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import socket
import struct
import typing
# Python's socket module does not have these constants
SO_ORIGINAL_DST = 80
SOL_IPV6 = 41
def original_addr(csock: socket.socket) -> typing.Tuple[str, int]:
# Get the original destination on Linux.
# In theory, this can be done using the following syscalls:
# sock.getsockopt(socket.SOL_IP, SO_ORIGINAL_DST, 16)
# sock.getsockopt(SOL_IPV6, SO_ORIGINAL_DST, 28)
#
# In practice, it is a bit more complex:
# 1. We cannot rely on sock.family to decide which syscall to use because of IPv4-mapped
# IPv6 addresses. If sock.family is AF_INET6 while sock.getsockname() is ::ffff:127.0.0.1,
# we need to call the IPv4 version to get a result.
# 2. We can't just try the IPv4 syscall and then do IPv6 if that doesn't work,
# because doing the wrong syscall can apparently crash the whole Python runtime.
# As such, we use a heuristic to check which syscall to do.
is_ipv4 = "." in csock.getsockname()[0] # either 127.0.0.1 or ::ffff:127.0.0.1
if is_ipv4:
# the struct returned here should only have 8 bytes, but invoking sock.getsockopt
# with buflen=8 doesn't work.
dst = csock.getsockopt(socket.SOL_IP, SO_ORIGINAL_DST, 16)
port, raw_ip = struct.unpack_from("!2xH4s", dst)
ip = socket.inet_ntop(socket.AF_INET, raw_ip)
else:
dst = csock.getsockopt(SOL_IPV6, SO_ORIGINAL_DST, 28)
port, raw_ip = struct.unpack_from("!2xH4x16s", dst)
ip = socket.inet_ntop(socket.AF_INET6, raw_ip)
return ip, port
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