| The material on this page was prepared using Sarge or Etch configured using our Installation and Packages pages. If you did not use our pages to set up your system, what you encounter on your system may be different than what is given here. |
As mentioned on the Networking page, every system on the Internet must have a unique IP address. (This does not include systems that are behind a NAT firewall because they are not directly on the Internet.) DNS acts as a directory service for all of these systems, allowing you to specify each one by its hostname. A telephone book allows you to look up an individual person by name and get their telephone number, their unique identifier on the telephone system's network. DNS allows you to look up individual server by name and get its IP address, its unique identifer on the Internet.
There are other hostname-to-IP directory services in use, mainly for LANs. Windows LANs can use WINS. UNIX LANs can use NIS. But because DNS is the directory service for the Internet (and can also be used for LANs) it is the most widely used. UNIX LANs could always use DNS instead of NIS, and starting with Windows 2000 Server, Windows LANs could use DNS instead of, or in addition to, WINS. And on small LANs where there are only a few machines you could just use HOSTS files on each system instead of setting up a server running DNS, NIS, or WINS.
As a service, DNS is critical to the operation of the Internet. When you enter www.some-domain.com in a Web browser, it's DNS that takes the www host name and translates it to an IP address. Without DNS, you could be connected to the Internet just fine, but you ain't goin' no where. Not unless you keep a record of the IP addresses of all of the resources you access on the Internet and use those instead of host/domain names.
So when you visit a Web site, you are actually doing so using the site's IP address even though you specified a host and domain name in the URL. In the background your computer quickly queried a DNS server to get the IP address that corresponds to the Web site's server and domain names. Now you know why you have to specify one or two DNS server IP addresses in the TCP/IP configuration on your desktop PC (in the resolv.conf file on a Linux system and the TCP/IP properties in the Network Control Panel on Windows systems).
A "cannot connect" error doesn't necessarily indicate there isn't a connection to the destination server. There may very well be. The error may indicate a failure in "resolving" the domain name to an IP address. I use the open source Firefox Web browser on Windows systems because the status bar gives more informational messages like "Resolving host", "Connecting to", and "Transferring data" rather than just the generic "Opening page" with IE. (It also seems to render pages faster than IE.)
In short, always check for correct DNS operation when troubleshooting a problem involving the inability to access an Internet resource. The ability to resolve names is critical, and later in this page we'll show you some tools you can use to investigate and verify this ability.
When you are surfing the Web viewing Web pages or sending an e-mail your workstation is sending queries to a DNS server to resolve server/domain names. (Back on the Modems page we showed you how to set up your resolv.conf file to do this.) When you have you own Web site that other people visit you need a DNS server to respond to the queries from their workstations.When you visit Web sites, the DNS server your workstation queries for name resolution is typically run by your ISP, but you could have one of your own. When you have your own Web site the DNS servers which respond to visitors queries are typically run by your Web hosting provider, but you could likewise have your own one of these too. Actually, if you set up your own DNS server it could be used to respond to both "internal" (from your workstation) and "external" (from your Web site's visitors) queries.
Even if you don't have your own domain name, or even your own LAN, you can still benefit from using a DNS server to allow others to access your Debian system. If you have a single system connected to the Interent via a cable or DSL connection, you can have it act as a Web/e-mail/FTP server using a neat service called "dynamic DNS" which we'll cover later. Dynamic DNS will even work with a modem if you want to play around with it.
DNS Server Functions
You can set up a DNS server for several different reasons:
Don't take from this that you need three different types of DNS servers. If you were to set up a couple authoritative DNS servers they could also provide the functionality of LAN and simple DNS servers. And a LAN DNS server can simultaneously provide the functionality of a simple DNS server. It's a progressive type of thing.
- Internet Domain Support: If you have a domain name and you're operating Web, e-mail, FTP, or other Internet servers, you'll use a DNS server ro respond to resolution queries so others can find and access your server(s). This is a serious undertaking and you'd have to set up a minimum of two of them. On this page we'll refer to these types of DNS servers as authoritative DNS servers for reasons you'll see later. However, there are alternatives to having your own authoritative DNS server if you have (or want to have) your own domain name. You can have someone else host your DNS records for you. Even if someone else is taking care of your domain's DNS records you could still set up one of the following types of DNS servers.
- Local Name Resolution: Similar to the above scenario, this type of DNS server would resolve the hostnames of systems on your LAN. Typically in this scenario there is one DNS server and it does both jobs. The first being that it receives queries from workstations and the second being that it serves as the authoritative source for the responses (this will be more clear as we progress). Having this type of DNS server would eliminate the need to have (and manually update) a HOSTS file on each system on your LAN. On this page we'll refer to these as LAN DNS servers.
