145 lines
4.9 KiB
Plaintext
145 lines
4.9 KiB
Plaintext
.\" Copyright (c) 1994, 1996, 1997
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that: (1) source code distributions
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.\" retain the above copyright notice and this paragraph in its entirety, (2)
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.\" distributions including binary code include the above copyright notice and
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.\" this paragraph in its entirety in the documentation or other materials
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.\" provided with the distribution, and (3) all advertising materials mentioning
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.\" features or use of this software display the following acknowledgement:
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.\" ``This product includes software developed by the University of California,
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.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
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.\" the University nor the names of its contributors may be used to endorse
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.\" or promote products derived from this software without specific prior
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.\" written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.TH PCAP_NEXT_EX 3PCAP "25 July 2018"
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.SH NAME
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pcap_next_ex, pcap_next \- read the next packet from a pcap_t
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.SH SYNOPSIS
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.nf
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.ft B
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#include <pcap/pcap.h>
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.ft
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.LP
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.ft B
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int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
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.ti +8
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const u_char **pkt_data);
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const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h);
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.ft
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.fi
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.SH DESCRIPTION
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.B pcap_next_ex()
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reads the next packet and returns a success/failure indication.
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If the packet was read without problems, the pointer pointed to by the
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.I pkt_header
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argument is set to point to the
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.I pcap_pkthdr
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struct for the packet, and the
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pointer pointed to by the
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.I pkt_data
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argument is set to point to the data in the packet. The
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.I struct pcap_pkthdr
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and the packet data are not to be freed by the caller, and are not
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guaranteed to be valid after the next call to
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.BR pcap_next_ex() ,
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.BR pcap_next() ,
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.BR pcap_loop(3PCAP) ,
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or
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.BR pcap_dispatch(3PCAP) ;
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if the code needs them to remain valid, it must make a copy of them.
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.PP
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.B pcap_next()
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reads the next packet (by calling
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.B pcap_dispatch()
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with a
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.I cnt
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of 1) and returns a
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.I u_char
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pointer to the data in that packet. The
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packet data is not to be freed by the caller, and is not
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guaranteed to be valid after the next call to
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.BR pcap_next_ex() ,
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.BR pcap_next() ,
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.BR pcap_loop() ,
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or
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.BR pcap_dispatch() ;
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if the code needs it to remain valid, it must make a copy of it.
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The
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.I pcap_pkthdr
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structure pointed to by
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.I h
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is filled in with the appropriate values for the packet.
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.PP
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The bytes of data from the packet begin with a link-layer header. The
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format of the link-layer header is indicated by the return value of the
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.B pcap_datalink(PCAP)
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routine when handed the
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.B pcap_t
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value also passed to
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.B pcap_loop()
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or
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.BR pcap_dispatch() .
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.I https://www.tcpdump.org/linktypes.html
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lists the values
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.B pcap_datalink()
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can return and describes the packet formats that
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correspond to those values. The value it returns will be valid for all
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packets received unless and until
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.B pcap_set_datalink(3PCAP)
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is called; after a successful call to
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.BR pcap_set_datalink() ,
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all subsequent packets will have a link-layer header of the type
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specified by the link-layer header type value passed to
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.BR pcap_set_datalink() .
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.PP
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Do
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.B NOT
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assume that the packets for a given capture or ``savefile`` will have
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any given link-layer header type, such as
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.B DLT_EN10MB
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for Ethernet. For example, the "any" device on Linux will have a
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link-layer header type of
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.B DLT_LINUX_SLL
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even if all devices on the system at the time the "any" device is opened
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have some other data link type, such as
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.B DLT_EN10MB
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for Ethernet.
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.SH RETURN VALUE
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.B pcap_next_ex()
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returns 1 if the packet was read without problems, 0 if packets are
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being read from a live capture and the packet buffer timeout expired,
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.B PCAP_ERROR
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if an error occurred while reading the packet, and
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.B PCAP_ERROR_BREAK
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if packets
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are being read from a ``savefile'' and there are no more packets to read
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from the savefile. If
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.B PCAP_ERROR
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is returned,
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.B pcap_geterr(3PCAP)
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or
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.B pcap_perror(3PCAP)
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may be called with
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.I p
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as an argument to fetch or display the error text.
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.PP
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.B pcap_next()
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returns a pointer to the packet data on success, and returns
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.B NULL
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if an error occurred, or if no packets were read from a live capture
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(if, for example, they were discarded because they didn't pass the
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packet filter, or if, on platforms that support a packet buffer timeout
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that starts before any packets arrive, the timeout expires before any
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packets arrive, or if the file descriptor for the capture device is in
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non-blocking mode and no packets were available to be read), or if no
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more packets are available in a ``savefile.'' Unfortunately, there is no
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way to determine whether an error occurred or not.
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.SH SEE ALSO
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pcap(3PCAP)
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