1.6 Network Services

Question 1

What is Domain Name System (DNS)?

Answer 1

A decentralised hierarchical database used for resolving fully qualified domain names (FQDNs) to IP addresses.

Question 2

What is a DNS A record (host record)?

Answer 2

They allow a host to resolve an FQDN to an IPv4 address, also called a forward DNS query.

Question 3

What is a DNS AAAA record (quad A)?

Answer 3

They are the IPv6 equivalent of an A record. It allows a host to resolve an FQDN to an IPv6 IP address.

Question 4

What are DNS TXT records?

Answer 4

TXT records are often used for proof of ownership of a domain name. Mail servers use TXT records for identifying the authorized IP addresses of originating mail servers.

Question 5

What is a Sender Policy Framework (SPF) record?

Answer 5

It is a TXT record and used to define originating mail servers IP addresses so that other mail servers can combat spam.

Question 6

What is a Domain Keys Identified Mail (DKIM) record?

Answer 6

They allow for organisations to authenticate messages sent through a cryptographic key stored in the TXT record.

Question 7

What are DNS SRV records (service locator)?

Answer 7

They are used for locating services such as LDAP, which is used by Active Directory and Linux for login services.

Question 8

What are DNS MX records (mail exchanger)?

Answer 8

They assist a mail server in identifying the mail server for your domain. You must publish an MX record for your mail server if you want to receive mail from other mail servers.

Question 9

What are DNS CNAME records (canonical name)?

Answer 9

It allows a FQDN to resolve to an A record. This is quite handy when you have one host that has multiple names.

Question 10

What are DNS NS records (name server)?

Answer 10

They are used by the DNS servers to define who can have a copy of the DNS zone file.

Question 11

What are DNS SOA records (start of authority)?

Answer 11

They are used to define the parameters of a zone file, such as the primary DNS server, refresh intervals, and TTL.

Question 12

What are PTR records (pointer records)?

Answer 12

They allow a client to resolve an IP address to an FQDN. PTR record queries are called reverse lookups. This is useful for learning the name of a host. PTR records are mainly used by administrators.

Question 13

What is External DNS used for?

Answer 13

The resolution of public IP addresses for services published externally on the internet.

Question 14

What is Internal DNs used for?

Answer 14

The resolution of private IP addresses for services published internally on the network.

Question 15

What is it called when Internal and External DNS servers are managed separately?

Answer 15

Split-Brain DNS Model. It is best practice to separate out the internal DNS servers from external DNS servers.

Question 16

What is a DNS Zone File?

Answer 16

They contain mappings between domain names and IP addresses and other resources.

Question 17

What is a Primary DNS Server?

Answer 17

They own the editable copy of the DNS zone file.

Question 18

What is a Secondary DNS Server?

Answer 18

They contain the copy of a particular DNS zone file, but the copy is read-only.

Question 19

What is a Forwarding DNS Server?

Answer 19

They may not contain any zone files, but they allow DNS requests to be forwarded to another DNS server such as an ISP’s server.

Question 20

What is a Reverse Zone Lookup?

Answer 20

It allows for IP address–to–FQDN resolution using PTR records.

Question 21

What is a Forward Zone Lookup?

Answer 21

It allows for all other name resolution, such as FQDN–to–IP address lookups, TXT lookups, and so on.

Question 22

What is a DHCP MAC reservation?

Answer 22

It allows for a host to be treated like a DHCP client yet always retain the same IP address.

Question 23

What is a DHCP pool?

Answer 23

The range of servable IP addresses that can be allocated via DHCP.

Question 24

What are IP address exclusions in DHCP?

Answer 24

They allow us to exclude a range from the DHCP pool for statically assigned hosts such as servers.

Question 25

What are DHCP Scope Options?

Answer 25

The allow the client to leave the network, obtain name resolution, and obtain the complete DNS name for the host.

Question 26

What are the most important DHCP Scope Options?

Answer 26

The Router, DNS server, and DNS suffix options.

Question 27

What is the first stage of the DHCP Lease process?

Answer 27

The lease time of the IP address is always the client’s responsibility to renew. DHCP clients renew the lease at 50 percent of the lease cycle.

Question 28

What is the second stage of the DHCP Lease process?

Answer 28

If the original server does not respond, then at 7/8ths of the lease cycle the client will send DHCP discover packets to all listening servers in hopes of a new lease.

Question 29

What is the third stage of the DHCP Lease process?

Answer 29

If the IP address cannot be renewed, then at the end of the lease cycle the client must release the IP address.

Question 30

What is an IP Helper?

Answer 30

A process on a router interface. The IP helper intercepts DHCP broadcast messages and sends them via unicast to a defined remote DHCP server.

Question 31

What is a DHCP relay?

Answer 31

It performs the same service as an IP helper and is often a server on the immediate network that is configured to intercept DHCP broadcasts and send them as unicast to the remote DHCP server.

Question 32

What is Network Time Protocol (NTP)?

Answer 32

It is used to synchronise time between hosts on the Internet. NTP uses a series of stratum levels; lower is more precise and higher is less precise.