// WSDG Chapter Capture

[[ChapterCapture]]

== Packet capturing

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This chapter needs to be reviewed and extended.
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[[ChCaptureAddLibpcap]]

=== How to add a new capture type to libpcap

The following is an updated excerpt from a developer mailing list mail about
adding ISO 9141 and 14230 (simple serial line card diagnostics) to Wireshark:

For libpcap, the first thing you’d need to do would be to get +{dlt-glob}+ values
for all the link-layer protocols you’d need. If ISO 9141 and 14230 use the same
link-layer protocol, they might be able to share a +{dlt-glob}+ value, unless the
only way to know what protocols are running above the link layer is to know
which link-layer protocol is being used, in which case you might want separate
+{dlt-glob}+ values.

For the rest of the libpcap discussion, I'll assume you're working with libpcap
1.0 or later and that this is on a UN*X platform. You probably don't want to
work with a version older than 1.0, even if whatever OS you're using happens to
include libpcap - older versions are not as friendly towards adding support for
devices other than standard network interfaces.

Then you’d probably add to the `pcap_open_live()` routine, for whatever
platform or platforms this code should work, something such as a check
for device names that look like serial port names and, if the check
succeeds, a call to a routine to open the serial port.

See, for example, the `#ifdef HAVE_DAG_API` code in _pcap-linux.c_ and
_pcap-bpf.c_.

The serial port open routine would open the serial port device, set the baud
rate and do anything else needed to open the device. It’d allocate a `pcap_t`,
set its `fd` member to the file descriptor for the serial device, set the
`snapshot` member to the argument passed to the open routine, set the `linktype`
member to one of the +{dlt-glob}+ values, and set the `selectable_fd` member to
the same value as the `fd` member. It should also set the `dlt_count` member to
the number of +{dlt-glob}+ values to support, and allocate an array of
`dlt_count` `u_int`s, assign it to the `dlt_list` member, and fill in that list
with all the +{dlt-glob}+ values.

You’d then set the various `_*_op` fields to routines to handle the operations in
question. `read_op` is the routine that’d read packets from the device. `inject_op`
would be for sending packets; if you don't care about that, you’d set it to a
routine that returns an error indication. `setfilter_op` can probably just be set
to `install_bpf_program`. `set_datalink` would just set the `linktype` member to the
specified value if it’s one of the values for OBD, otherwise it should return an
error. `getnonblock_op` can probably be set to `pcap_getnonblock_fd`. `setnonblock_op`
can probably be set to `pcap_setnonblock_fd`. `stats_op` would be set to a routine
that reports statistics. `close_op` can probably be set to `pcap_close_common`.

If there’s more than one +{dlt-glob}+ value, you definitely want a `set_datalink`
routine so that the user can select the appropriate link-layer type.

For Wireshark, you’d add support for those +{dlt-glob}+ values to
_wiretap/libpcap.c_, which might mean adding one or more _WTAP_ENCAP_ types to
_wtap.h_ and to the `encap_table[]` table in _wiretap/wtap.c_. You’d then
have to write a dissector or dissectors for the link-layer protocols or
protocols and have them register themselves with the `wtap_encap` dissector
table, with the appropriate _WTAP_ENCAP_ values by calling
`dissector_add_uint()`.

// End of WSDG Chapter Capture