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