lib/libpcap/pcap_open_live.3

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.Dd $Mdocdate: May 29 2020 $
.Dt PCAP_OPEN_LIVE 3
.Os
.Sh NAME
.Nm pcap_open_live ,
.Nm pcap_open_offline ,
.Nm pcap_dump_open ,
.Nm pcap_dump_fopen ,
.Nm pcap_lookupdev ,
.Nm pcap_lookupnet ,
.Nm pcap_dispatch ,
.Nm pcap_loop ,
.Nm pcap_dump ,
.Nm pcap_inject ,
.Nm pcap_sendpacket ,
.Nm pcap_compile ,
.Nm pcap_setfilter ,
.Nm pcap_freecode ,
.Nm pcap_next ,
.Nm pcap_next_ex ,
.Nm pcap_setdirection ,
.Nm pcap_datalink ,
.Nm pcap_snapshot ,
.Nm pcap_is_swapped ,
.Nm pcap_major_version ,
.Nm pcap_minor_version ,
.Nm pcap_stats ,
.Nm pcap_file ,
.Nm pcap_fileno ,
.Nm pcap_get_selectable_fd ,
.Nm pcap_perror ,
.Nm pcap_geterr ,
.Nm pcap_strerror ,
.Nm pcap_close ,
.Nm pcap_dump_file ,
.Nm pcap_dump_ftell ,
.Nm pcap_dump_flush ,
.Nm pcap_dump_close ,
.Nm pcap_breakloop ,
.Nm pcap_findalldevs ,
.Nm pcap_freealldevs ,
.Nm pcap_getnonblock ,
.Nm pcap_setnonblock ,
.Nm pcap_set_datalink ,
.Nm pcap_list_datalinks ,
.Nm pcap_free_datalinks ,
.Nm pcap_open_dead ,
.Nm pcap_fopen_offline ,
.Nm pcap_lib_version ,
.Nm pcap_datalink_val_to_name ,
.Nm pcap_datalink_val_to_description ,
.Nm pcap_datalink_name_to_val ,
.Nm pcap_create ,
.Nm pcap_set_snaplen ,
.Nm pcap_set_promisc ,
.Nm pcap_can_set_rfmon ,
.Nm pcap_set_rfmon ,
.Nm pcap_set_timeout ,
.Nm pcap_set_buffer_size ,
.Nm pcap_set_immediate_mode ,
.Nm pcap_activate ,
.Nm pcap_statustostr ,
.Nm pcap_offline_filter
.Nd Packet Capture library
.Sh SYNOPSIS
.In pcap.h
.Ft "pcap_t *"
.Fn pcap_open_live "const char *source" "int snaplen" "int promisc" "int to_ms" "char *errbuf"
.Ft "pcap_t *"
.Fn pcap_open_offline "char *fname" "char *errbuf"
.Ft "pcap_dumper_t *"
.Fn pcap_dump_open "pcap_t *p" "char *fname"
.Ft "pcap_dumper_t *"
.Fn pcap_dump_fopen "pcap_t *p" "FILE *f"
.Ft "char *"
.Fn pcap_lookupdev "char *errbuf"
.Ft int
.Fn pcap_lookupnet "const char *device" "bpf_u_int32 *netp" "bpf_u_int32 *maskp" "char *errbuf"
.Ft int
.Fn pcap_dispatch "pcap_t *p" "int cnt" "pcap_handler callback" "u_char *user"
.Ft int
.Fn pcap_loop "pcap_t *p" "int cnt" "pcap_handler callback" "u_char *user"
.Ft void
.Fn pcap_dump "u_char *user" "const struct pcap_pkthdr *h" "const u_char *sp"
.Ft int
.Fn pcap_inject "pcap_t *p" "const void *buf" "size_t len"
.Ft int
.Fn pcap_sendpacket "pcap_t *p" "const u_char *buf" "int size"
.Ft int
.Fn pcap_compile "pcap_t *p" "struct bpf_program *program" "const char *buf" "int optimize" "bpf_u_int32 netmask"
.Ft int
.Fn pcap_setfilter "pcap_t *p" "struct bpf_program *fp"
.Ft void
.Fn pcap_freecode "struct bpf_program *program"
