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574 Part IV . Implementing Network Services in SUSE Linux Listing 21-1 (continued) ;; QUESTION SECTION: ;www.palmcoder.net. IN A ;; ANSWER SECTION: www.palmcoder.net. 38396 zen.palmcoder.net. 38400 IN IN A CNAME zen.palmcoder.net. 212.13.208.115 ;; AUTHORITY SECTION: palmcoder.net. 38400 IN NS zen.palmcoder.net. ;; Query time: 443 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Tue Jul 6 10:02:21 2004 ;; MSG SIZE rcvd: 83 Tip In the past, the name resolver tool nslookup was used in place of dig; nslookup is now deprecated, but is included in most Linux distributions. If you want to do a quick name resolution, using nslookup hostname returns the IP address. The output of dig may be confusing if you have never used it before, but its response to a name query is logical when you understand what DNS does. The output contains three basic parts: . The first part of the response is the QUESTION section. The QUESTION is what was sent originally to the DNS server. In this case, you asked for the A record of www.palmcoder.net. (Note that we talk about record types later in the chapter.) . The second part is the ANSWER section. The answer is the response to your question. In this case, you have been told the www.palmcoder.net is a CNAME of zen.palmcoder.net, and zen s address is 212.13.208.115. . Finally, you have the AUTHORITY section. The authority is the controlling server s IP address in the NS record. Using host A user-friendly replacement for the nslookup program that enables you to query the DNS for information about a domain is host. Using host, you can query a specific domain record and by default look up the address mapping of a host name. bible:~ # host www.palmcoder.net www.palmcoder.net is an alias for zen.palmcoder.net. zen.palmcoder.net has address 212.13.208.115
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Chapter 21 . Configuring a DNS Server 573 Note When a BIND server has been configured as a forwarder, it automatically caches any answers it receives from the Internet DNS servers. No extra configuration is needed to enable this caching functionality. When the forwarders declaration is configured, start the nameserver with rcnamed: bible:~ # rcnamed start Starting name server BIND 9 done To automatically set named to start when your system boots, use chkconfig-a named. Before you start testing the DNS server, you need to configure the system resolver. To do this, you need to edit the /etc/resolv.conf file. Change the nameserver entry to point to your newly created DNS server. In this case, this is 127.0.0.1. search palmcoder.net nameserver 127.0.0.1 When the DNS server is started, you can look up a host name on the Internet to check that the forwarder and DNS are working correctly. The dig program is used to query a DNS server for information (see Listing 21-1), and we use it throughout this chapter for this purpose. Using dig The dig program is being phased in as a replacement for the nslookup application. With dig, you can query specific record types in a DNS zone. To query a specific record type, use the -t option. For example: . To query the MX record of palmcoder.net, use dig -tm x palmcoder.net. . For a nameserver (NS), use dig -tns palmcoder.net. . For an address record (A), use dig -a www.palmcoder.net. Listing 21-1: Using dig to Test a DNS Server bible:~ # dig www.palmcoder.net ;; Warning: ID mismatch: expected ID 23997, got 50172 ;; Warning: ID mismatch: expected ID 23997, got 50172 ; <<>> DiG 9.2.3 <<>> www.palmcoder.net ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23997 ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 1, ADDITIONAL: 0 Continued
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572 Part IV . Implementing Network Services in SUSE Linux Caching You can imagine that with all the machines in the world wanting the IP address of Google that it would severely impact the performance of the root, .com, and Google DNS servers. To combat this, most local DNS servers cache the result of a query for a period of time that is called a TTL (Time to Live). Until this countdown is reached, subsequent DNS lookups for www.google.comwill be answered by the local DNS server. In this chapter, we talk about configuring a forwarding/caching server, and we also create a small domain to control. Configuring BIND for Caching and Forwarding After installing the BIND DNS server using YaST, you need to edit its main configuration file /etc/named.conf. When you are configuring BIND to act as a central DNS server for your home or an organization, it is always a good idea to tap into your Internet service provider s DNS server as a resource to provide you with public DNS lookups. When you rely on another DNS server to go and find your DNS lookups, it