefiles will be able to read those captures.
捕获时，将捕获的时间戳精度设置为tstamp_precision。请注意，高精度时间戳（纳秒）的可用性及其实际精度取决于平台和硬件。还要注意的是，当以纳秒精度将捕获写入保存文件时，时间戳以纳秒分辨率写入，并且文件以不同的幻数写入，以指示时间戳以秒和纳秒为单位;并非所有读取pcap保存文件的程序都能够读取这些捕获。

When reading a savefile, convert time stamps to the precision specified by timestamp_precision, and display them with that resolution. If the precision specified is less than the precision of time stamps in the file, the conversion will lose precision.
当阅读保存文件时，将时间戳转换为timestamp_precision指定的精度，并以该分辨率显示它们。如果指定的精度小于文件中时间戳的精度，则转换将丢失精度。

The supported values for timestamp_precision are micro for microsecond resolution and nano for nanosecond resolution. The default is microsecond resolution.
timestamp_precision支持的值是micro（微秒分辨率）和nano（纳秒分辨率）。默认分辨率为微秒。

--micro

--nano

Shorthands for --time-stamp-precision=micro or --time-stamp-precision=nano, adjusting the time stamp precision accordingly. When reading packets from a savefile, using --micro truncates time stamps if the savefile was created with nanosecond precision. In contrast, a savefile created with microsecond precision will have trailing zeroes added to the time stamp when --nano is used.
缩写为--time-stamp-precision=micro或--time-stamp-precision=nano，相应地调整时间戳精度。从存储文件中阅读数据包时，如果存储文件是以纳秒精度创建的，则使用--micro会截断时间戳。相比之下，使用--nano时，以微秒精度创建的保存文件将在时间戳中添加尾随零。

-K

--dont-verify-checksums --dont-verify-checksum

Don't attempt to verify IP, TCP, or UDP checksums. This is useful for interfaces that perform some or all of those checksum calculation in hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
不要尝试验证IP、TCP或UDP校验和。这对于在硬件中执行部分或全部校验和计算的接口很有用;否则，所有传出的TCP校验和都将被标记为错误。

-l

Make stdout line buffered. Useful if you want to see the data while capturing it. E.g.,
使stdout行缓冲。如果您想在捕获数据时查看数据，则非常有用。例如，

tcpdump -l | tee dat

or

tcpdump -l > dat & tail -f dat

Note that on Windows,``line buffered'' means ``unbuffered'', so that WinDump will write each character individually if -l is specified.
请注意，在Windows上，“line buffered”表示“unbuffered”，因此如果指定了-l，WinDump将单独写入每个字符。

-U is similar to -l in its behavior, but it will cause output to be ``packet-buffered'', so that the output is written to stdout at the end of each packet rather than at the end of each line; this is buffered on all platforms, including Windows.
-U在行为上类似于-l，但它会导致输出被"packet-buffered“，因此输出在每个包的末尾而不是每行的末尾写入stdout;这在所有平台上都是缓冲的，包括Windows。

-L

--list-data-link-types

List the known data link types for the interface, in the specified mode, and exit. The list of known data link types may be dependent on the specified mode; for example, on some platforms, a Wi-Fi interface might support one set of data link types when not in monitor mode (for example, it might support only fake Ethernet headers, or might support 802.11 headers but not support 802.11 headers with radio information) and another set of data link types when in monitor mode (for example, it might support 802.11 headers, or 802.11 headers with radio information, only in monitor mode).
在指定模式下列出接口的已知数据链路类型，然后退出。已知数据链路类型的列表可以取决于指定的模式;例如，在某些平台上，Wi-Fi接口在不处于监控模式时可能支持一组数据链路类型（例如，它可能只支持假以太网报头，或者可能支持802.11报头但不支持具有无线电信息的802.11报头）和另一组数据链路类型（例如，它可能只在监视器模式下支持802.11报头，或具有无线电信息的802.11报头）。

--lengths

Print the captured and original packet lengths. The lengths are printed at the beginning of the line or after the packet number, if any. caplen is the captured packet length (See -s option). len is the original (on wire) packet length.
打印捕获的数据包长度和原始数据包长度。长度打印在行的开头或包号之后（如果有）。caplen是捕获的数据包长度（参见-s选项）。len是原始（在线）数据包长度。

-m module

Load SMI MIB module definitions from file module. This option can be used several times to load several MIB modules into tcpdump.
从文件模块加载SMI MIB模块定义。可以多次使用此选项将多个MIB模块加载到tcpdump中。

-M secret

Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5 option (RFC 2385), if present.
使用secret作为共享密钥，用于验证在TCP段中使用TCP-MD5选项（RFC 2385）找到的HTTP。

-n

Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
不转换地址（即，主机地址、端口号等）名字。

-N

Don't print domain name qualification of host names. E.g., if you give this flag then tcpdump will print ``nic'' instead of ``nic.ddn.mil''.
不打印主机名的域名限定。例如，在一个示例中，如果你给予这个标志，那么tcpdump将打印“nic”而不是"nic.ddn.mil“。

-#

--number

Print an optional packet number at the beginning of the line.
在行的开头打印可选的数据包编号。

-O

--no-optimize

Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer.
不要运行包匹配代码优化器。只有当您怀疑优化器中存在错误时，这才有用。

