Lesson 10: Explaining Network Services Flashcards
What is the first step in a DHCP request?
The client broadcasts a DHCPDISCOVER packet to find a DHCP server
What is the second step in a DHCP request?
If an IP address is available, the DHCP server responds to the client with a DHCPOFFER packet containing the ip address and other configuration information
What is the third step in a DHCP request?
The client can choose to accept the offer using a DHCPREQUEST packet that is broadcast onto the network
What is the fourth step in a DHCP request?
If the offer is still available, the server will respond with a DHCPACK packet, the client will broadcast an ARP message to check that the address is unused
Define a DHCP scope?
A range of IP addresses and options configured on a DHCP server to be server to clients requesting an IP address
What does is mean that a DHCP server maintains a one-to-one mapping of scopes to subnets?
No scope can cover more than one subnet and no subnet can contain more than one scope
What are some DHCP options available to a client?
- Default Gateway
- DNS Server
- DNS name
- NTP server
How are DHCP options presented to clients?
By a tag byte (1-254) in the DHCPOFFER and DHCPACK packets
What is the purpose of a DHCP relay agent/server?
Allows a server/router to forward DHCP traffic from one subnet to another
How does a DHCP relay server/agent function?
The DHCP relay server/agents intercepts broadcast DHCP frames, applies a unicast address for the appropriate DHCP server and forwards the frames to the the subnet the DHCP server is in
How does the DHCP server function after being forwarded a frame?
The DHCP server can identify the original IP subnet from the packet and offers a lease from the appropriate scope
What is the purpose of the IP helper command?
Command set in a router OS to support DHCP relay and other broadcast forwarding functionality
What is the command to set up broadcast forwading?
- int eth1
- ip helper-address x.x.x.x
What is a fully qualified domain name (FQDN)?
Unique label specified in a DNS hierarchy to identify a particular host within a subdomain within the internetwork
What is a fully qualified domain name (FQDN) made up of?
A host name assigned within the AS, and a domain name that is registered
What rules must a FQDN follow?
- Must be unique
- Can’t exceed 253 characters
- Letters, digits, and hyphens only
Define DNS
A global hierarchy of servers that contain information on domains and hosts in those domains that maps FQDNs to IP addresses
What is the DNS hierarchy consist of?
- Root (.)
- Top level domain (TLD)
- Second level domain (SLD)
- Sub domain
Define the root domain and its puropose
There are 13 root level servers in the world, each containing information about the top level domains
Define the top level domain (TLD) and its purpose
TLD’s initial purpose was to help classify websites based on their purposes, ownership, or geographical origin (.com, .org, .edu, .uk, .gov)
Define the second level domain (SLD) and its purpose
The official domain name of an entity (google, apple, paymentservicenetwork)
Define the sub domain and its purpose
A domain that is apart of a domain, in theory, each tier below the root domain is a sub domain (info.paymentservicenetwork, help.google)
Define the DNS resolution process
The client application (stub server) checks its local cache for a mapping, if no mapping is found it forwards the query to its local name server, if the local name server can’t resolve the query, its performs a recursive query until an IP addresses is returned
What is an iterative query/lookup?
A query performed between name servers in different domains that responds with a record from its own data store
What is a recursive query/lookup?
A query performed by a non-authoritative DNS server that takes on the task of querying other DNS servers until it finds the requested record or reaches a timeout
What type of DNS servers perform recursive queries?
Local DNS servers listed in a client’s DNS configuration
Define a DNS zone file
Contains numerous resource records about a DNS zone
What are the main records in a DNS zone file?
- A record
- AAAA record
- CNAME record
- MX record
- PTR record
Define the Start of Authority (SOA) record
Identifies the primary authoritative name server for the zone that maintains a full copy of all records as well as contact information for the zone
Define an Name Server (NS) record
Identifies secondary authoritative name server for the zone
What is an A record?
An address record used to resolve a hostname to an IPv4 address
What is an AAAA record?
An address record used to resolve hostname to an IPv6 address
What ports do DNS use?
UDP port 53
What is a CNAME/alias record?
A canonical name (CNAME) record is used to configure an alias for an existing A or AAAA record
What is round-robin DNS?
A load balancing technique using multiple A records that share the same hostname/domain name but map to different IP addresses within the network
What is a Mail Exchange (MX) record?
Record used to identify an email server for the domain, which requires there to be an associated A or AAAA record, no CNAME
What is a Service (SRV) record?
Specifies a host and port for specific services such as voice over IP (VoIP), media servers, and AD servers
What are the two types of text (TXT) records?
- Sender Policy Framework (SPF)
- DomainKeys Identified Mail (DKIM)
What is a Sender Policy Framework (SPF) record?
A TXT record used to list the IP addresses or names of servers that are permitted to send email from a particular domain; used to combat the sending of spam
What is a DomainKeys Identified Mail (DKIM) record?
A specially formatted DNS TXT record that stores the public key the receiving mail server will use to verify a message’s signature
What are the two lookup zones a DNS server may have?
- Forward lookup zone
- Reverse lookup zone
What is a forward lookup zone?
A zone dedicated to forwarding DNS queries and contains A records, AAAA, CNAME, MX, SPF, DKIM
What is a reverse lookup zone?
A zone dedicated to reverse DNS queries that resolves IP address to hostname and contains pointer (PTR) records
What does a pointer (PTR) record consist of?
The zone file has a special name containing the first 3 octets of the domain in reverse order appended to “in-addr.arpa”
When is DNS used over TCP port 53?
When DNS servers are on a network using IPv6 which requires record transfers over 512 bytes
What are the two types of DNS servers?
- Primary
- Secondary
What is the function of a primary DNS server?
The zone records held on the primary DNS server are editable and replicated to other DNS servers in the namespace
What is a secondary DNS sever?
The DNS server’s zone records are read only, and the server relies on a zone transfer form a primary DNS server
Define DNS caching
Data store holding results of recent DNS queries
What DNS configuration determines DNS caching?
By setting a time to live (TTL) value with the record to define how long a query can be kept in cache; the lower the cache the more frequent the record is updated.
What is the proper way to change a DNS record?
Reducing the TTL before the change, allowing the change to propagate once its made, and reverting to the original TTL
What is the windows cmd to troubleshoot DNS?
nslookup
What is the linux command to troubleshoot DNS?
dig
What does ICANN manage?
DNS and Generic TLDs
