Re: pcapng - packet comment terminator; packet list equiv for debug

[Guy Harris <gharris () sonic net>]

On Sep 24, 2020, at 5:41 PM, chuck c <bubbasnmp () gmail com> wrote:

> vmware has a packet capture utility (pktcap-uw) which adds packet comments when writing a capture as pcapng.
> Looks like the code that writes the comments is reusing a buffer so that when a smaller comment is written there are 
> leftover characters from the previous comment.
> The code is also adding a null terminator to the comment string.
> When the capture is reloaded in View->Reload as File Format/Capture, the comments are flagged with "Trailing stray 
> characters".
>
> Question #1. 
> The pcapng draft standard states:
> "If an option's value is a string, the value is not necessarily zero-terminated. Software that reads these files MUST 
> NOT assume that strings are zero-terminated, and MUST treat a zero-value octet as a string terminator."
>
> but also:
> "opt_comment:
>
> The opt_comment option is a UTF-8 string containing human-readable comment text that is associated to the current 
> block. Line separators SHOULD be a carriage-return + linefeed ('\r\n') or just linefeed ('\n'); either form may 
> appear and be considered a line separator. The string is not zero-terminated."

"The string is not zero-terminated" is a restatement of "If an option's value is a string, the value is not necessarily 
zero-terminated."  It should probably be removed, with the general statement about *all* string options covering 
comments.

> Is Wireshark handling comments with embedded nulls properly?

Prior to the master branch,

        If an option's value is a string, the value is not necessarily zero-terminated. Software that reads these files 
MUST NOT assume that strings are zero-terminated, and MUST treat a zero-value octet as a string terminator.

translates to FT_STRINGZPAD.  However, "frame.comment" is of type FT_STRING, so that's incorrect.

In the master branch, we have FT_STRINGZPAD and FT_STRINGZTRUNC; the former means "padded, if necessary, entirely with 
null characters" and the latter means "truncated, if necessary, with a null character, with no guarantee that anything 
that *follows* the null character is also null", so FT_STRINGZTRUNC would be appropriate there.

> Question #2.
> Viewing the pcapng internals in the Wireshark gui is great

Presumably you mean "opening a pcapng file using View > Reload as File Format/Capture".

> but ....
>
> I can view frame.comment in the "Capture" view but not pcapng.options.option.length

That's a file detail that's overkill for reading capture files.

> In the "File Format" view I can add option attributes as a column but get the values for all the blocks in a single 
> entry.

There is an entry called "Options", which includes all the options, each one as a separate entry.

> Has it ever been discussed to turn the Packet List pane into a Block List pane?

That's a third issue; comments wouldn't show up separately from the record in which they appear.

Any record (a capture-file-type-neutral term I've been using, and that's used in libwiretap) that has a time stamp 
should probably appear in the summary pane, as it's probably some sort of event.

For other records, it's less clear.

> Is #1 worthy of opening a bug/issue?

Yes.

> (or alternative, try to open bug with vmware - ugh)

They're not violating the current pcapng spec; we *could* change it to require null padding, but 1) Wireshark can 
handle null-truncated-but-not-null-padded strings and 2) most other software should be able to handle that with little 
difficulty, so I wouldn't be inclined to make that change.

> Is #2 worthy of an enhancement request?

As per the above, I don't see any need to display the details of comments when treating a pcapng file as a capture.  
Note that comments won't necessarily be pcapng-specific - other capture file formats may have them, and they might be 
in a format that doesn't have anything directly corresponding to the option type or length.  libwiretap is intended to 
abstract away as many file-type-specific details as possible.

When treating a pcapng file as "Fileshark" input ("View > Reload as File Format/Capture" can be thought of as a first 
step towards Fileshark), the only way not to have "all the blocks in a single entry" would be to remove the top-level 
"Options" entry and put the individual option items directly under the "Block Data" item.
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