tcpdump -i <ネットワークインターフェース>

13:45:32.123456 IP 192.168.1.10.12345 > 192.168.1.1.80: Flags [S], seq 0, win 65535, options [mss 1460], length 0

docker run alpine:3.21.0 cat /etc/services

# Network services, Internet style
#
# Updated from https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml .
#
# New ports will be added on request if they have been officially assigned
# by IANA and used in the real-world or are needed by a debian package.
# If you need a huge list of used numbers please install the nmap package.

tcpmux		1/tcp				# TCP port service multiplexer
echo		7/tcp
echo		7/udp
discard		9/tcp		sink null
discard		9/udp		sink null
systat		11/tcp		users
daytime		13/tcp
daytime		13/udp
netstat		15/tcp
qotd		17/tcp		quote
chargen		19/tcp		ttytst source
chargen		19/udp		ttytst source
ftp-data	20/tcp
ftp		21/tcp
fsp		21/udp		fspd
ssh		22/tcp				# SSH Remote Login Protocol
telnet		23/tcp
smtp		25/tcp		mail
time		37/tcp		timserver
time		37/udp		timserver
whois		43/tcp		nicname
tacacs		49/tcp				# Login Host Protocol (TACACS)
tacacs		49/udp
domain		53/tcp				# Domain Name Server
domain		53/udp
bootps		67/udp
bootpc		68/udp
tftp		69/udp
gopher		70/tcp				# Internet Gopher
finger		79/tcp
http		80/tcp		www		# WorldWideWeb HTTP
kerberos	88/tcp		kerberos5 krb5 kerberos-sec	# Kerberos v5
kerberos	88/udp		kerberos5 krb5 kerberos-sec	# Kerberos v5
iso-tsap	102/tcp		tsap		# part of ISODE
acr-nema	104/tcp		dicom		# Digital Imag. & Comm. 300
pop3		110/tcp		pop-3		# POP version 3
sunrpc		111/tcp		portmapper	# RPC 4.0 portmapper
sunrpc		111/udp		portmapper
auth		113/tcp		authentication tap ident
nntp		119/tcp		readnews untp	# USENET News Transfer Protocol
ntp		123/udp				# Network Time Protocol
epmap		135/tcp		loc-srv		# DCE endpoint resolution
netbios-ns	137/udp				# NETBIOS Name Service
netbios-dgm	138/udp				# NETBIOS Datagram Service
netbios-ssn	139/tcp				# NETBIOS session service
imap2		143/tcp		imap		# Interim Mail Access P 2 and 4
snmp		161/tcp				# Simple Net Mgmt Protocol
snmp		161/udp
snmp-trap	162/tcp		snmptrap	# Traps for SNMP
snmp-trap	162/udp		snmptrap
cmip-man	163/tcp				# ISO mgmt over IP (CMOT)
cmip-man	163/udp
cmip-agent	164/tcp
cmip-agent	164/udp
mailq		174/tcp			# Mailer transport queue for Zmailer
xdmcp		177/udp			# X Display Manager Control Protocol
bgp		179/tcp				# Border Gateway Protocol
smux		199/tcp				# SNMP Unix Multiplexer
qmtp		209/tcp				# Quick Mail Transfer Protocol
z3950		210/tcp		wais		# NISO Z39.50 database
ipx		213/udp				# IPX [RFC1234]
ptp-event	319/udp
ptp-general	320/udp
pawserv		345/tcp				# Perf Analysis Workbench
zserv		346/tcp				# Zebra server
rpc2portmap	369/tcp
rpc2portmap	369/udp				# Coda portmapper
codaauth2	370/tcp
codaauth2	370/udp				# Coda authentication server
clearcase	371/udp		Clearcase
ldap		389/tcp			# Lightweight Directory Access Protocol
ldap		389/udp
svrloc		427/tcp				# Server Location
svrloc		427/udp
https		443/tcp				# http protocol over TLS/SSL
https		443/udp				# HTTP/3
snpp		444/tcp				# Simple Network Paging Protocol
microsoft-ds	445/tcp				# Microsoft Naked CIFS
kpasswd		464/tcp
kpasswd		464/udp
submissions	465/tcp		ssmtp smtps urd # Submission over TLS [RFC8314]
saft		487/tcp			# Simple Asynchronous File Transfer
