X-Git-Url: https://git.tcpdump.org/tcpdump/blobdiff_plain/3bc62cd35f4c52f590605c73d92c92b6b4a4bdc5..b51a0dafc7861eb31d21524ec067d7c529a664b8:/tcpdump.1.in diff --git a/tcpdump.1.in b/tcpdump.1.in index eced6958..de5db995 100644 --- a/tcpdump.1.in +++ b/tcpdump.1.in @@ -20,7 +20,7 @@ .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" -.TH TCPDUMP 1 "2 February 2017" +.TH TCPDUMP 1 "29 November 2021" .SH NAME tcpdump \- dump traffic on a network .SH SYNOPSIS @@ -39,6 +39,9 @@ tcpdump \- dump traffic on a network .I count ] [ +.B \-\-count +] +[ .B \-C .I file_size ] @@ -135,6 +138,13 @@ tcpdump \- dump traffic on a network ] .ti +8 [ +.BI \-\-micro +] +[ +.BI \-\-nano +] +.ti +8 +[ .I expression ] .br @@ -142,7 +152,7 @@ tcpdump \- dump traffic on a network .SH DESCRIPTION .LP \fITcpdump\fP prints out a description of the contents of packets on a -network interface that match the boolean \fIexpression\fP; the +network interface that match the Boolean \fIexpression\fP; 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 @@ -212,9 +222,13 @@ 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. .LP +Using the SIGUSR2 signal along with the +.B \-w +flag will forcibly flush the packet buffer into the output file. +.LP Reading packets from a network interface may require that you have special privileges; see the -.B pcap (3PCAP) +.BR pcap (3PCAP) man page for details. Reading a saved packet file doesn't require special privileges. .SH OPTIONS @@ -238,6 +252,12 @@ units of KiB (1024 bytes). .BI \-c " count" Exit after receiving \fIcount\fP packets. .TP +.BI \-\-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, \fItcpdump\fP counts only packets that were matched by the filter +expression. +.TP .BI \-C " file_size" Before writing a raw packet to a savefile, check whether the file is currently larger than \fIfile_size\fP and, if so, close the current @@ -245,12 +265,43 @@ savefile and open a new one. Savefiles after the first savefile will have the name specified with the .B \-w flag, with a number after it, starting at 1 and continuing upward. -The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes, +The default unit of \fIfile_size\fP is millions of bytes (1,000,000 bytes, not 1,048,576 bytes). +.IP +By adding a suffix of k/K, m/M or g/G to the value, the unit +can be changed to 1,024 (KiB), 1,048,576 (MiB), or 1,073,741,824 (GiB) +respectively. .TP .B \-d Dump the compiled packet-matching code in a human readable form to standard output and stop. +.IP +Please mind that although code compilation is always DLT-specific, +typically it is impossible (and unnecessary) to specify which DLT to use +for the dump because \fItcpdump\fP uses either the DLT of the input pcap +file specified with +.BR -r , +or the default DLT of the network interface specified with +.BR -i , +or the particular DLT of the network interface specified with +.B -y +and +.B -i +respectively. In these cases the dump shows the same exact code that +would filter the input file or the network interface without +.BR -d . +.IP +However, when neither +.B -r +nor +.B -i +is specified, specifying +.B -d +prevents \fItcpdump\fP from guessing a suitable network interface (see +.BR -i ). +In this case the DLT defaults to EN10MB and can be set to another valid +value manually with +.BR -y . .TP .B \-dd Dump packet-matching code as a @@ -270,7 +321,7 @@ which .I tcpdump can capture packets. For each network interface, a number and an interface name, possibly followed by a text description of the -interface, is printed. The interface name or the number can be supplied +interface, are printed. The interface name or the number can be supplied to the .B \-i flag to specify an interface on which to capture. @@ -288,7 +339,7 @@ flag will not be supported if was built with an older version of .I libpcap that lacks the -.B pcap_findalldevs() +.BR pcap_findalldevs (3PCAP) function. .TP .B \-e @@ -317,7 +368,7 @@ with cryptography enabled. \fIsecret\fP is the ASCII text for ESP secret key. If preceded by 0x, then a hex value will be read. .IP -The option assumes RFC2406 ESP, not RFC1827 ESP. +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 @@ -357,6 +408,9 @@ Savefiles will have the name specified by which should include a time format as defined by .BR 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. .IP If used in conjunction with the .B \-C @@ -382,10 +436,13 @@ Attempt to detect 802.11s draft mesh headers. .TP .BI \-\-interface= interface .PD -Listen on \fIinterface\fP. -If unspecified, \fItcpdump\fP searches the system interface list for the -lowest numbered, configured up interface (excluding