netdev - Re: [net-next RFC 0/4] SO

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Date:	Mon, 7 Mar 2016 09:49:52 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Gilberto Bertin <gilberto.bertin@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next RFC 0/4] SO_BINDTOSUBNET

On Mon, Mar 7, 2016 at 9:22 AM, Gilberto Bertin
<gilberto.bertin@...il.com> wrote:
>
>> On 24 Feb 2016, at 05:06, Tom Herbert <tom@...bertland.com> wrote:
>>
>> On Tue, Feb 23, 2016 at 7:27 AM, Gilberto Bertin
>> <gilberto.bertin@...il.com> wrote:
>>> This series introduces support for the SO_BINDTOSUBNET socket option, which
>>> allows a listener socket to bind to a subnet instead of * or a single address.
>>>
>>> Motivation:
>>> consider a set of servers, each one with thousands and thousands of IP
>>> addresses. Since assigning /32 or /128 IP individual addresses would be
>>> inefficient, one solution can be assigning subnets using local routes
>>> (with 'ip route add local').
>>>
>> Hi Gilberto,
>>
>> The concept is certainly relevant, but allowing binds by subnet seems
>> arbitrary. I can imagine that someone might want to bind to a list of
>> addresses, list of interfaces, list of subnets, or complex
>> combinations like a subnet on one interface, and list of addresses on
>> another. So I wonder if this is another use case for a BPF program on
>> a listener socket, like a program for a scoring function. Maybe this
>> could even combined with  BPF SO_REUSERPORT somehow?
>>
>> Tom
>
> Hi Tom,
>
> I have a working POC of the patch that adds support for BPF into the
> compute_score function, and I would like to share some thoughts about
> advantages and disadvantages of both solutions.
>
Cool, thanks for implementing that!

> First, setup.
>
> SO_BINDTOSUBET:
> - add this to some_server.c:
>
>        subnet.net = addr.s_addr;
>        subnet.plen = 24
>        setsockopt(sock, SOL_SOCKET, SO_BINDTOSUBNET, &subnet, sizeof(subnet));
>
> and you are done. Your server will accept all connections from the
> specified subnet.
>
> BPF_LISTENER_FILTER:
> - write a bpf filter like this:
>
>         SEC("socket_bpf")
>         int bpf_prog1(struct __sk_buff *skb)
>         {
>               unsigned int daddr;
>               daddr = load_word(skb, ETH_HLEN + offsetof(struct iphdr, daddr));
>
>               if (/* daddr matches subnet */) {
>                       return -1; //accept
>               }
>
>               return 0; // reject
>         }
>
> - compile it:
>         $ clang -target bpf -c -o socket_bpf.o socket_bpf.c
>
> - add this to your server.c:
>         bpf_load_file("/path/to/socket_bpf.o");
>         setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, prog_fd, sizeof(prog_fd[0]));
>
> - link your server with a couple of libbpf libraries (I'm
>  using the kernel ones from samples/bpf) and -lelf
>
> And this is still simplified (since instead of hardcoding the subnet
> into the bpf filter it would be preferable to use maps).
>
>
> thoughts:
> - SO_BINDTOSUBNET is much simpler to configure than BPF
> - BPF requires some external C libraries and I think it would not be
>  trivial to get it working with other languages than C/C++.

Yes, but the direction seems to be to this type of potentially open
ended socket level filtering is done via BPF. The SO_REUSEPORT BPF
patches really demonstrates the potential.

>  As an example, I have two working servers for SO_BINDTOSUBNET written
>  in Ruby and Go (since both these languages expose setsockopt), but it
>  would be necessary to write something that wrap the C libbpf to use
>  BPF
> - I (personally) do not think SO_BINDTOSUBNET is that much arbitrary, I
>  see it more as the logical missing piece between * and a single
>  address when calling bind() (otherwise I think we should consider
>  arbitrary even SO_BINDTODEVICE)
>
Yes SO_BINDTODEVICE is arbitrary. It seems like we could just as
easily have BINDTODEVICES. Or, as I said SO_BINDTOADDRESSES also makes
perfect sense.

> That said, do you believe it could be an option to maybe have both these
> options? I think that the ability to run BPF in the listening path is
> really interesting, but it's probably an overkill for the bind-to-subnet
> use case.
>

Maybe. It will be quite common server configuration with IPv6 to
assign each server its own /64 prefix(es). From that POV I suppose
there is some value in having SO_BINDTOSUBNET.

Tom

> Thank you,
>  gilberto
>