Команда tcpdump в Linux

tcpdump — это утилита командной строки, которую вы можете использовать для захвата и проверки сетевого трафика, идущего в вашу систему и из нее. Это наиболее часто используемый сетевыми администраторами инструмент для устранения неполадок в сети и тестирования безопасности.

Несмотря на название, с помощью tcpdump вы также можете захватывать не-TCP трафик, такой как UDP, ARP или ICMP. Перехваченные пакеты можно записать в файл или на стандартный вывод. Одной из самых мощных функций команды tcpdump является ее способность использовать фильтры и собирать только те данные, которые вы хотите анализировать.

В этой статье мы рассмотрим основы использования команды tcpdump в Linux.

Установка tcpdump

tcpdump установлен по умолчанию в большинстве дистрибутивов Linux и macOS. Чтобы проверить, доступна ли команда tcpdump в вашей системе, введите:

Результат должен выглядеть примерно так:

Если tcpdump отсутствует в вашей системе, приведенная выше команда напечатает «tcpdump: команда не найдена». Вы можете легко установить tcpdump с помощью диспетчера пакетов вашего дистрибутива.

Установка tcpdump в Ubuntu и Debian

Установка tcpdump на CentOS и Fedora

Установка tcpdump в Arch Linux

Захват пакетов с помощью tcpdump

Общий синтаксис команды tcpdump следующий:

• Команда options позволяет управлять поведением команды.
• expression фильтра определяет, какие пакеты будут захвачены.

Только root или пользователь с привилегиями sudo может запускать tcpdump . Если вы попытаетесь запустить команду от имени непривилегированного пользователя, вы получите сообщение об ошибке: «У вас нет разрешения на захват на этом устройстве».

Самый простой вариант использования — вызвать tcpdump без каких-либо опций и фильтров:

tcpdump будет продолжать захватывать пакеты и записывать их на стандартный вывод, пока не получит сигнал прерывания. Используйте Ctrl+C чтобы отправить сигнал прерывания и остановить команду.

Для более подробного вывода передайте параметр -v или -vv для более подробного вывода:

Вы можете указать количество пакетов для захвата с помощью опции -c . Например, чтобы захватить только десять пакетов, введите:

После захвата пакетов tcpdump остановится.

Если интерфейс не указан, tcpdump использует первый найденный интерфейс и выгружает все пакеты, проходящие через этот интерфейс.

Используйте параметр -D чтобы распечатать список всех доступных сетевых интерфейсов, с которых tcpdump может собирать пакеты:

Для каждого интерфейса команда выводит имя интерфейса, краткое описание и соответствующий индекс (номер):

Приведенные выше выходные данные показывают, что ens3 — это первый интерфейс, обнаруженный tcpdump и используемый, когда команде не предоставлен интерфейс. Второй интерфейс any — это специальное устройство, позволяющее захватывать все активные интерфейсы.

Чтобы указать интерфейс, на котором вы хотите перехватывать трафик, вызовите команду с параметром -i за которым следует имя интерфейса или связанный индекс. Например, чтобы захватить все пакеты со всех интерфейсов, вы должны указать any интерфейс:

По умолчанию tcpdump выполняет обратное разрешение DNS для IP-адресов и переводит номера портов в имена. Используйте параметр -n чтобы отключить перевод:

Пропуск поиска DNS позволяет избежать генерации трафика DNS и делает вывод более читаемым. Рекомендуется использовать эту опцию всякий раз, когда вы вызываете tcpdump .

Вместо отображения вывода на экране вы можете перенаправить его в файл с помощью операторов перенаправления > и >> :

Вы также можете просматривать данные при сохранении в файл с помощью команды tee :

Параметр -l в приведенной выше команде сообщает tcpdump о необходимости буферизации выходной строки. Если этот параметр не используется, вывод не будет записан на экране при создании новой строки.