During the Debian installation you are asked to supply a domain name. This is an internal (private) domain name which is not visible to the outside world so, like the private IP address ranges you use on a LAN, it doesn't have to be registered with anyone. A LAN DNS server would be authoritative for this internal, private domain. For security reasons, the name for this internal domain should not be the same as any public domain name you have registered. Private domain names are not restricted to using one of the established public TLD (Top Level Domain) names such as .com or .net. You could use .corp or .inc or anything else for your TLD. Since a single DNS server can be authoritative for multiple domains, you could use the same DNS server for both your public and private domains. However, the server would need to be accessible from both the Internet and the LAN so you'd need to locate it in a DMZ. Though you want to use different public and private domain names, you can use the same name for the second-level domain. For example, my-domain.com for the public name and my-domain.inc for the private name.- Internet Name Resolution: LAN workstations and other desktop PCs need to send Internet domain name resolution queries to a DNS server. The DNS server most often used for this is the ISP's DNS servers. These are often the DNS servers you specify in your TCP/IP configuration. You can have your own DNS server respond to these resolution queries instead of using your ISP's DNS servers. My ISP recently had a problem where they would intermittently lose connectivity to the network segment that their DNS servers were connected to so they couldn't be contacted. It took me about 30 seconds to turn one of my Debian systems into this type of DNS server and I was surfing with no problems. On this page we'll refer to these as simple DNS servers. If a simple DNS server fails, you could just switch back to using your ISP's DNS servers. As a matter of fact, given that you typically specify two DNS servers in the TCP/IP configuration of most desktop PCs, you could have one of your ISP's DNS servers listed as the second (fallback) entry and you'd never miss a beat if your simple DNS server did go down. Turning your Debian system into a simple DNS server is simply a matter of entering a single command.
If you were going to set up authoritative DNS servers or a simple DNS server you'd have to have a 24/7 broadband connection to the Internet. Naturally, a LAN DNS server that didn't resolve Internet host/domain names wouldn't need this.
A DNS server is just a Debian system running a DNS application. The most widely used DNS application is BIND (Berkeley Internet Name Domain) and it runs a daemon called named that, among other things, responds to resolution queries. We'll see how to install it after we cover some basics.
| DNS Basics | |
Finding a single server out of all of the servers on the Internet is like trying to find a single file on drive with thousands of files. In both cases it helps to have some hierarchy built into the directory to logically group things. The DNS "namespace" is hierarchical in the same type of upside-down tree structure seen with file systems. Just as you have the root of a partition or drive, the DNS namespace has a root which is signified by a period.
When specifying the absolute path to a file in a file system you start at the root and go to the file:
/etc/bind/named.conf
When specifying the absolute path to a server in the DNS namespace you start at the server and go to the root:
www.aboutdebian.com.
Note that period after the 'com' as it's important. It's how you specify the root of the namespace. An absolute path in the DNS namespace is called a FQDN (Fully Qualified Domain Name). The use of FQDNs are prevalent in DNS configuration files and it's important that you always use that trailing period.
Internet resources are usually specified by a domain name and a server hostname. The www part of a URL is often the hostname of the Web server (or it could be an alias to a server with a different host name). DNS is basically just a database with records for these hostnames. The directory for the entire telephone system is not stored in one huge phone book. Rather, it is broken up into many pieces with each city having, and maintaining, its piece of the entire directory in its phone book. By the same token, pieces of the DNS directory database (the "zones") are stored, and maintained, on many different DNS servers located around the Internet. If you want to find the telephone number for a person in Poughkeepsie, you'd have to look in the Poughkeepsie telephone book. If you want to find the IP address of the www server in the some-domain.com domain, you'd have to query the DNS server that stores the DNS records for that domain.
The entries in the database map a host/domain name to an IP address. Here is a simplistic logical view of the type of information that is stored (we'll get to the A, CNAME, and MX designations in a bit).
| A | www.their-domain.com | 172.29.183.103 |
| MX | mail.their-domain.com | 172.29.183.217 |
| A | debian.your-domain.com | 10.177.8.3 |
| CNAME | www.your-domain.com | 10.177.8.3 |
| MX | debian.your-domain.com | 10.177.8.3 |
This is why a real Internet server needs a static (unchanging) IP address. The IP address of the server's NIC connected to the Internet has to match whatever address is in the DNS database. Dynamic DNS does provide a way around this for home servers however, which we'll see later.
When you want to browse to www.their-domain.com your DNS server (the one you specify in the TCP/IP configuration on your desktop computer) most likely won't have a DNS record for the their-domain.com domain so it has to contact the DNS server that does. When your DNS server contacts the DNS server that has the DNS records (referred to as "resource records" or "zone records") for their-domain.com your DNS server gets the IP address of the www server and relays that address back to your desktop computer. So which DNS server has the DNS records for a particular domain?
When you register a domain name with someone like Network Solutions, one of the things they ask you for are the server names and addresses of two or three "name servers" (DNS servers). These are the servers where the DNS records for your domain will be stored (and queried by the DNS servers of those browsing to your site). So where do you get the "name servers" information for your domain? Typically, when you host your Web site using a Web hosting service they not only provide a Web server for your domain's Web site files but they will also provide a DNS server to store your domain's DNS records. In other words, you'll want to know who your Web hosting provider is going to be before you register a domain name (so you can enter the provider's DNS server information in the name servers section of the domain name registration application).
| You'll see the term "zone" used in DNS references. Most of the time a zone just equates to a domain. The only times this wouldn't be true is if you set up subdomains and set up separate DNS servers to handle just those subdomains. For example, a company would set up the subdomains us.their-domain.com and europe.their-domain.com and would "delegate" a separate DNS server to each one of them. In the case of these two DNS servers their zone would be just the subdomains. The zone of the DNS server for the parent their-domain.com (which would contain the servers www.their-domain.com and mail.their-domain.com) would only contain records for those few machines in the parent domain. Note that in the above example "us" and "europe" are subdomains while "www" and "mail" are host names of servers in the parent domain. |
Once you've got your Web site up and running on your Web hosting provider's servers and someone surf's to your site, the DNS server they specified in their local TCP/IP configuration will query your hosting provider's DNS servers to get the IP address for your Web site. The DNS servers that host the DNS records for your domain, i.e. the DNS servers you specify in your domain name registration application, are the authoritative DNS servers for your domain. The surfer's DNS server queries one of your site's authoritative DNS servers to get an address and gets an authoritative response. When the surfer's DNS server relays the address information back to the surfer's local PC it is a "non-authoritaive" response because the surfer's DNS server is not an authoritative DNS server for your domain.
