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""" Parsing and composing domain names
This module contains two classes for converting domain names to and from bytes.
You won't have to use these classes. They're used internally in Message,
Question, ResourceRecord and RecordData. You can read section 4.1.4 of RFC 1035
if you want more info.
"""
import struct
class Composer(object):
""" Converts a string representation of a domain name to bytes """
def __init__(self):
self.offsets = dict()
def to_bytes(self, offset, dnames):
"""Convert each domain name in to bytes"""
result = b""
for dname in dnames:
# Split domain name into labels
labels = dname.split(".")
# Determine keys of subdomains in offset dict
keys = []
for label in reversed(labels):
name = label
if keys:
name += "." + keys[-1]
keys.append(name)
keys.reverse()
# Convert label to bytes
add_null = True
for j, label in enumerate(labels):
if keys[j] in self.offsets:
offset = self.offsets[keys[j]]
pointer = (3 << 14) + offset
result += struct.pack("!H", pointer)
add_null = False
offset += 2
break
else:
self.offsets[keys[j]] = offset
result += struct.pack("!B{}s".format(len(label)),
len(label),
label)
offset += 1 + len(label)
# Add null character at end
if add_null:
result += b"\x00"
offset += 1
return result
class Parser(object):
""" Convert byte representations of domain names to strings """
def __init__(self):
self.labels = dict()
def from_bytes(self, packet, offset, num):
""" Convert domain name from bytes to string
Args:
packet (bytes): packet containing the domain name
offset (int): offset of domain name in packet
num (int): number of domain names to decode
Returns:
str, int
"""
dnames = []
# Read the domain names
for _ in range(num):
# Read a new domain name
dname = ""
prev_offsets = []
done = False
while done is False:
# Read length of next label
llength = struct.unpack_from("!B", packet, offset)[0]
# Done reading domain when length is zero
if llength == 0:
offset += 1
break
# Compression label
elif (llength >> 6) == 3:
new_offset = offset + 2
target = struct.unpack_from("!H", packet, offset)[0]
target -= 3 << 14
label = self.labels[target]
done = True
# Normal label
else:
new_offset = offset + llength + 1
label = struct.unpack_from("{}s".format(llength),
packet, offset+1)[0]
# Add label to dictionary
self.labels[offset] = label
for prev_offset in prev_offsets:
self.labels[prev_offset] += "." + label
prev_offsets.append(offset)
# Update offset
offset = new_offset
# Append label to domain name
if len(dname) > 0:
dname += "."
dname += label
# Append domain name to list
dnames.append(dname)
return dnames, offset
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