Понимание вывода tcpdump

tcpdump выводит информацию для каждого захваченного пакета в новой строке. Каждая строка включает метку времени и информацию об этом пакете в зависимости от протокола.

Типичный формат строки протокола TCP выглядит следующим образом:

Пойдем по полю и объясним следующую строку:

15:47:24.248737 — 15:47:24.248737 метка захваченного пакета 15:47:24.248737 по местному времени и использует следующий формат: hours:minutes:seconds.frac , где frac — доли секунды с полуночи.

IP — пакетный протокол. В данном случае IP означает Интернет-протокол версии 4 (IPv4).

192.168.1.185.22 — IP-адрес и порт источника, разделенные точкой ( . ).

192.168.1.150.37445 — IP-адрес и порт назначения, разделенные точкой ( . ).

Flags [P.] — поле TCP Flags. В этом примере [P.] означает пакет подтверждения push, который используется для подтверждения предыдущего пакета и отправки данных. Другие типичные значения поля флага следующие:

• [.] — ACK (подтверждение)
• [S] — SYN (Начать соединение)
• [P] — PSH (Push-данные)
• [F] — FIN (Завершить соединение)
• [R] — RST (сбросить соединение)
• [S.] — SYN-ACK (пакет SynAcK)

seq 201747193:201747301 — Порядковый номер находится в seq 201747193:201747301 first:last . Он показывает количество данных, содержащихся в пакете. За исключением первого пакета в потоке данных, где эти числа являются абсолютными, все последующие пакеты используются как относительные позиции байтов. В этом примере номер 201747193:201747301 , что означает, что этот пакет содержит байты от 201747193 до 201747301 потока данных. Используйте параметр -S для вывода абсолютных порядковых номеров.

ack 1226568763 Номер подтверждения — это порядковый номер следующих данных, ожидаемых на другом конце этого соединения.

win 402 — Номер окна — это количество доступных байтов в приемном буфере.

options [nop,nop,TS val 1051794587 ecr 2679218230] — параметры TCP. nop , или «нет операции» — это заполнение, используемое для того, чтобы сделать заголовок TCP кратным 4 байтам. TS val — это временная метка TCP, а ecr — эхо-ответ. Посетите документацию IANA для получения дополнительной информации о параметрах TCP.

length 108 — длина данных полезной нагрузки

tcpdump фильтры

Когда tcpdump вызывается без фильтров, он захватывает весь трафик и производит огромное количество выходных данных, что очень затрудняет поиск и анализ интересующих пакетов.

Фильтры — одна из самых мощных функций команды tcpdump . Они позволяют захватывать только те пакеты, которые соответствуют выражению. Например, при устранении проблем, связанных с веб-сервером, вы можете использовать фильтры для получения только HTTP-трафика.

tcpdump использует синтаксис Berkeley Packet Filter (BPF) для фильтрации перехваченных пакетов с использованием различных параметров обработки, таких как протоколы, IP-адреса и порты источника и назначения и т. д.

В этой статье мы рассмотрим некоторые из наиболее распространенных фильтров. Список всех доступных фильтров можно найти на странице руководства pcap-filter .

Фильтрация по протоколу

Чтобы ограничить захват определенным протоколом, укажите этот протокол как фильтр. Например, чтобы захватить только трафик UDP, вы должны запустить:

Другой способ определить протокол — использовать квалификатор proto , за которым следует номер протокола. Следующая команда отфильтрует протокол номер 17 и выдаст тот же результат, что и приведенный выше:

Для получения дополнительной информации о числах проверьте список номеров IP-протоколов .

Фильтрация по хосту

Чтобы захватить только пакеты, относящиеся к определенному хосту, используйте квалификатор host :

Хостом может быть IP-адрес или имя.

