That is where dns comes in. In wireshark you can use the dns filter to see this traffic. To resolve names, computers had a flat file that contained a table to resolve hostnames to ip addresses. This local file was downloaded from hosts. In Paul Mockapetris created dns, a distributed treelike hierarchical database that will be explained in detail in these chapters. Today, dns or domain name system is a worldwide distributed hierarchical database controlled by ICANN.
Its primary function is to resolve names to ip addresses, and to point to internet servers providing smtp or ldap services. The old hosts. We will discuss this file later, as it can influence name resolution. When a client queries for an ip address, this is called a forward lookup query as seen in the previous drawing. The reverse, a query for the name of a host, is called a reverse lookup query. Below a picture of a reverse lookup query. Here is a screenshot of a reverse lookup query in nslookup.
This is what a reverse lookup looks like when sniffing with tcpdump. This is either provided by a dhcp server or manually entered. For example Google provides a public name server at 8. The root servers are usually represented by a dot. Currently about countries have a tld. Journalists often refer to these servers as the master servers of the internet, because if these servers go down, then nobody can use names to connect to websites.
The root servers are not thirteen physical machines, they are many more. For example the F root server consists of 46 physical machines that all behave as one using anycast. This screenshot shows a small portion of the root hints file that comes with bind 9.
Domains can have subdomains also called child domains. This picture shows dns domains like google. DNS domains are registered at the tld servers, the tld servers are registered at the dot servers.
Originally there were only seven defined: Table 4. See RFC for more info. Table 4. If for example a system is called gwen and it is in the domain linux-training. On Linux systems you can use the hostname and dnsdomainname commands to verify this information. The picture below represents zones as blue ovals.
Some zones will contain delegate authority over a child domain to another zone. A dns server can be authoritative over 0, 1 or more dns zones. We will see more details later on the relation between a dns server and a dns zone. A dns zone consists of records, also called resource records. We will list some of those resource records on the next page. An AAAA record is similar but contains an ipv6 address instead of ipv4. It contains the name of a computer and can be used to resolve an ip address to a hostname.
The contents of the SOA record is explained in detail in the section about zone transfers. There is exactly one SOA record for each zone. The name of the mail server is often aliased to mail or smtp, and the name of a web server to www.
MX record The MX record points to an smtp server. Caching only name servers do not have a zone database with resource records. Instead they connect to other name servers and cache that information. There are two kinds of caching only name servers. Those with a forwarder, and those that use the root servers. When it receives a query from a client, then it will consult one of the root servers. The root server will refer it to a tld server, which will refer it to another dns server. That last server might know the answer to the query, or may refer to yet another server.
In the end, our hard working dns server will find an answer and report this back to the client. In the picture below, the clients asks for the ip address of linux-training. Our caching only server will contact the root server, and be refered to the. It will then contact the. One of these name servers in this cas ns1. When our caching only server reports this to the client, then the client can connect to this website.
Sniffing with tcpdump will give you this the first 20 characters of each line are cut. The forwarder must be a dns server for example the dns server of an internet service provider. This picture shows a dns server on the company LAN that has set the dns server from their isp as a forwarder. If the ip address of the isp dns server is The definition of a conditional forwarder looks like this.
An iterative query is a DNS query where the client does not expect a complete answer the three black arrows originating from the DNS server in the picture above. Iterative queries usually take place between name servers.
The root name servers do not respond to recursive queries. Remember that a zone is a collection of resource records. This server will have a readable and writable copy of the zone database. For reasons of fault tolerance, performance or load balancing you may decide to set up another dns server with authority over that zone.
This is called a secondary dns server. Zone transfers are requested by the slave servers at regular intervals. Those intervals are defined in the soa record. You can force a refresh from a zone with rndc. The example below force a transfer of the fred. The primary server then becomes the master server of the slave server. Often the primary dns server is the master server of all slaves. Sometimes a slave server is master server for a second line slave server.
In the picture below ns1 is the primary dns server and ns2, ns3 and ns4 are secondaries. The master for slaves ns2 and ns3 is ns1, but the master for ns4 is ns2. SOA record The soa record contains a refresh value. If this is set to 30 minutes, then the slave server will request a copy of the zone file every 30 minutes. There is also a retry value. The retry value is used when the master server did not reply to the last zone transfer request. The value for expiry time says how long the slave server will answer to queries, without receiving a zone update.
