Fichas
System Admin- Domain Name Server
Fichas del conjunto (29)
- 1What is a DNS?a hierarchical naming structure used to identify comps, services or resources
- 2advantage of a hierarchical namespace ?it is easier to route information to the correct destination
- 3In DNS what are the parts of the domain name called?A domain name consists of two or more parts called labels that are separated by dots e.g. salford.ac.uk
Each label represents a different level in the hierarchy, with highest level label being the one on the right e.g. UK. and each further label to the left representing a successively lower level in the hierarchy. e.g. Salford is the lowest label
In a FULLY QUALIFIED DOMAIN NAME (FQDN) the rightmost label is the top-level domain (TLD) - 4The DNS Namespacelooks like an upside down tree
- 5Who are the names of these domains controlled by?IANA- internet assigned numbers association
- 6What is each TLD managed by?The top level domains (TLD's) are those directly below the root e.g .com .net .org .edu and country names such as .uk .de .au
Each TLD is managed by a TLD registry which authorises domain name registrars to assign names within the TLD - 7When requested a name below a TLD is valid and available it can be registered:labels may contain the letters a-z (upper/lower case)
the number 0-9 and the hyphen (labels may not start or end with a hyphen) - 8as the number of top level domains was limited, what were the only allowed TLD's?.com .net .org .mil .gov and .edu
- 9Today's the TLD's are grouped as follows:COUNTRY CODE TLDs(ccTLD)
.uk .de .fr
INTERNATIONALISED COUNTRY CODE TLDs
non-latin characters
GENERIC TLD's (gTLD)
there are 22 of these split into:
-UNSPONSORED TLDs: managed by ICANN for thr global internet community and are .com .net .org and .info
-SPONSORED TLDs (sTLDs): that are proposed and managed by private organisations with the names being based on community themes such as .aero .edu .jobs .mueseum
INFRASTRUCTURE TLD : which is .arpa - 10What is the DNS namespace cut up into?Zones
- 11Each DNS zone is looked after by one or more....authoritative name servers
- 12What does a nameserver store?resource records for the zone
- 13A given DNS nameserver may be responsible for....more than one zone
- 14Creating a sub-domain with a new authoritative nameserver is where....authority can be delegated for a portion of a zone to another nameserver
- 15DNS servers are responsible for one or more zones of the DNS namespace e.g.....the salford DNS server has been delegated to look after the salford part of the ac part of the uk part of the namespace
the info about salford resources is stored in a "zone file" called by default salford.ac.uk
in the zone file are the IP addresses for all resources, whose address ends salford.ac.uk, e.g.
IP address of webmail is 146.87.255.33
IP address of web server is 146.87.255.146
IP address of blackboard server is 146.87.255.16 - 16DNS from the client perspectiveIf a client has been given the IP address of a DNS server it can ask it to perform name resolution
e.g. PC on the UoS network connecting to Blackboard (vle.salford.ac.uk)
- 17DNS servers can also act as resolversExample 2- PC on the UoS network connecting to www.google.com
- 18problems with the mechanismPlaces a huge query load on the root DNS servers
To reduce the load, caching is used
DNS resolvers (servers&clients) store cache the result of query for a period of time defined by the Time to Live of the record.
one consequence of caching is that when changers are made to DNS servers, resolvers continue to use apparently valid, but actually out of date info not helped by the fact that resolvers can override TTL values and cache results up to 68 years - 19Types of Resource Recordsthere a a large number of different types of Resource Record (RR)
A- associates an IPv4 address with a host name
AAAA- associates an IPv6 address with a host name
CNAME- associates an alternative name (alias) with a host name
MX- associates a domain name with a list of mail servers serving that domain
NS- associates a domain name with the host name of a DNS server that resolves queries for that domain
SOA- associates a domain name with the hose name of the DNS server that holds the authoritative version of the records for that domain - 20Primary and Secondary serversFor security each zone must have at least two name servers
- one nameserver is the PRIMARY (master) server (the SOA) and the other nameservers are SECONDARY (slave) servers
-Secondary servers copy the zone file from their primary server
-info in the SOA record tells secondary servers how to interact with their primary server - 21How the interaction occurs:Serial Number
Tells a secondary server if it has the latest copy of the zone file
Refresh
Tells a secondary server how often it should check with the primary to see if the zone file has changed
if a secondary server cannot contact its primary then:
RETRY- tells the secondary how long to wait before trying again
EXPIRY- tells the secondary for how long it can continue to resolve query requests when out of reach with its primary
Minimum
the default time for which returned records should be cached by resolvers - 22What are Glue Records?The query process can become stuck if glue is not used
e.g.