-p

--no-promiscuous-mode

Don't put the interface into promiscuous mode. Note that the interface might be in promiscuous mode for some other reason; hence, `-p' cannot be used as an abbreviation for `ether host {local-hw-addr} or ether broadcast'.
不要将接口置于混杂模式。请注意，接口可能由于其他原因而处于混杂模式;因此，“-p”不能用作“ether host {local-hw-addr}或ether broadcast”的缩写。

--print

Print parsed packet output, even if the raw packets are being saved to a file with the -w flag.
打印解析的数据包输出，即使原始数据包被保存到带有-w标志的文件中。

--print-sampling=nth

Print every nth packet. This option enables the --print flag.
每隔n个包打印一次。此选项启用--print标志。

Unprinted packets are not parsed, which decreases processing time. Setting nth to 100 for example, will (counting from 1) parse and print the 100th packet, 200th packet, 300th packet, and so on.
未打印的数据包不会被解析，这减少了处理时间。例如，将nth设置为100，将（从1开始计数）解析并打印第100个数据包、第200个数据包、第300个数据包，依此类推。

This option also enables the -S flag, as relative TCP sequence numbers are not tracked for unprinted packets.
此选项还启用-S标志，因为不会跟踪未打印数据包的相对TCP序列号。

-Q direction

--direction=direction --direction=方向

Choose send/receive direction direction for which packets should be captured. Possible values are `in', `out' and `inout'. Not available on all platforms.
选择应捕获数据包的发送/接收方向。可能的值是“in”、“out”和“inout”。并非在所有平台上都可用。

-q

Quick (quiet?) output. Print less protocol information so output lines are shorter.
快（安静？）输出.打印更少的协议信息，因此输出行更短。

-r file

Read packets from file (which was created with the -w option or by other tools that write pcap or pcapng files). Standard input is used if file is ``-''.
从文件（使用-w选项或其他编写pcap或pcapng文件的工具创建的文件）中读取数据包。如果文件为"-“，则使用标准输入。

-S

--absolute-tcp-sequence-numbers

Print absolute, rather than relative, TCP sequence numbers.
打印绝对而非相对TCP序列号。

-s snaplen

--snapshot-length=snaplen

Snarf snaplen bytes of data from each packet rather than the default of 262144 bytes. Packets truncated because of a limited snapshot are indicated in the output with ``[|proto]'', where proto is the name of the protocol level at which the truncation has occurred.
Snarf从每个包中捕捉字节的数据，而不是默认的262144字节。由于有限的快照而被截断的数据包在输出中用``[|proto“，其中proto是发生截断的协议级别的名称。

Note that taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering. This may cause packets to be lost. Note also that taking smaller snapshots will discard data from protocols above the transport layer, which loses information that may be important. NFS and AFS requests and replies, for example, are very large, and much of the detail won't be available if a too-short snapshot length is selected.
请注意，拍摄较大的快照既会增加处理数据包所需的时间，也会有效地减少数据包缓冲量。这可能会导致数据包丢失。还请注意，拍摄较小的快照将丢弃来自传输层以上协议的数据，从而丢失可能重要的信息。例如，NFS和AFS请求和应答非常大，如果选择的快照长度太短，则无法获得大部分详细信息。

If you need to reduce the snapshot size below the default, you should limit snaplen to the smallest number that will capture the protocol information you're interested in. Setting snaplen to 0 sets it to the default of 262144, for backwards compatibility with recent older versions of tcpdump.
如果需要将快照大小减小到默认值以下，则应将snaplen限制为能够捕获您感兴趣的协议信息的最小值。将snaplen设置为0会将其设置为默认值262144，以便向后兼容最近的旧版本的tcpdump。

-T type

Force packets selected by "expression" to be interpreted the specified type. Currently known types are aodv (Ad-hoc On-demand Distance Vector protocol), carp (Common Address Redundancy Protocol), cnfp (Cisco NetFlow protocol), domain (Domain Name System), lmp (Link Management Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), ptp (Precision Time Protocol), quic (QUIC), radius (RADIUS), resp (REdis Serialization Protocol), rpc (Remote Procedure Call), rtcp (Real-Time Applications control protocol), rtp (Real-Time Applications protocol), snmp (Simple Network Management Protocol), someip (SOME/IP), tftp (Trivial File Transfer Protocol), vat (Visual Audio Tool), vxlan (Virtual eXtensible Local Area Network), wb (distributed White Board) and zmtp1 (ZeroMQ Message Transport Protocol 1.0).
强制由“expression”选择的数据包被解释为指定的类型。目前已知的类型是aodv（Ad-hoc按需距离向量协议），鲤鱼（公共地址冗余协议）（Cisco NetFlow协议），域（域名系统）（链路管理协议）（实用通用多播），pgm_zmtp1（ZMTP/1.0内置PGM/EPGM），ptp（精确时间协议）、quic（QUIC）、radius（RADIUS）、（REdis序列化协议），rpc（远程过程调用）（实时应用程序控制协议）（实时应用协议），snmp（简单网络管理协议）、someip（SOME/IP）、tftp（Trivial File Transfer Protocol）、vat（Visual Audio Tool）、vxlan（Virtual eXtensible Local Area Network）、fixed（Distributed白色Board）和zmtp 1（ZeroMQ Message Transport Protocol 1.0）。

