Python simple UDP server does not receive message with IP options - sockets

Question

I have a simple udp server/client setup where I send a message from the client and print it on the server. This works well for a regular IP packet but the message is not received when I add an IP options header to the packet, even though I can sniff the packet using scapy.
Here's the packet without IP options
###[ Ethernet ]###
dst = 00:04:00:00:04:01
src = 00:aa:00:02:00:04
type = 0x800
###[ IP ]###
version = 4L
ihl = 5L
tos = 0x0
len = 47
id = 1
flags =
frag = 0L
ttl = 61
proto = udp
chksum = 0x62f4
src = 10.0.2.101
dst = 10.0.4.101
\options \
###[ UDP ]###
sport = 10001
dport = 3478
len = 27
chksum = 0x2bd1
###[ Raw ]###
load = 'message from a game'
And here's the packet with IP options header:
###[ Ethernet ]###
dst = 00:04:00:00:04:01
src = 00:aa:00:02:00:04
type = 0x800
###[ IP ]###
version = 4L
ihl = 8L
tos = 0x0
len = 59
id = 1
flags =
frag = 0L
ttl = 61
proto = udp
chksum = 0x5fe8
src = 10.0.2.101
dst = 10.0.4.101
\options \
|###[ IPOption ]###
| copy_flag = 1L
| optclass = control
| option = 31L
| length = 12
| value = '\x00\x01\x00\x00RTGAME'
###[ UDP ]###
sport = 10001
dport = 3478
len = 27
chksum = 0x2bd1
###[ Raw ]###
load = 'message from a game'
And here's the UDP server:
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(('', args.port))
while True:
try:
data, addr = sock.recvfrom(1024)
print("received: %s" % data)
except KeyboardInterrupt:
sock.close()
break
I've been stuck on this for a few days and would love if someone could figure it out.
Thanks

Answer 1

have just been playing and the following works as a self-contained/minimal working example for me with Python 3.7.1 under both OSX and Linux
generating a valid set of IP Options:
from scapy.all import IPOption, raw
ipopts = raw(IPOption(
copy_flag=1, optclass='control', option=31,
value='\x00\x01\x00\x00RTGAME'))
(if you don't have Scapy, the above should generate: b'\x9f\x0c\x00\x01\x00\x00RTGAME')
client code:
import socket
from time import sleep
with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
s.connect(('127.0.0.1', 3478))
s.setsockopt(socket.IPPROTO_IP, socket.IP_OPTIONS, ipopts)
while True:
s.send(b'message from a game')
sleep(1)
server code:
import socket
with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
s.bind(('', 3478))
s.setsockopt(socket.IPPROTO_IP, socket.IP_RECVOPTS, 1)
while True:
print(*s.recvmsg(4096, 1024))
this should result in the "server" displaying lines like:
b'message from a game\n' [(0, 6, b'\x9f\x0c\x00\x01\x00\x00RTGAME')] 0 ('127.0.0.1', 46047)
furthermore, I can watch packets move over the network by running:
sudo tcpdump -i lo0 -vvv -n 'udp and port 3478'
at the command line, or this in Scapy:
sniff(iface='lo0', filter='udp and port 3478', prn=lambda x: x.show())
for some reason I don't actually receive the ancillary data containing the IP Options under OSX, but the data shows up in the packet sniffers.

Answer 2

The problem was due to an incorrect IPv4 checksum. I failed to mention in the question that I'm running this in a mininet environment with custom switches. The IP options get added in transit by a switch, but the checksum wasn't updated. Once I fixed that, the packet made it to the server.
Thanks for the help and pointers everyone!