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Integrating
with Other Protocols
Some types of DHCP options require that some
other package exchange data with the DHCP server. Frequently, this is a matter
of the DHCP client accepting and using additional information from the DHCP
server. Other times, some other server must consult the DHCP server to help
with its configuration. The preceding discussion has included an example in
which the DHCP server uses another server to obtain information: The get-lease-hostnames parameter allows the DHCP server to set clients' hostnames based on
a DNS server lookup of the IP address being assigned.
Including NetBIOS Information
NetBIOS, which serves as the underpinning for
the Server Message Block (SMB)/Common Internet Filesystem (CIFS) file- and
printer-sharing protocols described in
series of structures and tools atop TCP/IP to do its job. You can configure a
DHCP server to deliver information on some of these structures to Windows
clients to help these systems operate most efficiently. Specifically,
you can set the following dhcpd.conf
parameters: option
netbios-name-servers server-addresses Traditionally, NetBIOS has relied upon name resolution systems
that are independent of those used by most TCP/IP clients. NetBIOS computers
can use a broadcast model, in which computers send broadcasts on the local
network to locate clients, or they can use name servers independent of the DNS
name servers used by TCP/IP. These independent name servers are known as
NetBIOS Name Service (NBNS) servers or Windows Internet Name Service (WINS)
servers. You can give Windows clients the names of these servers with the option netbios-name-servers parameter. You'll usually list just one server here, although DHCP
can deliver multiple WINS server addresses. option
netbios-node-type type-code This option is used in conjunction with the preceding one to tell
clients whether to use broadcast name resolution or a WINS server. The type-code is a value that ranges from 1 to 8 . Values of 1 and 2 mean to use broadcasts or a WINS server, respectively. Values of 4 and 8 mean to use
both, with 4 causing the client to try broadcasts first and fall back on the
WINS server, and 8 to use WINS first and fall back on broadcasts. Most networks that
have WINS servers should use a type-code of 8 , because
this will reduce network broadcasts but provide the most reliable operation if the
WINS server goes down or lacks data on a particular computer. option
netbios-dd-server server-address This option sets one of the more obscure aspects of NetBIOS
networking: the address of the NetBIOS Datagram Distribution (NBDD) server. This
server relays broadcasts to clients that might not otherwise receive them. Chances
are you won't need to set this option. option
netbios-scope string
NetBIOS scope refers to a group of computers
across which a given NetBIOS name is known. Most NetBIOS systems leave their
scopes unchanged, so any NetBIOS computer on a network can resolve the name of
any other. If your network uses NetBIOS scopes, though, you may need to set
this option (possibly in a group declaration).It's often beneficial to set the first two of
these options globally on a DHCP server that delivers IP addresses to Windows
clients. (Linux's Samba package doesn't use the values delivered by DHCP, but
its Windows clients may.) For instance, you might include the following two
lines near the top of the dhcpd.conf file: option netbios-name-servers 192.168.1.1; option netbios-node-type 8;
To be sure that the Windows computers use this information, you should check the Use DHCP
for WINS Resolution option in the TCP/IP Properties dialog box in Windows, as
shown in
select Disable WINS Resolution, the system won't use WINS at all, and if you
select Enable WINS Resolution, you must manually enter the IP address of your
WINS server.
Figure 5.3. You must
explicitly configure Windows clients to use NetBIOS-specific options provided
by a DHCP server.
Communicating with a DNS Server
If you want to be able to directly address a
DHCP client from random systems, you have two choices: You can configure the
DHCP server to give the client a fixed IP address, as described earlier in this
chapter; or you can configure the DHCP server and the DNS server to communicate
with one another, in order to update the DNS server's records to reflect the
DHCP client's current address. This latter approach works best when the DHCP
clients' addresses don't change very often, and when you're interested
primarily in accessing the DHCP client from other systems on your local
network. When Internet access is needed, a changing IP address may take some
time to propagate through the Internet's DNS server collection, so there may be
attempts to reach the computer at its old address for some time after that
address has changed.Prior to version 3.0, dhcpd included
no explicit support for DNS updates. Version 3, though, implements two
preliminary methods of DNS updates. Another is on track to become an Internet
standard, and will be implemented in a future version of dhcpd , once
the standard is finalized. The two methods that are currently implemented are
the ad-hoc method and the interim method.NOTE
align=left border=0> Other update methods also exist, which are
usually implemented by third-party packages. These tools monitor DHCP log files
and then change the DNS server's configuration when the DHCP log files
indicate important actions.
