DNS HOWTO
Nicolai Langfeldt janl@math.uio.no
v2.2, 11 February 1999
HOWTO become a totally small time DNS admin.
______________________________________________________________________
Table of Contents
1. Preamble
1.1 Legal stuff
1.2 Credits and request for help.
1.3 Dedication
2. Introduction.
3. A caching only name server.
3.1 Starting named
3.2 Making it even better
3.3 Congratulations
4. A
4.1 But first some dry theory
4.2 Our own domain
4.3 The reverse zone
4.4 Words of caution
4.5 Why reverse lookups don't work.
4.5.1 The reverse zone isn't delegated.
4.5.2 You've got a classless subnet
5. A real domain example
5.1 /etc/named.conf (or /var/named/named.conf)
5.2 /var/named/root.hints
5.3 /var/named/zone/127.0.0
5.4 /var/named/zone/land-5.com
5.5 /var/named/zone/206.6.177
6. Maintenance
7. Converting from version 4 to version 8
8. Questions and Answers
9. How to become a bigger time DNS admin.
______________________________________________________________________
1. Preamble
Keywords: DNS, bind, bind-4, bind-8, named, dialup, ppp, slip, isdn,
Internet, domain, name, hosts, resolving, caching.
This document is part of the Linux Documentation Project.
1.1. Legal stuff
(C)opyright 1995-1999 Nicolai Langfeldt. Do not modify without
amending copyright, distribute freely but retain copyright message.
1.2. Credits and request for help.
I want to thank Arnt Gulbrandsen whom I cause to suffer through the
drafts to this work and whom provided many useful suggestions. I also
want to thank the numerous people that have e-mailed suggestions and
notes.
This will never be a finished document, please send me mail about your
problems and successes, it can make this a better HOWTO. So please
send comments and/or questions or money to janl@math.uio.no. If you
send e-mail and want an answer please show the simple courtesy of
making sure that the return address is correct and working. Also,
please read the ``QnA'' section before mailing me. Another thing, I
can only understand Norwegian and English.
If you want to translate this HOWTO please notify me so I can keep
track of what languages it has been published in, and also I can
notify you when the HOWTO has been updated.
1.3. Dedication
This HOWTO is dedicated to Anne Line Norheim Langfeldt. Though she
will probably never read it since she's not that kind of girl.
2. Introduction.
What this is and isn't.
DNS is is the Domain Name System. DNS converts machine names to the
IP addresses that all machines on the net have. It maps from name to
address and from address to name, and some other things. This HOWTO
documents how to define such mappings using a Linux system. A mapping
is simply a association between two things, in this case a machine
name, like ftp.linux.org, and the machines IP number (or address)
199.249.150.4.
DNS is, to the uninitiated (you ;-), one of the more opaque areas of
network administration. This HOWTO will try to make a few things
clearer. It describes how to set up a simple DNS name server.
Starting with a caching only server and going on to setting up a
primary DNS server for a domain. For more complex setups you can
check the ``QnA'' section of this document. If it's not described
there you will need to read the Real Documentation. I'll get back to
what this Real Documentation consists of in ``the last chapter''.
Before you start on this you should configure your machine so that you
can telnet in and out of it, and successfully make all kinds of
connections to the net, and you should especially be able to do telnet
127.0.0.1 and get your own machine (test it now!). You also need a
good /etc/nsswitch.conf (or /etc/host.conf), /etc/resolv.conf and
/etc/hosts files as a starting point, since I will not explain their
function here. If you don't already have all this set up and working
the NET-3-HOWTO and/or the PPP-HOWTO explains how to set it up. Read
them.
When I say `your machine' I mean the machine you are trying to set up
DNS on. Not any other machine you might have that's involved in your
networking effort.
I assume you're not behind any kind of firewall that blocks name
queries. If you are you will need a special configuration, see the
section on ``QnA''.
Name serving on Unix is done by a program called named. This is a
part of the ``bind'' package which is coordinated by Paul Vixie for
The Internet Software Consortium. Named is included in most Linux
distributions and is usually installed as /usr/sbin/named. If you
have a named you can probably use it; if you don't have one you can
get a binary off a Linux ftp site, or get the latest and greatest
source from ftp.isc.org:/isc/bind/src/cur/bind-8/. This HOWTO is
about bind version 8. The old version of the HOWTO, about bind 4 is
still available at http://www.math.uio.no/~janl/DNS/ in case you use
bind 4. If the named man page talks about (at the very end, the FILES
section) named.conf you have bind 8, if it talks about named.boot you
have bind 4. If you have 4 and are security conscious you really
ought to upgrade to a recent 8.
DNS is a net-wide database. Take care about what you put into it. If
you put junk into it, you, and others will get junk out of it. Keep
your DNS tidy and consistent and you will get good service from it.
Learn to use it, admin it, debug it and you will be another good admin
keeping the net from falling to it's knees by mismanagement.
In this document I state flatly a couple of things that are not
completely true (they are at least half truths though). All in the
interest of simplification. Things will (probably ;-) work if you
believe what I say.
Tip: Make backup copies of all the files I instruct you to change if
you already have them, so if after going through this nothing works
you can get it back to your old, working state.
3. A caching only name server.
A first stab at DNS config, very useful for dialup users.
A caching only name server will find the answer to name queries and
remember the answer the next time you need it. This will shorten the
waiting time the next time significantly, especially if you're on a
slow connection.
First you need a file called /etc/named.conf. This is read when named
starts. For now it should simply contain:
______________________________________________________________________
// Config file for caching only name server
options {
directory "/var/named";
// Uncommenting this might help if you have to go through a
// firewall and things are not working out:
// query-source port 53;
};
zone "." {
type hint;
file "root.hints";
};
zone "0.0.127.in-addr.arpa" {
type master;
file "pz/127.0.0";
};
______________________________________________________________________
The `directory' line tells named where to look for files. All files
named subsequently will be relative to this. Thus pz is a directory
under /var/named, i.e., /var/named/pz. /var/named is the right
directory according to the Linux File system Standard.