is called a forwarder because it literally forwards your DNS lookups to another DNS server with the added benefit of caching the response so that future lookups of the data are returned locally by your DNS server. BIND is available with both the Professional and Enterprise versions of SUSE. To edit DNS server settings from now on, you will have to be logged in as the root user. Tip You can also configure your DNS client and server through YaST. This chapter concentrates on direct configuration of the services so that you know how the system works. If you want to configure a DNS server, start YaST and navigate to Network Services . DNS Server. If you want to configure your DNS client, navigate to Network Services . DNS and Host Name. The important entry in /etc/named.conf is the forwarders declaration. A forwarder is a DNS server that your DNS server passes requests to when your DNS installation does not know the answer. forwarders { 194.106.56.6; 192.106.33.42; }; We have used two DNS servers for resilience purposes in this example, so that if the first DNS server cannot be contacted, the second one will be used. Note that a semicolon separates the forwarders, and the line is delimited with a semicolon. Note
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Chapter 21 . Configuring a DNS Server 571 Table 21-1 Top-Level Domains TLD Use .com Commercial organizations .net Network of machines .org Non-profit organizations .mil U.S. military .gov U.S. government .co.uk U.K. company How Does a DNS Search Work? We can take a look at how a DNS search takes place to put the idea of a distributed, hierarchical system into context. 1. Take our laptop as an example the name lookup system is configured to get all name translations from our DNS server at 192.168.1.1. When we go to www.google.com, the laptop contacts 192.168.1.1 and asks it for the address of www.google.com. As our DNS server is a caching server (we talk about this later in the chapter), it contacts a public DNS server to see if it knows it. 2. The public DNS server at 194.106.56.6 accepts our DNS request and attempts to find the IP address of Google for us. As this public DNS server does not know the address directly (it does not control the Google domain), it asks a root DNS server if it knows the IP address of Google. Because the root name- servers do not control the Google domain, they tell our Internet service provider s domain server that it should ask the DNS server that controls the .com domain. 3. The public DNS server then queries the .com DNS server for the IP address of www.google.com. As the .com DNS server does not know the address directly, it refers the ISP DNS to the DNS server that controls the google.com domain (which the .com server knows about). 4. The ISP then asks the google.com DNS server if it knows the address of www.google.com. This time we strike it lucky, as the Google nameserver knows the address and returns this IP address to our ISP s DNS server. 5. When the ISP s DNS server knows the IP address, it passes this address back to our local DNS server, which finally returns it to our laptop. As you can see, the DNS system works by servers cooperating with each other to find the answer to a DNS query. This helps to alleviate the burden of name resolution on a single root DNS server or a small group of root DNS servers.
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570 Part IV . Implementing Network Services in SUSE Linux Take our domain of palmcoder.net we control all the information about the palm- coder.net territory on the Internet, and any mistakes or misconfiguration in DNS are under our control. This takes away a huge management burden from a central authority. With millions of web sites in the world, it would take more than an army of monkeys to smoothly run the Internet. Even though domain control is up to the masses, a central control still needs to exist to allow everyone to query data in the distributed system. These centrally managed servers are called the root DNS servers. They control the top-level domains (TLDs) of the Internet, and this helps to facilitate the idea of a massively distributed network. Top-Level Domains Top-level domains (TLDs) encompass the .com, .net, .org, .co.uk, and other similar domains on the Internet. These top-level domains contain information about lower- level domains in the DNS address space. For example, palmcoder.net is under the control of the .net namespace (not the Microsoft programming architecture!). In Figure 21-1, you can see that palmcoder.net falls under the administrative domain of the .net TLD. palmcoder.net .net .org . .com Figure 21-1: Top-level domain organization There are many top-level domains that serve to segregate the domain name space. Table 21-1 lists some of the most common ones.
