Re: [PATCH v6] ip.7: Add "special and reserved addresses" section

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Seth,

On 4/14/23 20:45, Seth David Schoen wrote:
> Break out the discussion of special and reserved IPv4 addresses into
> a subsection, formatted as a pair of definition lists, and briefly
> describing three cases in which Linux no longer treats addresses
> specially, where other systems do or did.
> 
> Also add a specific example to the NOTES paragraph that discourages
> the use of IP broadcasting, so people can more easily understand
> what they are supposed to do instead.
> 
> Signed-off-by: Seth David Schoen <schoen@loyalty.org>
> Suggested-by: John Gilmore <gnu@toad.com>

Patch applied.

Thanks!

Alex

> ---
>  man7/ip.7 | 83 +++++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 75 insertions(+), 8 deletions(-)
> 
> diff --git a/man7/ip.7 b/man7/ip.7
> index 6c50d0281..6f1ee4dbe 100644
> --- a/man7/ip.7
> +++ b/man7/ip.7
> @@ -237,19 +237,82 @@ In particular, this means that you need to call
>  on the number that is assigned to a port.
>  All address/port manipulation
>  functions in the standard library work in network byte order.
> -.PP
> +.SS Special and reserved addresses
>  There are several special addresses:
> -.B INADDR_LOOPBACK
> -(127.0.0.1)
> +.TP
> +.BR INADDR_LOOPBACK " (127.0.0.1)"
>  always refers to the local host via the loopback device;
> +.TP
> +.BR INADDR_ANY " (0.0.0.0)"
> +means any address for socket binding;
> +.TP
> +.BR INADDR_BROADCAST " (255.255.255.255)"
> +has the same effect on
> +.BR bind (2)
> +as
>  .B INADDR_ANY
> -(0.0.0.0)
> -means any address for binding;
> +for historical reasons.
> +A packet addressed to
>  .B INADDR_BROADCAST
> -(255.255.255.255)
> -means any host and has the same effect on bind as
> +through a socket which has
> +.B SO_BROADCAST
> +set will be broadcast to all hosts on the local network segment,
> +as long as the link is broadcast-capable.
> +
> +.TP
> +Highest-numbered address
> +.TQ
> +Lowest-numbered address
> +On any locally-attached non-point-to-point IP subnet
> +with a link type that supports broadcasts,
> +the highest-numbered address
> +(e.g., the .255 address on a subnet with netmask 255.255.255.0)
> +is designated as a broadcast address.
> +It cannot usefully be assigned to an individual interface,
> +and can only be addressed with a socket on which the
> +.B SO_BROADCAST
> +option has been set.
> +Internet standards have historically
> +also reserved the lowest-numbered address
> +(e.g., the .0 address on a subnet with netmask 255.255.255.0)
> +for broadcast, though they call it "obsolete" for this purpose.
> +(Some sources also refer to this as the "network address.")
> +Since Linux 5.14,
> +.\" commit 58fee5fc83658aaacf60246aeab738946a9ba516
> +it is treated as an ordinary unicast address
> +and can be assigned to an interface.
> +
> +.PP
> +Internet standards have traditionally also reserved various addresses
> +for particular uses, though Linux no longer treats
> +some of these specially.
> +
> +.TP
> +[0.0.0.1, 0.255.255.255]
> +.TQ
> +[240.0.0.0, 255.255.255.254]
> +Addresses in these ranges (0/8 and 240/4) are reserved globally.
> +Since Linux 5.3
> +.\" commit 96125bf9985a75db00496dd2bc9249b777d2b19b
> +and Linux 2.6.25,
> +.\" commit 1e637c74b0f84eaca02b914c0b8c6f67276e9697
> +respectively,
> +the 0/8 and 240/4 addresses, other than
>  .B INADDR_ANY
> -for historical reasons.
> +and
> +.BR INADDR_BROADCAST ,
> +are treated as ordinary unicast addresses.
> +Systems that follow the traditional behaviors may not
> +interoperate with these historically reserved addresses.
> +.TP
> +[127.0.0.1, 127.255.255.254]
> +Addresses in this range (127/8) are treated as loopback addresses
> +akin to the standardized local loopback address
> +.B INADDR_LOOPBACK
> +(127.0.0.1);
> +.TP
> +[224.0.0.0, 239.255.255.255]
> +Addresses in this range (224/4) are dedicated to multicast use.
>  .SS Socket options
>  IP supports some protocol-specific socket options that can be set with
>  .BR setsockopt (2)
> @@ -1343,6 +1406,10 @@ with careless broadcasts.
>  For new application protocols
>  it is better to use a multicast group instead of broadcasting.
>  Broadcasting is discouraged.
> +See RFC 6762 for an example of a protocol (mDNS)
> +using the more modern multicast approach
> +to communicating with an open-ended
> +group of hosts on the local network.
>  .PP
>  Some other BSD sockets implementations provide
>  .B IP_RCVDSTADDR

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