The file named /var/named/root.hints is named in this.
/var/named/root.hints should contain this:
______________________________________________________________________
;
; There might be opening comments here if you already have this file.
; If not don't worry.
;
. 6D IN NS G.ROOT-SERVERS.NET.
. 6D IN NS J.ROOT-SERVERS.NET.
. 6D IN NS K.ROOT-SERVERS.NET.
. 6D IN NS L.ROOT-SERVERS.NET.
. 6D IN NS M.ROOT-SERVERS.NET.
. 6D IN NS A.ROOT-SERVERS.NET.
. 6D IN NS H.ROOT-SERVERS.NET.
. 6D IN NS B.ROOT-SERVERS.NET.
. 6D IN NS C.ROOT-SERVERS.NET.
. 6D IN NS D.ROOT-SERVERS.NET.
. 6D IN NS E.ROOT-SERVERS.NET.
. 6D IN NS I.ROOT-SERVERS.NET.
. 6D IN NS F.ROOT-SERVERS.NET.
G.ROOT-SERVERS.NET. 5w6d16h IN A 192.112.36.4
J.ROOT-SERVERS.NET. 5w6d16h IN A 198.41.0.10
K.ROOT-SERVERS.NET. 5w6d16h IN A 193.0.14.129
L.ROOT-SERVERS.NET. 5w6d16h IN A 198.32.64.12
M.ROOT-SERVERS.NET. 5w6d16h IN A 202.12.27.33
A.ROOT-SERVERS.NET. 5w6d16h IN A 198.41.0.4
H.ROOT-SERVERS.NET. 5w6d16h IN A 128.63.2.53
B.ROOT-SERVERS.NET. 5w6d16h IN A 128.9.0.107
C.ROOT-SERVERS.NET. 5w6d16h IN A 192.33.4.12
D.ROOT-SERVERS.NET. 5w6d16h IN A 128.8.10.90
E.ROOT-SERVERS.NET. 5w6d16h IN A 192.203.230.10
I.ROOT-SERVERS.NET. 5w6d16h IN A 192.36.148.17
F.ROOT-SERVERS.NET. 5w6d16h IN A 192.5.5.241
______________________________________________________________________
The file describes the root name servers in the world. This changes
over time and must be maintained. See the ``maintenance section'' for
how to keep it up to date.
The next section in named.conf is the last zone. I will explain its
use in a later chapter, for now just make this a file named 127.0.0 in
the subdirectory pz:
______________________________________________________________________
@ IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
1 ; Serial
8H ; Refresh
2H ; Retry
1W ; Expire
1D) ; Minimum TTL
NS ns.linux.bogus.
1 PTR localhost.
______________________________________________________________________
Next, you need a /etc/resolv.conf looking something like this:
______________________________________________________________________
search subdomain.your-domain.edu your-domain.edu
nameserver 127.0.0.1
______________________________________________________________________
The `search' line specifies what domains should be searched for any
host names you want to connect to. The `nameserver' line specifies
the address of your nameserver, in this case your own machine since
that is where your named runs (127.0.0.1 is right, no matter if your
machine has an other address too). If you want to list several name
servers put in one `nameserver' line for each. (Note: Named never
reads this file, the resolver that uses named does.)
To illustrate what this file does: If a client tries to look up foo,
then foo.subdomain.your-domain.edu is tried first, then foo.your-
fomain.edu, finally foo. If a client tries to look up
sunsite.unc.edu, sunsite.unc.edu.subdomain.your-domain.edu is tried
first (yes, it's silly, but that's the way it works), then
sunsite.unc.edu.your-domain.edu, and finally sunsite.unc.edu. You may
not want to put in too many domains in the search line, it takes time
to search them all.
The example assumes you belong in the domain subdomain.your-
domain.edu, your machine then, is probably called your-
machine.subdomain.your-domain.edu. The search line should not contain
your TLD (Top Level Domain, `edu' in this case). If you frequently
need to connect to hosts in another domain you can add that domain to
the search line like this:
______________________________________________________________________
search subdomain.your-domain.edu your-domain.edu other-domain.com
______________________________________________________________________
and so on. Obviously you need to put real domain names in instead.
Please note the lack of periods at the end of the domain names. This
is important, please note the lack of periods at the end of the domain
names.
Next, depending on your libc version you either need to fix
/etc/nsswitch.conf or /etc/host.conf. If you already have
nsswitch.conf that's what we'll fix, if not, we'll fix host.conf.
/etc/nsswitch.conf
This is a long file specifying where to get different kinds of data
types, from what file or database. It usually contains helpful
comments at the top, which you should consider reading. After that
find the line starting with `hosts:', it should read
______________________________________________________________________
hosts: files dns
______________________________________________________________________
If there is no line starting with `hosts:' then put in the one above.
It says that programs should first look in the /etc/hosts file, then
check DNS according to resolv.conf.
/etc/host.conf
It probably contains several lines, one should start with order and it
should look like this:
______________________________________________________________________
order hosts,bind
______________________________________________________________________
If there is no `order' line you should add one. It tells the host
name resolving routines to first look in /etc/hosts, then ask the name
server (which you in resolv.conf said is at 127.0.0.1).
3.1. Starting named
After all this it's time to start named. If you're using a dialup
connection connect first. Type `ndc start', and press return, no
options. If that does not work try `/usr/sbin/ndc start' instead. If
that back-fires see the ``QnA'' section. If you view your syslog
message file (usually called /var/adm/messages, but another directory
to look in is /var/log and another file to look in is syslog) while
starting named (do tail -f /var/log/messages) you should see something
like:
(the lines ending in \ continue on the next line)
Feb 15 01:26:17 roke named[6091]: starting. named 8.1.1 Sat Feb 14 \
00:18:20 MET 1998 ^Ijanl@roke.uio.no:/var/tmp/bind-8.1.1/src/bin/named
Feb 15 01:26:17 roke named[6091]: cache zone "" (IN) loaded (serial 0)
Feb 15 01:26:17 roke named[6091]: master zone "0.0.127.in-addr.arpa" \
(IN) loaded (serial 1)
Feb 15 01:26:17 roke named[6091]: listening [127.0.0.1].53 (lo)
Feb 15 01:26:17 roke named[6091]: listening [129.240.230.92].53 (ippp0)
Feb 15 01:26:17 roke named[6091]: Forwarding source address is [0.0.0.0].1040
Feb 15 01:26:17 roke named[6092]: Ready to answer queries.
