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Coding workflows · Backward-pointer DNS names
03
A concrete starting point
Decode a DNS name from a byte packet of at most 65535 bytes and return raw labels with the following offset. Test roots, truncation and a 255-byte expanded-name limit, rejecting invalid label tags, cycles and pointers that do not point backward.
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Backward-pointer DNS names
A concrete starting point
def dns_name(data, offset):
if type(data) is not bytes or len(data) > 65535 or type(offset) is not int or not 0 <= offset < len(data):
raise ValueError("dns.input")
labels, seen, pos, end, size = [], set(), offset, None, 1
while True:
if pos in seen or pos >= len(data):
raise ValueError("dns.pointer")
seen.add(pos)
count = data[pos]
if count & 192 == 192:
if pos+1 >= len(data):
raise ValueError("dns.truncated")
target = ((count & 63) << 8) | data[pos+1]
if target >= pos:
raise ValueError("dns.forward")
if end is None:
end = pos+2
pos = target
continue
if count & 192 or pos+1+count > len(data):
raise ValueError("dns.label")
pos += 1
if count == 0:
return labels, pos if end is None else end
size += count+1
if size > 255:
raise ValueError("dns.length")
labels.append(data[pos:pos+count])
pos += count
The example is a starting point for a real project, not a claim that generated code is already tested. Adjust paths, dependencies and expected behavior to match your application.