.Ft "u_char *"
.Fn pcap_next "pcap_t *p" "struct pcap_pkthdr *h"
.Ft int
.Fn pcap_next_ex "pcap_t *p" "struct pcap_pkthdr **pkt_header" "const u_char **pkt_data"
.Ft int
.Fn pcap_setdirection "pcap_t *p" "pcap_direction_t d"
.Ft int
.Fn pcap_datalink "pcap_t *p"
.Ft int
.Fn pcap_snapshot "pcap_t *p"
.Ft int
.Fn pcap_is_swapped "pcap_t *p"
.Ft int
.Fn pcap_major_version "pcap_t *p"
.Ft int
.Fn pcap_minor_version "pcap_t *p"
.Ft int
.Fn pcap_stats "pcap_t *p" "struct pcap_stat *ps"
.Ft "FILE *"
.Fn pcap_file "pcap_t *p"
.Ft int
.Fn pcap_fileno "pcap_t *p"
.Ft int
.Fn pcap_get_selectable_fd "pcap_t *p"
.Ft void
.Fn pcap_perror "pcap_t *p" "const char *prefix"
.Ft "char *"
.Fn pcap_geterr "pcap_t *p"
.Ft "const char *"
.Fn pcap_strerror "int errnum"
.Ft void
.Fn pcap_close "pcap_t *p"
.Ft "FILE *"
.Fn pcap_dump_file "pcap_dumper_t *p"
.Ft long
.Fn pcap_dump_ftell "pcap_dumper_t *p"
.Ft int
.Fn pcap_dump_flush "pcap_dumper_t *p"
.Ft void
.Fn pcap_dump_close "pcap_dumper_t *p"
.Ft void
.Fn pcap_breakloop "pcap_t *p"
.Ft int
.Fn pcap_findalldevs "pcap_if_t **alldevsp" "char *errbuf"
.Ft void
.Fn pcap_freealldevs "pcap_if_t *alldevs"
.Ft int
.Fn pcap_getnonblock "pcap_t *p" "char *errbuf"
.Ft int
.Fn pcap_setnonblock "pcap_t *p" "int nonblock" "char *errbuf"
.Ft int
.Fn pcap_set_datalink "pcap_t *p" "int dlt"
.Ft int
.Fn pcap_list_datalinks "pcap_t *p" "int **dlt_buffer"
.Ft void
.Fn pcap_free_datalinks "int *dlt_list"
.Ft pcap_t
.Fn pcap_open_dead "int linktype" "int snaplen"
.Ft pcap_t
.Fn pcap_fopen_offline "FILE *fp" "char *errbuf"
.Ft const char *
.Fn pcap_lib_version "void"
.Ft const char *
.Fn pcap_datalink_val_to_name "int dlt"
.Ft const char *
.Fn pcap_datalink_val_to_description "int dlt"
.Ft int
.Fn pcap_datalink_name_to_val "const char *name"
.Ft "pcap_t *"
.Fn pcap_create "const char *device" "char *errbuf"
.Ft int
.Fn pcap_set_snaplen "pcap_t *p" "int snaplen"
.Ft int
.Fn pcap_set_promisc "pcap_t *p" "int promisc"
.Ft int
.Fn pcap_can_set_rfmon "pcap_t *p"
.Ft int
.Fn pcap_set_rfmon "pcap_t *p" "int rfmon"
.Ft int
.Fn pcap_set_timeout "pcap_t *p" "int timeout_ms"
.Ft int
.Fn pcap_set_buffer_size "pcap_t *p" "int buffer_size"
.Ft int
.Fn pcap_set_immediate_mode "pcap_t *p" "int immediate"
.Ft int
.Fn pcap_activate "pcap_t *p"
.Ft const char *
.Fn pcap_statustostr "int errnum"
.Ft int
.Fn pcap_offline_filter "const struct bpf_program *fp" "const struct pcap_pkthdr *h" "const u_char *pkt"
.Sh DESCRIPTION
.Nm
provides a high level interface to packet capture systems.
All packets
on the network, even those destined for other hosts, are accessible
through this mechanism.
