netdev - Re: Documentation for IP

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Date:	Wed, 13 Oct 2010 07:16:42 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Balazs Scheidler <bazsi@...abit.hu>
Cc:	KOVACS Krisztian <hidden@....bme.hu>,
	David Miller <davem@...emloft.net>, linux-net@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: Documentation for IP_TRANSPARENT?

Hello Bazsi

A long time after your mail, I've now added the text you supplied
below. The changes are now available via git, and will be in
man-pages-3.29. However, I have a few small questions that you can
maybe clarify. See below.

On Tue, Dec 9, 2008 at 2:17 PM, Balazs Scheidler <bazsi@...abit.hu> wrote:
> Hi,
>
> On Mon, 2008-11-24 at 11:06 -0500, Michael Kerrisk wrote:
>> Hello Krisztian,
>>
>> Your 2.6.27 patch adds  the IP_TRANSPARENT socket option.  Could you
>> please supply some documentation (plain text is fine) suitable for
>> inclusion in the ip(7) man page.
>
> Although it was Krisztian who originally submitted the patches, but I
> also did some things about them, so here I gave the documentation part a
> spin.
>
> There are some other related options, that are undocumented right now,
> so I tried to give documentation on those as well.
>
> IP_FREEBIND
>        Enable binding to IP addresses that do not currently exist. When
>        enabled on a TCP or UDP socket, the bind(2) call referencing a
>        non-existing IP address will succeed. This functionality is useful
>        if the specified IP address is dynamic (e.g. assigned to a PPP device)
>        and is down at the time when the application is trying to bind to it.
>        This setsockopt is the per-socket equivalent to ip_nonlocal_bind sysctl.
>
> IP_TRANSPARENT
>        Enable transparent proxying on this socket. This socket option allows
>        the calling application to bind to a non-local IP address and operate
>        both as a client and a server with the foreign address as the local
>        endpoint. NOTE: this requires that routing be set up in a way that
>        packets going to the foreign address are routed through the tproxy box.

Can you say a few words explaining what is meant by "tproxy box".

Also, should one write "TProxy" here, analogous with "TProxy" below?

>        Enabling this socket option requires superuser privileges
>        (more specifically the CAP_NET_ADMIN capability).
>
>        TProxy redirection with the iptables TPROXY target also requires that
>        this option be set on the redirected socket.

In iptables(8), I find no mention of TPROXY. Is it just that the
iptables(8) man page is not up to date?

> IP_RECVORIGDSTADDR
>        Enables the IP_ORIGDSTADDR ancillary message in recvmsg(2) in which
>        the kernel returns the original destination address of a datagram. The
>        ancillary message contains a "struct sockaddr_in" instance.

Do you happen to know of any userspace test code that demonstrates the
use of IP_RECVORIGDSTADDR?

Thanks for this documentation, and sorry it took so long to make its
way into man-pages.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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