dhcp: remove 0.0.0.0s from the DNS serevr list.

tp-link routers pad the DNS server list with 0.0.0.0 to a fixed size :(

src/socket/dhcpv4.rs

@@ -2,7 +2,7 @@ use crate::{Error, Result};
 use crate::wire::{EthernetAddress, IpProtocol, IpAddress,
            Ipv4Cidr, Ipv4Address, Ipv4Repr,
            UdpRepr, UDP_HEADER_LEN,
-           DhcpPacket, DhcpRepr, DhcpMessageType, DHCP_CLIENT_PORT, DHCP_SERVER_PORT};
+           DhcpPacket, DhcpRepr, DhcpMessageType, DHCP_CLIENT_PORT, DHCP_SERVER_PORT, DHCP_MAX_DNS_SERVER_COUNT};
 use crate::wire::dhcpv4::{field as dhcpv4_field};
 use crate::socket::SocketMeta;
 use crate::time::{Instant, Duration};
@@ -36,7 +36,7 @@ pub struct Config {
     /// match the DHCP server's address.
     pub router: Option<Ipv4Address>,
     /// DNS servers
-    pub dns_servers: [Option<Ipv4Address>; 3],
+    pub dns_servers: [Option<Ipv4Address>; DHCP_MAX_DNS_SERVER_COUNT],
 }
 
 /// Information on how to reach a DHCP server.
@@ -256,10 +256,25 @@ impl Dhcpv4Socket {
 
         let lease_duration = dhcp_repr.lease_duration.unwrap_or(DEFAULT_LEASE_DURATION);
 
+        // Cleanup the DNS servers list, keeping only unicasts/
+        // TP-Link TD-W8970 sends 0.0.0.0 as second DNS server if there's only one configured :(
+        let mut dns_servers = [None; DHCP_MAX_DNS_SERVER_COUNT];
+        if let Some(received) = dhcp_repr.dns_servers {
+            let mut i = 0;
+            for addr in received.iter() {
+                if let Some(addr) = addr{
+                    if addr.is_unicast() {
+                        // This can never be out-of-bounds since both arrays have length DHCP_MAX_DNS_SERVER_COUNT
+                        dns_servers[i] = Some(*addr);
+                        i += 1;
+                    }
+                }
+            }
+        }
         let config = Config{
             address: Ipv4Cidr::new(dhcp_repr.your_ip, prefix_len),
             router: dhcp_repr.router,
-            dns_servers: dhcp_repr.dns_servers.unwrap_or([None; 3]),
+            dns_servers: dns_servers
         };
 
         // RFC 2131 indicates clients should renew a lease halfway through its expiration.

src/wire/dhcpv4.rs

@@ -8,6 +8,7 @@ use crate::wire::arp::Hardware;
 
 pub const SERVER_PORT: u16 = 67;
 pub const CLIENT_PORT: u16 = 68;
+pub const MAX_DNS_SERVER_COUNT: usize = 3;
 
 const DHCP_MAGIC_NUMBER: u32 = 0x63825363;
 
@@ -686,7 +687,7 @@ pub struct Repr<'a> {
     /// the client is interested in.
     pub parameter_request_list: Option<&'a [u8]>,
     /// DNS servers
-    pub dns_servers: Option<[Option<Ipv4Address>; 3]>,
+    pub dns_servers: Option<[Option<Ipv4Address>; MAX_DNS_SERVER_COUNT]>,
     /// The maximum size dhcp packet the interface can receive
     pub max_size: Option<u16>,
     /// The DHCP IP lease duration, specified in seconds.
@@ -780,7 +781,7 @@ impl<'a> Repr<'a> {
                     parameter_request_list = Some(data);
                 }
                 DhcpOption::Other {kind: field::OPT_DOMAIN_NAME_SERVER, data} => {
-                    let mut servers = [None; 3];
+                    let mut servers = [None; MAX_DNS_SERVER_COUNT];
                     for (server, chunk) in servers.iter_mut().zip(data.chunks(4)) {
                         *server = Some(Ipv4Address::from_bytes(chunk));
                     }

src/wire/mod.rs

@@ -225,4 +225,5 @@ pub use self::dhcpv4::{Packet as DhcpPacket,
                        Repr as DhcpRepr,
                        MessageType as DhcpMessageType,
                        CLIENT_PORT as DHCP_CLIENT_PORT,
-                       SERVER_PORT as DHCP_SERVER_PORT};
+                       SERVER_PORT as DHCP_SERVER_PORT,
+                       MAX_DNS_SERVER_COUNT as DHCP_MAX_DNS_SERVER_COUNT};