Вы также можете фильтровать вывод по заданному диапазону IP-адресов, используя квалификатор net . Например, чтобы выгрузить только пакеты, относящиеся к 10.10.0.0/16 вы должны использовать:

Фильтрация по порту

Чтобы ограничить захват только пакетами от или к определенному порту, используйте квалификатор port . Приведенная ниже команда захватывает пакеты, связанные со службой SSH (порт 22), с помощью этой команды:

portrange позволяет захватывать трафик в диапазоне портов:

Фильтрация по источнику и назначению

Вы также можете фильтровать пакеты на основе порта или хоста источника или назначения, используя квалификаторы are src , dst , src and dst , а также src or dst .

Следующая команда захватывает приходящие пакеты от хоста с IP 192.168.1.185:

Чтобы найти трафик, поступающий из любого источника на порт 80, вы должны использовать:

Комплексные фильтры

Фильтры можно комбинировать с помощью операторов and ( && ), or ( || ), но not ( ! ).

Например, чтобы захватить весь HTTP-трафик, поступающий с исходного IP-адреса 192.168.1.185, вы должны использовать эту команду:

Вы также можете использовать круглые скобки для группировки и создания более сложных фильтров:

Чтобы избежать ошибок синтаксического анализа при использовании специальных символов, заключайте фильтры в одинарные кавычки.

Вот еще один пример команды для захвата всего трафика, кроме SSH, с исходного IP-адреса 192.168.1.185:

Инспекция пакетов

По умолчанию tcpdump захватывает только заголовки пакетов. Однако иногда вам может потребоваться проверить содержимое пакетов.

tcpdump позволяет печатать содержимое пакетов в ASCII и HEX.

Параметр -A указывает tcpdump печатать каждый пакет в ASCII и -x в HEX:

Чтобы показать содержимое пакета как в HEX, так и в ASCII, используйте параметр -X :

Чтение и запись снимков в файл

Еще одна полезная функция tcpdump — записывать пакеты в файл. Это удобно, когда вы захватываете большое количество пакетов или захватываете пакеты для последующего анализа.

Чтобы начать запись в файл, используйте параметр -w за которым следует выходной файл захвата:

Эта команда выше сохранит захват в файл с именем data.pcap . Вы можете назвать файл по .pcap , но обычно используется расширение .pcap (захват пакетов).

Когда используется опция -w , вывод не отображается на экране. tcpdump записывает необработанные пакеты и создает двоичный файл, который невозможно прочитать обычным текстовым редактором.

Чтобы проверить содержимое файла, вызовите tcpdump с параметром -r :

Если вы хотите запустить tcpdump в фоновом режиме , добавьте символ амперсанда ( & ) в конце команды.

Файл захвата также можно проверить с помощью других инструментов анализатора пакетов, таких как Wireshark.

При захвате пакетов в течение длительного периода времени вы можете включить ротацию файлов. tcpdump позволяет создавать новые файлы и вращать файл дампа через указанный интервал времени или фиксированного размера. Следующая команда создаст до десяти файлов размером 200 file.pcap0 именами file.pcap0 , file.pcap1 и т. Д. Перед перезаписью старых файлов.

После создания десяти файлов старые файлы будут перезаписаны.

Обратите внимание, что запускать tcpdump только во время устранения неполадок.

Если вы хотите запустить tcpdump в определенное время, вы можете использовать cronjob . tcpdump не имеет возможности выйти через заданное время. Вы можете использовать команду timeout чтобы остановить tcpdump через некоторое время. Например, чтобы выйти через 5 минут, вы должны использовать:

Выводы

tcpdump — это инструмент командной строки для анализа и устранения проблем, связанных с сетью.

Эта статья познакомила вас с основами использования и синтаксиса tcpdump . Для получения более подробной документации посетите веб-сайт tcpdump .

Если у вас есть какие-либо вопросы или отзывы, не стесняйтесь оставлять комментарии.

Источник

Man page of TCPDUMP

This man page documents tcpdump version 5.0.0-PRE-GIT.