Note that the pgm type above affects UDP interpretation only, the native PGM is always recognised as IP protocol 113 regardless. UDP-encapsulated PGM is often called "EPGM" or "PGM/UDP".
请注意，上述pgm类型仅影响UDP解释，无论如何，本地PGM始终被识别为IP协议113。UDP封装的PGM通常称为“EPGM”或“PGM/UDP”。

Note that the pgm_zmtp1 type above affects interpretation of both native PGM and UDP at once. During the native PGM decoding the application data of an ODATA/RDATA packet would be decoded as a ZeroMQ datagram with ZMTP/1.0 frames. During the UDP decoding in addition to that any UDP packet would be treated as an encapsulated PGM packet.
请注意，上述pgm_zmtp1类型同时影响原生PGM和UDP的解释。在本机PGM解码期间，ODATA/RDATA分组的应用数据将被解码为具有ZMTP/1.0帧的ZeroMQ数据报。在UDP解码期间，除此之外，任何UDP分组将被视为封装的PGM分组。

-t

Don't print a timestamp on each dump line.
不要在每个转储行上打印时间戳。

-tt

Print the timestamp, as seconds since January 1, 1970, 00:00:00, UTC, and fractions of a second since that time, on each dump line.
在每个转储行上打印时间戳，以秒为单位，自1970年1月1日00：00：00 UTC起，以及自该时间起的几分之一秒。

-ttt

Print a delta (microsecond or nanosecond resolution depending on the --time-stamp-precision option) between current and previous line on each dump line. The default is microsecond resolution.
在每个转储行上打印当前行和前一行之间的增量（微秒或纳秒分辨率，取决于--time-stamp-precision选项）。默认分辨率为微秒。

-tttt

Print a timestamp, as hours, minutes, seconds, and fractions of a second since midnight, preceded by the date, on each dump line.
在每个转储行上打印时间戳，以小时、分钟、秒和自午夜以来的几分之一秒表示，并在日期之前打印。

-ttttt

Print a delta (microsecond or nanosecond resolution depending on the --time-stamp-precision option) between current and first line on each dump line. The default is microsecond resolution.
在每个转储行上打印当前行和第一行之间的增量（微秒或纳秒分辨率，取决于--time-stamp-precision选项）。默认分辨率为微秒。

-u

Print undecoded NFS handles.  打印未解码的NFS句柄。

-U

--packet-buffered

If the -w option is not specified, or if it is specified but the --print flag is also specified, make the printed packet output ``packet-buffered''; i.e., as the description of the contents of each packet is printed, it will be written to the standard output, rather than, when not writing to a terminal, being written only when the output buffer fills.
如果未指定-w选项，或者指定了-w选项但还指定了--print标志，则将打印的数据包输出为“packet-buffered”;即，当打印每个分组的内容的描述时，它将被写入标准输出，而不是当不写入终端时，仅当输出缓冲器填满时才被写入。

If the -w option is specified, make the saved raw packet output ``packet-buffered''; i.e., as each packet is saved, it will be written to the output file, rather than being written only when the output buffer fills.
如果指定了-w选项，则将保存的原始数据包输出为“packet-buffered”;即，当每个分组被保存时，它将被写入输出文件，而不是仅在输出缓冲器填满时才被写入。

The -U flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_dump_flush(3PCAP) function.
如果tcpdump是用缺少pcap_dump_flush（3 PCAP）函数的旧版本libpcap构建的，则不支持-U标志。

-v

When parsing and printing, produce (slightly more) verbose output. For example, the time to live, identification, total length and options in an IP packet are printed. Also enables additional packet integrity checks such as verifying the IP and ICMP header checksum.
解析和打印时，产生（稍微多一点的）冗长输出。例如，可以打印IP数据包中的生存时间、标识、总长度和选项。还启用其他数据包完整性检查，例如验证IP和IP报头校验和。

When writing to a file with the -w option and at the same time not reading from a file with the -r option, report to stderr, once per second, the number of packets captured. In Solaris, FreeBSD and possibly other operating systems this periodic update currently can cause loss of captured packets on their way from the kernel to tcpdump.
当使用-w选项写入文件，同时不使用-r选项从文件中阅读时，每秒向stderr报告一次捕获的数据包数量。在Solaris、FreeBSD和其他可能的操作系统中，这种定期更新目前可能会导致捕获的数据包在从内核到tcpdump的过程中丢失。

-vv

Even more verbose output. For example, additional fields are printed from NFS reply packets, and SMB packets are fully decoded.
更详细的输出。例如，从NFS应答数据包打印附加字段，并对SMB数据包进行完全解码。

-vvv

Even more verbose output. For example, telnet SB ... SE options are printed in full. With -X Telnet options are printed in hex as well.
更详细的输出。例如，telnet SB. SE选项将完整打印。使用-X时，选项也以十六进制打印。

-V file

Read a list of filenames from file. Standard input is used if file is ``-''.
从文件中读取文件名列表。如果文件为"-“，则使用标准输入。

-w file

Write the raw packets to file rather than parsing and printing them out. They can later be printed with the -r option. Standard output is used if file is ``-''.
将原始数据包写入文件，而不是解析和打印它们。稍后可以使用-r选项打印它们。如果文件为"-“，则使用标准输出。

