[PATCH] Don't treat 0/8 and 240/4 specially in IPv4 classification

[Daniel Suchy]

With respect to (for example) RFC 8212, such features should have 
reverse logic - default behavior should be blocking that, but there 
might be configuration option to change default prefix clasification 
explicitly, if needed for any reason...

In such cases, mind is changing. And it's more secure to have strict 
defaults here...

Your patch doesn't care about security here...

For example - Junos has for these special cases different behavior ( 
routing-options martians x.x.x.x/y allow ). Such way of handling of 
special prefixes should be generally preffered...

- Daniel

On 11/19/22 00:38, Seth David Schoen via Bird-users wrote:
> With the exception of 0.0.0.0 and 255.255.255.255, which have additional
> special meanings, treat 0/8 and 240/4 as normal unicast addresses by
> default. This is because some people are experimenting with using these
> addresses as regular unicast (either for private addresses or for potential
> future public addresses).
> 
> On the public Internet, they would still currently be regarded as bogons and
> one could make (maybe by default) a bogon-filtering rule in bird.conf that
> would not permit these addresses to be routed, e.g. with a pair of static
> routes
> 
> route 0.0.0.0/8 prohibit;
> route 240.0.0.0/4 prohibit;
> 
> or simply
> 
> route 0.0.0.0/8 blackhole;
> route 240.0.0.0/4 blackhole;
> 
> Dave Taht, who wrote a prior version of this patch, suggested that in
> any case it is better to have bogons defined in a configuration file
> than hard-coded in software.
> ---
>   lib/ip.c | 7 +++++--
>   lib/ip.h | 2 +-
>   2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/ip.c b/lib/ip.c
> index 4c5fa47f..e13bbce0 100644
> --- a/lib/ip.c
> +++ b/lib/ip.c
> @@ -87,8 +87,10 @@ ip4_classify(ip4_addr ad)
>   
>     if (b < 0xe0)
>     {
> -    if (b == 0x00)				/* 0.0.0.0/8        This network */
> +    if (a == 0x00000000)			/* 0.0.0.0/32       Unset address */
>         return IADDR_INVALID;
> +						/* 0.0.0.0/8 is otherwise reserved, but
> +						 * some people are using it or trying to */
>   
>       if (b == 0x7f)				/* 127.0.0.0/8      Loopback address */
>         return IADDR_HOST | SCOPE_HOST;
> @@ -107,7 +109,8 @@ ip4_classify(ip4_addr ad)
>     if (a == 0xffffffff)				/* 255.255.255.255  Broadcast address */
>       return IADDR_BROADCAST | SCOPE_LINK;
>   
> -  return IADDR_HOST | SCOPE_SITE;		/* 240.0.0.0/4      Reserved / private */
> +  return IADDR_HOST | SCOPE_UNIVERSE;		/* 240.0.0.0/4      Reserved / private, but
> +						 * some people are using it or trying to */
>   }
>   
>   int
> diff --git a/lib/ip.h b/lib/ip.h
> index 9eef2e16..875b9f5e 100644
> --- a/lib/ip.h
> +++ b/lib/ip.h
> @@ -245,7 +245,7 @@ static inline int ip6_is_v4mapped(ip6_addr a)
>   #define ipa_is_link_local(x) ip6_is_link_local(x)
>   
>   static inline int ip4_is_unicast(ip4_addr a)
> -{ return _I(a) < 0xe0000000; }
> +{ return _I(a) < 0xe0000000 || (_I(a) >= 0xf0000000 && _I(a) != 0xffffffff); }
>   
>   /* XXXX remove */
>   static inline int ipa_classify_net(ip_addr a)