Re: Support NAT-ed expect entries from user space

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

BORBELY Zoltan wrote:
> Hi,
> 
> I'm developing a transparent user space ftp proxy, and I'm using expect
> entries to accept incoming data connections. It works quite well, but I
> had to patch the kernel to get it work. It's quite small, so I put it
> inline:

Just a wild thought. What if you create a new conntrack (instead of an
expectation) using a small timeout in state TCP SYN_SENT that represents
the new flow that it is expected to arrive? The first packet would seen
as a resent by the connection tracking so it would accept it. However,
the TCP sequence tracking may complain about it. Nevertheles, we can
also access to the internal TCP flags to enable   IP_CT_TCP_FLAG_BE_LIBERAL.

> --- nf_conntrack_netlink-orig-2.6.22.9.c        2007-09-26 20:03:01.000000000 +0
> +++ nf_conntrack_netlink-new-2.6.22.9.c 2008-06-13 17:14:17.000000000 +0200
> @@ -39,6 +39,7 @@
>  #ifdef CONFIG_NF_NAT_NEEDED
>  #include <net/netfilter/nf_nat_core.h>
>  #include <net/netfilter/nf_nat_protocol.h>
> +#include <net/netfilter/nf_nat_helper.h>
>  #endif
> 
>  #include <linux/netfilter/nfnetlink.h>
> @@ -1439,7 +1440,11 @@
>                 goto out;
>         }
> 
> -       exp->expectfn = NULL;
> +       exp->expectfn = nf_nat_follow_master;
> +#ifdef CONFIG_NF_NAT_NEEDED
> +       exp->saved_proto.tcp.port = tuple.dst.u.tcp.port;       //!!!FIXME
> +       exp->dir = IP_CT_DIR_ORIGINAL;
> +#endif
>         exp->flags = 0;
>         exp->master = ct;
>         exp->helper = NULL;
> 
> It's ugly as hell in the current form, but I have some ideas how to
> improve it to integrate it into the main kernel tree. Maybe we can add a
> new CTA_EXPECT_SAVED_PROTO attribute and get it from user space. What
> is your opinion?

Would this change be generic enough for userspace transparent proxies or
only for FTP?

-- 
"Los honestos son inadaptados sociales" -- Les Luthiers