Example: If you surf to MIT's Web site the DNS server you have specified in your TCP/IP configuration queries one of MIT's authoritative DNS servers and gets an authoritative response with the IP address for the 'www' server. Your DNS server then sends a non-authoritative response back to your PC. You can easily see this for yourself. At a shell prompt, or a DOS window on a newer Windows system, type in:
nslookup www.mit.edu
First you'll see the name and IP address of your locally-specified DNS server. Then you'll see the non-authoritative response your DNS server sent back containing the name and IP address of the MIT Web server.
If you're on a Linux system you can also see which name server(s) your DNS server contacted to get the IP address. At a shell prompt type in:
whois mit.edu
and you'll see three authoritative name servers listed with the hostnames STRAWB, W20NS, and BITSY. The 'whois' command simply returns the contents of a site's domain record.
| Don't confuse DNS zone records with domain records. Your domain record is created when you fill out a domain name registration application and is maintained by the domain registration service (like Network Solutions) you used to register the domain name. A domain only has one domain record and it contains administrative and technical contact information as well as entries for the authoritative DNS servers (aka "name servers") that are hosting the DNS records for the domain. You have to enter the hostnames and addresses for multiple DNS servers in your domain record for redundancy (fail-over) purposes. DNS records (aka zone records) for a domain are stored in the domain's zone file on the authoritative DNS servers. Typically, it is stored on the DNS servers of whatever Web hosting service is hosting your domain's Web site. However, if you have your own Web server (rather than using a Web hosting service) the DNS records could be hosted by you using your own authoritative DNS servers (as in MIT's case), or by a third party like EasyDNS. In short, the name servers you specified in your domain record host the domain's zone file containing the zone records. The name servers, whether they be your Web hosting provider's, those of a third party like EasyDNS, or your own, which host the domain's zone file are auhoritative DNS servers for the domain. |
Because DNS is so important to the operation of the Internet, when you register a domain name you must specify a minimum of two name servers. If you set up your own authoritative DNS servers for your domain you must set up a minimum of two of them (for redundency) and these would be the servers you specify in your domain record. While the multiple servers you specify in your domain record are authoritative for your domain, only one DNS server can be the primary DNS server for a domain. Any others are "secondary" servers. The zone file on the primary DNS server is "replicated" (transferred) to all secondary servers. As a result, any changes made to DNS records must be made on the primary DNS server. The zone files on secondary servers are read-only. If you made changes to the records in a zone file on a secondary DNS server they would simply be overwritten at the next replication. As you will see below, the primary server for a domain and the replication frequency are specified in a special type of zone record.
Early on in this page we said that the DNS zone records are stored in a DNS database which we now know is called a zone file. The term "database" is used quite loosely. The zone file is actually just a text file which you can edit with any text editor. A zone file is domain-specific. That is, each domain has its own zone file. Actually, there are two zone files for each domain but we're only concerned with one right now. The DNS servers for a Web hosting provider will have many zone files, two for each domain it's hosting zone records for. A zone "record" is, in most cases, nothing more than a single line in the text zone file.
There are different types of DNS zone records. These numerous record types give you flexibility in setting up the servers in your domain. The most common types of zone records are:
Several important points to note about the records in a zone file:
- An A (Address) record is a "host record" and it is the most common type. It is simply a static mapping of a hostname to an IP address. A common hostname for a Web server is 'www' so the A record for this server gives the IP address for this server in the domain.
- An MX (Mail eXchanger) record is specifically for mail servers. It's a special type of service-specifier record. It identifies a mail server for the domain. That's why you don't have to enter a hostname like 'www' in an e-mail address. If you're running Sendmail (mail server) and Apache (Web server) on the same system (i.e. the same system is acting as both your Web server and e-mail server), both the A record for the system and the MX record would refer to the same server.
To offer some fail-over protection for e-mail, MX records also have a Priority field (numeric). You can enter two or three MX records each pointing to a different mail server, but the server specified in the record with the highest priority (lowest number) will be chosen first. A mail server with a priority of 10 in the MX record will receive e-mail before a server with a priority of 20 in its MX record. Note that we are only talking about receiving mail from other Internet mail servers here. When a mail server is sending mail, it acts like a desktop PC when it comes to DNS. The mail server looks at the domain name in the recipient's e-mail address and the mail server then contacts its local DNS server (specified in the resolv.conf file) to get the IP address for the mail server in the recipient's domain. When an authoriative DNS server for the recipient's domain receives the query from the sender's DNS server it sends back the IP addresses from the MX records it has in that domain's zone file.- A CNAME (Canonical Name) record is an alias record. It's a way to have the same physical server respond to two different hostnames. Let's say you're not only running Sendmail and Apache on your server, but you're also running WU-FTPD so it also acts as an FTP server. You could create a CNAME record with the alias name 'ftp' so people would use ftp.your-domain.com and www.your-domain.com to access different services on the same server.