SYNOPSIS

tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]

[ -c count ] [ —count ] [ -C file_size ]
[ -E spi@ipaddr algo:secret. ]
[ -F file ] [ -G rotate_seconds ] [ -i interface ]
[ —immediate-mode ] [ -j tstamp_type ] [ -m module ]
[ -M secret ] [ —number ] [ —print ] [ -Q in|out|inout ]
[ -r file ] [ -s snaplen ] [ -T type ] [ —version ]
[ -V file ] [ -w file ] [ -W filecount ] [ -y datalinktype ]
[ -z postrotate-command ] [ -Z user ]
[ —time-stamp-precision= tstamp_precision ]
[ —micro ] [ —nano ]
[ expression ]

DESCRIPTION

Tcpdump prints out a description of the contents of packets on a network interface that match the Boolean expression ; 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 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.

When tcpdump finishes capturing packets, it will report counts of: packets «captured» (this is the number of packets that tcpdump has received and processed); 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 ); 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).

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.

Using the SIGUSR2 signal along with the -w flag will forcibly flush the packet buffer into the output file.

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.

OPTIONS

For the expression syntax, see 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.

EXAMPLES

To print all packets arriving at or departing from sundown :

To print traffic between helios and either hot or ace :

To print all IP packets between ace and any host except helios :

To print all traffic between local hosts and hosts at Berkeley:

To print all ftp traffic through internet gateway snup : (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses):

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).

To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host.

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)

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.)

To print IP packets longer than 576 bytes sent through gateway snup :

To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:

To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):

OUTPUT FORMAT

The output of tcpdump is protocol dependent. The following gives a brief description and examples of most of the formats.

By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form and is as accurate as the kernel’s clock. The timestamp reflects the time the kernel applied a time stamp to the packet. No attempt is made to account for the time lag between when the network interface finished receiving the packet from the network and when the kernel applied a time stamp to the packet; that time lag could include a delay between the time when the network interface finished receiving a packet from the network and the time when an interrupt was delivered to the kernel to get it to read the packet and a delay between the time when the kernel serviced the `new packet’ interrupt and the time when it applied a time stamp to the packet.

Link Level Headers

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.

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.

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.

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.

(N.B.: The following description assumes familiarity with the SLIP compression algorithm described in RFC-1144.)

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.

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:

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 : The first line says that rtsg sent an ARP packet asking for the Ethernet address of internet host csam. Csam replies with its Ethernet address (in this example, Ethernet addresses are in caps and internet addresses in lower case).

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

If we had done tcpdump -e , the fact that the first packet is broadcast and the second is point-to-point would be visible: For the first packet this says the Ethernet source address is RTSG, the destination is the Ethernet broadcast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.

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 is the type of service field; if the ECN bits are non-zero, those are reported as ECT(1) , ECT(0) , or CE . ttl is the time-to-live; it is not reported if it is zero. id is the IP identification field. offset is the fragment offset field; it is printed whether this is part of a fragmented datagram or not. flags are the MF and DF flags; + is reported if MF is set, and DF is reported if F is set. If neither are set, . is reported. proto is the protocol ID field. length is the total length field. options are the IP options, if any.

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.

(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 and dst are the source and destination IP addresses and ports. Tcpflags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or `.’ (ACK), or `none’ if no flags are set. Data-seqno describes the portion of sequence space covered by the data in this packet (see example below). Ackno is sequence number of the next data expected the other direction on this connection. Window is the number of bytes of receive buffer space available the other direction on this connection. Urg indicates there is `urgent’ data in the packet. Opts are TCP options (e.g., mss 1024). Len is the length of payload data.

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 . The first line says that TCP port 1023 on rtsg sent a packet to port login on csam. The S indicates that the SYN flag was set. The packet sequence number was 768512 and it contained no data. (The notation is `first:last’ which means `sequence numbers first up to but not including last ‘.) There was no piggy-backed ACK, the available receive window was 4096 bytes and there was a max-segment-size option requesting an MSS of 1024 bytes.

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 PUSH 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 ]».

