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socket.setsockopt() sets an optlen of '4' in the setsockopt system call
for options IP_MULTICAST_TTL and IP_MULTICAST_LOOP. On OpenBSD, this
causes the kernel to hit an error condition and set errno 22 when these
options are set:
socket.error: (22, 'Invalid argument')
Yielded by e.g. socket.setsockopt(socket.IPPROTO_IP,
socket.IP_MULTICAST_TTL, 1)
According to both FreeBSD and OpenBSD manual pages (see e.g.
http://www.freebsd.org/cgi/man.cgi?query=ip&apropos=0&sektion=0&manpath=FreeBSD+7.0-RELEASE&format=html),
these fields are in fact both 8 bit unsigned, and the manuals suggest
the following:
u_char ttl; /* range: 0 to 255, default = 1 */ 
 
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
While '4' (sizeof int) is correct for many options, special casing is
needed in Modules/socketmodule.c. The following diff fixes the issue
for me on OpenBSD:
 @@ -1716,12 +1719,8 @@ sock_setsockopt(PySocketSockObject *s, PyObject
*args)
 
 if (PyArg_ParseTuple(args, "iii:setsockopt",
 &level, &optname, &flag)) {
- buflen = sizeof flag;
- /* Multi cast options take shorter arguments */
- if (optname == IP_MULTICAST_TTL
- || optname == IP_MULTICAST_LOOP)
- buflen = sizeof(u_char);
 buf = (char *) &flag;
+ buflen = sizeof flag;
 }
 else {
 PyErr_Clear();

This also appears to be a bug in Python 2.4.

This is already fixed in release24-maint, release25-maint, trunk and py3k.

This is still an issue as of OpenBSD 5.6. Here is an updated patch for the latest 2.7 branch:
diff -r 88de50c1696b Modules/socketmodule.c
--- a/Modules/socketmodule.c Sun Dec 28 18:51:25 2014 +0200
+++ b/Modules/socketmodule.c Sun Dec 28 21:24:41 2014 -0500
@@ -1881,24 +1881,29 @@
 {
 int level;
 int optname;
 int res;
 char *buf;
 int buflen;
 int flag;
 
 if (PyArg_ParseTuple(args, "iii:setsockopt",
 &level, &optname, &flag)) {
 buf = (char *) &flag;
 buflen = sizeof flag;
+ /* Multi cast options take shorter arguments */
+ if (optname == IP_MULTICAST_TTL
+ || optname == IP_MULTICAST_LOOP)
+ buflen = sizeof(u_char);
+ buf = (char *) &flag;
 }
 else {
 PyErr_Clear();
 if (!PyArg_ParseTuple(args, "iis#:setsockopt",
 &level, &optname, &buf, &buflen))
 return NULL;
 }
 res = setsockopt(s->sock_fd, level, optname, (void *)buf, buflen);
 if (res < 0)
 return s->errorhandler();
 Py_INCREF(Py_None);
 return Py_None;