IP_ADD_MEMBERSHIP fails when set both on interface and its subinterface; is that expected? - sockets

Question

I'm debugging a 3rd-party network application and trying to figure out why it reports errors when calling setsockopt with IP_ADD_MEMBERSHIP to set up a multicast group. The application is in C++, but I've written an MWE in python that replicates the same syscalls:
import socket
import struct
ETH0_IP = "192.168.88.85"
ETH0_1_IP = "192.168.88.254"
MULTICAST_IP = "224.0.0.7"
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
ip = socket.inet_aton(ETH0_IP)
s.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_IF, ip)
group = struct.pack("4s4s", socket.inet_aton(MULTICAST_IP), ip)
s.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, group)
# s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
# s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
s2 = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
ip2 = socket.inet_aton(ETH0_1_IP)
s2.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_IF, ip2)
group2 = struct.pack("4s4s", socket.inet_aton(MULTICAST_IP), ip2)
# the second group is added to the first socket so that we can only bind to one socket and read data from it
s.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, group2)
At the second IP_ADD_MEMBERSHIP call I get error OSError: [Errno 98] Address already in use.
I found out this only happens when ETH0_1_IP is a subinterface of ETH_0_IP. And I'm not sure if this is expected. If it is, is there a way to actually detect this situation and discard subinterfaces of already bound interfaces? Further, would my multicast socket receive data sent to the subinterface if registration for it fails with the above error?
For the sake of completeness:
$ cat /etc/network/interfaces
auto lo
iface lo inet loopback
iface lo inet6 loopback
auto eth0:1
iface eth0:1 inet static
address 192.168.88.254
netmask 255.255.240.0

Answer 1

Linux is tracking your alias interface as the same interface and so rejecting the attempt to re-use the interface.
In a bit more detail, I have run your code successfully on CentOS 7 using two separate physical interfaces with no changes. If I then change the code to use an alias on the same physical address, it fails with the same error that you see.
Digging a little further, I see that if I dump the interface indeces (using SIOCGIFINDEX) for the physical adaptor and the alias, they do indeed have the same index.
If you want to use Python to check this for yourself, have a quick look at https://gist.github.com/firaxis/0e538c8e5f81eaa55748acc5e679a36e for some code (missing imports of ctypes and socket) and then try something like this:
print(Interface(name="eth0").index)
print(Interface(name="eth0:1").index)