DHCP option domain name server

Question

I am new to networking. I got struck while having a look at the dhcp configuration file. I just tried to make a WiFi connection for data transmission using this link. I am using linux OS.

I have successfully established Wi-Fi connection and it is working fine. But some times the ICMP Destination Port unreachable message is displayed.

dhcpd.conf

ddns-update-style none;
ignore client-updates;
authoritative;
option local-wpad code 252 = text;

subnet
10.0.0.0 netmask 255.255.255.0 {
# --- default gateway
option routers
10.0.0.1;
# --- Netmask
option subnet-mask
255.255.255.0;
# --- Broadcast Address
option broadcast-address
10.0.0.255;
# --- Domain name servers, tells the clients which DNS servers to use.
option domain-name-servers
10.0.0.1, 8.8.8.8, 8.8.4.4;
option time-offset
0;
range 10.0.0.3 10.0.0.13;
default-lease-time 1209600;
max-lease-time 1814400;
}

For the specified subnet,

1) Should the router IP address (10.0.0.1 in this case ) must match the domain-name server's IP ?

2)Why do we provide a list of 3 or more IP address in domain-name servers (10.0.0.1, 8.8.8.8, 8.8.4.4) if providing one Server alone is sufficient

option domain-name-servers ip-address [, ip-address... ];

The domain-name-servers option specifies a list of Domain Name System (STD 13, RFC 1035) name servers available to the client. Servers should be listed in order of preference.

3)In some connection, I could find the client connecting to second or third (8.8.8.8 or 8.8.4.4) server IPs. In such case as these IPs are not in the range of the defined Subnet mask (10.0.0.0) will it not create any connection issues ?

Answer 1

1. The router is commonly used as the local DNS server which then sends the requests to the DNS servers it gets assigned through its connection (possibly with DHCP too).
2. Different systems can deal with the list of servers in different fashions in case a lookup fails. You are right that a single DNS server is often sufficient, especially when that DNS server is local (such as your router) and will deal with DNS issues with its own list of DNS servers.
3. The subnet mask will simply tell the computer that the request needs to be sent to its gateway for processing since it can not route it itself locally.

In regards to your intermittent Destination Port unreachable error message, the first thing to suspect while using a wifi connection is wifi communication errors. If you get those errors while using Ethernet, you would need to do more advanced troubleshooting, but I doubt it's justified in this case.