Wireshark definitely reads and stores the byte-order from the pcap header when opening the file. I don't think that is exposed currently, but it should be relatively easy to do (from wiretap).
> On Jan 10, 2014, at 7:33 AM, Michal Labedzki <michal.labedzki@xxxxxxxxx> wrote:
>
> Hello,
>
> Is there option to dissector know byte order of... "interface"? I am
> thinking about:
> 1) PCAP File
> 2) PCAPNG file
> 3) live capturing
>
> Please look at packet-usb.c.
> There is:
> -----------------------
> /* Adds the Linux USB pseudo header fields to the tree.
> * NOTE: The multi-byte fields in this header, and the pseudo-header
> * extension, are in host-endian format so we can't
> * use proto_tree_add_item() nor the tvb_get_xyz() routines and is
> * the reason for the tvb_memcpy() and proto_tree_add_uint[64]()
> * pairs below. */
> static void
> dissect_linux_usb_pseudo_header(tvbuff_t *tvb, packet_info *pinfo,
> proto_tree *tree,
> guint *bus_id, guint *device_address)
> {
> ...
> tvb_memcpy(tvb, (guint8 *)&val64, 0, 8);
> proto_tree_add_uint64(tree, hf_usb_urb_id, tvb, 0, 8, val64);
> ...
> }
> -----------------------
>
> And: http://www.tcpdump.org/linktypes.html
> -----------------------
> LINKTYPE_USB_LINUX_MMAPPED 220 DLT_USB_LINUX_MMAPPED
> USB packets, beginning with a Linux USB header, as specified by the
> struct usbmon_packet in the Documentation/usb/usbmon.txt file in the
> Linux source tree. All 64 bytes of the header are present. All fields
> in the header are in the host byte order for the pcap file, as
> specified by the file's magic number, or for the section of the
> pcap-ng file, as specified by the Section Header Block. For
> isochronous transfers, the ndesc field specifies the number of
> isochronous descriptors that follow.
> -----------------------
>
>> "All fields in the header are in the host byte order for the pcap file"
>
> This USB header is in host order of capture's program. So when you
> capturing on Big-endian machine and open capture file on Little-endian
> machine this USB code should fail.
>
> Probably PCAP/PCAPNG have ordering info by magic bytes, but I do not
> know how to do that while live capturing (current code work for this
> case)
>
> Possible solutions:
> 1. Wireshark already support byte-ordering information for dissectors
> (anyone seen, anyone knows?)
>
> 2. Drop LINKTYPE_USB*s (LINKTYPE_NULL?) and request for replacement
> with specified byte order (network order?) - this does not solve
> backward compatibility with already existing captures...
>
> 3. Wireshark already use magic and bytes are all the time in right
> order (host order)
>
>
> --
>
> Pozdrawiam / Best regards
> -------------------------------------------------------------------------------------------------------------
> Michał Łabędzki, Software Engineer
> Tieto Corporation
>
> Product Development Services
>
> http://www.tieto.com / http://www.tieto.pl
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