Re: [PLUG] Weird Bash Question

[Jeff Abrahamson <jeff@purple.com>]

On Thu, Mar 04, 2004 at 11:02:58AM -0500, gabriel rosenkoetter wrote:
>   [29 lines, 198 words, 1094 characters]  Top characters: teonaisr
> 
> On Thu, Mar 04, 2004 at 10:40:13AM -0500, Martin DiViaio wrote:
> > Anyone know of an easy way to find an open, non-priviledged TCP port in 
> > bash?
> > 
> > I could troll through the output from netstat and then pick a port that 
> > isn't listed, but I'm hoping there is an easier solution.
> 
> I don't think there is, since bash doesn't (or, at least, I sure
> HOPE it doesn't) have any networking code internal to it. netstat or
> lsof would be your best bets.

It does if you compile with --enable-net-redirections.  The bash man
page doesn't mention that this is a compile-time option, but here's
what it says in bash(1):

    Bash handles several filenames specially when they are used in redirec-
    tions, as described in the following table:

           /dev/fd/fd
                  If  fd  is  a valid integer, file descriptor fd is dupli-
                  cated.
           /dev/stdin
                  File descriptor 0 is duplicated.
           /dev/stdout
                  File descriptor 1 is duplicated.
           /dev/stderr
                  File descriptor 2 is duplicated.
           /dev/tcp/host/port
                  If host is a valid hostname or Internet address, and port
                  is  an integer port number or service name, bash attempts
                  to open a TCP connection to the corresponding socket.
           /dev/udp/host/port
                  If host is a valid hostname or Internet address, and port
                  is  an integer port number or service name, bash attempts
                  to open a UDP connection to the corresponding socket.

    A failure to open or create a file causes the redirection to fail.

On the other hand, some degree of decorum is preserved for Debian
users.  In /usr/share/doc/bash/README.Debian.gz, we can read many
interesting items, including this:

    9. Why is bash configured with --disable-net-redirections?

        It can produce completely unexpected results. This kind of
        feature should not be part of a shell but a special. tool. And
        that tool has existed for years already, it's called netcat.


> Why do you want to do this, though? Binding to "0" gets you an
> unused socket which you then own, meaning there's no chance for a
> race condition between when you get the number and start using the
> socket.

Hmm, that's cool, I didn't know that.  Is that a bash-ism, a
linux-ism, or can I really say this:

    sock = socket(AF_INET, SOCK_DGRAM, 0);
    memset(&saddr, 0, sizeof(saddr));
    saddr.sin_family = AF_INET;
    saddr.sin_addr.s_addr = htonl(INADDR_ANY);
    saddr.sin_port = 0;      /* Gabe said I could do this */
    ret = bind(sock, (struct sockaddr *)&saddr, sizeof(saddr));

and expect it to work on most unices?


> What problem are you trying to solve?

He's trying to waste my time, because he knows I've been doing socket
programming this week and am easily distracted by socket questions!

;-)

-- 
 Jeff

 Jeff Abrahamson  <http://www.purple.com/jeff/>
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