Re: ulogd packet based logging with CT info

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Aug 18, 2021 at 10:06:40PM +0200, Blažej Krajňák wrote:
> I'm really confused from searching a bug.
> 
> Getting nf_conntrack via nflog_nlmsg_parse(ph, attrs); is (I think)
> bad because ph parameter must be nlmsghdr not nfulnl_msg_packet_hdr

Right, nflog_nlmsg_parse() should take the nlh parameter.

> So different way. I added new getters to libnetfilter_log.c:
> 
> struct nf_conntrack *nflog_get_ct(struct nflog_data *nfad)
> {
>     return nfnl_get_pointer_to_data(nfad->nfa, NFULA_CT, struct nf_conntrack);
> }

This will not work (as you noticed). The kernel does not store a
struct in the NFULA_CT attribute.

Better to stick to use nflog_nlmsg_parser(), my suggestion is:

#1 msg_cb() provides struct nfgenmsg *nfmsg, you could retrieve the nlmsg
   from there since the nlmsghdr comes before nfgenmsg:

        struct nlmsghdr *nlh;

        nlh = (struct nlmsghdr *)((void *)nfg - sizeof(*nlh));

        err = nflog_nlmsg_parse(nlh, attrs);
        if (err < 0)
                ... error path

#2 once you have access to attrs[NFULA_CT], from there on:

        struct nf_conntrack *ct;

        ct = nfct_new();
        if (!ct)
                ... error path

        err = nfct_nlmsg_parse(nlh, ct);
        if (err < 0)
                ... error path

Then, you get the pointer to conntrack object.