If there are any messages about errors then there is a mistake. Named
will name the file it is in (one of named.conf and root.hints I hope
:-) Kill named and go back and check the file.
Now you can test your setup. Start nslookup to examine your work.
$ nslookup
Default Server: localhost
Address: 127.0.0.1
>
If that's what you get it's working. We hope. Anything else, go back
and check everything. Each time you change the named.conf file you
need to restart named using the ndc restart command.
Now you can enter a query. Try looking up some machine close to you.
pat.uio.no is close to me, at the University of Oslo:
> pat.uio.no
Server: localhost
Address: 127.0.0.1
Name: pat.uio.no
Address: 129.240.130.16
nslookup now asked your named to look for the machine pat.uio.no. It
then contacted one of the name server machines named in your
root.hints file, and asked its way from there. It might take tiny
while before you get the result as it may need to search all the
domains you named in /etc/resolv.conf.
If you ask the same again you get this:
> pat.uio.no
Server: localhost
Address: 127.0.0.1
Non-authoritative answer:
Name: pat.uio.no
Address: 129.240.2.50
Note the ``Non-authoritative answer:'' line we got this time around.
That means that named did not go out on the network to ask this time,
the information is in the cache now. But the cached information might
be out of date (stale). So you are informed of this (very slight)
possibility by it saying `Non-authorative answer:'. When nslookup
says this the second time you ask for a host it's a sure sign that
named caches the information and that it's working. You exit nslookup
by giving the command `exit'.
3.2. Making it even better
In large, well organized, academic or ISP (Internet Service Provider)
networks you will sometimes find that the network people has set up a
forwarder hierarchy of DNS servers which helps lighten the internal
network load and on the outside servers as well. It's not easy to
know if you're inside such a network or not. It is however not
important and by using the DNS server of your network provider as a
``forwarder'' you can make the responses to queries faster and less of
a load on your network. If you use a modem this can be quite a win.
For the sake of this example we assume that your network provider has
two name servers they want you to use, with IP numbers 10.0.0.1 and
10.1.0.1. Then, in your named.conf file, inside the opening section
called ``options'' insert these lines:
______________________________________________________________________
forward first;
forwarders {
10.0.0.1;
10.1.0.1;
};
______________________________________________________________________
Restart your nameserver and test it with nslookup. Should work fine.
3.3. Congratulations
Now you know how to set up a caching named. Take a beer, milk, or
whatever you prefer to celebrate it.
4. A simple domain.
How to set up your own domain.
4.1. But first some dry theory
Before we really start this section I'm going to serve you some theory
on and an example of how DNS works. And you're going to read it
because it's good for you. If you don't want to you should at least
skim it very quickly. Stop skimming when you get to what should go in
your named.conf file.
DNS is a hierarchical, tree structured, system. The top is written
`.' and pronounced `root'. Under . there are a number of Top Level
Domains (TLDs), the best known ones are ORG, COM, EDU and NET, but
there are many more. Just like a tree it has a root and it branches
out. If you have any computer science background you will recognize
DNS as a search tree, and you will be able to find nodes, leaf nodes
and edges.
When looking for a machine the query proceeds recursively into the
hierarchy starting at the top. If you want to find out the address of
prep.ai.mit.edu your name server has to find a name server that serves
edu. It asks a . server (it already knows the . servers, that's what
the root.hints file is for), the . server gives a list of edu
servers:
$ nslookup
Default Server: localhost
Address: 127.0.0.1
Start asking a root server:
> server c.root-servers.net.
Default Server: c.root-servers.net
Address: 192.33.4.12
Set the Query type to NS (name server records):
> set q=ns
Ask about edu:
> edu.
The trailing . here is significant, it tells nslookup we're asking
that edu is right under . (and not under any of our search domains, it
speeds the search).
edu nameserver = A.ROOT-SERVERS.NET
edu nameserver = H.ROOT-SERVERS.NET
edu nameserver = B.ROOT-SERVERS.NET
edu nameserver = C.ROOT-SERVERS.NET
edu nameserver = D.ROOT-SERVERS.NET
edu nameserver = E.ROOT-SERVERS.NET
edu nameserver = I.ROOT-SERVERS.NET
edu nameserver = F.ROOT-SERVERS.NET
edu nameserver = G.ROOT-SERVERS.NET
A.ROOT-SERVERS.NET internet address = 198.41.0.4
H.ROOT-SERVERS.NET internet address = 128.63.2.53
B.ROOT-SERVERS.NET internet address = 128.9.0.107
C.ROOT-SERVERS.NET internet address = 192.33.4.12
D.ROOT-SERVERS.NET internet address = 128.8.10.90
E.ROOT-SERVERS.NET internet address = 192.203.230.10
I.ROOT-SERVERS.NET internet address = 192.36.148.17
F.ROOT-SERVERS.NET internet address = 192.5.5.241
G.ROOT-SERVERS.NET internet address = 192.112.36.4
This tells us that all ROOT-SERVERS.NET servers serves EDU., so we can
go on asking any of them. We'll continue asking C. Now we want to
know who serves the next level of the domain name: mit.edu.:
> mit.edu.
Server: c.root-servers.net
Address: 192.33.4.12
Non-authoritative answer:
mit.edu nameserver = W20NS.mit.edu
mit.edu nameserver = BITSY.mit.edu
mit.edu nameserver = STRAWB.mit.edu
Authoritative answers can be found from:
W20NS.mit.edu internet address = 18.70.0.160
BITSY.mit.edu internet address = 18.72.0.3
STRAWB.mit.edu internet address = 18.71.0.151
steawb, w20ns and bitsy all serves mit.edu, we select one and inquire
about the name one more level up: ai.mit.edu:
> server W20NS.mit.edu.