Another use for a CNAME record was illustrated in the example near the top of the page. Suppose you name your Web server 'debian' instead of 'www'. You could simply create a CNAME record with the alias name 'www' but with the hostname 'debian' and debian's IP address.- NS (Name Server) records specify the authoritative DNS servers for a domain.
- There can multiples of all of the above record types. There is one special record type of which there is only one record in the zone file. That's the SOA (Start Of Authority) record and it's the first record in the zone file. An SOA record is only present in a zone file located on authoritative DNS servers (non-authoritative DNS servers can cache zone records). It specifies such things as:
- The primary authoritative DNS server for the zone (domain).
- The e-mail address of the zone's (domain's) administrator. In zone files, the '@' has a specific meaning so the e-mail address is written as me.my-domain.com.
- Timing information as to when secondary DNS servers should refresh or expire a zone file and a serial number to indicate the version of the zone file for the sake of comparison.
The SOA record is the one that takes up several lines.
- Records can specify servers in other domains. This is most commonly used with MX and NS records when backup servers are located in a different domain but receive mail or resolve queries for your domain.
- There must be an A record for systems specified in all MX, NS, and CNAME records.
- A and CNAME records can specify workstations as well as servers (which you'll see when we set up a LAN DNS server).
$TTL 86400 |
Several things to take note of when evaluating this example zone file:
|
If you had a simpler setup with only one server with the hostname 'debian' that operated as a Web, e-mail, and FTP server and you had your DNS records hosted by someone like EasyDNS, your zone file would look a lot simpler:
$TTL 86400 |
Naturally, the 192.168.1.51 private address in this example would have to be an ISP-assigned public address for an Internet-accessible server. We just used a private address as an example.
Notice that the last CNAME record is a little different from the others. It specifies which server should handle requests when no hostname is specified, i.e. requests going to simply my-name.com in a URL, etc. Notice also that you can specify other domains in your zone file which is where the long-hand way of specifying a FQDN is useful.
| Dynamic DNS | |
If you set up a Debian system to act as a combination firewall, NAT, and home Web server you (and others if you wish) can access the Web pages on it (such as your Web cam images) from a remote location by entering the system's IP address in the URL. The IP address would be whatever is assigned to you by your ISP. The problem is that, unless you pay extra to have a static IP address, the IP address assigned by your ISP will change from time to time and trying keeping up with these changes can be a pain. You can get around this by using a host and domain name to access your system instead of an IP address. Being able to access your system using a consistent name in the URL even though the IP address changes is a major benefit of dynamic DNS.
Dynamic DNS (DDNS) is the ability for a host (your Debian server) to update its own DNS A record. A host's IP address (or what appears to be its IP address) can change when you use a home broadband service such as cable or DSL, or when you dial into an ISP (PPP connection) using a modem. If you have a broadband connection, DDNS allows you to have a full-time Internet server even though you don't have a static IP address.
You run a small DDNS client on your server that sends DNS record update requests to the DDNS server. If you have your own domain name, the DDNS server is the one that's listed as the primary name server in your domain record. Most DNS servers do not support dynamic updates by default. They have to be configured to listen for dynamic updates. When your server is booted up (or you run the client software manually) it sends a request to the DDNS server to check/update the IP address in the A record for your server. If you've pulled a different IP address from your ISP since the last time a request was sent, the A record is updated with this new IP address.
When you use a firewall router, what appears to be your server's IP address is actually the IP address on the "external" router interface. As mentioned on the Networking page, the router does NAT and this address translation can cause difficulties for dynamic DNS. ddclient is a DDNS client that works with firewalls, is compatible with a number of DDNS services, and is available as a Debian package.
Dynamic DNS with Your Own Domain
You can use dynamic DNS if you already have, or want to have, your own registered domain name. You may want your own domain name for several reasons:
|
For this I use the Domain Name+DNS Only Service bundle from EasyDNS.com because it kills two birds with one stone (and because they have toll-free telephone tech support). EasyDNS will not only host your zone files on their DNS server, but register your domain name (and annually renew the domain name registration) all for $35/year. That's a pretty good deal as well as being convenient. You don't have to go to one place to register/renew your domain name, and then go to another place to host your DNS records. When you register a domain name with EasyDNS they'll set up some preliminary zone records for you and you just go in and add/modify/delete records.
EasyDNS provides a Web interface for DNS management so you can play around with the settings, change server names, create alias records, etc.
The best part is support for dynamic DNS is included in their DNS offerings so you can use them for home and test servers that don't have static IP addresses (and ddclient will work with them too).
As a side note, what if you already have a domain name with servers that have static IP addresses? Most places that will host your DNS records (like your ISP or Web hosting provider) won't let you even see them much less work with them. Having EasyDNS host your DNS records will allow you to have some control in the management of your DNS. Their straight DNS records hosting service (without the domain registration/renewal piece) costs $20/year. Just be sure to update your domain record with the EasyDNS name servers information once you sign up for the service and get your DNS records set up. (You also have the option of transferring your domain to them, i.e. making them the domain name registrar, if you want to take advantage of the single-payment convenience thing.)
Having your own non-production domain will not only let you play around with zone records, but you can experiment with having your own authoritative DNS server. One of EasyDNS's servers would be the primary authoritative name server and you could set up your Debian server as a secondary. Then you'd just use the 'nameservers' link in the Web interface to enter your server's hostname and IP address as a secondary server entry. This way you could play with the "zone transfers" that take place between authoritative servers.
