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@xxxxxxxxxxx> > Suggested-by: John Gilmore <gnu@xxxxxxxx> 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 -- <http://www.alejandro-colomar.es/> GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
Attachment:
OpenPGP_signature
Description: OpenPGP digital signature