isakmp		500/udp				# IPSEC key management
rtsp		554/tcp			# Real Time Stream Control Protocol
rtsp		554/udp
nqs		607/tcp				# Network Queuing system
asf-rmcp	623/udp		# ASF Remote Management and Control Protocol
qmqp		628/tcp
ipp		631/tcp				# Internet Printing Protocol
ldp		646/tcp				# Label Distribution Protocol
ldp		646/udp
#
# UNIX specific services
#
exec		512/tcp
biff		512/udp		comsat
login		513/tcp
who		513/udp		whod
shell		514/tcp		cmd syslog	# no passwords used
syslog		514/udp
printer		515/tcp		spooler		# line printer spooler
talk		517/udp
ntalk		518/udp
route		520/udp		router routed	# RIP
gdomap		538/tcp				# GNUstep distributed objects
gdomap		538/udp
uucp		540/tcp		uucpd		# uucp daemon
klogin		543/tcp				# Kerberized `rlogin' (v5)
kshell		544/tcp		krcmd		# Kerberized `rsh' (v5)
dhcpv6-client	546/udp
dhcpv6-server	547/udp
afpovertcp	548/tcp				# AFP over TCP
nntps		563/tcp		snntp		# NNTP over SSL
submission	587/tcp				# Submission [RFC4409]
ldaps		636/tcp				# LDAP over SSL
ldaps		636/udp
tinc		655/tcp				# tinc control port
tinc		655/udp
silc		706/tcp
kerberos-adm	749/tcp				# Kerberos `kadmin' (v5)
#
domain-s	853/tcp				# DNS over TLS [RFC7858]
domain-s	853/udp				# DNS over DTLS [RFC8094]
rsync		873/tcp
ftps-data	989/tcp				# FTP over SSL (data)
ftps		990/tcp
telnets		992/tcp				# Telnet over SSL
imaps		993/tcp				# IMAP over SSL
pop3s		995/tcp				# POP-3 over SSL
#
# From ``Assigned Numbers'':
#
#> The Registered Ports are not controlled by the IANA and on most systems
#> can be used by ordinary user processes or programs executed by ordinary
#> users.
#
#> Ports are used in the TCP [45,106] to name the ends of logical
#> connections which carry long term conversations.  For the purpose of
#> providing services to unknown callers, a service contact port is
#> defined.  This list specifies the port used by the server process as its
#> contact port.  While the IANA can not control uses of these ports it
#> does register or list uses of these ports as a convienence to the
#> community.
#
socks		1080/tcp			# socks proxy server
proofd		1093/tcp
rootd		1094/tcp
openvpn		1194/tcp
openvpn		1194/udp
rmiregistry	1099/tcp			# Java RMI Registry
lotusnote	1352/tcp	lotusnotes	# Lotus Note
ms-sql-s	1433/tcp			# Microsoft SQL Server
ms-sql-m	1434/udp			# Microsoft SQL Monitor
ingreslock	1524/tcp
datametrics	1645/tcp	old-radius
datametrics	1645/udp	old-radius
sa-msg-port	1646/tcp	old-radacct
sa-msg-port	1646/udp	old-radacct
kermit		1649/tcp
groupwise	1677/tcp
l2f		1701/udp	l2tp
radius		1812/tcp
radius		1812/udp
radius-acct	1813/tcp	radacct		# Radius Accounting
radius-acct	1813/udp	radacct
cisco-sccp	2000/tcp			# Cisco SCCP
nfs		2049/tcp			# Network File System
nfs		2049/udp			# Network File System
gnunet		2086/tcp
gnunet		2086/udp
rtcm-sc104	2101/tcp			# RTCM SC-104 IANA 1/29/99
rtcm-sc104	2101/udp
gsigatekeeper	2119/tcp
gris		2135/tcp		# Grid Resource Information Server
cvspserver	2401/tcp			# CVS client/server operations
venus		2430/tcp			# codacon port
venus		2430/udp			# Venus callback/wbc interface
venus-se	2431/tcp			# tcp side effects
venus-se	2431/udp			# udp sftp side effect
codasrv		2432/tcp			# not used
codasrv		2432/udp			# server port
codasrv-se	2433/tcp			# tcp side effects