.Pp
Note that
.Fa errbuf
in
.Fn pcap_open_live ,
.Fn pcap_open_offline ,
.Fn pcap_findalldevs ,
.Fn pcap_lookupdev ,
.Fn pcap_lookupnet ,
.Fn pcap_getnonblock ,
.Fn pcap_setnonblock ,
.Fn pcap_fopen_offline ,
and
.Fn pcap_create
is assumed to be able to hold at least
.Dv PCAP_ERRBUF_SIZE
chars.
.Pp
.Fn pcap_open_live
is used to obtain a packet capture descriptor to look
at packets on the network.
.Fa source
is a string that specifies the network device to open.
.Fa snaplen
specifies the maximum number of bytes to capture from one packet.
.Fa promisc
specifies if the interface is to be put into promiscuous mode.
(Note that even if this parameter is false, the interface
could well be in promiscuous mode for some other reason.)
.Fa to_ms
specifies the read timeout in milliseconds.
.Fa errbuf
is used to return error text and is only set when
.Fn pcap_open_live
fails and returns
.Dv NULL .
.Pp
.Fn pcap_open_offline
is called to open a
.Dq savefile
for reading.
.Fa fname
specifies the name of the file to open.
The file has the same format as those used by
.Xr tcpdump 8 .
.\" and
.\" .BR tcpslice(1) .
The name
.Ql -
is a synonym for
.Dv stdin .
.Fa errbuf
is used to return error text and is only set when
.Fn pcap_open_offline
fails and returns
.Dv NULL .
.Pp
.Fn pcap_dump_open
is called to open a
.Dq savefile
for writing.
The name
.Ql -
is a synonym for
.Dv stdout .
.Dv NULL
is returned on failure.
.Fa p
is a
.Fa pcap
struct as returned by
.Fn pcap_open_offline
or
.Fn pcap_open_live .
.Fa fname
specifies the name of the file to open.
If
.Dv NULL
is returned,
.Fn pcap_geterr
can be used to get the error text.
.Pp
.Fn pcap_dump_fopen
allows the use of savefile functions on the already-opened stream
.Fa f .
.Pp
.Fn pcap_lookupdev
returns a pointer to a network device suitable for use with
.Fn pcap_open_live
and
.Fn pcap_lookupnet .
If there is an error,
.Dv NULL
is returned and
.Fa errbuf
is filled in with an appropriate error message.
.Pp
.Fn pcap_lookupnet
is used to determine the network number and mask
associated with the network device
.Fa device .
Both
.Fa netp
and
.Fa maskp
are
.Fa bpf_u_int32
pointers.
A return of \-1 indicates an error in which case
.Fa errbuf
is filled in with an appropriate error message.
.Pp
.Fn pcap_dispatch
is used to collect and process packets.
.Fa cnt
specifies the maximum number of packets to process before returning.
A
.Fa cnt
of \-1 processes all the packets received in one buffer.
A
.Fa cnt
of 0 processes all packets until an error occurs, EOF is reached,
or the read times out (when doing live reads and a non-zero
read timeout is specified).
.Fa callback
specifies a routine to be called with three arguments: a
.Fa u_char
pointer which is passed in from
.Fn pcap_dispatch ,
a pointer to the
.Fa pcap_pkthdr
struct (which precedes the actual network headers and data),
and a
.Fa u_char
pointer to the packet data.
The number of packets read is returned.
Zero is returned when EOF is reached in a savefile.
A return of \-1 indicates an error in which case
.Fn pcap_perror
or
.Fn pcap_geterr
may be used to display the error text.
.Pp
.Fn pcap_dump
outputs a packet to a save file previously opened using
.Fn pcap_dump_open
or
.Fn pcap_dump_fopen .
Note that the
.Fa user
argument contains the handle to the save file and should be of type
.Ft "pcap_dumper_t *" .
This makes
.Fn pcap_dump
a suitable callback function for use as an argument to
.Fn pcap_dispatch .
.Pp
.Fn pcap_inject
uses
.Xr write 2
to inject a raw packet through the network interface.
It returns the number of bytes written or \-1 on failure.