Host names are not case sensitive, but I use my mouse to cut and paste
so it gets copied as-is from the screen.
Server: W20NS.mit.edu
Address: 18.70.0.160
> ai.mit.edu.
Server: W20NS.mit.edu
Address: 18.70.0.160
Non-authoritative answer:
ai.mit.edu nameserver = ALPHA-BITS.AI.MIT.EDU
ai.mit.edu nameserver = GRAPE-NUTS.AI.MIT.EDU
ai.mit.edu nameserver = TRIX.AI.MIT.EDU
ai.mit.edu nameserver = MUESLI.AI.MIT.EDU
ai.mit.edu nameserver = LIFE.AI.MIT.EDU
ai.mit.edu nameserver = BEET-CHEX.AI.MIT.EDU
ai.mit.edu nameserver = MINI-WHEATS.AI.MIT.EDU
ai.mit.edu nameserver = COUNT-CHOCULA.AI.MIT.EDU
ai.mit.edu nameserver = MINTAKA.LCS.MIT.EDU
Authoritative answers can be found from:
AI.MIT.EDU nameserver = ALPHA-BITS.AI.MIT.EDU
AI.MIT.EDU nameserver = GRAPE-NUTS.AI.MIT.EDU
AI.MIT.EDU nameserver = TRIX.AI.MIT.EDU
AI.MIT.EDU nameserver = MUESLI.AI.MIT.EDU
AI.MIT.EDU nameserver = LIFE.AI.MIT.EDU
AI.MIT.EDU nameserver = BEET-CHEX.AI.MIT.EDU
AI.MIT.EDU nameserver = MINI-WHEATS.AI.MIT.EDU
AI.MIT.EDU nameserver = COUNT-CHOCULA.AI.MIT.EDU
AI.MIT.EDU nameserver = MINTAKA.LCS.MIT.EDU
ALPHA-BITS.AI.MIT.EDU internet address = 128.52.32.5
GRAPE-NUTS.AI.MIT.EDU internet address = 128.52.36.4
TRIX.AI.MIT.EDU internet address = 128.52.37.6
MUESLI.AI.MIT.EDU internet address = 128.52.39.7
LIFE.AI.MIT.EDU internet address = 128.52.32.80
BEET-CHEX.AI.MIT.EDU internet address = 128.52.32.22
MINI-WHEATS.AI.MIT.EDU internet address = 128.52.54.11
COUNT-CHOCULA.AI.MIT.EDU internet address = 128.52.38.22
MINTAKA.LCS.MIT.EDU internet address = 18.26.0.36
So museli.ai.mit.edu is a nameserver for ai.mit.edu:
> server MUESLI.AI.MIT.EDU
Default Server: MUESLI.AI.MIT.EDU
Address: 128.52.39.7
Now I change query type, we've found the name server so now we're
going to ask about everything wheaties knows about prep.ai.mit.edu.
> set q=any
> prep.ai.mit.edu.
Server: MUESLI.AI.MIT.EDU
Address: 128.52.39.7
prep.ai.mit.edu CPU = dec/decstation-5000.25 OS = unix
prep.ai.mit.edu
inet address = 18.159.0.42, protocol = tcp
ftp telnet smtp finger
prep.ai.mit.edu preference = 1, mail exchanger = gnu-life.ai.mit.edu
prep.ai.mit.edu internet address = 18.159.0.42
ai.mit.edu nameserver = beet-chex.ai.mit.edu
ai.mit.edu nameserver = alpha-bits.ai.mit.edu
ai.mit.edu nameserver = mini-wheats.ai.mit.edu
ai.mit.edu nameserver = trix.ai.mit.edu
ai.mit.edu nameserver = muesli.ai.mit.edu
ai.mit.edu nameserver = count-chocula.ai.mit.edu
ai.mit.edu nameserver = mintaka.lcs.mit.edu
ai.mit.edu nameserver = life.ai.mit.edu
gnu-life.ai.mit.edu internet address = 128.52.32.60
beet-chex.ai.mit.edu internet address = 128.52.32.22
alpha-bits.ai.mit.edu internet address = 128.52.32.5
mini-wheats.ai.mit.edu internet address = 128.52.54.11
trix.ai.mit.edu internet address = 128.52.37.6
muesli.ai.mit.edu internet address = 128.52.39.7
count-chocula.ai.mit.edu internet address = 128.52.38.22
mintaka.lcs.mit.edu internet address = 18.26.0.36
life.ai.mit.edu internet address = 128.52.32.80
So starting at . we found the successive name servers for the each
level in the domain name. If you had used your own DNS server instead
of using all those other servers, your named would of-course cache all
the information it found while digging this out for you, and it would
not have to ask again for a while.
In the tree analogue each ``.'' in the name is a branching point. And
each part between the ``.''s are the names of individual branches in
the tree.
We climb the tree by taking the name we want (prep.ai.mit.edu) first
finding the root (.) and then looking for the next branch to climb, in
this case edu. Once we have found it we climb it by switching to the
server that knows about that part of the name. Next we look for the
mit branch over the edu branch (the combined name is mit.edu) and
climb it by switching to a server that knows about mit.edu. Again we
look for the next branch, it's ai.mit.edu and again we switch to the
server that knows about it. Now we have arrived at the right server,
at the right branching point. The last part is finding
prep.ai.mit.edu, which is simple. In computer science we usually call
prep a leaf on the tree.
A much less talked about, but just as important domain is in-
addr.arpa. It too is nested like the `normal' domains. in-addr.arpa
allows us to get the hosts name when we have its address. A important
thing here is to note that ip addresses are written in reverse order
in the in-addr.arpa domain. If you have the address of a machine:
192.128.52.43 named proceeds just like for the prep.ai.mit.edu
example: find arpa. servers. Find in-addr.arpa. servers, find 192.in-
addr.arpa. servers, find 128.192.in-addr.arpa. servers, find
52.128.192.in-addr.arpa. servers. Find needed records for
43.52.128.192.in-addr.arpa. Clever huh? (Say `yes'.) The reversion
of the numbers can be confusing for years though.
