Add DNS support

On 04 Oct 2008 Pieter posted a dns implementation for U-Boot.

http://www.mail-archive.com/u-boot-users@lists.sourceforge.net/msg10216.html
>
> DNS can be enabled by setting CFG_CMD_DNS. After performing a query,
> the serverip environment var is updated.
>
> Probably there are some cosmetic issues with the patch. Unfortunatly I
> do not have the time to correct these. So if anybody else likes DNS
> support in U-Boot and has the time, feel free to patch it in the main tree.

Here it is again - slightly modified & smaller:
  - update to 2009-06 (Pieter's patch was for U-Boot 1.2.0)
  - README.dns is added
  - syntax is changed (now takes a third option, the env var to store
    the result in)
  - add a random port() function in net.c
  - sort Makefile in ./net/Makefile
  - dns just returns unless a env var is given
  - run through checkpatch, and clean up style issues
  - remove packet from stack
  - cleaned up some comments
  - failure returns much faster (if server responds, don't wait for
    timeout)
  - use built in functions (memcpy) rather than byte copy.

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Signed-off-by: Pieter Voorthuijsen <pieter.voorthuijsen@prodrive.nl>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>

common/cmd_net.c

@@ -353,3 +353,52 @@ U_BOOT_CMD(
 	"[NTP server IP]\n"
 );
 #endif
+
+#if defined(CONFIG_CMD_DNS)
+int do_dns(cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
+{
+	if (argc == 1) {
+		cmd_usage(cmdtp);
+		return -1;
+	}
+
+	/*
+	 * We should check for a valid hostname:
+	 * - Each label must be between 1 and 63 characters long
+	 * - the entire hostname has a maximum of 255 characters
+	 * - only the ASCII letters 'a' through 'z' (case-insensitive),
+	 *   the digits '0' through '9', and the hyphen
+	 * - cannot begin or end with a hyphen
+	 * - no other symbols, punctuation characters, or blank spaces are
+	 *   permitted
+	 * but hey - this is a minimalist implmentation, so only check length
+	 * and let the name server deal with things.
+	 */
+	if (strlen(argv[1]) >= 255) {
+		printf("dns error: hostname too long\n");
+		return 1;
+	}
+
+	NetDNSResolve = argv[1];
+
+	if (argc == 3)
+		NetDNSenvvar = argv[2];
+	else
+		NetDNSenvvar = NULL;
+
+	if (NetLoop(DNS) < 0) {
+		printf("dns lookup of %s failed, check setup\n", argv[1]);
+		return 1;
+	}
+
+	return 0;
+}
+
+U_BOOT_CMD(
+	dns,	3,	1,	do_dns,
+	"lookup the IP of a hostname",
+	"hostname [envvar]"
+);
+
+#endif	/* CONFIG_CMD_DNS */
+