.Pp
.Fn pcap_sendpacket
is an alternate interface for packet injection (provided for compatibility).
It returns 0 on success or \-1 on failure.
.Pp
.Fn pcap_compile
is used to compile the string
.Fa buf
into a filter program.
.Fa program
is a pointer to a
.Fa bpf_program
struct and is filled in by
.Fn pcap_compile .
.Fa optimize
controls whether optimization on the resulting code is performed.
.Fa netmask
specifies the netmask of the local net.
.Pp
.Fn pcap_setfilter
is used to specify a filter program.
.Fa fp
is a pointer to an array of
.Fa bpf_program
struct, usually the result of a call to
.Fn pcap_compile .
\-1
is returned on failure;
0
is returned on success.
.Pp
.Fn pcap_freecode
is used to free up allocated memory pointed to by a
.Fa bpf_program
struct generated by
.Fn pcap_compile
when that BPF program is no longer needed, for example after it has
been made the filter program for a pcap structure by a call to
.Fn pcap_setfilter .
.Pp
.Fn pcap_loop
is similar to
.Fn pcap_dispatch
except it keeps reading packets until
.Fa cnt
packets are processed or an error occurs.
It does
.Em not
return when live read timeouts occur.
Rather, specifying a non-zero read timeout to
.Fn pcap_open_live
and then calling
.Fn pcap_loop
allows the reception and processing of any packets that arrive when the
timeout occurs.
A negative
.Fa cnt
causes
.Fn pcap_loop
to loop forever (or at least until an error occurs).
.Fn pcap_loop
may be terminated early through an explicit call to
.Fn pcap_breakloop .
In this case, the return value of
.Fn pcap_loop
will be \-2.
.Pp
.Fn pcap_next
returns a
.Fa u_char
pointer to the next packet.
.Pp
.Fn pcap_next_ex
reads the next packet and returns a success/failure indication: a
return value of 1 indicates success, 0 means that the timeout was exceeded
on a live capture, \-1 indicates that an error occurred whilst reading
the packet and \-2 is returned when there are no more packets to read in a
savefile.
.Pp
.Fn pcap_datalink
returns the link layer type, e.g., DLT_EN10MB.
.Pp
.Fn pcap_snapshot
returns the snapshot length specified when
.Fn pcap_open_live
was called.
.Pp
.Fn pcap_is_swapped
returns true if the current savefile
uses a different byte order than the current system.
.Pp
.Fn pcap_major_version
returns the major number of the version of the pcap used to write the savefile.
.Pp
.Fn pcap_minor_version
returns the minor number of the version of the pcap used to write the savefile.
.Pp
.Fn pcap_file
returns the stream associated with the savefile.
.Pp
.Fn pcap_stats
returns 0 and fills in a
.Fa pcap_stat
struct.
The values represent packet statistics from the start of the
run to the time of the call.
If there is an error or the underlying
packet capture doesn't support packet statistics, \-1 is returned and
the error text can be obtained with
.Fn pcap_perror
or
.Fn pcap_geterr .
.Pp
.Fn pcap_fileno
and
.Fn pcap_get_selectable_fd
return the file descriptor number of the savefile.
.Pp
.Fn pcap_perror
prints the text of the last pcap library error on
.Dv stderr ,
prefixed by
.Fa prefix .
.Pp
.Fn pcap_geterr
returns the error text pertaining to the last pcap library error.
.Pp
.Fn pcap_strerror
is provided in case
.Xr strerror 3
isn't available.
.Pp
.Fn pcap_close
closes the files associated with
.Fa p
and deallocates resources.
.Pp
.Fn pcap_dump_file
returns the stream associated with a savefile.
.Pp
.Fn pcap_dump_ftell
returns the current file offset within a savefile.
.Pp
.Fn pcap_dump_flush
ensures that any buffered data has been written to a savefile.
.Pp
.Fn pcap_dump_close
closes the savefile.
.Pp
.Fn pcap_findalldevs
constructs a linked list of network devices that are suitable for
opening with
.Fn pcap_open_live .
.Pp
.Fn pcap_freealldevs
frees a list of interfaces built by
.Fn pcap_findalldevs .