codasrv-se	2433/udp			# udp sftp side effect
mon		2583/tcp			# MON traps
mon		2583/udp
dict		2628/tcp			# Dictionary server
f5-globalsite	2792/tcp
gsiftp		2811/tcp
gpsd		2947/tcp
gds-db		3050/tcp	gds_db		# InterBase server
icpv2		3130/udp	icp		# Internet Cache Protocol
isns		3205/tcp			# iSNS Server Port
isns		3205/udp			# iSNS Server Port
iscsi-target	3260/tcp
mysql		3306/tcp
ms-wbt-server	3389/tcp
nut		3493/tcp			# Network UPS Tools
nut		3493/udp
distcc		3632/tcp			# distributed compiler
daap		3689/tcp			# Digital Audio Access Protocol
svn		3690/tcp	subversion	# Subversion protocol
suucp		4031/tcp			# UUCP over SSL
sysrqd		4094/tcp			# sysrq daemon
sieve		4190/tcp			# ManageSieve Protocol
epmd		4369/tcp			# Erlang Port Mapper Daemon
remctl		4373/tcp		# Remote Authenticated Command Service
f5-iquery	4353/tcp			# F5 iQuery
ntske		4460/tcp	# Network Time Security Key Establishment
ipsec-nat-t	4500/udp			# IPsec NAT-Traversal [RFC3947]
iax		4569/udp			# Inter-Asterisk eXchange
mtn		4691/tcp			# monotone Netsync Protocol
radmin-port	4899/tcp			# RAdmin Port
sip		5060/tcp			# Session Initiation Protocol
sip		5060/udp
sip-tls		5061/tcp
sip-tls		5061/udp
xmpp-client	5222/tcp	jabber-client	# Jabber Client Connection
xmpp-server	5269/tcp	jabber-server	# Jabber Server Connection
cfengine	5308/tcp
mdns		5353/udp			# Multicast DNS
postgresql	5432/tcp	postgres	# PostgreSQL Database
freeciv		5556/tcp	rptp		# Freeciv gameplay
amqps		5671/tcp			# AMQP protocol over TLS/SSL
amqp		5672/tcp
amqp		5672/sctp
x11		6000/tcp	x11-0		# X Window System
x11-1		6001/tcp
x11-2		6002/tcp
x11-3		6003/tcp
x11-4		6004/tcp
x11-5		6005/tcp
x11-6		6006/tcp
x11-7		6007/tcp
gnutella-svc	6346/tcp			# gnutella
gnutella-svc	6346/udp
gnutella-rtr	6347/tcp			# gnutella
gnutella-rtr	6347/udp
redis		6379/tcp
sge-qmaster	6444/tcp	sge_qmaster	# Grid Engine Qmaster Service
sge-execd	6445/tcp	sge_execd	# Grid Engine Execution Service
mysql-proxy	6446/tcp			# MySQL Proxy
babel		6696/udp			# Babel Routing Protocol
ircs-u		6697/tcp		# Internet Relay Chat via TLS/SSL
bbs		7000/tcp
afs3-fileserver 7000/udp
afs3-callback	7001/udp			# callbacks to cache managers
afs3-prserver	7002/udp			# users & groups database
afs3-vlserver	7003/udp			# volume location database
afs3-kaserver	7004/udp			# AFS/Kerberos authentication
afs3-volser	7005/udp			# volume managment server
afs3-bos	7007/udp			# basic overseer process
afs3-update	7008/udp			# server-to-server updater
afs3-rmtsys	7009/udp			# remote cache manager service
font-service	7100/tcp	xfs		# X Font Service
http-alt	8080/tcp	webcache	# WWW caching service
puppet		8140/tcp			# The Puppet master service
bacula-dir	9101/tcp			# Bacula Director
bacula-fd	9102/tcp			# Bacula File Daemon
bacula-sd	9103/tcp			# Bacula Storage Daemon
xmms2		9667/tcp	# Cross-platform Music Multiplexing System
nbd		10809/tcp			# Linux Network Block Device
zabbix-agent	10050/tcp			# Zabbix Agent
zabbix-trapper	10051/tcp			# Zabbix Trapper
amanda		10080/tcp			# amanda backup services
dicom		11112/tcp
hkp		11371/tcp			# OpenPGP HTTP Keyserver
db-lsp		17500/tcp			# Dropbox LanSync Protocol
dcap		22125/tcp			# dCache Access Protocol
gsidcap		22128/tcp			# GSI dCache Access Protocol
wnn6		22273/tcp			# wnn6