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Configuring a 21DNS Server CHAPTER …. We have talked about a few backbone services of the Internet in this book, but one of the most important ones is the role of the nameserver. Imagine life without names; going to your favorite search engine (e.g., www.google.com) would involve your having to type http://66.102.11.104. Not so bad, but if you had ten favorite sites, you would have to remember all of those addresses. As TCP/IP needs to use IP addresses to make a connection to another machine, the Domain Name System (DNS) is important to bridge the gap between the human and the computer. The role of DNS is not just to translate names to IP addresses, but also to form the basis for mail server lookups and reverse name lookups (IP addresses to names). The most popular DNS server on the Internet is BIND (Berkeley Internet Name Daemon). BIND truly is an open source success, on the par with Apache. Both are terribly important to the function of the Internet, and both are very well-written pieces of software. Some DNS Theory As is customary in the Unix world, DNS is distributed and hierarchical in design. The DNS management is controlled locally in a domain, and this is something that may need to be explained. The dictionary tells us that domain is a territory over which rule or control is exercised. The term domain has been used to describe a domain name, but its meaning is still the same, a territory over which control is exercised. In This Chapter DNS theory Configuring caching and forwarding Examining DNS record types Constructing zones ….
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568 Part IV . Implementing Network Services in SUSE Linux This output shows two packets that are identified as DHCP traffic. The first shows an attempt to contact the DHCP server at the IP address 192.168.6.200, which was successful and which is offering the IP address 192.168.6.245 to the client. The second, unrelated packet shows broadcast DHCP traffic from a TiVo on the author s home network, which is requesting SMTP server contact information. Wrapping Up DHCP is something that any administrator needs to know when it is no longer practical to assign static IP addresses to the hosts on your network. Thanks to the power and flexibility of YaST and the other administrative tools provided with SUSE Linux, DHCP can be quickly and easily configured to provide TCP/IP information to all of the hosts on your network, eliminating the need to do SABWA system administration by walking around. …
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Chapter 20 . Configuring and Using DHCP Services 567 Listing 20-5: Using tcpdump # tcpdump -lenx -s 1500 | dhcpdump Error in packet: . TIME: 01:27:46.168382 IP: 192.168.6.200.1024 (00:30:65:3c:7e:22) > 255.255.255.255.514 (ff:ff:ff:ff:ff:ff) n OP: 60 (Boot file size) HTYPE: 49 ((null)) HLEN: 52 HOPS: 50 XID: 3e303537 SECS: 14112 FLAGS: 33fc CIADDR: 48.32.68.72 YIADDR: 67.80.32.83 SIADDR: 69.82.86.69 GIADDR: 82.32.79.102 CHADDR: 66:65:72:65:64:20:20:20:20:20:20:20:20:20:7c:20 SNAME: Offering: 192.168.6.243 To: 00112F8D94B8 By: 192.168.6.200. FNAME: . Error in packet: : TIME: 01:28:15.044104 IP: 192.168.6.246.2190 (00:06:25:07:f7:0e) > 192.168.6.255.2190 (ff:ff:ff:ff:ff:ff) OP: 116 (SMTP server) HTYPE: 105 (Static route) HLEN: 118 HOPS: 111 XID: 636f6e6e SECS: 25955 FLAGS: 73c9 CIADDR: 49.10.115.119 YIADDR: 118.101.114.115 SIADDR: 105.111.110.61 GIADDR: 55.46.49.97 CHADDR: 2d:30:32:2d:32:2d:32:34:30:0a:6d:65:74:68:6f:64 SNAME: =broadcast identity=2400000804DC8FA machine=Living Room platform=tcd/Series2 services=TiVo-ServeTcdVideo-1:2191/tvbus_v3,TiVoMediaServer:80/http . FNAME: =tcd/Series2 services=TiVo-ServeTcdVideo-1:2191/tvbus_v3,TiVoMediaServer:80/http . 2 packets captured 2 packets received by filter 0 packets dropped by kernel