This output will be buffered if written to a file or pipe, so a program reading from the file or pipe may not see packets for an arbitrary amount of time after they are received. Use the -U flag to cause packets to be written as soon as they are received.
如果将此输出写入文件或管道，则将对其进行缓冲，因此从文件或管道阅读的程序可能在收到数据包后的任意时间内看不到数据包。使用-U标志可使数据包在收到后立即写入。

The MIME type application/vnd.tcpdump.pcap has been registered with IANA for pcap files. The filename extension .pcap appears to be the most commonly used along with .cap and .dmp. tcpdump itself doesn't check the extension when reading capture files and doesn't add an extension when writing them (it uses magic numbers in the file header instead). However, many operating systems and applications will use the extension if it is present and adding one (e.g. .pcap) is recommended.
MIME类型application/vnd.tcpdump.pcap已在IANA注册为pcap文件。文件扩展名.pcap似乎是最常用的沿着.cap和. dmp。tcpdump本身在阅读捕获文件时不检查扩展名，并且在写入捕获文件时不添加扩展名（而是在文件头中使用幻数）。但是，许多操作系统和应用程序将使用扩展名，如果它存在，建议添加一个（例如.pcap）。

See pcap-savefile(5) for a description of the file format.
有关文件格式的描述，请参见pcap-savefile（5）。

-W filecount

Used in conjunction with the -C option, this will limit the number of files created to the specified number, and begin overwriting files from the beginning, thus creating a 'rotating' buffer. In addition, it will name the files with enough leading 0s to support the maximum number of files, allowing them to sort correctly.
与-C选项一起使用，这将把创建的文件数量限制在指定的数量，并从开始开始重新创建文件，从而创建“旋转”缓冲区。此外，它将使用足够的前导0来命名文件，以支持最大数量的文件，从而使它们能够正确排序。

Used in conjunction with the -G option, this will limit the number of rotated dump files that get created, exiting with status 0 when reaching the limit.
与-G选项一起使用，这将限制创建的旋转转储文件的数量，当达到限制时以状态0退出。

If used in conjunction with both -C and -G, the -W option will currently be ignored, and will only affect the file name.
如果与-C和-G一起使用，-W选项当前将被忽略，并且只影响文件名。

-x

When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex. The smaller of the entire packet or snaplen bytes will be printed. Note that this is the entire link-layer packet, so for link layers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer packet is shorter than the required padding. In the current implementation this flag may have the same effect as -xx if the packet is truncated.
在解析和打印时，除了打印每个数据包的报头之外，还要以十六进制打印每个数据包的数据（减去其链路级报头）。将打印整个数据包或对线字节中较小的一个。请注意，这是整个链路层数据包，因此对于填充的链路层（例如以太网），当高层数据包比所需的填充短时，填充字节也将被打印。在当前实现中，如果数据包被截断，则该标志可以具有与-xx相同的效果。

-xx

When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex.
在解析和打印时，除了打印每个数据包的报头之外，还要以十六进制打印每个数据包的数据，包括其链路级报头。

-X

When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex and ASCII. This is very handy for analysing new protocols. In the current implementation this flag may have the same effect as -XX if the packet is truncated.
在解析和打印时，除了打印每个数据包的报头之外，还要以十六进制和ASCII格式打印每个数据包的数据（减去其链路级报头）。这对于分析新协议非常方便。在当前实现中，如果数据包被截断，则该标志可以具有与-XX相同的效果。

-XX

When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex and ASCII.
在解析和打印时，除了打印每个数据包的报头之外，还要打印每个数据包的数据，包括其链接级报头（十六进制和ASCII）。

-y datalinktype

--linktype=datalinktype

Set the data link type to use while capturing packets (see -L) or just compiling and dumping packet-matching code (see -d) to datalinktype.
设置数据链路类型，以便在捕获数据包（参见-L）或仅编译和转储数据包匹配代码（参见-d）时使用datalinktype。

-z postrotate-command

Used in conjunction with the -C or -G options, this will make tcpdump run " postrotate-command file " where file is the savefile being closed after each rotation. For example, specifying -z gzip or -z bzip2 will compress each savefile using gzip or bzip2.
与-C或-G选项一起使用，这将使tcpdump运行“postrotate-command file“，其中file是每次旋转后关闭的保存文件。例如，指定-z gzip或-z bzip 2将使用gzip或bzip 2压缩每个保存文件。

Note that tcpdump will run the command in parallel to the capture, using the lowest priority so that this doesn't disturb the capture process.
请注意，tcpdump将使用最低优先级与捕获并行运行命令，这样就不会干扰捕获过程。

And in case you would like to use a command that itself takes flags or different arguments, you can always write a shell script that will take the savefile name as the only argument, make the flags & arguments arrangements and execute the command that you want.
如果你想使用一个本身带有标志或不同参数的命令，你可以编写一个shell脚本，它将把保存文件名作为唯一的参数，安排标志和参数，并执行你想要的命令。

-Z user

--relinquish-privileges=user

If tcpdump is running as root, after opening the capture device or input savefile, but before opening any savefiles for output, change the user ID to user and the group ID to the primary group of user.
如果tcpdump以root身份运行，在打开捕获设备或输入保存文件之后，但在打开任何保存文件进行输出之前，请将用户ID更改为user，将组ID更改为主组user。