- Your DNS server is trying to resolve the IP address of the widgets web server www.widgets.com
- The .com DNS server returns the info that it needs to talk to the widgets.com DNS server but NOT the IP address of this server- the IP address of the widgets.com DNS server is ONLY held in the zone file on the widgets.com DNS server itself
-Your DNS server is now stuck because it needs to query the widgets.com DNS server to find out the IP address of the widgets.com DNS server, so it can query it, which cannot be done
The problem is solved by adding an A(host) RR for the widgets.com DNS server to the zone file on the .com DNS server. This is called a GLUE record.
Glue Records may also be used to speed up queries even when the records for a DNS server are not held within the DNS servers own zone file - 23Q1: A server has the name mail5.salford.ac.uk, for this name identify:a. The FQDN is mail5.salford.ac.uk
b. The TLD is .uk - 24Q2: you are trying to register the domain name Kate's whizzba@ng ideas.com but you cannot. Nobody else has registered this name so why are you having troublr?The name includes several not allowed characters- spaces, apostrophe and the @ symbol
- 25Q3: what is a zone and what is an authoritative nameserver?A Zone is a logical portion of the DNS namespace e.g. google.com (part of the .com part of the namespace)
A nameserver is a DNS server that stores resource records for a given zone.
A authoritative name server is a DNS server that stores the definitive version of the resource records for the zone. Resource records can be modified, deleted, and added in this copy of the zone file - 26Q4: you are trying to connect to the www.bbc.co.uk webpage from your PC at home. Describe the process that relevant DNS servers will go through to enable this connection. State any assumptions that you make about the DNS zone structure?Assume that:
your PC has been given the IP address of your ISP’s DNS server, which does not hold the zone file for bbc.co.uk.
also that neither your PC nor any other client of this server has previously made this request (ie, the answer has not been cached anywhere).
the DNS namespace has been divided up such that there is a DNS server looking after the zone file for bbc.co.uk. The process is then as follows: - 27Q4: continued....a. Your PC asks your ISP’s DNS server to resolve the name www.bbc.co.uk into an IP address. It does not look after this zone file so it can’t. It will therefore resolve the IP address by working its way along the name:
i. It will ask a root (dot) server for the IP address of a .uk DNS server
ii. It will ask the .uk DNS server for the IP address of a .co.uk DNS server
iii. It will ask the .co.uk DNS server for the IP address of the bbc.co.uk server
iv. It will ask the bbc.co.uk DNS server for the IP address of the www.bbc.co.uk web server.
b. The ISP DNS server will cache the result and then send it back to your PC
c. Your PC will cache the result and then use the IP address to contact the bbc.co.uk web server. - 28Q5: what are the advantages and disadvantages of caching query results?Advantages
reduces the load on DNS server, because queries can be answered from the cache, rather than having to contact the DNS servers. This is particularly important for the root nameservers as these have the highest load during the recursive query process.
Disadvantages
information will become out of date. For example, if Salford changes the IP address of its DNS server, and updates this information on the .ac.uk DNS servers, DNS resolvers that are holding a resolution to the old IP address will not update it because it is still, apparently, ‘in-date’. - 29Q6: A DNS zone file has the following entries, using the info in the zone file answer the questions below?a. the IP address of the web server- 201.45.213.6
b. the IP address if the mail server 201.45.213.20
c. the IP address of the primary DNS server
201.45.213.6
d. the IP address of the secondary DNS server
201.45.213.104
e. 172800 seconds
f. 108000 Expiry in seconds
g. 3600 in seconds minimum
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- Descripción
- Fichas sobre System Admin- Domain Name Server, creado por Robena786 el 17/05/2015.
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- Robena786
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