#
# Datagram Delivery Protocol services
#
rtmp		1/ddp			# Routing Table Maintenance Protocol
nbp		2/ddp			# Name Binding Protocol
echo		4/ddp			# AppleTalk Echo Protocol
zip		6/ddp			# Zone Information Protocol

#=========================================================================
# The remaining port numbers are not as allocated by IANA.
#=========================================================================

# Kerberos (Project Athena/MIT) services
kerberos4	750/udp		kerberos-iv kdc	# Kerberos (server)
kerberos4	750/tcp		kerberos-iv kdc
kerberos-master	751/udp		kerberos_master	# Kerberos authentication
kerberos-master	751/tcp
passwd-server	752/udp		passwd_server	# Kerberos passwd server
krb-prop	754/tcp		krb_prop krb5_prop hprop # Kerberos slave propagation
zephyr-srv	2102/udp			# Zephyr server
zephyr-clt	2103/udp			# Zephyr serv-hm connection
zephyr-hm	2104/udp			# Zephyr hostmanager
iprop		2121/tcp			# incremental propagation
supfilesrv	871/tcp			# Software Upgrade Protocol server
supfiledbg	1127/tcp		# Software Upgrade Protocol debugging

#
# Services added for the Debian GNU/Linux distribution
#
poppassd	106/tcp				# Eudora
moira-db	775/tcp		moira_db	# Moira database
moira-update	777/tcp		moira_update	# Moira update protocol
moira-ureg	779/udp		moira_ureg	# Moira user registration
spamd		783/tcp				# spamassassin daemon
skkserv		1178/tcp			# skk jisho server port
predict		1210/udp			# predict -- satellite tracking
rmtcfg		1236/tcp			# Gracilis Packeten remote config server
xtel		1313/tcp			# french minitel
xtelw		1314/tcp			# french minitel
zebrasrv	2600/tcp			# zebra service
zebra		2601/tcp			# zebra vty
ripd		2602/tcp			# ripd vty (zebra)
ripngd		2603/tcp			# ripngd vty (zebra)
ospfd		2604/tcp			# ospfd vty (zebra)
bgpd		2605/tcp			# bgpd vty (zebra)
ospf6d		2606/tcp			# ospf6d vty (zebra)
ospfapi		2607/tcp			# OSPF-API
isisd		2608/tcp			# ISISd vty (zebra)
fax		4557/tcp			# FAX transmission service (old)
hylafax		4559/tcp			# HylaFAX client-server protocol (new)
munin		4949/tcp	lrrd		# Munin
rplay		5555/udp			# RPlay audio service
nrpe		5666/tcp			# Nagios Remote Plugin Executor
nsca		5667/tcp			# Nagios Agent - NSCA
canna		5680/tcp			# cannaserver
syslog-tls	6514/tcp			# Syslog over TLS [RFC5425]
sane-port	6566/tcp	sane saned	# SANE network scanner daemon
ircd		6667/tcp			# Internet Relay Chat
zope-ftp	8021/tcp			# zope management by ftp
tproxy		8081/tcp			# Transparent Proxy
omniorb		8088/tcp			# OmniORB
clc-build-daemon 8990/tcp			# Common lisp build daemon
xinetd		9098/tcp
git		9418/tcp			# Git Version Control System
zope		9673/tcp			# zope server
webmin		10000/tcp
kamanda		10081/tcp			# amanda backup services (Kerberos)
amandaidx	10082/tcp			# amanda backup services
amidxtape	10083/tcp			# amanda backup services
sgi-cmsd	17001/udp		# Cluster membership services daemon
sgi-crsd	17002/udp
sgi-gcd		17003/udp			# SGI Group membership daemon
sgi-cad		17004/tcp			# Cluster Admin daemon
binkp		24554/tcp			# binkp fidonet protocol
asp		27374/tcp			# Address Search Protocol
asp		27374/udp
csync2		30865/tcp			# cluster synchronization tool
dircproxy	57000/tcp			# Detachable IRC Proxy
tfido		60177/tcp			# fidonet EMSI over telnet
fido		60179/tcp			# fidonet EMSI over TCP

