Compatibility Issue with `socket` Module and OpenBSD Multicast Options

[emphyrioio]

Bug report

Bug description:

Dear Python Developers,

I am writing to report a compatibility issue with the socket module in Python 3.11.10 when running on OpenBSD 7.6. Specifically, the issue occurs with the handling of multicast socket options, such as IP_MULTICAST_TTL and IP_MULTICAST_LOOP.

---

Problem Description

On OpenBSD, the setsockopt system call expects specific types for certain multicast socket options:

• IP_MULTICAST_TTL and IP_MULTICAST_LOOP expect an unsigned 8-bit integer (u_char), with a length of 1 byte.
• IP_ADD_MEMBERSHIP expects a structure consisting of two IPv4 addresses, typically represented as struct { 4s, 4s }.

These expectations are defined in the OpenBSD source code, particularly in the file sys/netinet/in.h.

In contrast, Python's socket module passes these options using a 4-byte integer by default. This discrepancy causes the following error when attempting to use these options on OpenBSD:

socket.error: [Errno 22] Invalid argument

---

Steps to Reproduce

1. Run the following code on OpenBSD 7.6:

   import socket
   
   MCAST_GRP = "224.0.0.251"
   MCAST_PORT = 5353
   
   sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
   sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 1)  # This will raise an error

2. This will raise the error:

   socket.error: [Errno 22] Invalid argument
   

---

Expected Behavior

The setsockopt call should successfully set the IP_MULTICAST_TTL option with the provided value.

---

Root Cause

The issue arises because Python's socket module passes a 4-byte integer for the IP_MULTICAST_TTL and IP_MULTICAST_LOOP options, which is incompatible with OpenBSD's requirements.

On OpenBSD:

• IP_MULTICAST_TTL and IP_MULTICAST_LOOP require a u_char type (1 byte).
• The length of the option value is validated against this type in the OpenBSD kernel.

This discrepancy leads to the EINVAL error when setsockopt is called with these options.

---

Suggested Solution

The socket module should adapt to use the correct type (u_char) for these options when running on OpenBSD. This could involve special-casing these options to ensure the expected type and length are passed.

For example, the following workaround resolves the issue:

from ctypes import CDLL, c_ubyte, c_int, byref
import socket

libc = CDLL("libc.so")

def set_multicast_ttl(sock, ttl):
    u_char_ttl = c_ubyte(ttl)
    libc.setsockopt(
        sock.fileno(),
        socket.IPPROTO_IP,
        socket.IP_MULTICAST_TTL,
        byref(u_char_ttl),
        c_int(1)
    )

This workaround ensures that the correct type and size are passed to the setsockopt system call on OpenBSD.

---

Request for Upstream Fix

I believe this issue should be resolved upstream in the Python socket module. The current behavior prevents multicast functionality from working out of the box on OpenBSD and requires platform-specific workarounds.

I kindly request that the development team:

1. Investigate this issue further.
2. Adapt the socket module to handle platform-specific requirements for these multicast options.

This change should ensure compatibility with OpenBSD while maintaining backward compatibility on other platforms.

---

Thank you for your attention to this matter.

Best regards.

CPython versions tested on:

3.11

Operating systems tested on:

Other

[ZeroIntensity]

Would you like to submit a PR?

[vadmium]

There is already Issue #67316 specifically about the TTL and LOOP options.

The documentation currently says

socket.setsockopt(level, optname, value: int)
socket.setsockopt(level, optname, value: buffer)

The value can be an integer, . . . or a bytes-like object representing a buffer. In the later case it is up to the caller to ensure that the bytestring contains the proper bits.

So on one hand, you should be able to pass your TTL, LOOP and membership values as byte strings, e.g. b'\x01' or by encoding with struct.pack. That may be more desirable than using ctypes.

On the other hand, one could read the documentation I quoted as implying some mechanism already exists to ensure “the proper bits” are passed to the OS when setting an option to a Python integer value.

Peering into the implementation, there is already precedent for special-casing integer encoding with Linux AF_VSOCK sockets (commit effc12f). And commit b5aa271 emulates setting Linux’s TCP_QUICKACK on Windows, without any underlying setsockopt calls. The documentation should be clearer how a Python integer is handled (normally passed as a C int; what exceptions are implemented).

It looks like people are already calling setsockopt(IPPROTO_IP, IP_MULTICAST_TTL, 1), which should work on some platforms, like Linux. Where it already works on one platform, it might be reasonable to add special cases for compatibility with other platforms, like Open/Free BSD. But I also wonder if setsockopt should stay low-level only without special cases, and a higher-level function/method (supplied by Python or the user) should be responsible for deciding the correct platform- and option-dependent arguments.

[ZeroIntensity]

IMO, I would rather see a PR so we can actually discuss the implementation and precedence instead of speculating (which is why I suggested that the author send one). Philosophical questions about what to do are generally a dead end. If it's no good, we can always close it.

[rruuaanng]

cpython/Modules/socketmodule.c

Lines 3146 to 3167 in db5c576

	int level;
	int optname;
	int res;
	Py_buffer optval;
	int flag;
	unsigned int optlen;
	PyObject *none;
	#ifdef AF_VSOCK
	if (s->sock_family == AF_VSOCK) {
	uint64_t vflag; // Must be set width of 64 bits
	/* setsockopt(level, opt, flag) */
	if (PyArg_ParseTuple(args, "iiK:setsockopt",
	&level, &optname, &vflag)) {
	// level should always be set to AF_VSOCK
	res = setsockopt(s->sock_fd, level, optname,
	(void*)&vflag, sizeof vflag);
	goto done;
	}
	return NULL;
	}
	#endif

It seems that the problem is here, int is interpreted as 4-bytes on your platform. Maybe you should fix it :)

Edit

I would actually like to fix this, but I'm working on my existing PR. You can try the solution I proposed.

[mtelka]

Please note that both Solaris and illumos are affected similarly too.