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566 Part IV . Implementing Network Services in SUSE Linux Listing 20-4: (continued) DOMAIN= vonhagen.org DNS=204.127.202.4,216.148.227.68 DHCPSID=192.168.6.200 DHCPGIADDR=0.0.0.0 DHCPSIADDR=192.168.6.200 DHCPCHADDR=00:02:2D:31:B8:C8 DHCPSHADDR=00:30:65:3C:7E:22 DHCPSNAME= LEASETIME=4080 RENEWALTIME=2040 REBINDTIME=3570 INTERFACE= eth1 CLASSID= Linux 2.6.11.4-20a-default i686 CLIENTID=00:02:2D:31:B8:C8 Pay particular information to the DHCPSIADDR value, which is the IP address of the DHCP server from which this information was retrieved. If this is not the IP address of the system on which your official DHCP server is running, track down that system and either terminate or reconfigure its DHCP server or restart it on the correct Ethernet interface. Troubleshooting DHCP Servers If DHCP clients are not able to contact your DHCP server, use YaST to install the dhcp-tools package discussed earlier in this chapter in the SUSE DHCP Server Packages section. This package provides the dhcping utility, which you can use to attempt to contact your DHCP server and retrieve an IP address. The basic syntax of this command is the following: dhcping -c valid-IP-address -s DHCP-server-IP-address You must substitute the IP address of your client for valid-IP-address and the IP address of the DHCP server that you are trying to contact for DHCP-server-IPaddress. This command will either respond with the message no answer if it was unable to contact the DHCP server or the message Got answer from: DHCP-server- IP-address if the command was successful. You can use the dhcpdump command to obtain more detailed information about DHCP traffic on your network. The dhcpdump command, also provided in the dhcptools package, filters DHCP-related information from packet capture information retrieved by the tcpdump command, as in Listing 20-5.
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Chapter 20 . Configuring and Using DHCP Services 565 Troubleshooting DHCP Clients and Servers You will rarely encounter problems with starting or using a DHCP server, which makes these problems even more frustrating when they occur. On the other hand, DHCP client problems are quite common, usually resulting from the presence of multiple, unsynchronized DHCP servers on your network. This section provides some tips for identifying and resolving DHCP client and server problems. Troubleshooting DHCP Clients Most of the problems that you may see in DHCP environments are related to DHCP clients that somehow retrieve erroneous information from a DHCP server. This is almost always the result of people starting DHCP servers on other systems that either serve the same range of IP addresses as your DHCP server or serve an entirely different set of IP addresses. If a DHCP client on your system retrieves an IP address that is in the same range as those delivered by your DHCP server but any other aspect of its network configuration differs, you will have to look for another DHCP server on your network that delivers the same range of IP addresses but provides slightly different network configuration information. If a DHCP client on your system retrieves an IP address that is in a completely different range of IP addresses, some user on your network has probably either started a DHCP server that is misconfigured or has accidentally started a DHCP meant for one Ethernet interface on another Ethernet interface. The latter is a common problem in network environments that include multihomed systems, which are systems with multiple Ethernet interfaces, each connected to a separate network. Regardless of the type of erroneous information that DHCP clients are retrieving, you can use the dhcpcd-test utility (part of the dhcpcd package) to examine the information that they are retrieving from a DHCP server. To use this utility, execute the dhcpcd-test command followed by the name of an Ethernet interface on your system. Listing 20-4 is an example of the information retrieved from a DHCP server for the Ethernet interface, which is a wireless card on my laptop. Listing 20-4: Retrieving Information from a DHCP Server # dhcpcd-test eth1 dhcpcd: MAC address = 00:02:2d:31:b8:c8 IPADDR=192.168.6.242 NETMASK=255.255.255.0 NETWORK=192.168.6.0 BROADCAST=192.168.6.255 GATEWAY=192.168.6.1 Continued
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