This behavior can also be enabled by default at compile time.
这个行为也可以在编译时默认启用。

expression

selects which packets will be dumped. If no expression is given, all packets on the net will be dumped. Otherwise, only packets for which expression is `true' will be dumped.
选择哪些数据包将被转储。如果没有给出表达式，网络上的所有数据包都将被转储。否则，只有表达式为“true”的数据包才会被转储。

For the expression syntax, see pcap-filter(7).
有关表达式语法，请参见pcap-filter（7）。

The expression argument can be passed to tcpdump as either a single Shell argument, or as multiple Shell arguments, whichever is more convenient. Generally, if the expression contains Shell metacharacters, such as backslashes used to escape protocol names, it is easier to pass it as a single, quoted argument rather than to escape the Shell metacharacters. Multiple arguments are concatenated with spaces before being parsed.
表达式参数可以作为单个Shell参数或多个Shell参数传递给tcpdump，以更方便的方式进行传递。通常，如果表达式包含Shell元字符，例如用于转义协议名称的反斜杠，则将其作为单个带引号的参数传递比转义Shell元字符更容易。多个参数在解析之前用空格连接起来。

To print all packets arriving at or departing from sundown:
要打印日落到达或离开的所有数据包，请执行以下操作：

tcpdump host sundown

To print traffic between helios and either hot or ace:
打印helios与hot或ace之间的流量：

tcpdump host helios and \( hot or ace \)

To print all IP packets between ace and any host except helios:
打印ace和除helios以外的任何主机之间的所有IP数据包：

tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at Berkeley:
打印本地主机和伯克利主机之间的所有流量：

tcpdump net ucb-ether

To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses):
打印所有通过互联网网关snup的ftp流量：（注意，表达式被引用以防止shell（错误）解释括号）：

tcpdump 'gateway snup and (port ftp or ftp-data)'

To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this stuff should never make it onto your local net).
打印既不来自本地主机也不去往本地主机的流量（如果你网关到另一个网络，这些东西永远不应该进入你的本地网络）。

tcpdump ip and not net localnet

To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host.
打印涉及非本地主机的每个TCP会话的开始和结束数据包（SYN和FIN数据包）。

tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

To print the TCP packets with flags RST and ACK both set. (i.e. select only the RST and ACK flags in the flags field, and if the result is "RST and ACK both set", match)
打印TCP数据包，同时设置了标志位“否”和“确认”。(i.e.在标志字段中只选择ACK和NACK标志，如果结果是“设置了ACK和NACK”，则匹配）

tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'

To print all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for example, SYN and FIN packets and ACK-only packets. (IPv6 is left as an exercise for the reader.)
打印所有来往于端口80的IPv4 HTTP数据包，即仅打印包含数据的数据包，而不打印SYN和FIN数据包以及仅ACK数据包。(IPv6留给读者作为练习）。

tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

To print IP packets longer than 576 bytes sent through gateway snup:
要打印通过网关snup发送的长度超过576字节的IP数据包：

tcpdump 'gateway snup and ip[2:2] > 576'

To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:
要打印不是通过以太网广播或多播发送的IP广播或多播数据包，请执行以下操作：

tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
要打印所有不是回显请求/回复的HTTP数据包（即，不ping数据包）：

tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

The output of tcpdump is protocol dependent. The following gives a brief description and examples of most of the formats.
tcpdump的输出依赖于协议。下面给出了大多数格式的简要说明和示例。

Timestamps

By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form
默认情况下，所有输出行前面都有一个时间戳。时间戳是当前时钟时间，格式为

hh:mm:ss.frac

Interface

When the any interface is selected on capture or when a link-type LINUX_SLL2 capture file is read the interface name is printed after the timestamp. This is followed by the packet type with In and Out denoting a packet destined for this host or originating from this host respectively. Other possible values are B for broadcast packets, M for multicast packets, and P for packets destined for other hosts.
在捕获时选择any接口或读取链接类型的LINUX_SLL2捕获文件时，接口名称将在时间戳之后打印。后面是数据包类型，In和Out分别表示发往该主机或源自该主机的数据包。其他可能的值是B（用于广播数据包）、M（用于多播数据包）和P（用于发往其他主机的数据包）。

Link Level Headers 链路级报头

If the '-e' option is given, the link level header is printed out. On Ethernets, the source and destination addresses, protocol, and packet length are printed.
如果给出了“-e”选项，则打印出链接级标题。在以太网上，源地址和目的地址、协议和数据包长度都被打印出来。

On FDDI networks, the '-e' option causes tcpdump to print the `frame control' field, the source and destination addresses, and the packet length. (The `frame control' field governs the interpretation of the rest of the packet. Normal packets (such as those containing IP datagrams) are `async' packets, with a priority value between 0 and 7; for example, `async4'. Such packets are assumed to contain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet.
在FDDI网络上，“-e”选项使tcpdump打印“帧控制”字段、源地址和目的地址以及数据包长度。(The“帧控制”字段控制对分组其余部分的解释。普通数据包（如包含IP数据报的数据包）是“Pencc”数据包，其优先级值在0和7之间;例如，“Penc 4”。这样的数据包被假定包含802.2逻辑链路控制（LLC）数据包;如果它不是ISO数据报或所谓的SNAP数据包，则打印LLC报头。