Dynamic DNS updates are enabled through the ddns-update-style parameter, which takes one of three values: ad-hoc , interim , or none (the last
of these is the default). To use dynamic DNS, enable one of the first two
values in the global section of the dhcpd.conf fileit won't
work if you try to enable it for just some clients.WARNING
align=left border=0> Both these methods of DNS updates rely upon
the DNS server accepting the updates. This topic is covered in
Administering a Domain via DNS.
Using the
Ad-Hoc Update Method
The ad-hoc method obtains a hostname for a
client in one of four ways, in the following priority:1. If a host declaration contains
a ddns-hostname option, the value of that option is used.2. If the client passes a fully-qualified domain
name (FQDNthat is, a hostname complete with domain name portion), the DHCP
server uses the hostname portion of the FQDN.3. If the client passes a hostname without a domain
name, the DHCP server uses it.4. The name of the host declaration for the
client may be used.In all these cases, only the local hostname
portion is used. If none of these methods yields a hostname, the DHCP server
does not attempt to perform a DNS update for
the client. If the DHCP server can find a hostname, the server combines the
hostname with the value of the ddns-domainname option, if it's present,
or with the domain-name option if there's no ddns-domainname option.The DHCP server uses the FQDN it's derived to
update the DNS server. It first updates the A recordthe record that controls forward lookups, in which a user enters a hostname
and obtains an IP address from that. If the A record change was successful, the
DHCP server then updates the PTR record, which is used for reverse lookups, in which a user enters an IP address
to obtain a hostname.
Using the Interim Update Method
The interim update method works much like the
ad-hoc method in many ways, but it provides support for allowing the client to
update the DNS server's A record, if the client makes this request. You may
include the allow
client-updates or ignore client-updates statements in the dhcpd. conf file to tell DHCP to allow clients to do this job themselves, or to
ignore such requests, respectively. The default is to honor client requests on
this matter.If the DHCP server is configured to honor
client DNS updates, then the DHCP server uses the FQDN provided by the client
to perform PTR updates. If the DHCP server disallows this option, then the DHCP
server constructs an FQDN from the client's hostname and the domain name set in
dhcpd.conf , and uses this name to update both the A and PTR records.When configured to disallow client-initiated
DNS changes, the interim method creates results that are similar to those of
the ad-hoc method. When the interim method permits client-initiated DNS
changes, though, there's a difference: The client might specify a domain name
that's something other than what the DHCP server is configured to use. For
instance, suppose the DHCP server hands out hostnames in the threeroomco.com domain. Using the ad-hoc method, all clients that receive DNS
updates will receive them in this domain, even if the client presents an FQDN
in another domain. With the interim method configured to permit client updates,
the client could claim a hostname of, say, dino.pangaea.edu . Assuming that this domain's DNS servers accept the update, the
client could then be reached by this name. If your own DHCP and DNS servers
interoperate fully, the PTR record for reverse lookups would also then return
the name dino.pangaea.edu when asked about the name associated with the dynamically granted
IP address for the client. Such a configuration isn't possible with the ad-hoc
method. Whether or not it's a desirable
configuration is something you'll have to decide for yourself. If your network
connections are tightly controlled and your users are likely to need hostnames
in foreign domains, such a configuration might be desirable. You might prefer
to prevent such configurations if your network is relatively easy to access
physically, though, because a reverse lookup of one of your IP addresses linked
to a foreign domain might cause confusion or help a cracker mask illegal
activities.
Dynamic DNS Services
Some users of broadband Internet access,
such as Digital Subscriber Line (DSL) and cable modems, receive dynamic IP
addresses from their ISPs using DHCP or other protocols. If you're in this
situation and want a fixed hostname for your computer, you probably won't be
able to use the dynamic DNS features of DHCP and DNS on Linux, since what you
really need to do is to interoperate with an Internet-accessible DNS server. The
solution is to use one of many dynamic DNS services
that are available for free or at low cost.A dynamic DNS service operates its own DNS
servers, which provide lookups on names on the Internet at large. Once you
contract with such a service, you can use a client package on your computer
to contact the provider's DNS server whenever your IP address changes. You
can then have a fixed hostname (either within a domain owned by the dynamic
DNS provider or within your own domain) that points to your dynamic IP
address. Many dynamic DNS providers offer update software written in Perl or
other common scripting languages, so most of them are Linux-compatible. You
should check on this detail to be sure, though.There are many dynamic DNS providers today.
Indeed, there are a large number of lists of
dynamic DNS providers. These include
provider that suits your needs, if you don't operate your own externally
accessible DNS server.
