gdhcp: set secs in a RFC 951- and 2131-compliant manner

This patch sets the BOOTP secs field in a RFC 951- and 2131-compliant
manner for DHCPv4 DISCOVER and REQUEST/SELECT packets.

Certain DHCP servers, such as that implemented in Mac OS X (< 10.7)
for its "Internet Sharing" feature, refuse to issue a DHCP lease to
clients that have not set a non-zero value in their DISCOVER or
REQUEST packets. In fact, based on http://hints.macworld.com/article.php?
story=20071223001432304, it's not non-zero but a value greater than four (4)
seconds to allow another "authoritative" DHCP server on the subnet to reply
first.

Side-by-side packet analysis of Mac OS X, iOS, Android, ISC and
Windows clients show that these clients set the BOOTP 'secs' field and
are successfully issued a DHCP lease by Mac OS X. By contrast, a
connman-based client will issue 10 back-to-back DISCOVER packets and
will not be returned a DHCP OFFER from the server.

diff --git a/gdhcp/client.c b/gdhcp/client.c
index a3b4b8a..cf4cc3c 100644 (file)
--- a/gdhcp/client.c
+++ b/gdhcp/client.c
@@ -88,6 +88,7 @@ struct _GDHCPClient {
        uint32_t server_ip;
        uint32_t requested_ip;
        char *assigned_ip;
+       time_t start;
        uint32_t lease_seconds;
        ListenMode listen_mode;
        int listener_sockfd;
@@ -339,6 +340,17 @@ static void add_send_options(GDHCPClient *dhcp_client,
                                add_binary_option, packet);
 }
 
+/*
+ * Return an RFC 951- and 2131-complaint BOOTP 'secs' value that
+ * represents the number of seconds elapsed from the start of
+ * attempting DHCP to satisfy some DHCP servers that allow for an
+ * "authoritative" reply before responding.
+ */
+static uint16_t dhcp_attempt_secs(GDHCPClient *dhcp_client)
+{
+       return htons(MIN(time(NULL) - dhcp_client->start, UINT16_MAX));
+}
+
 static int send_discover(GDHCPClient *dhcp_client, uint32_t requested)
 {
        struct dhcp_packet packet;
@@ -348,6 +360,7 @@ static int send_discover(GDHCPClient *dhcp_client, uint32_t requested)
        init_packet(dhcp_client, &packet, DHCPDISCOVER);
 
        packet.xid = dhcp_client->xid;
+       packet.secs = dhcp_attempt_secs(dhcp_client);
 
        if (requested)
                dhcp_add_simple_option(&packet, DHCP_REQUESTED_IP, requested);
@@ -374,6 +387,7 @@ static int send_select(GDHCPClient *dhcp_client)
        init_packet(dhcp_client, &packet, DHCPREQUEST);
 
        packet.xid = dhcp_client->xid;
+       packet.secs = dhcp_attempt_secs(dhcp_client);
 
        dhcp_add_simple_option(&packet, DHCP_REQUESTED_IP,
                                        dhcp_client->requested_ip);
@@ -2240,6 +2254,7 @@ int g_dhcp_client_start(GDHCPClient *dhcp_client, const char *last_address)
                        return re;
 
                dhcp_client->xid = rand();
+               dhcp_client->start = time(NULL);
        }
 
        if (last_address == NULL) {