On Token Ring networks, the '-e' option causes tcpdump to print the `access control' and `frame control' fields, the source and destination addresses, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet. Regardless of whether the '-e' option is specified or not, the source routing information is printed for source-routed packets.
在令牌环网络上，“-e”选项使tcpdump打印“访问控制”和“帧控制”字段、源地址和目的地址以及数据包长度。如在FDDI网络上，假定分组包含LLC分组。无论是否指定了“-e”选项，源路由信息都将为源路由的数据包打印。

On 802.11 networks, the '-e' option causes tcpdump to print the `frame control' fields, all of the addresses in the 802.11 header, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet.
在802.11网络上，'-e'选项使tcpdump打印“帧控制”字段、802.11报头中的所有地址和数据包长度。如在FDDI网络上，假定分组包含LLC分组。

(N.B.: The following description assumes familiarity with the SLIP compression algorithm described in RFC 1144.)
(N.B.：下面的描述假定您熟悉RFC 1144中描述的SLIP压缩算法。

On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound), packet type, and compression information are printed out. The packet type is printed first. The three types are ip, utcp, and ctcp. No further link information is printed for ip packets. For TCP packets, the connection identifier is printed following the type. If the packet is compressed, its encoded header is printed out. The special cases are printed out as *S+n and *SA+n, where n is the amount by which the sequence number (or sequence number and ack) has changed. If it is not a special case, zero or more changes are printed. A change is indicated by U (urgent pointer), W (window), A (ack), S (sequence number), and I (packet ID), followed by a delta (+n or -n), or a new value (=n). Finally, the amount of data in the packet and compressed header length are printed.
在SLIP链路上，方向指示符（"I“表示入站，" O”表示出站）、数据包类型和压缩信息被打印出来。首先打印数据包类型。这三种类型是ip、utcp和ctcp。不再为ip数据包打印链接信息。对于TCP数据包，连接标识符将在类型之后打印。如果数据包被压缩，它的编码报头被打印出来。特殊情况打印为 *S+n和 *SA+n，其中n是序列号（或序列号和ack）的变化量。如果不是特殊情况，则打印零个或多个更改。改变由U（紧急指针）、W（窗口）、A（确认）、S（序列号）和I（数据包ID）指示，后跟增量（+n或-n）或新值（=n）。最后，打印数据包中的数据量和压缩报头长度。

For example, the following line shows an outbound compressed TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header:

O ctcp * A+6 S+49 I+6 3 (6)

ARP/RARP Packets

ARP/RARP output shows the type of request and its arguments. The format is intended to be self explanatory. Here is a short sample taken from the start of an `rlogin' from host rtsg to host csam:

arp who-has csam tell rtsg
arp reply csam is-at CSAM

This would look less redundant if we had done tcpdump -n:

arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4

If we had done tcpdump -e, the fact that the first packet is broadcast and the second is point-to-point would be visible:

RTSG Broadcast 0806  64: arp who-has csam tell rtsg
CSAM RTSG 0806  64: arp reply csam is-at CSAM

IPv4 Packets

If the link-layer header is not being printed, for IPv4 packets, IP is printed after the time stamp.

If the -v flag is specified, information from the IPv4 header is shown in parentheses after the IP or the link-layer header. The general format of this information is:

tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)

Next, for TCP and UDP packets, the source and destination IP addresses and TCP or UDP ports, with a dot between each IP address and its corresponding port, will be printed, with a > separating the source and destination. For other protocols, the addresses will be printed, with a > separating the source and destination. Higher level protocol information, if any, will be printed after that.

For fragmented IP datagrams, the first fragment contains the higher level protocol header; fragments after the first contain no higher level protocol header. Fragmentation information will be printed only with the -v flag, in the IP header information, as described above.

TCP Packets

(N.B.:The following description assumes familiarity with the TCP protocol described in RFC 793. If you are not familiar with the protocol, this description will not be of much use to you.)

The general format of a TCP protocol line is:

src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len

Iptype, Src, dst, and flags are always present. The other fields depend on the contents of the packet's TCP protocol header and are output only if appropriate.

Here is the opening portion of an rlogin from host rtsg to host csam.

IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1

Csam replies with a similar packet except it includes a piggy-backed ACK for rtsg's SYN. Rtsg then ACKs csam's SYN. The `.' means the ACK flag was set. The packet contained no data so there is no data sequence number or length. Note that the ACK sequence number is a small integer (1). The first time tcpdump sees a TCP `conversation', it prints the sequence number from the packet. On subsequent packets of the conversation, the difference between the current packet's sequence number and this initial sequence number is printed. This means that sequence numbers after the first can be interpreted as relative byte positions in the conversation's data stream (with the first data byte each direction being `1'). `-S' will override this feature, causing the original sequence numbers to be output.

On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the rtsg → csam side of the conversation). The PSH flag is set in the packet. On the 7th line, csam says it's received data sent by rtsg up to but not including byte 21. Most of this data is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg.

