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Re: smb.time || smb2.time is valid in a filter string but not in a custom column field?
From: Guy Harris <guy () alum mit edu>
Date: Thu, 13 Jun 2013 17:46:24 -0700

On Jun 13, 2013, at 5:16 PM, Richard Sharpe <realrichardsharpe () gmail com> wrote:

I thought I would try to work around my issue with smb.time vs
smb2.time by using a custom column field and putting in an expression,
but it seems not to work. An expression that is acceptable in the
Filter field is not acceptable in a custom column.

Why is this?

Because "||" is a Boolean "or", not a human-language "or". :-)

I.e., "a || b" is a packet-matching expression that matches if the expression "a" matches or the expression "b" 
matches, not a field expression that evaluates to whichever of the fields "a" or "b" is present (with "a" chosen if 
both are present, presumably).

The Filter field takes packet-matching expressions; the custom column field takes field names.  Field names are valid 
packet-matching expressions, which match if the field is present in the packet and don't match if it's not, but 
arbitrary packet-matching expressions aren't valid as custom columns.

Having a separate feature allowing a set of field names for a custom column could be useful.  I'm not sure an "||" 
operator, given its existing meaning in C (and in packet-matching expressions), would be the right syntax for that.

(Other expressions for custom columns might be useful, e.g. (ip.ttl % 17) + (1/137.035999074)*ip.proto - OK, well, 
*that* expression might not be useful, but....)
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