After more investigation, I think the error is a mix of some faulty code
and a bad assumption on my part.
The code I was using to decode the SRV target field is approximately
identical to dns_rdata_to_name (nss_mdns.c:2559), where it is used to
decode PTR RDATA into a single string.
For some reason which I cannot now recall, the code strips whitespace
before processing the rdata, according to the following code:
while (isspace (*rdata_curr))
{
rdata_curr ++;
if (rdata_curr > rdata + rdlen)
{
return DNS_RDATA_TO_NAME_BAD_FORMAT;
}
}
Now, isspace returns TRUE for *rdata_curr in "\t\n\v\f\r ", which is (I
think) values 9-13 or 32. Thus, an length byte in the range 9-13 or 32
will be discarded and the code begin parsing with the next value, which is
quite likely meaningless. If this ascii value was greater than 63 the code
would then fail with a pointer error! Otherwise, it is likely to fail with
a format error.
Because the error was occurring on some embedded hardware that I didn't
have direct access to (and thus couldn't line-by-line debug), I incorrectly
assumed the parse failure was due to the compressed SRV records rather than
the broken parsing code (which had previously worked due to being lucky
enough not to get one of the 'broken' length values).
I'm a little surprised this error hasn't been noticed as mysterious
failures when doing PTR lookups via nss_mdns. The fix is easy: delete
lines 2565 to 2574 of nss_mdns.c (the above loop). Valid RDATA doesn't
have leading whitespace.
Thanks for your time.
--
Andrew White
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