Below an example of how to use nslookup to query the soa record of a zone linuxtraining. When both serial numbers are the same, then no update is needed because no records were added, removed or deleted. When the slave has a lower serial number than its master, then a zone transfer is requested. Below a zone transfer captured in wireshark. The decision depends on the size of the transfer that is needed to completely update the zone on the slave server.
An incremental zone transfer is prefered when the total size of changes is smaller than the size of the zone database. Full zone transfers use the axfr protocol, incremental zone transfer use the ixfr protocol. When a client queries its local DNS server, and the local DNS server is not authoritative for the query, then this server will go looking for an authoritative name server in the DNS tree.
The local name server will first query a root server, then a tld server and then a domain server. When the local name server resolves the query, then it will relay this information to the client that submitted the query, and it will also keep a copy of these queries in its cache. So when a nother client submits the same query to this name server, then it will retrieve this information form its cache. This is the first query ever received by this local server. The local server checks that it is not authoritative for the linux-training.
So the local server will use the root hints to send an iterative query to a root server. The root server will reply with a reference to the server that is authoritative for the. The local server will then sent an iterative query to the authoritative server for the. This server will respond with a reference to the name server that is authoritative for the linux-training.
Discover the default configuration files. Can you define the purpose of each file? Setup caching only dns server. This is normally the default setup. A caching-only name server will look up names for you and cache them. Many tutorials will tell you to add a forwarder, but we first try without this! Hey this seems to work without a forwarder. Using a sniffer you can find out what really happens.
So where is this information coming from? And what can you learn from sniffing this dns traffic? Explain in detail what happens when you enable a caching only dns server without forwarder. This wireshark screenshot can help, but you learn more by sniffing the traffic yourself.
You should see traffic to a root name server whenever you try a new tld for the first time. Remember that dns is a caching protocol, which means that repeating a query will generate a lot less traffic since your dns server will still have the answer in its memory. Add the public Google dns server as a forwarder. The ip address of this server is 8. Starting domain name service…: bind9. Explain the purpose of adding the forwarder. What is our dns server doing when it receives a query? What happens when you query for the same domain name more than once?
When is a dns server authoritative? You can also use dig instead of nslookup. How can we avoid having to set the server in dig or nslookup? When you use dig for the first time for a domain, where is the answer coming from? And the second time? How can you tell? Instead of only cachng the information from other servers, we will now make our server authoritative for our own domain. I choose the top level domain. Also add a zone database file, similar to this one add some A records for testing.
Set the Refresh and Retry values not too high so you can sniff this traffic this example makes the slave server contact the master every hour. You can accomplish this feature automatically by adjusting dhclient. Restart the DNS server and check your zone in the error log. Use dig or nslookup or even ping to test your A records.
Our primary server appears to be up and running. Note the information here: server os : Debian 7 ip address : A slave server transfers zone information over the network from a master server a slave can also be a master. A primary server maintains zone records in its local file system.
As an exercise, and to verify the work of all students, set up a slave server of all the master servers in the classroom. Before configuring the slave server, we may have to allow transfers from our zone to this server.
Remember that this is not very secure since transfers are in clear text and limited to an ip address. This example follows our demo from above. Imagine a student named Jesse having completed the setup as shown before, with the domain name jesse.
The goal is to have a slave server of paul. Below is an example of an allow-transfer statement. Careful, maybe the default allows transfer to any. With the configuration below I can make my server a slave for the jesse. Restarting bind on the slave server should transfer the zone database file.
Install bind9 and verify with a sniffer how it works. Add a forwarder and verify that it works. Create a primary forward lookup zone named yourname. Use dig and nslookup to verify your NS and A records. Create a slave of your primary zone on another server and verify the zone transfer. Set up two primary zones on two servers and implement a conditional forwarder you can use the two servers from before. You should see queries to the root name servers with tcpdump or wireshark.
The forwarder van be added in named. This is literally explained in the theory. A conditional forwarder is set in named. There is more to dns, content will be added rsn.
This allows the use of DNS as a load balancer between hosts, since clients will usually take the first ip-address offered. There are two A records for www pointing to two distinct ip addresses. Notice the order of ip addresses returned. The child domain then becomes a new zone, with authority at the new dns server.
