linux: support IPv6 destinations in transparent mode

This fixes #2869 with the help of @aniketpanjwani.

1 files changed, 28 insertions, 6 deletions

diff --git a/mitmproxy/platform/linux.py b/mitmproxy/platform/linux.py
index 4fa3191a..f446bb72 100644
--- a/mitmproxy/platform/linux.py
+++ b/mitmproxy/platform/linux.py
@@ -1,12 +1,34 @@
 import socket
 import struct
+import typing
 
-# Python socket module does not have this constant
+# Python's socket module does not have these constants
 SO_ORIGINAL_DST = 80
+SOL_IPV6 = 41
 
 
-def original_addr(csock: socket.socket):
-    odestdata = csock.getsockopt(socket.SOL_IP, SO_ORIGINAL_DST, 16)
-    _, port, a1, a2, a3, a4 = struct.unpack("!HHBBBBxxxxxxxx", odestdata)
-    address = "%d.%d.%d.%d" % (a1, a2, a3, a4)
-    return address, port
+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