From: Reid Thompson <Reid.Thompson@...>
Date: 2007-06-15T02:22:21+09:00
Subject: Re: How can I get the local machine's netmask programmatically?

On Fri, 2007-06-15 at 01:48 +0900, list.rb@gmail.com wrote:
> I spent quite a bit of time last night trying to figure this out to no
> avail.
> 
> Reid, how long did it take you to come up with this? Could anyone
> without C experience figured this out?
> 
> >
> > see if this works....????
> >
> > rthompso@shienar ~ $ cat getnetmask.rb
> > require 'rubygems'
> > require "inline"
> >
> > class NetMask
> >   inline do |builder|
> >     builder.include '<sys/types.h>'
> >     builder.include '<sys/socket.h>'
> >     builder.include '<sys/ioctl.h>'
> >     builder.include '<netinet/in.h>'
> >     builder.include '<net/if.h>'
> >     builder.c "
> >
> >     char * nmask() {
> >         int fd;
> >         struct ifreq ifr;
> >
> >         fd = socket(AF_INET, SOCK_DGRAM, 0);
> >         ifr.ifr_addr.sa_family = AF_INET;
> >         strncpy(ifr.ifr_name, \"eth0\", IFNAMSIZ-1);
> >         ioctl(fd, SIOCGIFNETMASK, &ifr);
> >         close(fd);
> >
> >         return (char *)inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr);
> >   }"
> >   end
> > end
> >
> > nm = NetMask.new()
> > nmaa = nm.nmask()
> > puts nmaa
> >
> > rthompso@shienar ~ $ ruby getnetmask.rb
> > 255.255.255.0
> 

1) I was previously aware of RubyInline  -- so didn't have to
'find/research' it

2) I knew that getting the info in C code was doable
3) I googled for examples of getting socket info
4) it took me about 15 minutes probably

4a) I think a non-C programmer could have figured it out in time
I basically copied the C RubyInline example ( the factorial one )
and pasted in the socket code.

The gotcha's would probably have been
    a)figuring out to use the builder.include, described in the C++
example, to get the header files included
    b)getting rid of a warning message due to the original C code not
casting to (char *) the return value of inet_ntoa()

5) Ruby's various socket classes probably provide access to the same
info - but I couldn't find out how to get at it ( like you, I spent
waaaay more time trying to find a pure ruby way of getting the info than
coding the example )  i.e. Ruby's Socket class has a getsockname which
returns the struct sockaddr packed into a string -- It may have all the
info needed, but I'm not familiar enough with unpacking a sockaddr
structure to try to figure it out ( or unpacking anything in Ruby for
that matter :) ).