# Local services

-t	タイムスタンプを非表示にする。
-tt	秒単位で絶対タイムスタンプを表示（エポックタイム）。
-ttt	キャプチャの開始からの経過時間をミリ秒単位で表示。
-tttt	日付と時刻を含むタイムスタンプを表示（標準的な形式）。
-ttttt	日付を含まず、時刻のみで「経過時間」形式（見やすい形式）。

-X パケットデータを16進数（左側）とASCII（右側）の両方で表示。
-x パケットデータを16進数のみで表示（ASCIIは表示しない）。
-XX -X の機能に加えて、パケットのリンク層ヘッダー（Ethernetなど）も16進数とASCIIで表示。

-w ファイルパス
ex: tcpdump -i eth0 -w file.pcap

A. wireshark file.pcap
B. tcpdump -r file.pcap

from scapy.all import rdpcap
packets = rdpcap('file.pcap')
for packet in packets:
    print(packet.summary())

TCPDUMP(8)                  System Manager's Manual                 TCPDUMP(8)

NAME
       tcpdump - dump traffic on a network

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 net‐
       work 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 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 sig‐
       nal (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 ex‐
              pression 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  (in‐
       cluding 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  captur‐
       ing  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 spe‐
       cial privileges; see the pcap(3PCAP) man page for details.   Reading  a
       saved packet file doesn't require special privileges.

OPTIONS
       -A     Print each packet (minus its link level header) in ASCII.  Handy
              for capturing web pages.

       -b     Print the AS number in BGP packets in ASDOT notation rather than
              ASPLAIN notation.

       -B buffer_size
       --buffer-size=buffer_size
              Set  the operating system capture buffer size to buffer_size, in
              units of KiB (1024 bytes).

       -c count
              Exit after receiving count packets.

       --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.

       -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 units
              of  file_size  are  millions  of  bytes  (1,000,000  bytes,  not
              1,048,576 bytes).

              Note that when used with -Z option (enabled by default),  privi‐
              leges are dropped before opening first savefile.

       -d     Dump  the compiled packet-matching code in a human readable form
              to standard output and stop.

              Please mind that although code compilation  is  always  DLT-spe‐
              cific,  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  respec‐
              tively.  In  these cases the dump shows the same exact code that
              would filter the input file or the network interface without -d.

              However, when neither -r nor -i is specified, specifying -d pre‐
              vents 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.

       -dd    Dump packet-matching code as a C program fragment.

       -ddd   Dump packet-matching code as decimal numbers  (preceded  with  a
              count).

       -D
       --list-interfaces
              Print the list of the network interfaces available on the system
              and  on which tcpdump can capture packets.  For each network in‐
              terface, 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.

              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.

              The  -D  flag will not be supported if tcpdump was built with an
              older version of libpcap that lacks the  pcap_findalldevs(3PCAP)
              function.

       -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.

       -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 sep‐
              aration.

              Note  that  setting the secret for IPv4 ESP packets is supported
              at this time.