Capturing TCP packets with particular 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:

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:

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

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:

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

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:

Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is
00010010

which translates to decimal

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:

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.

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 format is illustrated by this rwho packet: This says that port who on host actinide sent a UDP datagram to port who on host broadcast , the Internet broadcast address. The packet contained 84 bytes of user data.

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 Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucbvax.berkeley.edu. The query id was `3′. The `+’ indicates the recursion desired flag was set. The query length was 37 bytes, excluding the TCP or UDP and IP protocol headers. The query operation was the normal one, Query , so the op field was omitted. If the op had been anything else, it would have been printed between the `3′ and the `+’. Similarly, the qclass was the normal one, C_IN , and omitted. Any other qclass would have been printed immediately after the `A’.

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 `[ n a]’, `[ n n]’ or `[ n au]’ 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 In the first example, helios responds to query id 3 from h2opolo with 3 answer records, 3 name server records and 7 additional records. The first answer record is type A (address) and its data is internet address 128.32.137.3. The total size of the response was 273 bytes, excluding TCP or UDP and IP headers. The op (Query) and response code (NoError) were omitted, as was the class (C_IN) of the A record.

In the second example, helios responds to query 2 with a response code of non-existent 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, `[ n q]’ is printed.

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: In the first line, host sushi sends a transaction with id 26377 to wrl . The request was 112 bytes, excluding the UDP and IP headers. The operation was a readlink (read symbolic link) on file handle ( fh ) 21,24/10.731657119. (If one is lucky, as in this case, the file handle can be interpreted as a major,minor device number pair, followed by the inode number and generation number.) In the second line, wrl replies `ok’ with the same transaction id and the contents of the link.

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: (-v also prints the IP header TTL, ID, length, and fragmentation fields, which have been omitted from this example.) In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl replies `ok’; the packet shown on the second line is the first fragment of the reply, and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not have NFS or even UDP headers and so might not be printed, depending on the filter expression used). Because the -v flag is given, some of the file attributes (which are returned in addition to the file data) are printed: the file type («REG», for regular file), the file mode (in octal), the UID and GID, and the file size.

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:

In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call. The RPC call was a rename, with the old directory file id of 536876964/1/1 and an old filename of `.newsrc.new’, and a new directory file id of 536876964/1/1 and a new filename of `.newsrc’. The host pike responds with a RPC reply to the rename call (which was successful, because it was a data packet and not an abort packet).

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 The first two lines give the names of AppleTalk networks. The third line gives the name of a particular host (a host is distinguished from a net by the 3rd octet in the number — a net number must have two octets and a host number must have three octets.) The number and name should be separated by whitespace (blanks or tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a `#’).

AppleTalk addresses are printed in the form (If the /etc/atalk.names doesn’t exist or doesn’t contain an entry for some AppleTalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (`office’). The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name with no host number — for this reason it’s a good idea to keep node names and net names distinct in /etc/atalk.names).

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 are formatted like the following examples: The first line is a name lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag. The nbp id for the lookup is 190. The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named «RM1140» registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter «techpit» registered on port 186.

ATP packet formatting is demonstrated by the following example: Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets (the ` ‘). The hex number at the end of the line is the value of the `userdata’ field in the request.

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.

SEE ALSO

AUTHORS

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

It is currently being maintained by tcpdump.org.

The current version is available via HTTPS:

The original distribution is available via anonymous ftp:

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 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.

On Linux systems with 2.0[.x] kernels: packets on the loopback device will be seen twice; packet filtering cannot be done in the kernel, so that all packets must be copied from the kernel in order to be filtered in user mode; all of a packet, not just the part that’s within the snapshot length, will be copied from the kernel (the 2.0[.x] packet capture mechanism, if asked to copy only part of a packet to userspace, will not report the true length of the packet; this would cause most IP packets to get an error from tcpdump ); capturing on some PPP devices won’t work correctly.

We recommend that you upgrade to a 2.2 or later kernel.

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.

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