.Pp
.Fn pcap_getnonblock
returns 1 if the capture file descriptor is in non-blocking mode, 0
if it is in blocking mode, or \-1 on error.
.Pp
.Fn pcap_setnonblock
sets or resets non-blocking mode on a capture file descriptor.
.Pp
.Fn pcap_set_datalink
sets the datalink type on a live capture device that supports multiple
datalink types.
.Pp
.Fn pcap_setdirection
is used to limit the direction that packets must be flowing in order
to be captured.
The direction is either
.Dv PCAP_D_INOUT ,
.Dv PCAP_D_IN
or
.Dv PCAP_D_OUT .
Direction is only relevant to live captures.
When reading from a dump file,
.Fn pcap_setdirection
has no effect.
.Pp
.Fn pcap_list_datalinks
returns an array of the supported datalink types for an opened live capture
device as a \-1 terminated array.
It is the caller's responsibility to free this list with
.Fn pcap_free_datalinks ,
which frees the list of link-layer header types pointed to by
.Dv dlt_list .
.Pp
.Fn pcap_breakloop
safely breaks out of a
.Fn pcap_loop .
This function sets an internal flag and is safe to be called from inside a
signal handler.
.Pp
.Fn pcap_open_dead
is used for creating a pcap_t structure to use when calling the
other functions in libpcap.
It is typically used when just using libpcap for compiling BPF code.
.Pp
.Fn pcap_fopen_offline
may be used to read dumped data from an existing open stream
.Fa fp .
.Pp
.Fn pcap_lib_version
returns a string describing the version of libpcap.
.Pp
.Fn pcap_datalink_val_to_name
and
.Fn pcap_datalink_val_to_description
look up the name or description of a datalink type by number.
These functions return
.Dv NULL
if the specified datalink type is not known.
.Fn pcap_datalink_name_to_val
finds the datalink number for a given datalink name.
Returns \-1 if the name is not known.
.Pp
.Fn pcap_create
is used to create a packet capture handle to look at
packets on the network.
The returned handle must be activated with
.Fn pcap_activate
before packets can be captured with it; options for the
capture, such as promiscuous mode, can be set on the handle
before activating it.
.Pp
.Fn pcap_set_snaplen
sets the snapshot length to be used on a capture handle when the
handle is activated to
.Fa snaplen .
.Pp
.Fn pcap_set_promisc
sets whether promiscuous mode should be set on a capture handle
when the handle is activated.
If
.Fa promisc
is non-zero, promiscuous mode will be set, otherwise it will not be set.
.Pp
.Fn pcap_can_set_rfmon
checks whether monitor mode could be set on a capture handle when the
handle is activated.
.Pp
.Fn pcap_set_rfmon
sets whether monitor mode should be set on a capture handle
when the handle is activated.
If
.Fa rfmon
is non-zero, monitor mode will be set, otherwise it will not be set.
.Pp
.Fn pcap_set_timeout
sets the read timeout that will be used on a capture handle when the
handle is activated to
.Fa to_ms ,
which is in units of milliseconds.
.Pp
.Fn pcap_set_buffer_size
sets the buffer size that will be used on a capture handle when the
handle is activated to
.Fa buffer_size ,
which is in units of bytes.
.Pp
.Fn pcap_set_immediate_mode
sets whether immediate mode should be set on a capture handle when
the handle is activated.
If
.Fa immediate
is non-zero, immediate mode will be set, otherwise it will not be set.
.Pp
.Fn pcap_activate
is used to activate a packet capture handle to look at
packets on the network, with the options that were set on the handle
being in effect.
.Pp
.Fn pcap_statustostr
converts a PCAP_ERROR_ or PCAP_WARNING_ value returned by a libpcap
routine to an error string.
.Pp
.Fn pcap_offline_filter
checks whether a filter matches a packet.
.Sh SEE ALSO
.Xr pcap-filter 5 ,
.Xr tcpdump 8
.\" , tcpslice(1)
.Sh AUTHORS
.An -nosplit
.An Van Jacobson ,
.An Craig Leres ,
and
.An Steven McCanne ,
all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.