Hi Duncan,
thank you very much for your tips!
On Sun 12/Apr/2020 10:21:53 +0200 Duncan Roe wrote:
> 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?
>>
>
> 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)
I don't have mnl_socket_open(). I have struct nfq_handle *h = nfq_open(); and
then fd = nfq_fd(h);
After replacing the call to nfnl_rcvbufsiz() with setsockopt(), I can actually
link without -lnfnetlink. However, I'm not sure it is sane to fiddle with
configure macros trying to avoid it. On my system I have:
ale@pcale:~$ pkg-config --libs libnetfilter_queue
-lnetfilter_queue -lnfnetlink
ale@pcale:~$ pkg-config --modversion libnetfilter_queue
1.0.2
Should a future version drop that dependency, my code is ready :-)
>> 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.
It's stranger than that. The default value is 0x34000. If I set that same
value or higher, I seem to always get 0x68000. However, if I set 0x33fff I get
0x67ffe, the double, as you say. This strange behavior apparently was the same
when using nfnl_rcvbufsiz().
Best
Ale
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