But the benefits of having your own non-production domain go beyond just DNS. It also comes in handy for testing Sendmail e-mail server and Apache Web server configurations, etc. For instance, you can see if your Debian system properly sends and receives e-mail for your non-production domain. Or you could install a test cerfificate (available for free from most certifying authorities like Thawte or Verisign) on your Debian system acting as a Web server so you can investigate SSL functionality. Just about any type of Internet server you want to play with will have more functionality when you can give it a registered domain name that has DNS resolution capabilities. And if you don't have any plans to eventually use it as a production domain, just let it expire after the first year is up and the knowledge gained will be well worth the 35 bucks.
ddclient Configuration File for EasyDNS
We'll install ddclient in a bit. It'll prompt you for the necessary configuration information during the install. When it's finished it'll create the /etc/ddclient.conf file and it should look something like this (the information you enter during the client install is in blue):
# Configuration file for ddclient generated by debconf |
If your server is behind a cable/DSL router (such as a Linksys, DLink, or Netgear) or some other type of firewall or proxy server, replace the line:
use=if, if=eth0
with the line:
use=web, web=support.easydns.com/utils/get_ip.php
This simply uses a page on EasyDNS's Web site to display your 'outside' IP address. The ddclient software will read the IP address off the returned HTML code and send it to EasyDNS. It'll do this periodically which is necessary with the changing IP addresses you get with cable and DSL services.
Support for dynamic DNS is disabled by default which is fine if you do have static IP address(es) on your server(s). Enabling dynamic DNS using the EasyDNS Web interface is simply a matter of clicking on the "disabled" link as illustrated below and acknowledging the change on the subsequent confirmation page.
Once you've got your configuration file set up and you've set your domain for dynamic DNS, you can test your ddclient configuration to make sure it's working with the command:
ddclient -daemon=0 -noquiet -debug -verbose
If you use Apache's virtual hosts feature to host multiple Web sites on your server and you have multiple domain names registered with EasyDNS you can update the dynamic DNS for all the domains simultaneously by separating each of the domains with a comma (,) like so:
# Configuration file for ddclient generated by debconf |
The EasyDNS Web interface allows you to add/modify A records, MX records and priorities, aliases (CNAME records), and even the time intervals in the SOA record, all by clicking on the "dns" link shown in the figure above. The "nameservers" link takes you to a page which lists the authoritative DNS server information (EasyDNS's name servers) for your domain.
Your Own DNS Server ?
If you have your own domain name and you also want to try running your own DNS server, EasyDNS.com has a Secondary DNS Service for $15/year which takes some of the risk out of running your own DNS. You set their servers up to transfer zone information from your DNS server. You would then enter your DNS server address as the primary in your domain record, and the EasyDNS DNS server addresses as the secondary DNS servers in your domain record. Then, should your DNS server ever fail, name resolution queries will go to the EasyDNS servers.
| Would you like your own domain name and receive e-mail and Web traffic to your domain without all the work of setting up your own e-mail and Web servers? Piece of cake! Just get the Domain Name+DNS Plus Service bundle from EasyDNS.com for $55/year. This service includes the domain name registration (and renewals) and you'll be able to:
Your own domain name is nice to have for several reasons. You may want to use your last name for your domain name (if it's available). Some of the benefits of having your own domain name include:
Because so many sites use your e-mail address as a login ID it's a real pain to change your e-mail address. Not to mention notifying everyone that your e-mail address is different. With e-mail forwarding using your own domain name, if you switch ISPs you simply change the forwarding address. The ability to have a consistent e-mail address is valuable for students as their e-mail providers change while they go from high school to college to their first job (and having a lastname.com e-mail address looks good on a resume too). It allows you to have the same e-mail address if you're forced to change ISPs because you move to a different city. It also protects you from ISP mergers, failures, and name changes. And having a consistent Web URL via Web forwarding means you won't lose all the search engine traffic to your Web pages if the URL to your personal Web space changes. |
One valuable thing about the EasyDNS.com service is that it supports SPF TXT records. This allows your to set up Sender Policy Framework for your mail server which prevents spammers from sending e-mails using your domain name. SPF is fast becoming the standard for spam prevention. You use it to specify the IP addresses of servers which are allowed to send e-mails on behalf of your domain (usually only one or two). When a spammer sends an e-mail using your domain name, and the receiving e-mail (SMTP) server is configured to use SPF, the receiving e-mail server checks the source address of the e-mail against the allowed addresses listed in your SPF TXT record in your DNS. If there's no match the mail is discarded.
Domain-less DNS For Free
If you have a broadband Internet connection without a static IP and have no desire to have your own domain name, you can use a free service offered by dyndns.org to set up a home Web/e-mail/ftp server. It offers a dynamic DNS service which will redirect traffic to your server using their domain name.
With this free service you use your server's hostname but dyndns.org's domain name. You're basically just adding/modifying an A record for your server in their zone file. Your Web server would have a URL like:
http://your-hostname.dyndns.org
E-mail addressed to your server would have to have an address like:
you@your-hostname.dyndns.org
Because you'll be using your hostname with dyndns.org's domain name, you have to make sure your hostname isn't the same as that of anyone else using their service. As a result, you'll want to come up with a hostname for your server that's really unique. Recall that you set the hostname during the installation. You can always change it by editing the /etc/hosts file. However, you'll also need to check for the current hostname in the configuration files of any server applications that may use it, such as Sendmail and Apache, and edit those files as well.
