Fix #367: custom providers with printf(3) format specifiers broken

A custom provider definition with printf() style format specifiers *and*
multiple hostname was broken, as reported in issue #367.

    custom namecheap {
        username    = YOURDOMAIN.TLD
        password    = mypass
        ddns-server = dynamicdns.park-your-domain.com
        ddns-path   = "/update?domain=%u&password=%p&host=%h"
        hostname    = { "host1", "host2", "host3" }
    }

The resulting HTTP requests were garbled, like this:

    GET /update?domain=YOURDOMAIN.TLD&password=mypass&host=host1 HTTP/1.0
    GET /update?domain=YOURDOMAIN.TLD&password=mypass&host=host1host2 HTTP/1.0
    GET /update?domain=YOURDOMAIN.TLD&password=mypass&host=host1host3 HTTP/1.0

The root cause was a legacy limitation causing the ddns-path to be
overwritten on first entry with the first host.  This patch fixes the
limitation by always composing the HTTP request in a temporary buffer,
using the custom ddns-path only as a template.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

plugins/generic.c

@@ -86,11 +86,11 @@ static void skip_fmt(char *fmt, size_t len)
  * %h - hostname
  * %i - IP address
  */
-static int custom_server_url(ddns_info_t *info, ddns_alias_t *alias)
+static int custom_server_url(char *buf, ddns_info_t *info, ddns_alias_t *alias)
 {
 	char *ptr;
 
-	while ((ptr = strchr(info->server_url, '%'))) {
+	while ((ptr = strchr(buf, '%'))) {
 		if (!strncmp(ptr, "%u", 2)) {
 			if (strnlen(info->creds.username, USERNAME_LEN) <= 0) {
 				logit(LOG_ERR, "Format specifier in ddns-path used: '%%u',"
@@ -135,7 +135,7 @@ static int custom_server_url(ddns_info_t *info, ddns_alias_t *alias)
 		return -1;
 	}
 
-	return strlen(info->server_url);
+	return strlen(buf);
 }
 
 static char tohex(char code)
@@ -204,17 +204,21 @@ static char *url_encode(char *str)
  */
 static int request(ddns_t *ctx, ddns_info_t *info, ddns_alias_t *alias)
 {
-	int ret;
-	char *url;
+	char  buf[2 * sizeof(info->server_url)];
 	char *arg = "";
+	char *url;
+	int ret;
+
+	/* Operate of copy of URL, may contain %-modifiers */
+	strlcpy(buf, info->server_url, sizeof(buf));
 
 	/*
-	 * if the user has specified modifiers, then he is probably
-	 * aware of how to append his hostname or IP, otherwise just
-	 * append the hostname or ip (depending on the append_myip option)
+	 * if the user has specified modifiers, then they probably know
+	 * how to append his hostname or IP, otherwise just append the
+	 * hostname or ip (depending on the append_myip option)
 	 */
-	if (strchr(info->server_url, '%')) {
-		if (custom_server_url(info, alias) <= 0)
+	if (strchr(buf, '%')) {
+		if (custom_server_url(buf, info, alias) <= 0)
 			logit(LOG_ERR, "Invalid server URL: %s", info->server_url);
 	} else {
 		/* Backwards compat, default to append hostname */
@@ -224,7 +228,7 @@ static int request(ddns_t *ctx, ddns_info_t *info, ddns_alias_t *alias)
 			arg = alias->address;
 	}
 
-	url = url_encode(info->server_url);
+	url = url_encode(buf);
 	if (!url)
 		return 0;