              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.

              secret is the ASCII text for ESP secret key.  If preceded by 0x,
              then a hex value will be read.

              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.

              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 per‐
              missions that tcpdump may have been given  should  already  have
              been given up.

       -f     Print  `foreign' IPv4 addresses numerically rather than symboli‐
              cally (this option is intended to get around serious brain  dam‐
              age  in  Sun's NIS server — usually it hangs forever translating
              non-local internet numbers).

              The test for `foreign' IPv4 addresses is done using the IPv4 ad‐
              dress and netmask of the interface  on  that  capture  is  being
              done.   If that address or netmask are not available, either be‐
              cause 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.

       -F file
              Use file as input for the filter expression.  An additional  ex‐
              pression given on the command line is ignored.

       -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 pre-existing data; providing
              a time specification that is coarser than the capture period  is
              therefore not advised.

              If  used  in conjunction with the -C option, filenames will take
              the form of `file<count>'.

       -h
       --help Print the tcpdump and libpcap version  strings,  print  a  usage
              message, and exit.

       --version
              Print the tcpdump and libpcap version strings and exit.

       -H     Attempt to detect 802.11s draft mesh headers.

       -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''.

              On Linux systems with 2.2 or later kernels and  on  recent  ver‐
              sions of macOS and Solaris, an interface argument of ``any'' can
              be  used to capture packets from all interfaces.  Note that cap‐
              tures on the  ``any''  pseudo-interface  will  not  be  done  in
              promiscuous mode.

              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.

       -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 operat‐
              ing systems.

              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.

              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.

       --immediate-mode
              Capture in "immediate mode".  In this mode, packets  are  deliv‐
              ered  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.

       -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.

       -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.

       --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.

              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 preci‐
              sion of time stamps in the file, the conversion will lose preci‐
              sion.

              The supported values for timestamp_precision are micro  for  mi‐
              crosecond  resolution  and  nano for nanosecond resolution.  The
              default is microsecond resolution.

       --micro
       --nano Shorthands for --time-stamp-precision=micro or --time-stamp-pre‐
              cision=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  preci‐
              sion.   In  contrast, a savefile created with microsecond preci‐
              sion will have trailing zeroes added  to  the  time  stamp  when
              --nano is used.

       -K
       --dont-verify-checksums
              Don't attempt to verify IP, TCP, or UDP checksums.  This is use‐
              ful  for  interfaces  that perform some or all of those checksum
              calculation in hardware; otherwise, all outgoing  TCP  checksums
              will be flagged as bad.

       -l     Make  stdout  line buffered.  Useful if you want to see the data
              while capturing it.  E.g.,

                     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.

              -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.

       -L
       --list-data-link-types
              List the known data link types for the interface, in the  speci‐
              fied  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  sup‐
              port  802.11  headers with radio information) and another set of
              data link types when in monitor mode (for example, it might sup‐
              port 802.11 headers, or 802.11 headers with  radio  information,
              only in monitor mode).

       -m module
              Load  SMI  MIB module definitions from file module.  This option
              can be used several times to load several MIB modules into  tcp‐
              dump.

       -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.

       -n     Don't convert addresses (i.e.,  host  addresses,  port  numbers,
              etc.) to names.

       -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''.

       -#
       --number
              Print an optional packet number at the beginning of the line.

       -O
       --no-optimize
              Do  not  run the packet-matching code optimizer.  This is useful
              only if you suspect a bug in the optimizer.

       -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'.

       --print
              Print  parsed  packet  output, even if the raw packets are being
              saved to a file with the -w flag.

       -Q direction
       --direction=direction
              Choose send/receive direction direction for which packets should
              be captured. Possible values are `in', `out'  and  `inout'.  Not
              available on all platforms.

       -q     Quick  (quiet?) output.  Print less protocol information so out‐
              put lines are shorter.

       -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 ``-''.

       -S
       --absolute-tcp-sequence-numbers
              Print absolute, rather than relative, TCP sequence numbers.

       -s snaplen
       --snapshot-length=snaplen
              Snarf snaplen bytes of data from each packet rather than the de‐
              fault  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.