If you connect your Linux server to the Internet using a modem (we show you how on the Modems page), you'll need to a way to keep your connection up long enough for any dynamic DNS changes to take effect and this could take up to 45 minutes. Most ISPs will drop an inactive connection before that. You can use the ping command to keep your PPP connection up. The trick is to run it in the background and set it so it only sends a ping once every five minutes. Pick a Web site and enter:
ping -i 300 www.chosen-site.com > /dev/null &
Just don't forget to bring it to the foreground and stop it once you've disconnected your modem connection. To bring it to the foreground simply type:
fg ping
and then press Ctrl-C to exit the ping program.
ddclient Configuration File for dyndns.org
If you selected the dyndns.org service when you installed ddclient your /etc/ddclient.conf file should look something like this:
# Configuration file for ddclient generated by debconf |
Note that this file indicates the ppp0 (dialup modem) interface was entered during the installation rather than the 'eth0' that you would use for a network card.
If your server is behind a cable/DSL router (such as a Linksys, DLink, or Netgear) or some other type of firewall or proxy server, replace the line:
use=if, if=ppp0
with the line:
use=web, web=checkip.dyndns.com/, web-skip='Current IP Address:'
This simply uses a page on dyndns.org's Web site to display your 'outside' IP address. The ddclient software will read the IP address off the returned HTML code.
Dynamic DNS is OK for home servers, but it's not really appropriate for businesses. Static IP addresses and having your ISP or a third party like EasyDNS host your DNS records would be more appropriate for Internet server implementations by businesses.Security Note: Even home Web/e-mail servers need to be set up securely. Spammers have a talent for quickly locating improperly secured e-mail servers and using them as spam relay points. This not only puts your server at risk but gobbles up all your bandwidth. If you are going to set up a home Web/e-mail server, be sure to do it securely. That not only involves setting up the server in a secure fashion during the initial install, but also includes configuring Apache and Sendmail in a secure manner. The procedures on these pages do not result in secure servers. If you are going to set up your own Web/e-mail server you'll need to buy some books and do some research to learn how to do it securely. More information is given on theSecuring Servers page. You'll also want to take a look at the Firewall page for information on how to use IPTABLES entries to help protect your server and your home network. (Remember that if you only have one server Apache and Sendmail are going to be running on the same system that is acting as your NAT/firewall system.) In addition, the Packages page shows you how to use the cron scheduler and a shell script to automatically keep your system up to date with the latest security patches.
Installing ddclient
Before installing this package be sure to sign up for an account with EasyDNS or dyndns.org. You'll need your account username and password when you install the package. With your account set up you install the package by typing in
apt-get install ddclient
at the shell prompt. You'll then be prompted for the following:
The client will now be installed and the appropriate configuration file like the ones shown above will be created. Even though the file was created for you, we showed you the typical files for both dyndns.org and EasyDNS services in case you need to edit them at a later point. If you want to examine your config file you can do so using the nano text editor with the command:
- Select the service you want to use.
- The next screen may seem confusing if you selected EasyDNS in
Step 1 because it prompts you for "your DynDNS fully qualified domain names" and then gives examples for dyndns.org. What they mean by the "DynDNS" is "Dynamic DNS", not "DynDNS.org". The "fully qualified" is also a bit misleading. You don't need to enter a trailing period after the TLD (.com, .net, or .org Top Level Domain). All you need to do is enter your server's hostname followed by your, or dyndns.org's, domain name. Examples:or
debian.gates.comvery-unique-hostname.dyndns.org - Enter the username you chose when you signed up with your service.
- Enter the password you chose when you signed up with your service.
- Enter the interface that will be connecting to the service. This will most likely be 'eth0' for an ethernet card (even if it is connected to a LAN which has a firewall router) or 'ppp0' for modem use (note that that's a zero on the end, not the letter O).
- If you entered 'ppp0' you'll be asked if you want ddcleint to run automatically every time you connect. You may want to select No here so you have the option of running it or not.
- You'll then be asked if you want to run ddclient as a daemon. If this server is going to be a full-time Web or e-mail server with a broadband connection you should answer Yes to this.
nano /etc/ddclient.conf
If you're using a modem connection you'll want to first connect to your ISP with the pon command. If you didn't set ddclient to run as a daemon then just type in:
ddclient
at the shell prompt once you're connected. The resulting message will tell you what IP address your external interface has (and what the DNS record will be updated with.
As mentioned earlier, it will take awhile for this update to take affect. To see if it has taken affect yet, try pinging using your domain name and see if the returned IP address matches what was indicated in the message when you started ddclient. Note that even if you used the above ping command in the background to keep your connection up you can still issue a second ping command in the foreground to check the returned IP address.
| Other DNS Server Files | |
Given that a DNS server can host the zone files for many different domains, each having two zone files, it needs a way to tell which zone files are for which domains. It does this in the named.conf file which, like the zone files themselves, is located in the /etc/bind directory (which you'll see when we install Bind shortly).
Of the two zone files for each domain the one we've been talking about all along has been for forward lookups (resolving names to IP addresses). This zone file is typically named
DNS also offers a "reverse lookup" function that allows you to translate IP addresses to host/domain names. The information that allows this to happen is stored in the second zone file. Here's a reverse-lookup zone file that corresponds to the simpler zone file we showed earlier:
$TTL 86400 |
Note that the NS records are the same but there's no A records. And since we only have one system handling all three Web, e-mail, and FTP server functions we only need one PTR record. A PTR (Pointer) record is the opposite of an A record. It has the host part of the IP address and gives the corresponding hostname. Typically you want a PTR record for every A record in the forward-lookup file provided the server is in the domain. We don't have PTR records for the name servers above because they're in a different domain (and thus in a different address space).
