Do not send a hostname option when we don't have a hostname.

The minimum length of a hostname is 1 according to RFC 2132.
Found the hard way by Ibrahim Khalifa (ibo AT trukonsult.se) with DHCP
Relay on a Cisco ASA in the installer where we don't have a hostname
yet.
Ibrahim also provided a diff which I simplified a bit.
OK tb

diff --git a/sbin/dhcpleased/frontend.c b/sbin/dhcpleased/frontend.c
index 58c6153..2959a6c 100644 (file)
--- a/sbin/dhcpleased/frontend.c
+++ b/sbin/dhcpleased/frontend.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: frontend.c,v 1.28 2022/01/04 06:20:37 florian Exp $   */
+/*     $OpenBSD: frontend.c,v 1.29 2022/04/26 14:50:04 florian Exp $   */
 
 /*
  * Copyright (c) 2017, 2021 Florian Obser <florian@openbsd.org>
@@ -971,7 +971,8 @@ build_packet(uint8_t message_type, char *if_name, uint32_t xid,
 #endif /* SMALL */
        {
                if (gethostname(dhcp_hostname + 2,
-                   sizeof(dhcp_hostname) - 2) == 0) {
+                   sizeof(dhcp_hostname) - 2) == 0 &&
+                   dhcp_hostname[2] != '\0') {
                        if ((c = strchr(dhcp_hostname + 2, '.')) != NULL)
                                *c = '\0';
                        dhcp_hostname[1] = strlen(dhcp_hostname + 2);