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Re: ncat - UNIX-domain sockets support
From: David Fifield <david () bamsoftware com>
Date: Tue, 27 Nov 2012 13:53:07 -0800

On Mon, Nov 26, 2012 at 10:44:53AM -0500, Tomas Hozza wrote:
----- Original Message -----
I used tempnam() function which uses TMPDIR and TMP, so "/tmp" is
not
hardcoded. It was done so also in the last bunch of patches.

http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/avoid-race.html#TEMPORARY-FILES

There is example of how to use tempnam function to be more safe,
but unfortunately this can not be used in this case. The problem is
that after the temporary name is generated, we are not
opening/creating
any file/socket. It is done later when calling bind() on the source
socket FD
together with the temporary name (so the socket binds to the
path/name).

This is the way I was thinking before too. But netcat-openbsd calls
mkstemp, at least if I understand correctly. Apparently it is no
problem for the file to exist as a regular file before binding to it?

If the file already exists it is a real problem. The bind() call will fail
with errno 98 "Address already in use". Therefore when creating source socket
"file" (when bind() is called) file with the given/generated path MUST NOT exist.

I have made some changes and sommitted your patch. Thank you for
spending time on it. My comments are below.

I see now that netcat-openbsd calls unlink just before bind, so the path
doesn't exist:

int
unix_bind(char *path)
{
        ...

        unlink(path);

        if (bind(s, (struct sockaddr *)&sun, SUN_LEN(&sun)) < 0) {
                close(s);
                return (-1);
        }
        return (s);
}

Another thing I found out after discussion with my colleagues is, that
Ncat's configure script should let user specify the directory for creating
temporary source DGRAM Unix sockets generated by Ncat. Therefore I added
new argument into Ncat configure script called TMP_UNIX_SOCK_DIR. If it is
specified, then configure will define macro in config.h header. Depending on
whether it is specified, tempnam() is compiled with or without specific
directory set:

#ifdef TMP_UNIX_SOCK_DIR 
                if ((tmp_name = tempnam(TMP_UNIX_SOCK_DIR, "ncat.")) == NULL)
#else
                if ((tmp_name = tempnam(NULL, "ncat.")) == NULL)
#endif

Even if TMP_UNIX_SOCK_DIR is specified, tempnam() prefers TMPDIR environment
variable if it is set.

My motivation for this is, that regarding to FSH 2.3 standard 
(http://www.pathname.com/fhs/pub/fhs-2.3.html#VARRUNRUNTIMEVARIABLEDATA)
System programs that maintain transient UNIX-domain sockets must place
them in "/var/run". Therefore we would like to have a way to specify
the directory in compilation time.

This seems like good reasoning. However I have removed this part of the
patch. The name TMP_UNIX_SOCK_DIR hardly seems standard; what do other
packages do in this regard? Also since one of the ideas behind adding
Unix domain support was compatibility with openbsd-netcat, I guess that
creating files in /tmp is fine. Distributors might like to patch the
source code rather than rely on an environment variable. You are welcome
to resubmit this part, but I will ask that you show me some other
package that deals with the same issue.

I modified the code, so Ncat does not call unlink on socket, that
it did not create.

I checked this part of the patch, but I had to remove it. Nsock is not
allowed to look into userdata in this way. Only the calling program is
allowed to read and write userdata.

+#if HAVE_SYS_UN_H
+    if (ss->ss_family == AF_UNIX) {
+      if (type == SOCK_DGRAM && nse->userdata)
+        *(int *)nse->userdata = 1;
+    } else
+#endif

I'd also like to be sure to unlink the socket even if there is an
error.
Would you check if it's possible to replace
        exit(1);
with
        nsock_loop_quit(nsp);
        return;
in the ncat_connect callbacks?

Thank you for this idea. I used it in every handler function, so now
the created socket is deleted if any ERROR occurs.

Thanks for checking this. This change appears to have broken some of the
tests:

FAIL Connect connection refused exit code
     Exit code was 0, not 1 at ./ncat-test.pl line 594.
FAIL Connect connection interrupted exit code
     Exit code was 0, not 1 at ./ncat-test.pl line 622.

I reverted this part of the patch because I'd rather have a new bug
(Unix domain socket failing to clean up) than a regression.

Regarding to this I discovered one Bug in Nsock library, that caused
bad statistic output when client tried to send some data to server,
but the server was not running any more. I this case it caused output
like the following (where you can see "18446744073709551615 bytes sent"):

Thanks, I committed this separately.

David Fifield
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