I have just told a lie. DNS does not work precisely the way I just
told you. But it's close enough.
4.2. Our own domain
Now to define our own domain. We're going to make the domain
linux.bogus and define machines in it. I use a totally bogus domain
name to make sure we disturb no-one Out There.
One more thing before we start: Not all characters are allowed in host
names. We're restricted to the characters of the English alphabet: a-
z, and numbers: 0-9 and the character '-' (dash). Keep to those
characters. Upper and lower-case characters are the same for DNS, so
pat.uio.no is identical to Pat.UiO.No.
We've already started this part with this line in named.conf:
______________________________________________________________________
zone "0.0.127.in-addr.arpa" {
type master;
file "pz/127.0.0";
};
______________________________________________________________________
Please note the lack of `.' at the end of the domain names in this
file. This says that now we will define the zone 0.0.127.in-
addr.arpa, that we're the master server for it and that it is stored
in a file called pz/127.0.0. We've already set up this file, it
reads:
______________________________________________________________________
@ IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
1 ; Serial
8H ; Refresh
2H ; Retry
1W ; Expire
1D) ; Minimum TTL
NS ns.linux.bogus.
1 PTR localhost.
______________________________________________________________________
Please note the `.' at the end of all the full domain names in this
file, in contrast to the named.conf file above. Some people like to
start each zone file with a $ORIGIN directive, but this is
superfluous. The origin (where in the DNS hierarchy it belongs) of a
zone file is specified in the zone section of the named.conf file, in
this case it's 0.0.127.in-addr.arpa.
This `zone file' contains 3 `resource records' (RRs): A SOA RR. A NS
RR and a PTR RR. SOA is short for Start Of Authority. The `@' is a
special notation meaning the origin, and since the `domain' column for
this file says 0.0.127.in-addr.arpa the first line really means
0.0.127.in-addr.arpa. IN SOA ...
NS is the Name Server RR. There is no '@' at the start of this line,
it is implicit since the last line started with a '@'. Saves some
typing that. So the NS line could also be written
0.0.127.in-addr.arpa. IN NS ns.linux.bogus
It tells DNS what machine is the name server of the domain 0.0.127.in-
addr.arpa, it is ns.linux.bogus. 'ns' is a customary name for name-
servers, but as with web servers who are customarily named
www.something the name may be anything.
And finally the PTR record says that the host at address 1 in the
subnet 0.0.127.in-addr.arpa, i.e., 127.0.0.1 is named localhost.
The SOA record is the preamble to all zone files, and there should be
exactly one in each zone file. It describes the zone, where it comes
from (a machine called ns.linux.bogus), who is responsible for its
contents (hostmaster@linux.bogus, you should insert your e-mail
address here), what version of the zone file this is (serial: 1), and
other things having to do with caching and secondary DNS servers. For
the rest of the fields (refresh, retry, expire and minimum) use the
numbers used in this HOWTO and you should be safe.
Now restart your named (the command is ndc restart) and use nslookup
to examine what you've done:
$ nslookup
Default Server: localhost
Address: 127.0.0.1
> 127.0.0.1
Server: localhost
Address: 127.0.0.1
Name: localhost
Address: 127.0.0.1
so it manages to get localhost from 127.0.0.1, good. Now for our main
task, the linux.bogus domain, insert a new 'zone' section in
named.conf:
______________________________________________________________________
zone "linux.bogus" {
notify no;
type master;
file "pz/linux.bogus";
};
______________________________________________________________________
Note again the lack of ending `.' on the domain name in the named.conf
file.
In the linux.bogus zone file we'll put some totally bogus data:
______________________________________________________________________
;
; Zone file for linux.bogus
;
; The full zone file
;
@ IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
199802151 ; serial, todays date + todays serial #
8H ; refresh, seconds
2H ; retry, seconds
1W ; expire, seconds
1D ) ; minimum, seconds
;
NS ns ; Inet Address of name server
MX 10 mail.linux.bogus ; Primary Mail Exchanger
MX 20 mail.friend.bogus. ; Secondary Mail Exchanger
;
localhost A 127.0.0.1
ns A 192.168.196.2
mail A 192.168.196.4
______________________________________________________________________
Two things must be noted about the SOA record. ns.linux.bogus must be
a actual machine with a A record. It is not legal to have a CNAME
record for he machine mentioned in the SOA record. It's name need not
be `ns', it could be any legal host name. Next,
hostmaster.linux.bogus should be read as hostmaster@linux.bogus, this
should be a mail alias, or a mailbox, where the person(s) maintaining
DNS should read mail frequently. Any mail regarding the domain will
be sent to the address listed here. The name need not be
`hostmaster', it can be your normal e-mail address, but the e-mail
address `hostmaster' is often expected to work as well.
There is one new RR type in this file, the MX, or Mail eXchanger RR.
It tells mail systems where to send mail that is addressed to
someone@linux.bogus, namely too mail.linux.bogus or mail.friend.bogus.
The number before each machine name is that MX RRs priority. The RR
with the lowest number (10) is the one mail should be sent to if
possible. If that fails the mail can be sent to one with a higher
number, a secondary mail handler, i.e., mail.friend.bogus which has
priority 20 here.
