Re: nfnetlink: This library is not meant as a public API for application developers.

[Duncan Roe <duncan_roe@optusnet.com.au>]

Hi Ale, and sorry for very late reply

On Thu, Feb 13, 2020 at 12:27:41PM +0100, Alessandro Vesely wrote:
> Has that disclaimer always been in libnfnetlink home page[*]?
>
> It is the first time I see it.
>
> I have a userspace filter[???] working with it, and it currently works well.
>
> If I remove -lnfnetlink from the link command, I get just one undefined
> reference to symbol 'nfnl_rcvbufsiz'.  It is used only if there is a command
> line option to set the buffer size to a given size, to avoid enobufs.  For the
> rest, the daemon uses libnetfilter_queue.
>
> Should I rewrite that?  How?
>
>
> Best
> Ale
> --

Yes you can code to avoid using nfnl_rcvbufsiz() from libnfnetlink.

Thre is no libmnl or libnetfilter_queue function to do it at present, but
libmnl/examples/netfilter/nfct-daemon.c has the code.
In case you haven't git cloned libmnl, here is a summary:

> socklen_t buffersize; // Set by your command-line option
Your code likely already has:
> struct mnl_socket *nl;
> nl = mnl_socket_open(NETLINK_NETFILTER);
(after mnl_socket_bind)
> setsockopt(mnl_socket_get_fd(nl), SOL_SOCKET, SO_RCVBUFFORCE, // You should
>   &buffersize, sizeof(socklen_t));     // check the return code (not shown)
If you like, you can check how big a buffer the kernel gave you
> socklen_t socklen = sizeof buffersize;
> socklen_t read_size = 0;
> getsockopt(mnl_socket_get_fd(nl), SOL_SOCKET, SO_RCVBUF, &read_size, &socklen);
From testing it seems you get a buffer of twice buffersize bytes.

HTH

Cheers ... Duncan.