loopback), which may turn -out to be, for example, ``eth0''. +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 +\fIinterface\fP. If unspecified and if the +.B -d +flag is not given, \fItcpdump\fP searches the system +interface list for the lowest numbered, configured up interface +(excluding loopback), which may turn out to be, for example, ``eth0''. .IP On Linux systems with 2.2 or later kernels, an .I interface @@ -439,7 +496,7 @@ terminal rather than to a file or pipe. .PD Set the time stamp type for the capture to \fItstamp_type\fP. The names to use for the time stamp types are given in -.BR pcap-tstamp (@MAN_MISC_INFO@); +.BR \%pcap-tstamp (@MAN_MISC_INFO@); not all the types listed there will necessarily be valid for any given interface. .TP @@ -471,6 +528,19 @@ The supported values for \fItimestamp_precision\fP are \fBmicro\fP for microsecond resolution and \fBnano\fP for nanosecond resolution. The default is microsecond resolution. .TP +.B \-\-micro +.PD 0 +.TP +.B \-\-nano +.PD +Shorthands for \fB\-\-time\-stamp\-precision=micro\fP or +\fB\-\-time\-stamp\-precision=nano\fP, adjusting the time stamp +precision accordingly. When reading packets from a savefile, using +\fB\-\-micro\fP truncates time stamps if the savefile was created with +nanosecond precision. In contrast, a savefile created with microsecond +precision will have trailing zeroes added to the time stamp when +\fB\-\-nano\fP is used. +.TP .B \-K .PD 0 .TP @@ -646,21 +716,25 @@ Currently known types are \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol), \fBcarp\fR (Common Address Redundancy Protocol), \fBcnfp\fR (Cisco NetFlow protocol), +\fBdomain\fR (Domain Name System), \fBlmp\fR (Link Management Protocol), \fBpgm\fR (Pragmatic General Multicast), \fBpgm_zmtp1\fR (ZMTP/1.0 inside PGM/EPGM), -\fBresp\fR (REdis Serialization Protocol), +\fBptp\fR (Precision Time Protocol), +\fBquic\fR (QUIC), \fBradius\fR (RADIUS), +\fBresp\fR (REdis Serialization Protocol), \fBrpc\fR (Remote Procedure Call), -\fBrtp\fR (Real-Time Applications protocol), \fBrtcp\fR (Real-Time Applications control protocol), +\fBrtp\fR (Real-Time Applications protocol), \fBsnmp\fR (Simple Network Management Protocol), +\fBsomeip\fR (SOME/IP), \fBtftp\fR (Trivial File Transfer Protocol), \fBvat\fR (Visual Audio Tool), -\fBwb\fR (distributed White Board), -\fBzmtp1\fR (ZeroMQ Message Transport Protocol 1.0) +\fBvxlan\fR (Virtual eXtensible Local Area Network), +\fBwb\fR (distributed White Board) and -\fBvxlan\fR (Virtual eXtensible Local Area Network). +\fBzmtp1\fR (ZeroMQ Message Transport Protocol 1.0). .IP Note that the \fBpgm\fR type above affects UDP interpretation only, the native PGM is always recognised as IP protocol 113 regardless. UDP-encapsulated PGM is @@ -680,16 +754,20 @@ 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. .TP .B \-ttt -Print a delta (micro-second resolution) between current and previous line -on each dump line. +Print a delta (microsecond or nanosecond resolution depending on the +.B \-\-time\-stamp-precision +option) between current and previous line on each dump line. +The default is microsecond resolution. .TP .B \-tttt Print a timestamp, as hours, minutes, seconds, and fractions of a second since midnight, preceded by the date, on each dump line. .TP .B \-ttttt -Print a delta (micro-second resolution) between current and first line -on each dump line. +Print a delta (microsecond or nanosecond resolution depending on the +.B \-\-time\-stamp-precision +option) between current and first line on each dump line. +The default is microsecond resolution. .TP .B \-u Print undecoded NFS handles. @@ -723,7 +801,7 @@ flag will not be supported if was built with an older version of .I libpcap that lacks the -.B pcap_dump_flush() +.BR pcap_dump_flush (3PCAP) function. .TP .B \-v @@ -735,7 +813,12 @@ IP and ICMP header checksum. .IP When writing to a file with the .B \-w -option, report, once per second, the number of packets captured. +option and at the same time not reading from a file with the +.B \-r +option, report to stderr, once per second, the number of packets captured. In +Solaris, FreeBSD and possibly other operating systems this periodic update +currently can cause loss of captured packets on their way from the kernel to +tcpdump. .TP .B \-vv Even more verbose output. @@ -777,10 +860,10 @@ operating systems and applications will use the extension if it is present and adding one (e.g. .pcap) is recommended. .IP See -.BR pcap-savefile (@MAN_FILE_FORMATS@) +.BR \%pcap-savefile (@MAN_FILE_FORMATS@) for a description of the file format. .TP -.B \-W +.BI \-W " filecount" Used in