Why is only the host part of the IP address needed in this file? Because the network portion of the IP address is used when naming the reverse-lookup zone file, and it's reversed. Because 192.168.1.x is a Class C network, the first three octets make up the network portion of the IP address so it's used in the zone file name. Only the last octet specifies the individual host so it's used to specify the host in PTR records. With the above example IP address, the zone file would be named:
db.1.168.192
The reverse-lookup zone file is also located in the /etc/bind directory. There's another place this naming convention is used. Take a look at the start of the SOA record. The domain is specified as
1.168.192.in-addr.arpa
in-addr.arpa is the default domain for all reverse lookups. As you'll see below, the shorthand method of specifying this with the '@' is normally used.
| DNS Tools, Testing, and Troubleshooting | |
When you're testing changes to your DNS records things may not act the way you expect them to. What you need is some patience. Most DNS servers cache lookups. If you make a change to a zone record on EasyDNS or dyndns.org, or the IP address you pulled from your ISP changes and ddclient sends the update, it'll take the DNS servers at EasyDNS or dyndns.org up to 15 minutes to update. Then the DNS server that your desktop system is using to resolve names may cache the old information for another 20 to 30 minutes.
If you're using a Windows system to test DNS changes don't forget that it also has a DNS cache. You can clear it manually in a DOS window with the command:
ipconfig /flushdns
As a result, if you make a change to your zone records give it at least 45 minutes before you try to see if the changes had the desired effect. Web browsers also cache name-to-address information. If you're using a Web browser to test your changes, you may want to go and delete all the files in the browser's cache directory as well.
The above makes playing around with dynamic DNS when using a modem kind of a pain. You have to keep the connection up for for at least 45 minutes because if you disconnect, you'll pull a different IP address when you reconnect and your DNS records will have invalid IP addresses. That's why I showed you how to run the ping command in the background to keep the dial-up connection alive.
A DNS problem will likely be in one of three places:
- The DNS server addresses specified in the TCP/IP configuration on the PC you are using to do the pinging are not correct.
- The registrar's domain record does not contain the correct name server hostnames and/or addresses.
- The authoritative DNS servers for the domain do not have the domain's zone records configured correctly.
The most basic tool for testing DNS is the ping command. If you can ping a Web server using its IP address but not it's domain name, you have a DNS problem. If you can ping a server using its domain name you'll notice that the server's IP address is also displayed. Verifying that this is the correct IP address will verify that DNS is working properly. Another thing ping can tell you is if you're pinging an actual server or an alias. Using the MIT example again, you may type in
ping www.mit.edu
but the response will be something like
Pinging DANDELION-PATCH.mit.edu
Another common tool for testing DNS is nslookup (name server lookup) and it's available on Linux systems and NT-class Windows systems (NT-WS, 2000 Pro). As you saw earlier in this page this command will show you what name server your PC is using to resolve names, as well as return hostname and address information on the server that's specified as the target of the command. However, it also has an interactive mode that increase its usefulness. If you simply type in:
nslookup
and you'll get a > prompt. There are several statements that you can enter at his prompt. One helpful one is when you want your system to send queries to a different, other than the default, name server. At the prompt type in the 'server' command followed by the IP address of the DNS server to use:
server 192.168.10.10
Then you just type in the domain name you want information on at the prompt. You'll see in the response that the name server being queried has changed to the one you specified. Type 'exit' at the prompt when you're done. Another similar tool on Linux systems is the dig command. You can specify the alternate DNS server to use on the command line:
dig 192.168.10.10 mit.edu any
The any parameter tells it to return information on all record types. Check the man pages for dig and nslookup for more information.
If you want to make sure that BIND isn't having a problem with your zone files, you can check the syslog after you boot the system (which is when BIND starts up and reads the zone files). At a shell prompt just type in:
nano /var/log/syslog
and look near the bottom of the file. You'll see messages when BIND was started. Check to see if any of them refer to any errors that were encountered. If it didn't have a problem with the zone file you'll see it referenced along with:
loaded serial 1
indicating that it has set the serial number (version) to 1.
| Your Own DNS Server | |
Don't set up your Debian system as a DNS server if it doesn't have access to the Internet. It will try and use external DNS servers (called "root hints" which we explain later) to resolve names and they won't be accessible. This will cause problems trying to FTP or telnet to your Debian server even over a local LAN using only IP addresses.
DNS is simply another server application. You can use your Linux system as an authoritative, LAN, or simple DNS server. Simple DNS servers and LAN servers which also provide simple DNS services (resolving Internet host/domain names) need to be connected to the Internet but being behind a firewall should not present a problem as long as you have UDP port 53 is open on the firewall. If you're going to set up and test a secondary authoritative DNS server you'll also need to have TCP port 53 open on the firewall as well for zone transfers.
