Use renewal time from DHCP server ACK, if given

Per RFC 2132 section 9.11 the server can manually specify a renewal (T1)
time different from the default of half the lease time through option
code 58. This PR updates the behavior of the dhcp client to use that
value, if provided, and only if not provided does it default to half of
the lease duration.

Since the current state of smoltcp does not seem to follow the REBINDING
state, I also made it look for a value in option code 59 (which should
be the rebinding (T2) interval) and use that if no T1 interval interval
is provided. This behavior seems sensible to me, given that we're not
following the REBINDING part of the spec, but I can change it to ignore
option code 59, or any other handling, if that is preferred.

src/socket/dhcpv4.rs

@@ -439,8 +439,23 @@ impl<'a> Socket<'a> {
             packet: None,
         };
 
-        // RFC 2131 indicates clients should renew a lease halfway through its expiration.
-        let renew_at = now + lease_duration / 2;
+        // Set renew time as per RFC 2131:
+        // The renew time (T1) can be specified by the server using option 58:
+        let renew_duration = dhcp_repr
+            .renew_duration
+            .map(|d| Duration::from_secs(d as u64))
+            // Since we don't follow the REBINDING part of the spec, when no
+            // explicit T1 time is given, we will also consider the rebinding
+            // time if it is given and less than the default.
+            .or_else(|| {
+                dhcp_repr
+                    .rebind_duration
+                    .map(|d| Duration::from_secs(d as u64).min(lease_duration / 2))
+            })
+            // Otherwise, we use the default T1 time, which is half the lease
+            // duration.
+            .unwrap_or(lease_duration / 2);
+        let renew_at = now + renew_duration;
         let expires_at = now + lease_duration;
 
         Some((config, renew_at, expires_at))
@@ -497,6 +512,8 @@ impl<'a> Socket<'a> {
             ),
             max_size: Some((cx.ip_mtu() - MAX_IPV4_HEADER_LEN - UDP_HEADER_LEN) as u16),
             lease_duration: None,
+            renew_duration: None,
+            rebind_duration: None,
             dns_servers: None,
             additional_options: self.outgoing_options,
         };

src/wire/dhcpv4.rs

@@ -651,6 +651,10 @@ pub struct Repr<'a> {
     pub max_size: Option<u16>,
     /// The DHCP IP lease duration, specified in seconds.
     pub lease_duration: Option<u32>,
+    /// The DHCP IP renew duration (T1 interval), in seconds, if specified in the packet.
+    pub renew_duration: Option<u32>,
+    /// The DHCP IP rebind duration (T2 interval), in seconds, if specified in the packet.
+    pub rebind_duration: Option<u32>,
     /// When returned from [`Repr::parse`], this field will be `None`.
     /// However, when calling [`Repr::emit`], this field should contain only
     /// additional DHCP options not known to smoltcp.
@@ -735,6 +739,8 @@ impl<'a> Repr<'a> {
         let mut dns_servers = None;
         let mut max_size = None;
         let mut lease_duration = None;
+        let mut renew_duration = None;
+        let mut rebind_duration = None;
 
         for option in packet.options() {
             let data = option.data;
@@ -767,6 +773,12 @@ impl<'a> Repr<'a> {
                 (field::OPT_MAX_DHCP_MESSAGE_SIZE, 2) => {
                     max_size = Some(u16::from_be_bytes([data[0], data[1]]));
                 }
+                (field::OPT_RENEWAL_TIME_VALUE, 4) => {
+                    renew_duration = Some(u32::from_be_bytes([data[0], data[1], data[2], data[3]]))
+                }
+                (field::OPT_REBINDING_TIME_VALUE, 4) => {
+                    rebind_duration = Some(u32::from_be_bytes([data[0], data[1], data[2], data[3]]))
+                }
                 (field::OPT_IP_LEASE_TIME, 4) => {
                     lease_duration = Some(u32::from_be_bytes([data[0], data[1], data[2], data[3]]))
                 }
@@ -808,6 +820,8 @@ impl<'a> Repr<'a> {
             dns_servers,
             max_size,
             lease_duration,
+            renew_duration,
+            rebind_duration,
             message_type: message_type?,
             additional_options: &[],
         })