conjunction with the .B \-C option, this will limit the number @@ -793,9 +876,15 @@ files, allowing them to sort correctly. Used in conjunction with the .B \-G option, this will limit the number of rotated dump files that get -created, exiting with status 0 when reaching the limit. If used with +created, exiting with status 0 when reaching the limit. +.IP +If used in conjunction with both .B \-C -as well, the behavior will result in cyclical files per timeslice. +and +.B \-G, +the +.B \-W +option will currently be ignored, and will only affect the file name. .TP .B \-x When parsing and printing, @@ -807,6 +896,9 @@ bytes will be printed. Note that this is the entire link-layer packet, so for link layers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer packet is shorter than the required padding. +In the current implementation this flag may have the same effect as +.B \-xx +if the packet is truncated. .TP .B \-xx When parsing and printing, @@ -820,6 +912,9 @@ 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 +.B \-XX +if the packet is truncated. .TP .B \-XX When parsing and printing, @@ -833,7 +928,11 @@ its link level header, in hex and ASCII. .TP .BI \-\-linktype= datalinktype .PD -Set the data link type to use while capturing packets to \fIdatalinktype\fP. +Set the data link type to use while capturing packets (see +.BR -L ) +or just compiling and dumping packet-matching code (see +.BR -d ) +to \fIdatalinktype\fP. .TP .BI \-z " postrotate-command" Used in conjunction with the @@ -883,7 +982,7 @@ Otherwise, only packets for which \fIexpression\fP is `true' will be dumped. .LP For the \fIexpression\fP syntax, see -.BR pcap-filter (@MAN_MISC_INFO@). +.BR \%pcap-filter (@MAN_MISC_INFO@). .LP The \fIexpression\fP argument can be passed to \fItcpdump\fP as either a single Shell argument, or as multiple Shell arguments, whichever is more convenient. @@ -952,6 +1051,16 @@ tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocal .fi .RE .LP +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) +.RS +.nf +.B +tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)' +.fi +.RE +.LP 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.) @@ -1054,7 +1163,7 @@ As on FDDI networks, packets are assumed to contain an LLC packet. .LP \fI(N.B.: The following description assumes familiarity with -the SLIP compression algorithm described in RFC-1144.)\fP +the SLIP compression algorithm described in RFC 1144.)\fP .LP On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound), packet type, and compression information are printed out. @@ -1086,7 +1195,7 @@ data and 6 bytes of compressed header: .HD ARP/RARP Packets .LP -Arp/rarp output shows the type of request and its arguments. +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 @@ -1099,7 +1208,7 @@ arp reply csam is-at CSAM\fR .sp .5 .fi .RE -The first line says that rtsg sent an arp packet asking +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 @@ -1152,7 +1261,7 @@ those are reported as \fBECT(1)\fP, \fBECT(0)\fP, or \fBCE\fP. \fIoffset\fP is the fragment offset field; it is printed whether this is part of a fragmented datagram or not. \fIflags\fP are the MF and DF flags; \fB+\fP is reported if MF is set, -and \fBDF\P is reported if F is set. If neither are set, \fB.\fP is +and \fBDF\fP is reported if F is set. If neither are set, \fB.\fP is reported. \fIproto\fP is the protocol ID field. \fIlength\fP is the total length field. @@ -1175,7 +1284,7 @@ flag, in the IP header information, as described above. TCP Packets .LP \fI(N.B.:The following description assumes familiarity with -the TCP protocol described in RFC-793. +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.)\fP @@ -1213,7 +1322,7 @@ host \fIcsam\fP. .RS .nf .sp .5 -\s-2\f(CWIP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024] +\f(CWIP 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 @@ -1221,7 +1330,7 @@ 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\fR\s+2 +IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1\fR .sp .5 .fi .RE @@ -1233,16 +1342,16 @@ The packet sequence number was 768512 and it contained no data. (The notation is `first:last' which means `sequence numbers \fIfirst\fP up to but not including \fIlast\fP'.) -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 +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. .LP Csam replies with a similar packet except it includes a piggy-backed -ack for rtsg's SYN. -Rtsg then acks csam's SYN. +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 +Note that the ACK sequence number is a small integer (1). The first time \fItcpdump\fP sees a TCP `conversation', it prints the sequence number from the packet. @@ -1494,23 +1603,23 @@ UDP format is illustrated by this rwho packet: .sp .5 .fi .RE -This says that port \fIwho\fP on host \fIactinide\fP sent a udp +This says that port \fIwho\fP on host \fIactinide\fP sent a UDP datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet broadcast address. The packet contained 84 bytes of user data. .LP 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. +In particular, Domain Name service requests (RFC 1034/1035) and Sun +RPC calls (RFC 1050) to NFS. .HD -UDP Name Server Requests +TCP or UDP Name Server Requests .LP \fI(N.B.:The following description assumes familiarity with -the Domain Service protocol described in RFC-1035. +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.)\fP +in Greek.)\fP .LP Name server requests are formatted as .RS @@ -1527,7 +1636,7 @@ address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag was set. -The query length was 37 bytes, not including the UDP and +The query length was 37 bytes, excluding the TCP or UDP and IP protocol headers. The query operation was the normal one, \fIQuery\fP, so the op field was omitted. @@ -1551,7 +1660,7 @@ 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=\fIx\fP]' is printed, where \fIx\fP is the hex value of header bytes two and three. .HD -UDP Name Server Responses +TCP or UDP Name Server Responses .LP Name server responses are formatted as .RS @@ -1569,7 +1678,7 @@ 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 UDP and IP headers. +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. .LP @@ -1601,8 +1710,7 @@ may take up a page or more, so only use -v if you really want all the gory details. .LP For information on SMB packet formats and what all the fields mean see -www.cifs.org or the pub/samba/specs/ directory on your favorite -samba.org mirror site. +\%https://download.samba.org/pub/samba/specs/ and other online resources. The SMB patches were written by Andrew Tridgell (tridge@samba.org). .HD @@ -1678,7 +1786,7 @@ 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. +the file mode (in octal), the UID and GID, and the file size. .LP If the \-v flag is given more than once, even more details are printed. .LP @@ -1822,9 +1930,9 @@ protocol) and packet size. .RS .nf .sp .5 -\s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*" +\f(CWicsd-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\fR\s+2 +techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR .sp .5 .fi .RE @@ -1842,7 +1950,7 @@ another reply to the same request saying host techpit has laserwriter .RS .nf .sp .5 -\s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001 +\f(CWjssmag.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 @@ -1855,7 +1963,7 @@ 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\fR\s+2 +jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR .sp .5 .fi .RE @@ -1868,7 +1976,7 @@ 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. +excluding the ATP header. The `*' on packet 7 indicates that the EOM bit was set. .LP @@ -1881,11 +1989,18 @@ The `*' on the request indicates that XO (`exactly once') was \fInot\fP set. .SH "SEE ALSO" -stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@), -pcap-filter(@MAN_MISC_INFO@), pcap-tstamp(@MAN_MISC_INFO@) +.BR stty (1), +.BR pcap (3PCAP), +.BR bpf (4), +.BR nit (4P), +.BR \%pcap-savefile (@MAN_FILE_FORMATS@), +.BR \%pcap-filter (@MAN_MISC_INFO@), +.BR \%pcap-tstamp (@MAN_MISC_INFO@) .LP .RS -.I http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap +.na +.I https://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap +.ad .RE .LP .SH AUTHORS @@ -1898,10 +2013,10 @@ Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. .LP It is currently being maintained by tcpdump.org. .LP -The current version is available via http: +The current version is available via HTTPS: .LP .RS -.I http://www.tcpdump.org/ +.I https://www.tcpdump.org/ .RE .LP The original distribution is available via anonymous ftp: @@ -1916,7 +2031,7 @@ This program uses OpenSSL/LibreSSL, under specific configurations. To report a security issue please send an e-mail to \%security@tcpdump.org. .LP To report bugs and other problems, contribute patches, request a -feature, provide generic feedback etc please see the file +feature, provide generic feedback etc. please see the file .I CONTRIBUTING in the tcpdump source tree root. .LP @@ -1932,7 +2047,7 @@ be copied from the kernel in order to be filtered in user mode; .IP 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 userland, will not report the +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 .BR tcpdump );