Subject: Re: [RFD] First draft of RDNSS-in-RA support for IPv6 DNS autoconfiguration

From: Michael Buesch <[email-masked]>
Date: Sat, 23 Jun 2007 01:39:49 +0200

On Saturday 23 June 2007 01:26:19 C. Scott Ananian wrote:
> Attached is my first draft of a patch to implement RDNSS-in-Router
> Advertisements support for IPv6 (
> http://tools.ietf.org/html/draft-jeong-dnsop-ipv6-dns-discovery-12 )
> as implemented in radvd ( http://www.litech.org/radvd/ ).  It
> currently exports the autoconfigured DNS list as /proc/net/ipv6_dns --
> ultimately it ought to (a) implement inotify on this file, so that
> glibc could use it like /etc/resolv.conf and get notifications when
> the DNS list changes, and (b) export the DNS list via netlink as well.
> 
> Comments & discussion, please!
>  --scott

> diff -ruHpN -X dontdiff linux-2.6.22-rc5-orig/include/net/ip6_rdnss.h
> linux-2.6.22-rc5/include/net/ip6_rdnss.h
> --- linux-2.6.22-rc5-orig/include/net/ip6_rdnss.h       1969-12-31
> 19:00:00.000000000 -0500
> +++ linux-2.6.22-rc5/include/net/ip6_rdnss.h    2007-06-21
> 18:16:33.000000000 -0400 @@ -0,0 +1,58 @@
> +#ifndef _NET_IP6_RDNSS_H
> +#define _NET_IP6_RDNSS_H
> +
> +#ifdef __KERNEL__
> +
> +#include <linux/in6.h>
> +
> +struct nd_opt_rdnss {
> +       __u8                    type;
> +       __u8                    length;
> +#if defined(__BIG_ENDIAN_BITFIELD)
> +       __u8                    priority:4,
> +                               open:1,
> +                               reserved1:3;
> +#elif defined(__LITTLE_ENDIAN_BITFIELD)
> +       __u8                    reserved1:3,
> +                               open:1,
> +                               priority:4;
> +#else
> +# error not little or big endian
> +#endif

That is not endianess-safe. Don't use foo:x at all
for stuff where a specific endianess is needed. The
compiler doesn't make any guarantee about it.

Please do
__u8 flags;
#define FOOBAR_RESERVED	0x07
#define FOOBAR_OPEN	0x08
#define FOOBAR_PRIORITY	0xF0
....
and use them in the code.

In general I try to avoid the foo:x stuff, as it
has little or no gain. It just generates worse code.

-- 
Greetings Michael.