              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 avail‐
              able if a too-short snapshot length is selected.

              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 compatibil‐
              ity with recent older versions of tcpdump.

       -T type
              Force  packets  selected  by  "expression" to be interpreted the
              specified type.  Currently known types are aodv  (Ad-hoc  On-de‐
              mand  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), radius (RADIUS), resp (REdis Serialization  Pro‐
              tocol),  rpc  (Remote  Procedure Call), rtcp (Real-Time Applica‐
              tions 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).

              Note that the pgm type above affects  UDP  interpretation  only,
              the  native  PGM is always recognised as IP protocol 113 regard‐
              less. UDP-encapsulated PGM is often called "EPGM" or "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 de‐
              coding in addition to that any UDP packet would be treated as an
              encapsulated PGM packet.

       -t     Don't print a timestamp on each dump line.

       -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.

       -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.

       -tttt  Print a timestamp, as hours, minutes, seconds, and fractions  of
              a  second  since  midnight,  preceded  by the date, on each dump
              line.

       -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.

       -u     Print undecoded NFS handles.

       -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  out‐
              put,  rather than, when not writing to a terminal, being written
              only when the output buffer fills.

              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.

              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.

       -v     When  parsing and printing, produce (slightly more) verbose out‐
              put.  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.

              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  up‐
              date  currently  can cause loss of captured packets on their way
              from the kernel to tcpdump.

       -vv    Even more verbose output.  For example,  additional  fields  are
              printed  from  NFS  reply packets, and SMB packets are fully de‐
              coded.

       -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.

       -V file
              Read  a  list  of filenames from file. Standard input is used if
              file is ``-''.

       -w file
              Write the raw packets to file rather than parsing  and  printing
              them  out.  They can later be printed with the -r option.  Stan‐
              dard output is used if file is ``-''.

              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  re‐
              ceived.

              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.

              See pcap-savefile(5) for a description of the file format.

       -W filecount
              Used in conjunction with the -C option, this will limit the num‐
              ber  of  files  created to the specified number, and begin over‐
              writing 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.

              Used in conjunction with the -G option, this will limit the num‐
              ber  of rotated dump files that get created, exiting with status
              0 when reaching the limit.

              If used in conjunction with both -C and -G, the -W  option  will
              currently be ignored, and will only affect the file name.

       -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 im‐
              plementation  this  flag  may have the same effect as -xx if the
              packet is truncated.

       -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.

       -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.

       -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.

       -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.

       -z postrotate-command
              Used in conjunction with the -C or -G options,  this  will  make
              tcpdump  run " postrotate-command file " where file is the save‐
              file being closed after each rotation. For  example,  specifying
              -z  gzip  or  -z bzip2 will compress each savefile using gzip or
              bzip2.

              Note that tcpdump will run the command in parallel to  the  cap‐
              ture, using the lowest priority so that this doesn't disturb the
              capture process.

              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.

       -Z user
       --relinquish-privileges=user
              If  tcpdump is running as root, after opening the capture device
              or input savefile, change the user ID to user and the  group  ID
              to the primary group of user.

              This  behavior  is  enabled  by default (-Z tcpdump), and can be
              disabled by -Z root.

        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.

              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 argu‐
              ments are concatenated with spaces before being parsed.

EXAMPLES
       To print all packets arriving at or departing from sundown:
              tcpdump host sundown

       To print traffic between helios and either hot or ace:
              tcpdump host helios and \( hot or ace \)

       To print all IP packets between ace and any host except helios:
              tcpdump ip host ace and not helios

       To print all traffic between local hosts and hosts at Berkeley:
              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):
              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).
              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.
              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)
              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.)
              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:
              tcpdump 'gateway snup and ip[2:2] > 576'

       To  print IP broadcast or multicast packets that were not sent via Eth‐
       ernet broadcast or multicast:
              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):
              tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

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

       Timestamps

       By  default,  all  output lines are preceded by a timestamp.  The time‐
       stamp is the current clock time in the form
              hh:mm:ss.frac
       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 ker‐
       nel  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 data‐
       grams) are `async' packets, with a priority value between 0 and 7;  for
       example,  `async4'.  Such packets are assumed to contain an 802.2 Logi‐
       cal 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 as‐
       sumed 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 com‐
       pression algorithm described in RFC 1144.)