If the snapshot was small enough that tcpdump didn't capture the full TCP header, it interprets as much of the header as it can and then reports ``[|tcp]'' to indicate the remainder could not be interpreted. If the header contains a bogus option (one with a length that's either too small or beyond the end of the header), tcpdump reports it as ``[bad opt]'' and does not interpret any further options (since it's impossible to tell where they start). If the header length indicates options are present but the IP datagram length is not long enough for the options to actually be there, tcpdump reports it as ``[bad hdr length]''.

Particular TCP Flag Combinations (SYN-ACK, URG-ACK, etc.)

There are 8 bits in the control bits section of the TCP header:

CWR | ECE | URG | ACK | PSH | RST | SYN | FIN

Let's assume that we want to watch packets used in establishing a TCP connection. Recall that TCP uses a 3-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is

1) Caller sends SYN

2) Recipient responds with SYN, ACK

3) Caller sends ACK

Now we're interested in capturing packets that have only the SYN bit set (Step 1). Note that we don't want packets from step 2 (SYN-ACK), just a plain initial SYN. What we need is a correct filter expression for tcpdump.

Recall the structure of a TCP header without options:

 0                            15                              31
-----------------------------------------------------------------
|          source port          |       destination port        |
-----------------------------------------------------------------
|                        sequence number                        |
-----------------------------------------------------------------
|                     acknowledgment number                     |
-----------------------------------------------------------------
|  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
-----------------------------------------------------------------
|         TCP checksum          |       urgent pointer          |
-----------------------------------------------------------------

A TCP header usually holds 20 octets of data, unless options are present. The first line of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc.

Starting to count with 0, the relevant TCP control bits are contained in octet 13:

 0             7|             15|             23|             31
----------------|---------------|---------------|----------------
|  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
----------------|---------------|---------------|----------------
|               |  13th octet   |               |               |

Let's have a closer look at octet no. 13:

                |               |
                |---------------|
                |C|E|U|A|P|R|S|F|
                |---------------|
                |7   5   3     0|

These are the TCP control bits we are interested in. We have numbered the bits in this octet from 0 to 7, right to left, so the PSH bit is bit number 3, while the URG bit is number 5.

Recall that we want to capture packets with only SYN set. Let's see what happens to octet 13 if a TCP datagram arrives with the SYN bit set in its header:

                |C|E|U|A|P|R|S|F|
                |---------------|
                |0 0 0 0 0 0 1 0|
                |---------------|
                |7 6 5 4 3 2 1 0|

Looking at the control bits section we see that only bit number 1 (SYN) is set.

Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the binary value of this octet is

00000010

and its decimal representation is

   7     6     5     4     3     2     1     0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2  =  2

We're almost done, because now we know that if only SYN is set, the value of the 13th octet in the TCP header, when interpreted as a 8-bit unsigned integer in network byte order, must be exactly 2.

This relationship can be expressed as

tcp[13] == 2

We can use this expression as the filter for tcpdump in order to watch packets which have only SYN set:

tcpdump -i xl0 'tcp[13] == 2'

The expression says "let the 13th octet of a TCP datagram have the decimal value 2", which is exactly what we want.

Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time. Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:

     |C|E|U|A|P|R|S|F|
     |---------------|
     |0 0 0 1 0 0 1 0|
     |---------------|
     |7 6 5 4 3 2 1 0|

Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is

     00010010

which translates to decimal

   7     6     5     4     3     2     1     0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2   = 18

Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression, because that would select only those packets that have SYN-ACK set, but not those with only SYN set. Remember that we don't care if ACK or any other control bit is set as long as SYN is set.

In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know that we want SYN to be set in any case, so we'll logically AND the value in the 13th octet with the binary value of a SYN:

          00010010 SYN-ACK              00000010 SYN
     AND  00000010 (we want SYN)   AND  00000010 (we want SYN)
          --------                      --------
     =    00000010                 =    00000010

We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set. The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we know that for packets with SYN set the following relation must hold true:

( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression

     tcpdump -i xl0 'tcp[13] & 2 == 2'

Some offsets and field values may be expressed as names rather than as numeric values. For example tcp[13] may be replaced with tcp[tcpflags]. The following TCP flag field values are also available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg, tcp-ece and tcp-cwr.

This can be demonstrated as:

     tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'

Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell.

UDP Packets

UDP format is illustrated by this rwho packet:

actinide.who > broadcast.who: udp 84

Some UDP services are recognized (from the source or destination port number) and the higher level protocol information printed. In particular, Domain Name service requests (RFC 1034/1035) and Sun RPC calls (RFC 1050) to NFS.

TCP or UDP Name Server Requests

(N.B.:The following description assumes familiarity with the Domain Service protocol described in RFC 1035. If you are not familiar with the protocol, the following description will appear to be written in Greek.)

Name server requests are formatted as

src > dst: id op? flags qtype qclass name (len)

h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)

A few anomalies are checked and may result in extra fields enclosed in square brackets: If a query contains an answer, authority records or additional records section, ancount, nscount, or arcount are printed as `[na]', `[nn]' or `[nau]' where n is the appropriate count. If any of the response bits are set (AA, RA or rcode) or any of the `must be zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where x is the hex value of header bytes two and three.