When delegation is properly set up, then clients that query your parent zone will also be able to resolve the delegated child zones. Note the name of the servers in the screenshots are either debian7 hosting the parent domain or debian7b hosting the child domain.
IN NS ns2. IN SOA ns2. IN NS debian7b. For example when someone from the But when someone from the A split-horizon setup can be used to redirect people to local copies of certain services. We start by creating three view clauses in named. It is mandatory to put all zones inside views when using a view.
The zone files are identical copies, except for the www record. You can see that the round robin is still active for internal users, computers from We can add ip to name resolution to our dns-server using a reverse dns zone. Start by adding a. Also create a zone database file for this reverse lookup zone.
When you have more than one DNS server authoritative for a zone, you can spread queries amongst all server. One way to do this is by creating NS records for all servers that participate in the load balancing of external queries. You could also configure different name servers on internal clients.
This can result in a lot of useless pull requests, or in a significant lag between updates. For this reason dns notify rfc was designed. The server will now notify slaves whenever there is an update. By default this feature is activated in bind. Notify can be disabled as in this screenshot. With nsupdate you can update a zone database and initiate an incremental zone transfer.
You need DDNS allowed for nsupdate to work. This can be cumbersome to maintain. This is called dynamic updates or DDNS. This domain has child domains from 0. And this twice more to a structure of over four billion 2 to the power 32 domains. There is the AAAA record for ipv6 hosts on the network, and there is the ip6. Nevermind that ip-addresses can be spoofed, still use this. This can be configured on master and slave servers.
Do not run any application daemon as root. Part IV. Practice: dhcp ……………………………………………………………………………………………………………… Chapter 6. The alternative to dhcp is manually entering the ip configuration on each client computer. A dhcp client that starts, will send a dhcp discover on the network. All dhcp servers that have a lease available will respond with a dhcp offer. The client will choose one of those offers and will send a dhcp request containing the chosen offer.
The dhcp server usually responds with a dhcp ack knowledge. In wireshark it looks like this. When this procedure is finished, then the client is allowed to use that ip-configuration until the end of its lease time. All computers are connected by a hub or switch pictured in the middle.
All four computers have a cable to the hub cables not pictured. The client SunWS1 sends a dhcp discover on the network. All computers receive this broadcast. Both dhcp servers answer with a dhcp offer. All four broadcasts or five when you count both offers can be layer 2 ethernet broadcast to mac address ff:ff:ff:ff:ff:ff and a layer 3 ip broadcast to The same story can be read in rfc Time to start the server.
NetSec Antwerp Network introduction to dhcp subnet Let us discuss redundant dhcp server setups. DDNS can be used with or without secure keys. Practice: dhcp 1. Decide on a valid scope and activate it. Test with a client that your DHCP server works. Use wireshark to capture the four broadcasts when a client receives an ip for the first time. Reserve a configuration for a particular client using mac address. Make sure your DNS server still works, and is master over at least one domain.
There are several ways to do steps Google is your friend in exploring DDNS with keys, with key-files or without keys. Test the working of Dynamic DNS. Part V. A firewall is a device that besides acting as a router, also contains and implements rules to determine whether packets are allowed to travel from one network to another. A firewall can be configured to block access based on networks, hosts, protocols and ports.
Firewalls can also change the contents of packets while forwarding them. When a multihomed host is connected to two different networks, and it allows packets to travel from one network to another through its two network interfaces, it is said to have enabled packet forwarding. The rules are stored by iptables. A stateful firewall will keep a table of active connections, and is knowledgeable enough to recognise when new connections are part of an active session.
Linux iptables is a stateful firewall. It is typically used to connect multiple computers in a private address range rfc with the public internet. A nat can hide private addresses from the internet. It is important to understand that people and vendors do not always use the right term when referring to a certain type of nat. Be sure you talk about the same thing. We can distuinguish several types of nat. A device sold to you as a nat-device will probably do nat and pat. Masquerade configuration with iptables uses a dynamic target source address.
The following screenshot shows how to enable packet forwarding on Linux. To enable packet forwarding whenever the system starts, change the net. You have the option to select or create an internal network when adding a network card in VirtualBox or VMWare. Use this option to create two internal networks. I named them leftnet and rightnet, but you can choose any other name. Set up two Linux machines, one on leftnet, the other on rightnet. Make sure they both get an ip-address in the correct subnet.