Restart named by running ndc restart. Examine the results with
nslookup:
$ nslookup
> set q=any
> linux.bogus
Server: localhost
Address: 127.0.0.1
linux.bogus
origin = ns.linux.bogus
mail addr = hostmaster.linux.bogus
serial = 199802151
refresh = 28800 (8 hours)
retry = 7200 (2 hours)
expire = 604800 (7 days)
minimum ttl = 86400 (1 day)
linux.bogus nameserver = ns.linux.bogus
linux.bogus preference = 10, mail exchanger = mail.linux.bogus.linux.bogus
linux.bogus preference = 20, mail exchanger = mail.friend.bogus
linux.bogus nameserver = ns.linux.bogus
ns.linux.bogus internet address = 192.168.196.2
mail.linux.bogus internet address = 192.168.196.4
Upon careful examination you will discover a bug. The line
linux.bogus preference = 10, mail exchanger = mail.linux.bogus.linux.bogus
is all wrong. It should be
linux.bogus preference = 10, mail exchanger = mail.linux.bogus
I deliberately made a mistake so you could learn from it :-) Looking
in the zone file we find that the line
MX 10 mail.linux.bogus ; Primary Mail Exchanger
is missing a period. Or has a 'linux.bogus' too many. If a machine
name does not end in a period in a zone file the origin is added to
its end causing the double linux.bogus.linux.bogus. So either
______________________________________________________________________
MX 10 mail.linux.bogus. ; Primary Mail Exchanger
______________________________________________________________________
or
______________________________________________________________________
MX 10 mail ; Primary Mail Exchanger
______________________________________________________________________
is correct. I prefer the latter form, it's less to type. There are
some bind experts that disagree, and some that agree with this. In a
zone file the domain should either be written out and ended with a `.'
or it should not be included at all, in which case it defaults to the
origin.
I must stress that in the named.conf file there should not be `.'s
after the domain names. You have no idea how many times a `.' too
many or few have fouled up things and confused the h*ll out of people.
So having made my point here is the new zone file, with some extra
information in it as well:
______________________________________________________________________
;
; Zone file for linux.bogus
;
; The full zone file
;
@ IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
199802151 ; serial, todays date + todays serial #
8H ; refresh, seconds
2H ; retry, seconds
1W ; expire, seconds
1D ) ; minimum, seconds
;
TXT "Linux.Bogus, your DNS consultants"
NS ns ; Inet Address of name server
NS ns.friend.bogus.
MX 10 mail ; Primary Mail Exchanger
MX 20 mail.friend.bogus. ; Secondary Mail Exchanger
localhost A 127.0.0.1
gw A 192.168.196.1
HINFO "Cisco" "IOS"
TXT "The router"
ns A 192.168.196.2
MX 10 mail
MX 20 mail.friend.bogus.
HINFO "Pentium" "Linux 2.0"
www CNAME ns
donald A 192.168.196.3
MX 10 mail
MX 20 mail.friend.bogus.
HINFO "i486" "Linux 2.0"
TXT "DEK"
mail A 192.168.196.4
MX 10 mail
MX 20 mail.friend.bogus.
HINFO "386sx" "Linux 1.2"
ftp A 192.168.196.5
MX 10 mail
MX 20 mail.friend.bogus.
HINFO "P6" "Linux 2.1.86"
______________________________________________________________________
There are a number of new RRs here: HINFO (Host INFOrmation) has two
parts, it's a good habit to quote each. The first part is the
hardware or CPU on the machine, and the second part the software or OS
on the machine. The machine called 'ns' has a Pentium CPU and runs
Linux 2.0. CNAME (Canonical NAME) is a way to give each machine
several names. So www is an alias for ns.
CNAME record usage is a bit controversial. But it's safe to follow
the rule that a MX, CNAME or SOA record should never refer to a CNAME
record, they should only refer to something with a A record, so it is
inadvisable to have
______________________________________________________________________
foobar CNAME www ; NO!
______________________________________________________________________
but correct to have
______________________________________________________________________
foobar CNAME ns ; Yes!
______________________________________________________________________
It's also safe to assume that a CNAME is not a legal host name for a
e-mail address: webmaster@www.linux.bogus is an illegal e-mail address
given the setup above. You can expect quite a few mail admins Out
There to enforce this rule even if it works for you. The way to avoid
this is to use A records (and perhaps some others too, like a MX
record) instead:
______________________________________________________________________
www A 192.168.196.2
______________________________________________________________________
A number of the arch-bind-wizards, recommend not using CNAME at all.
But the discussion of why or why not is beyond this HOWTO.
But as you see, this HOWTO and many sites does not follow this rule.
Load the new database by running ndc reload, this causes named to read
its files again.
$ nslookup
Default Server: localhost
Address: 127.0.0.1
> ls -d linux.bogus
This means that all records should be listed. It results in this:
[localhost]
$ORIGIN linux.bogus.
@ 1D IN SOA ns hostmaster (
199802151 ; serial
8H ; refresh
2H ; retry
1W ; expiry
1D ) ; minimum
1D IN NS ns
1D IN NS ns.friend.bogus.
1D IN TXT "Linux.Bogus, your DNS consultants"
1D IN MX 10 mail
1D IN MX 20 mail.friend.bogus.
gw 1D IN A 192.168.196.1
1D IN HINFO "Cisco" "IOS"
1D IN TXT "The router"
mail 1D IN A 192.168.196.4
1D IN MX 10 mail
1D IN MX 20 mail.friend.bogus.
1D IN HINFO "386sx" "Linux 1.0.9"
localhost 1D IN A 127.0.0.1
www 1D IN CNAME ns
donald 1D IN A 192.168.196.3
1D IN MX 10 mail
1D IN MX 20 mail.friend.bogus.
1D IN HINFO "i486" "Linux 1.2"
1D IN TXT "DEK"
ftp 1D IN A 192.168.196.5
1D IN MX 10 mail
1D IN MX 20 mail.friend.bogus.
1D IN HINFO "P6" "Linux 1.3.59"
ns 1D IN A 192.168.196.2
1D IN MX 10 mail
1D IN MX 20 mail.friend.bogus.
1D IN HINFO "Pentium" "Linux 1.2"
That's good. As you see it looks a lot like the zone file itself.
Let's check what it says for www alone:
> set q=any
> www.linux.bogus.
Server: localhost
Address: 127.0.0.1
www.linux.bogus canonical name = ns.linux.bogus
linux.bogus nameserver = ns.linux.bogus
linux.bogus nameserver = ns.friend.bogus
ns.linux.bogus internet address = 192.168.196.2
In other words, the real name of www.linux.bogus is ns.linux.bogus,
and it gives you some of the information it has about ns as well,
enough to connect to it if you were a program.