We'll show you how to set up simple and LAN DNS servers in this section. Setting up production ("real") authoritative DNS servers (remember that you need at least two) is beyond the scope of this page because you'll need to do quite a bit more reading to learn about zone transfers (insecure and secure) between primary and secondary servers and you'll need to know a lot more about the named.conf file. The issue of server security also becomes more important. However, seeing how to set up DNS server files for a LAN DNS server will be a good start.
| Where to learn more - The best of our bookshelves: | ||
 More info... | DNS and BIND is another case where an O'Reilly book is considered the bible in the industry. I doubt there's a DNS server admin out there that doesn't have a copy. The 4th Edition covers BIND 9 with its security enhancements. The first three chapters provide a detailed foundation in the basics of DNS operation from zone files to root name servers. From there it's all about server configuration. Setting up multiple servers, incremental zone transfers, and round-robin load distribution are just a few of the things covered. It also covers how to set up a server to respond to DDNS requests from clients and DHCP servers as well as how to control which systems have this ability through ACLs (Access Control Lists). How to use BIND's debugging levels and debugger output to solve problems is also covered. | |
A Simple DNS Server
As mentioned earlier, the most widely used DNS application is called BIND and installing it is simply a matter of entering the command:
apt-get install bind9
Congratulations! You now have a simple DNS server. Now just change the DNS server settings in the TCP/IP configuration files on the workstations on your LAN so that they start using this server as their preferred DNS server. You can use your ISP's DNS server(s) as alternate servers as this will provide some redundancy if your server ever goes down. You'll also want to modify the /etc/resolv.conf file on the DNS server itself so that it points to itself. Do that by opening the file in a text editor with the command:
nano /etc/resolv.conf
and making sure the first nameserver line is:
nameserver 127.0.0.1
Why is setting up a simple DNS server so easy? Because of things called "root hints". The root hints are a list of root-level DNS servers in the /etc/bind/db.root file. Your simple DNS server will query a root server to get the addresses of authoritative DNS servers for each given domain (so it can contact those authoritative DNS servers to get the IP addresses of the desired hosts).
Just remember that your simple DNS server needs a 24/7 connection to the Internet. Or it at least needs to be connected to the Internet any time any system on your LAN needs to access anything on the Internet.
A LAN DNS Server
We'll cover setting up a LAN DNS server for a small LAN where the workstation addresses are statically assigned. If you have a larger LAN that uses DHCP, you'll need to set up the server to respond to DDNS update requests because a system's A record will need to be updated when DHCP assigns the system a different address.
In setting up a LAN DNS server we need to:
- Create the forward and reverse zone files.
- Update the named.conf configuration file with things called "forwarders"
- Update the named.conf configuration file so that the server knows it's authoritative for the LAN domain.
Here's the forward-lookup zone file for a LAN with the domain name kplan.net. Note that the A records are grouped together, as are the other record types, and that there are no blank lines. However, when trying to get my DNS server to work I did see an error in the syslog file about the reverse-lookup zone file not ending in a "new line" so make sure there's a blank line at the bottom of the file.
$TTL 86400 |
And here's the reverse-lookup zone file for the same domain:
$TTL 86400 |
Notice that instead of using 10.168.192.in-addr.arpa at the start of the SOA record I just used the shortcut. Now when I add a new system to my network I can just add entries to these two files rather than editing the HOSTS files on all of the servers and workstations.
If you created these files on a Windows system using Notepad and FTPed them to your Debian server, go into the directory you FTPed them into and move/rename them like so:
mv forward.txt /etc/bind/db.kplan.net
While the zone file naming convention that BIND uses by default is db. followed by the domain name, and the reverse-lookup zone file is similar except that the domain name is replaced by the reversed network address, you can actually name them whatever you want. You tell the server what zone files to use in the named.conf file.
named.conf
The named.conf file is the main configuration file for a DNS server. In it you tell the server what, if any, forwarders to use, what domains it's authoritative for, and which zone files it should use for each domain.
Forwarders let you specify other DNS servers to use when your DNS server receives a query for a domain it isn't authoritative for. Your LAN DNS server will be authoritative for your LAN's domain name, but it won't know about domains on the Internet. When it gets a query for an Internet domain it will forward the request out to a DNS server specified in the forwarders section of the named.conf file.
Open the /etc/bind/named.conf file using the ee text editor. In the options section you'll see an indented block of text like this:
// forwarders { |
You typically want to put your ISP's DNS servers here. The '//' are comment characters in this file so you'll need to remove those also. You should end up with a block of text that looks like this:
forwarders { |
We used private addresses in the above example but naturally these would be publically-accessible DNS servers (your ISP's). Now we have to add the content to the file so the server knows it knows it's authoritative for the kplan.net domain. At the bottom of the file you'll see the line:
// add entries for other zones below here
Below this line we'll enter the following for the forward and reverse zone files:
zone "kplan.net" { |
Save the file and we're in business from a server perspective. The named daemon is running, we already have a root hints database, our zone files our set up, and our forwarders are set up in the configuration file. Now just change the /etc/resolv.conf file on any Debian and UNIX systems so it looks like this:
search kplan.net |
On Windows systems you'd have to change the "Preferred DNS server" in the TCP/IP properties to the 192.168.10.50 address.
Now that you've got a feel for what DNS does for you, and possibly have your own domain name with name resolution capabilities, it's time to start setting up some servers.
| Do NOT plan to use the system you will create using these guide pages as a "production" (real) server. It will NOT be secure! There are many steps involved in creating a secure Internet or LAN server. While we do refer to some things you can do to make your system more secure, there are many other measures related to system security that also need to be taken into consideration and they are not covered on these pages. These guide pages are meant as a learning tool only. The knowledge gained on these pages will help you understand the material covered in security-related publications when you are ready to consider setting up a production server. |
No comments:
Post a Comment