Set up a third Linux computer with three network cards, one on leftnet, the other on rightnet. Complete the table below with the relevant names, ip-addresses and mac-addresses. Table 7. How can you verify whether the router will allow packet forwarding by default or not? Test that you can ping from the router to the two other machines, and from those two machines to the router.
Use arp -a to make sure you are connected with the correct mac addresses. Ping from the leftnet computer to the rightnet computer. If you do not succeed in pinging between the two networks on different subnets , then use a sniffer like wireshark or tcpdump to discover the problem. Use wireshark or tcpdump -xx to answer the following questions.
Does the source MAC change when a packet passes through the filter? And the destination MAC? What about source and destination IP-addresses? Remember the third network card on the router? Connect this card to a LAN with internet connection. What needs to be done to give internet access to leftnet and rightnet. The ip configuration on your computers should be similar to the following two screenshots.
Both machines must be in a different subnet here I created a little script on both machines to configure the interfaces. Packet Forwarding Solution leftnet computer the router rightnet computer f6:ab:b9 f:5a be:4a:6b b Did you forget to add a default gateway to the LAN machines? The ping will not work when packet forwarding is disabled or when gateways are not configured correctly.
Both MAC addresses are changed when passing the router. The clients on leftnet and rightnet need a working dns server. The filter table is used for packet filtering. Series of rules in each table are called a chain. We will discuss chains and the nat table later in this chapter. While packet forwarding uses only a routing table to make decisions, packet filtering also uses a list of rules.
The kernel will inspect packets and decide based on these rules what to do with each packet. The screenshot below shows how to list the filter table and all its rules. This is not the most secure firewall.
A more secure setup would be to DROP everything. A package that is dropped will not continue in any chain, and no warning or error will be sent anywhere. The below commands lock down a computer. Do not execute these commands inside a remote ssh shell. Note that you might lose your connection when typing this over ssh ;-. Allowing ssh over eth0 This example show how to add two rules to allow ssh access to your system from outside. Allowing access from a subnet This example shows how to allow access from any computer in the There is no port application limitation here.
Below is an example script that implements the firewall rules that you saw before in this chapter. The following command enables routing of icmp messages between networks. Make sure you can ssh to your router-system when iptables is active. Bookshelves can now be found generated in libraries in villages , and libraries in strongholds.
Bookshelves now drop 3 books when broken by a non- Silk Touch item. They are now necessary to enchant with higher levels for better enchanted items. The top texture of bookshelves have now been updated as a result of the planks texture update. Librarian villagers now sell 1 bookshelf for 3 emeralds. The number of bookshelves required to reach the maximum enchantment level has been now decreased to Librarian villagers now sell 1 bookshelf for 3—4 emeralds. Bookshelves now generate in woodland mansions.
Prior to The Flattening , this block 's numeral ID was The texture of bookshelves has now been changed. The texture of bookshelves has now been changed, once again. Bookshelves are currently obtainable only in Creative mode. Bookshelves now naturally spawn in strongholds and villages. Bookshelves can now be used to increase enchantment table levels. Bookshelves can now be obtained by using a tool with the Silk Touch enchantment. Librarian villagers now sell 1 bookshelf for emeralds as their second tier trade.
Bookshelves now get inspected by librarian villagers. The texture of bookshelves has been changed. Whether they actually used this texture is unknown. Kelp Dried. Elements Hardened Glass Pane. Sculk Catalyst Sculk Shrieker. Creative only All editions Player Head. Send to my phone. Choo Choo! All aboard the Candyland Express! Hop on a train made of sweets and treats and venture onward from Chocolate Town, where all your candy fantasies come true!
Explore cupcake villages and scale up ice cream mountains before finally reaching the airport, where you can fly through the skies in the Candy Airplane and reach new horizons! Cyclone did a great job creating The Candyland Express for the community. The Candyland Express by Cyclone. The Candyland Express has the following product ID: e0faddbbc7-fd14c8e. Mojang and Minecraft are trademarks of Mojang AB. Privacy Policy Manage Preferences. Nice work! You crafted a Zombie House!
Drag the Ghost Spirit onto the Crafting Table.
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