Now we're halfway.
4.3. The reverse zone
Now programs can convert the names in linux.bogus to addresses which
they can connect to. But also required is a reverse zone, one making
DNS able to convert from an address to a name. This name is used buy
a lot of servers of different kinds (FTP, IRC, WWW and others) to
decide if they want to talk to you or not, and if so, maybe even how
much priority you should be given. For full access to all services on
the Internet a reverse zone is required.
Put this in named.conf:
______________________________________________________________________
zone "196.168.192.in-addr.arpa" {
notify no;
type master;
file "pz/192.168.196";
};
______________________________________________________________________
This is exactly as with the 0.0.127.in-addr.arpa, and the contents are
similar:
______________________________________________________________________
@ IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
199802151 ; Serial, todays date + todays serial
8H ; Refresh
2H ; Retry
1W ; Expire
1D) ; Minimum TTL
NS ns.linux.bogus.
1 PTR gw.linux.bogus.
2 PTR ns.linux.bogus.
3 PTR donald.linux.bogus.
4 PTR mail.linux.bogus.
5 PTR ftp.linux.bogus.
______________________________________________________________________
Now you restart your named (ndc restart) and examine your work with
nslookup again:
______________________________________________________________________
> 192.168.196.4
Server: localhost
Address: 127.0.0.1
Name: mail.linux.bogus
Address: 192.168.196.4
______________________________________________________________________
so, it looks OK, dump the whole thing to examine that too:
______________________________________________________________________
> ls -d 196.168.192.in-addr.arpa
[localhost]
$ORIGIN 196.168.192.in-addr.arpa.
@ 1D IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
199802151 ; serial
8H ; refresh
2H ; retry
1W ; expiry
1D ) ; minimum
1D IN NS ns.linux.bogus.
1 1D IN PTR gw.linux.bogus.
2 1D IN PTR ns.linux.bogus.
3 1D IN PTR donald.linux.bogus.
4 1D IN PTR mail.linux.bogus.
5 1D IN PTR ftp.linux.bogus.
@ 1D IN SOA ns.linux.bogus. hostmaster.linux.bogus. (
199802151 ; serial
8H ; refresh
2H ; retry
1W ; expiry
1D ) ; minimum
______________________________________________________________________
Looks good! If your output didn't look like that look for error-
messages in your syslog, I explained how to do that at the very
beginning of this chapter.
4.4. Words of caution
There are some things I should add here. The IP numbers used in the
examples above are taken from one of the blocks of 'private nets',
i.e., they are not allowed to be used publicly on the internet. So
they are safe to use in an example in a HOWTO. The second thing is
the notify no; line. It tells named not to notify its secondary
(slave) servers when it has gotten a update to one of its zone files.
In bind-8 the named can notify the other servers listed in NS records
in the zone file when a zone is updated. This is handy for ordinary
use, but for private experiments with zones this feature should be
off, we don't want the experiment to pollute the Internet do we?
And, of course, this domain is highly bogus, and so are all the
addresses in it. For a real example of a real-life domain see the
next main-section.
4.5. Why reverse lookups don't work.
There are a couple of ``gotchas'' that normally are avoided with name
lookups that are often seen when setting up reverse zones. Before you
go on you need reverse lookups of your machines working on your own
nameserver. If it isn't go back and fix it before continuing.
I will discuss two failures of reverse lookups as seen from outside
your network:
4.5.1. The reverse zone isn't delegated.
When you ask a service provider for a network-address range and a
domain name the domain name is normally delegated as a matter of
course. A delegation is the glue NS record that helps you get from
one nameserver to another as explained in the dry theory section
above. You read that, right? If your reverse zone dosn't work go
back and read it. Now.
The reverse zone also needs to be delegated. If you got the
192.168.196 net with the linux.bogus domain from your provider they
need to put NS records in for your reverse zone as well as for your
forward zone. If you follow the chain from in-addr.arpa and up to
your net you will probably find a break in the chain. Most probably
at your service provider. Having found the break in the chain contact
your service-provider and ask them to correct the error.
4.5.2. You've got a classless subnet
This is a somewhat advanced topic, but classless subnets are very
common these days and you probably have one unless you're a medium
sized company.
A classless subnet is what keeps the Internet going these days. Some
years ago there was much ado about the shortage of ip numbers. The
smart people in IETF (the Internet Engineering Task Force, they keep
the Internet working) stuck their heads together and solved the
problem. At a price. The price is that you'll get less than a ``C''
subnet and some things may break. Please see Ask Mr. DNS at
http://www.acmebw.com/askmrdns/00007.htm for an good explanation of
this and how to handle it.
Did you read it? I'm not going to explain it so please read it.
The first part of the problem is that your ISP must understand the
technique described by Mr. DNS. Not all small ISPs have a working
understanding of this. If so you might have to explain to them and be
persistent. But be sure you understand it first ;-). They will then
set up a nice reverse zone at their server which you can examine for
correctness with nslookup.
The second and last part of the problem is that you must understand
the technique. If you're unsure go back and read about it again.
Then you can set up your own classless reverse zone as described by
Mr. DNS.
There is another trap lurking here. Old resolvers will not be able to
follow the CNAME trick in the resolving chain and will fail to
reverse-resolve your machine. This can result in the service
assigning it an incorrect access class, deny access or something along
those lines. If you stumble into such a service the only solution
(that I know of) is for your ISP to insert your PTR record directly
into their trick classless zone file instead of the trick CNAME
record.
Some ISPs will offer other ways to handle this, like Web based forms
for you to input your reverse-mappings in or other automagical
systems.
5. A real domain example
Where we list some real zone files
Users have suggested that I include a real example of a working domain
as well as the tutorial example.
I use this example with permission from David Bullock of LAND-5.
