fix the domain name length limit checks

A domain name is at most 255 bytes long (RFC 1035), but the string
representation is two bytes smaller so the strlen maximum is 253.

src/network/lookup_name.c

@@ -14,7 +14,7 @@
 static int is_valid_hostname(const char *host)
 {
 	const unsigned char *s;
-	if (strnlen(host, 256)-1 > 254 || mbstowcs(0, host, 0) > 255) return 0;
+	if (strnlen(host, 254)-1 >= 253 || mbstowcs(0, host, 0) == -1) return 0;
 	for (s=(void *)host; *s>=0x80 || *s=='.' || *s=='-' || isalnum(*s); s++);
 	return !*s;
 }
@@ -153,7 +153,7 @@ int __lookup_name(struct address buf[static MAXADDRS], char canon[static 256], c
 	*canon = 0;
 	if (name) {
 		size_t l;
-		if ((l = strnlen(name, 256))-1 > 254)
+		if ((l = strnlen(name, 254))-1 >= 253)
 			return EAI_NONAME;
 		memcpy(canon, name, l+1);
 	}

src/network/res_mkquery.c

@@ -10,9 +10,9 @@ int __res_mkquery(int op, const char *dname, int class, int type,
 	int id, i, j;
 	unsigned char q[280];
 	struct timespec ts;
-	size_t l = strnlen(dname, 256);
+	size_t l = strnlen(dname, 254);
 
-	if (l-1>=254 || buflen<18+l || op>15u || class>255u || type>255u)
+	if (l-1>=253 || buflen<18+l || op>15u || class>255u || type>255u)
 		return -1;
 
 	/* Construct query template - ID will be filled later */

src/network/res_querydomain.c

@@ -3,10 +3,10 @@
 
 int res_querydomain(const char *name, const char *domain, int class, int type, unsigned char *dest, int len)
 {
-	char tmp[256];
-	size_t nl = strnlen(name, 256);
-	size_t dl = strnlen(domain, 256);
-	if (nl+dl+1 > 255) return -1;
+	char tmp[254];
+	size_t nl = strnlen(name, 254);
+	size_t dl = strnlen(domain, 254);
+	if (nl+dl+1 > 253) return -1;
 	memcpy(tmp, name, nl);
 	tmp[nl] = '.';
 	memcpy(tmp+nl+1, domain, dl+1);