IPv6 destination options header

Question

I'm working on a software-defined networking research project, and what I need is to make a simple UDP server that puts a data tag into the destination options field (IPv6) of the UDP packet. I was expecting to either the sendmsg() recvmsg() commands, or by using setsockopt() and getsockopt(). So, Python 2.7 doesn't have sendmsg() or recvmsg(), and while I can get setsockopt() to correctly load a tag into the packet (I see it in Wireshark), the getsockopt() command just returns a zero, even if the header is there.

#Python 2.7 client
#This code does put the dest opts header onto the packet correctly
#dst_header is a packed binary string (construction details irrelevant--
#  it appears correctly formatted and parsed in Wireshark)

addr = ("::", 5000, 0, 0)
s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_DSTOPTS, dst_header)
s.sendto('This is my message ', addr)

#Python 2.7 server
addr = ("::", 5000, 0, 0)
s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_RECVDSTOPTS, 1)
s.bind(addr)
data, remote_address = s.recvfrom(MAX)
header_data = s.getsockopt(socket.IPPROTO_IPV6, socket.IPPROTO_DSTOPTS, 1024)

I also tried this in Python 3.4, which does have sendmsg() and recvmsg(), but I just get an error message of "OSError: [Errno 22]: Invalid argument", even though I'm passing it (apparently) correct types:

s.sendmsg(["This is my message"], (socket.IPPROTO_IPV6, socket.IPV6_DSTOPTS, dst_header), 0, addr)  #dst_header is same string as for 2.7 version

It looks like 99% of the usage of sendmsg() and recvmsg() is for passing UNIX file descriptors, which isn't what I want to do. Anybody got any ideas? I thought this would be just a four or five line nothing-special program, but I'm stumped.

Answer 1

OK, I'm going to partially answer my own question here, on the off chance that a search engine will bring somebody here with the same issues as I had.

I got the Python 3.4 code working. The problem was not the header, it was the message body. Specifically, both the message body and the header options value fields must be bytes (or bytearray) objects, stored in an iterable container (here, a list). By passing it ["This is my message"] I was sending in a string, not a bytes object; Python let it go, but the OS couldn't cope with that.

You might say I was "byted" by the changes in the handling of strings in Python 3.X...