Source identification list. This identifies which
sources will match this rule. If non-null, The octet
string contains one or more 3-tuples of this form:
first octet: exact (1) or range (2)
next 4 octets: source address
next 4 octets: source mask
An entry with an 'exact' tag means to only match the
specific source (number & mask). An
entry with a 'range' tag means to match any source
that falls in the range indicated by the source
and mask.
A null string also means 'match any source'.