TCP or UDP Name Server Responses

Name server responses are formatted as

src > dst:  id op rcode flags a/n/au type class data (len)

helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)

In the second example, helios responds to query 2 with a response code of nonexistent domain (NXDomain) with no answers, one name server and no authority records. The `*' indicates that the authoritative answer bit was set. Since there were no answers, no type, class or data were printed.

Other flag characters that might appear are `-' (recursion available, RA, not set) and `|' (truncated message, TC, set). If the `question' section doesn't contain exactly one entry, `[nq]' is printed.

SMB/CIFS Decoding

tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and NetBEUI SMB data is also done.

By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory details.

For information on SMB packet formats and what all the fields mean see https://download.samba.org/pub/samba/specs/ and other online resources. The SMB patches were written by Andrew Tridgell (tridge@samba.org).

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are printed as:

src.sport > dst.nfs: NFS request xid xid len op args
src.nfs > dst.dport: NFS reply xid xid reply stat len op results


sushi.1023 > wrl.nfs: NFS request xid 26377
        112 readlink fh 21,24/10.73165
wrl.nfs > sushi.1023: NFS reply xid 26377
        reply ok 40 readlink "../var"
sushi.1022 > wrl.nfs: NFS request xid 8219
        144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.1022: NFS reply xid 8219
        reply ok 128 lookup fh 9,74/4134.3150

In the third line, sushi asks (using a new transaction id) wrl to lookup the name `xcolors' in directory file 9,74/4096.6878. In the fourth line, wrl sends a reply with the respective transaction id.

Note that the data printed depends on the operation type. The format is intended to be self explanatory if read in conjunction with an NFS protocol spec. Also note that older versions of tcpdump printed NFS packets in a slightly different format: the transaction id (xid) would be printed instead of the non-NFS port number of the packet.

If the -v (verbose) flag is given, additional information is printed. For example:

sushi.1023 > wrl.nfs: NFS request xid 79658
        148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1023: NFS reply xid 79658
        reply ok 1472 read REG 100664 ids 417/0 sz 29388

If the -v flag is given more than once, even more details are printed.

NFS reply packets do not explicitly identify the RPC operation. Instead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the transaction ID. If a reply does not closely follow the corresponding request, it might not be parsable.

AFS Requests and Replies

Transarc AFS (Andrew File System) requests and replies are printed as:

src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args


elvis.7001 > pike.afsfs:
        rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
        new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename

In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the arguments decoded (generally only the `interesting' arguments, for some definition of interesting).

The format is intended to be self-describing, but it will probably not be useful to people who are not familiar with the workings of AFS and RX.

If the -v (verbose) flag is given twice, acknowledgement packets and additional header information is printed, such as the RX call ID, call number, sequence number, serial number, and the RX packet flags.

If the -v flag is given twice, additional information is printed, such as the RX call ID, serial number, and the RX packet flags. The MTU negotiation information is also printed from RX ack packets.

If the -v flag is given three times, the security index and service id are printed.

Error codes are printed for abort packets, with the exception of Ubik beacon packets (because abort packets are used to signify a yes vote for the Ubik protocol).

AFS reply packets do not explicitly identify the RPC operation. Instead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the call number and service ID. If a reply does not closely follow the corresponding request, it might not be parsable.

KIP AppleTalk (DDP in UDP)

AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all the UDP header information is discarded). The file /etc/atalk.names is used to translate AppleTalk net and node numbers to names. Lines in this file have the form

number  name

1.254           ether
16.1            icsd-net
1.254.110       ace

AppleTalk addresses are printed in the form

net.host.port

144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2

NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size.

NBP Packets

icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186

ATP Packets

jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002

Helios responds with 8 512-byte packets. The `:digit' following the transaction id gives the packet sequence number in the transaction and the number in parens is the amount of data in the packet, excluding the ATP header. The `*' on packet 7 indicates that the EOM bit was set.

Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios resends them then jssmag.209 releases the transaction. Finally, jssmag.209 initiates the next request. The `*' on the request indicates that XO (`exactly once') was not set.

https://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap

Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.

It is currently maintained by The Tcpdump Group.

The current version is available via HTTPS:

https://www.tcpdump.org/

The original distribution is available via anonymous ftp:

ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z

IPv6/IPsec support is added by WIDE/KAME project. This program uses OpenSSL/LibreSSL, under specific configurations.

To report bugs and other problems, contribute patches, request a feature, provide generic feedback etc. please see the file CONTRIBUTING.md in the tcpdump source tree root.

NIT doesn't let you watch your own outbound traffic, BPF will. We recommend that you use the latter.

Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher level protocol.

Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program generating them rather than tcpdump.

A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored).

Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed Token Ring packets.

Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data packets with both To DS and From DS set.

ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this behavior.

Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It only looks at IPv4 packets.

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

EXAMPLES

OUTPUT FORMAT

Timestamps

Interface

Link Level Headers

ARP/RARP Packets

IPv4 Packets

TCP Packets

Particular TCP Flag Combinations (SYN-ACK, URG-ACK, etc.)

UDP Packets

TCP or UDP Name Server Requests

TCP or UDP Name Server Responses

SMB/CIFS Decoding

NFS Requests and Replies

AFS Requests and Replies

KIP AppleTalk (DDP in UDP)

NBP Packets

ATP Packets

BACKWARD COMPATIBILITY

SEE ALSO

AUTHORS

BUGS