Lets look at the main code part first:

Code:

/^Outgoing update/ {A=procDNS(A)}
{A=A "\n" $0}

Line 2 above is building up variable A with your file contents, it does this by appending A with a newline plus the current line's contents for every line of the file.

As for Line 1: function procDNS always returns blank, this is just a small trick that allows us to process the contents of A and then reset A back to the empty string. This could have been written as:

Code:

/^Outgoing update/ {procDNS(A); A=""}

You might prefer the above version for clarity. If you look at your datafile you will note that it starts with an "outgoing update" line. As no other lines have been read yet A will still be blank, procDNS needs to deal with blank input and not output a blank result, that is what the first line of the function is doing with the split command. Another elegant way to deal with this is to test for no updt_line as I guess output without this info is pretty meaningless:

Code:

function procDNS(rec) {
    updt_line=getfield(rec, "UPDATE SECTION:\n[^\n]+", 16)
    if (updt_line == "") return ""

Now remember the input to procDNS is the whole block between the "Outgoing update" lines. Your assessment of getfield() is pretty well spot on, note the value 16 above is the number of characters to discard from the front of the matched string. i.e. "UPDATE SECTION:" plus newline, which leaves the data required. You can easily extract other info you may need from the block using this technique.

Code:

    if(length(fail)) {
       gsub("YXRRSET", "CNAME already exist",fail)
       gsub("REFUSED", "Change refused",fail)
       gsub("SERVFAIL", "Failed to connect to server",fail)
       gsub("NOTAUTH", "Not authorized",fail)
       print "FAIL: " fail
    }
    else print "UPDATE SUCCEDED"

See above how you can add further human readable versions of the error codes returned, I don't know what ones you actually see in the "wild", of course any fail codes not replaced will remain as they are eg "FORMERR", "BADVERS", "NOTZONE", etc.

Finally when you reach the end of the file A will have the last record built up in it but there is no "Outgoing update" line to trigger it's processing, so we call procDNS(A) in the END section (a blank file could will result in a blank block being sent to procDNS but we already know procDNS deals with this fine).