ExSandbox.Egress.OriginalDst (ExSandbox v1.0.1)

Recovers the destination a sandbox's connection was originally aimed at, before the netns redirect sent it to the pool (005 T060a3, contracts/egress.md).

Why this exists at all

Enforcement is transparent (Principle VI): the tenant application connects straight to its destination and is never told it is sandboxed, so there is no protocol frame in which it states where it wants to go — and deliberately so. Anything the sandbox tells us is a claim it controls. The kernel records the pre-redirection destination against the socket, and that record is the only account of the destination the sandbox cannot influence.

Reading it, and the measurement that shaped this module

SO_ORIGINAL_DST (SOL_IP, option 80) is readable from plain :gen_tcp via a :raw getopt — no NIF and no port driver. Measured on Linux 6.10 aarch64:

inet:getopts(S, [{raw,0,80,16}])
%=> {ok,[{raw,0,80,<<2,0,148,227,127,0,0,1,0,0,0,0,0,0,0,0>>}]}

⚠️ {:ok, []} does not mean "no destination" — it means the option was not readable, and it is returned for an option that cannot exist. Measured on the same OTP, a deliberately nonsensical {:raw, 999, 999, 16} also returns {:ok, []}, and on macOS SO_ORIGINAL_DST silently vanishes from the result list while a control option in the same call returns its bytes:

inet:getopts(S, [{raw,6,1,4}, {raw,0,80,16}])   # macOS
%=> {:ok, [{:raw, 6, 1, <<0,0,0,0>>}]}          # ORIGINAL_DST just gone

So absence, invalidity, and "read successfully but empty" are one value. That is why this module returns {:error, reason} for every non-decodable shape and never a partially-trusted result: there is no bit pattern here that earns the benefit of the doubt. Had this been written as {:ok, [{:raw, _, _, dst}]} -> parse(dst) with a catch-all, it would compile, read correctly, and — on any host where the option is unavailable — take the catch-all for every connection while looking like working enforcement.

What is decoded, and what is refused

A sockaddr_in is 16 bytes: family (2), port (2, network order), address (4), padding (8). This decodes only AF_INET (family 2). Anything else is an error rather than a best effort:

a truncated buffer cannot be a destination
AF_INET6 (family 10) in a 16-byte sockaddr_in buffer is a contradiction, not an address to salvage
port 0 is not connectable, so a decode yielding it is a malformed read rather than a destination

⚠️ Each refusal matters because the caller's next move is an allowlist check. A decoder that guessed would hand Policy.permits?/2 a destination the sandbox never asked for, and the verdict — permit or refuse — would be about the wrong address either way.

Summary

Types

destination()

A decoded IPv4 destination.

error()

Why a buffer could not be decoded into a destination.

Functions

decode(buffer)

Decodes a sockaddr_in buffer into {address, port}.

getopt_request()

The :inet.getopts/2 request that reads SO_ORIGINAL_DST.

read(socket)

Reads the pre-redirection destination from an accepted socket.

Types

destination()

@type destination() :: {:inet.ip4_address(), :inet.port_number()}

A decoded IPv4 destination.

error()

@type error() ::
  :unavailable
  | {:truncated, non_neg_integer()}
  | {:unsupported_family, non_neg_integer()}
  | :unconnectable_port

Why a buffer could not be decoded into a destination.

Functions

decode(buffer)

@spec decode(binary()) :: {:ok, destination()} | {:error, error()}

Decodes a sockaddr_in buffer into {address, port}.

Split from read/1 so every rejected shape is testable without a socket, and without needing a host where the redirect exists.

getopt_request()

@spec getopt_request() :: [
  {:raw, non_neg_integer(), non_neg_integer(), non_neg_integer()}
]

The :inet.getopts/2 request that reads SO_ORIGINAL_DST.

Exposed so tests can issue the same request the pool does, rather than restating the three magic numbers and drifting from it.

read(socket)

@spec read(:gen_tcp.socket()) :: {:ok, destination()} | {:error, error()}

Reads the pre-redirection destination from an accepted socket.

Returns {:error, :unavailable} on any host or socket where the option does not come back — which, per the module note, is the same value the OS gives for an option that does not exist. The caller must treat that as "no destination established", never as "no restriction".