       On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out‐
       bound),  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 pack‐
       ets, 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  in‐
       dicated  by  U  (urgent pointer), W (window), A (ack), S (sequence num‐
       ber), 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:
              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 for‐
       mat 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
       The  first line says that rtsg sent an ARP packet asking for the Ether‐
       net address of internet host csam.  Csam replies with its Ethernet  ad‐
       dress (in this example, Ethernet addresses are in caps and internet ad‐
       dresses in lower case).

       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
       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.

       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 for‐
       mat of this information is:
              tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
       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 re‐
       ported 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 to‐
       tal 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 corre‐
       sponding port, will be printed, with a > separating the source and des‐
       tination.  For other protocols, the addresses will be printed, with a >
       separating the source and destination.  Higher level protocol  informa‐
       tion, 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 proto‐
       col 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
       Src  and  dst  are  the  source and destination IP addresses and ports.
       Tcpflags are some combination of S (SYN), F (FIN), P (PSH), R (RST),  U
       (URG),  W  (CWR),  E (ECE) 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 direc‐
       tion 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  de‐
       pend 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
       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  includ‐
       ing  last'.)  There was no piggy-backed ACK, the available receive win‐
       dow 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 se‐
       quence 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  sit‐
       ting  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 re‐
       ports ``[|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:

        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 dec‐
       imal 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 representa‐
       tion of the AND value as well as the result of this operation is 2 (bi‐
       nary  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
       This says that port who on host actinide sent a UDP  datagram  to  port
       who on host broadcast, the Internet broadcast address.  The packet con‐
       tained 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 particu‐
       lar, 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)
       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
       `[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 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  ad‐
       dress  128.32.137.3.  The total size of the response was 273 bytes, ex‐
       cluding 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, `[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 Net‐
       BEUI 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 ([email protected]).

       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 first line, host sushi sends a transaction with id 26377 to wrl.
       The request was 112 bytes, excluding the UDP and IP headers.  The oper‐
       ation  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:
              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
       (-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  off‐
       set  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.  In‐
       stead, tcpdump keeps track of ``recent'' requests, and matches them  to
       the replies using the transaction ID.  If a reply does not closely fol‐
       low 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 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  success‐
       ful, 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 ne‐
       gotiation 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.  In‐
       stead, 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 dis‐
       carded).  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
       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
              net.host.port

              144.1.209.2 > icsd-net.112.220
              office.2 > icsd-net.112.220
              jssmag.149.235 > icsd-net.2
       (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 (`of‐
       fice').   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:
              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
       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  an‐
       other  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.209.165 > 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
       Jssmag.209 initiates transaction id 12266 with host helios by  request‐
       ing  up  to  8 packets (the `<0-7>').  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, jss‐
       mag.209 initiates the next request.  The `*' on the  request  indicates
       that XO (`exactly once') was not set.

BACKWARD COMPATIBILITY
       The  TCP  flag  names tcp-ece and tcp-cwr became available when linking
       with libpcap 1.9.0 or later.

SEE ALSO
       stty(1),    pcap(3PCAP),     bpf(4),     nit(4P),     pcap-savefile(5),
       pcap-filter(7), pcap-tstamp(7)

              https://www.iana.org/assignments/media-types/applica‐
              tion/vnd.tcpdump.pcap

AUTHORS
       The original authors are:

       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.

BUGS
       To    report    a   security   issue   please   send   an   e-mail   to
       [email protected].

       To report bugs and other problems, contribute patches, request  a  fea‐
       ture, 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 rec‐
       ommend 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 pack‐
              ets  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 cap‐
              ture mechanism, if asked to copy only part of a packet to  user‐
              space, 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) ques‐
       tion 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.

                                 12 March 2023                      TCPDUMP(8)