These files were current 24th of September 1996, and were then edited
to fit bind 8 restrictions and use extensions by me. So, what you see
here differs a bit from what you find if you query LAND-5's name
servers now.
5.1. /etc/named.conf (or /var/named/named.conf)
Here we find master zone sections for the two reverse zones needed:
the 127.0.0 net, as well as LAND-5's 206.6.177 subnet. And a primary
line for land-5's forward zone land-5.com. Also note that instead of
stuffing the files in a directory called pz, as I do in this HOWTO, he
puts them in a directory called zone.
______________________________________________________________________
// Boot file for LAND-5 name server
options {
directory "/var/named";
};
zone "." {
type hint;
file "root.hints";
};
zone "0.0.127.in-addr.arpa" {
type master;
file "zone/127.0.0";
};
zone "land-5.com" {
type master;
file "zone/land-5.com";
};
zone "177.6.206.in-addr.arpa" {
type master;
file "zone/206.6.177";
};
______________________________________________________________________
If you put this in your named.conf file to play with PLEASE put
``notify no;'' in the zone sections for the two land-5 zones so as to
avoid accidents.
5.2. /var/named/root.hints
Keep in mind that this file is dynamic, and the one listed here is
old. You're better off using one produced now, with dig, as explained
earlier.
______________________________________________________________________
; <<>> DiG 8.1 <<>> @A.ROOT-SERVERS.NET.
; (1 server found)
;; res options: init recurs defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10
;; flags: qr aa rd; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 13
;; QUERY SECTION:
;; ., type = NS, class = IN
;; ANSWER SECTION:
. 6D IN NS G.ROOT-SERVERS.NET.
. 6D IN NS J.ROOT-SERVERS.NET.
. 6D IN NS K.ROOT-SERVERS.NET.
. 6D IN NS L.ROOT-SERVERS.NET.
. 6D IN NS M.ROOT-SERVERS.NET.
. 6D IN NS A.ROOT-SERVERS.NET.
. 6D IN NS H.ROOT-SERVERS.NET.
. 6D IN NS B.ROOT-SERVERS.NET.
. 6D IN NS C.ROOT-SERVERS.NET.
. 6D IN NS D.ROOT-SERVERS.NET.
. 6D IN NS E.ROOT-SERVERS.NET.
. 6D IN NS I.ROOT-SERVERS.NET.
. 6D IN NS F.ROOT-SERVERS.NET.
;; ADDITIONAL SECTION:
G.ROOT-SERVERS.NET. 5w6d16h IN A 192.112.36.4
J.ROOT-SERVERS.NET. 5w6d16h IN A 198.41.0.10
K.ROOT-SERVERS.NET. 5w6d16h IN A 193.0.14.129
L.ROOT-SERVERS.NET. 5w6d16h IN A 198.32.64.12
M.ROOT-SERVERS.NET. 5w6d16h IN A 202.12.27.33
A.ROOT-SERVERS.NET. 5w6d16h IN A 198.41.0.4
H.ROOT-SERVERS.NET. 5w6d16h IN A 128.63.2.53
B.ROOT-SERVERS.NET. 5w6d16h IN A 128.9.0.107
C.ROOT-SERVERS.NET. 5w6d16h IN A 192.33.4.12
D.ROOT-SERVERS.NET. 5w6d16h IN A 128.8.10.90
E.ROOT-SERVERS.NET. 5w6d16h IN A 192.203.230.10
I.ROOT-SERVERS.NET. 5w6d16h IN A 192.36.148.17
F.ROOT-SERVERS.NET. 5w6d16h IN A 192.5.5.241
;; Total query time: 215 msec
;; FROM: roke.uio.no to SERVER: A.ROOT-SERVERS.NET. 198.41.0.4
;; WHEN: Sun Feb 15 01:22:51 1998
;; MSG SIZE sent: 17 rcvd: 436
______________________________________________________________________
5.3. /var/named/zone/127.0.0
Just the basics, the obligatory SOA record, and a record that maps
127.0.0.1 to localhost. Both are required. No more should be in this
file. It will probably never need to be updated, unless your
nameserver or hostmaster address changes.
______________________________________________________________________
@ IN SOA land-5.com. root.land-5.com. (
199609203 ; Serial
28800 ; Refresh
7200 ; Retry
604800 ; Expire
86400) ; Minimum TTL
NS land-5.com.
1 PTR localhost.
______________________________________________________________________
5.4. /var/named/zone/land-5.com
Here we see the mandatory SOA record, the needed NS records. We can
see that he has a secondary name server at ns2.psi.net. This is as it
should be, always have a off site secondary server as backup. We can
also see that he has a master host called land-5 which takes care of
many of the different Internet services, and that he's done it with
CNAMEs (a alternative is using A records).
As you see from the SOA record, the zone file originates at
land-5.com, the contact person is root@land-5.com. hostmaster is
another oft used address for the contact person. The serial number is
in the customary yyyymmdd format with todays serial number appended;
this is probably the sixth version of zone file on the 20th of
September 1996. Remember that the serial number must increase
monotonically, here there is only one digit for todays serial#, so
after 9 edits he has to wait until tomorrow before he can edit the
file again. Consider using two digits.
______________________________________________________________________
@ IN SOA land-5.com. root.land-5.com. (
199609206 ; serial, todays date + todays serial #
8H ; refresh, seconds
2H ; retry, seconds
1W ; expire, seconds
1D ) ; minimum, seconds
NS land-5.com.
NS ns2.psi.net.
MX 10 land-5.com. ; Primary Mail Exchanger
TXT "LAND-5 Corporation"
localhost A 127.0.0.1
router A 206.6.177.1
land-5.com. A 206.6.177.2
ns A 206.6.177.3
www A 207.159.141.192
ftp CNAME land-5.com.
mail CNAME land-5.com.
news CNAME land-5.com.
funn A 206.6.177.2
;
; Workstations
;
ws-177200 A 206.6.177.200
MX 10 land-5.com. ; Primary Mail Host
ws-177201 A 206.6.177.201
MX 10 land-5.com. ; Primary Mail Host
ws-177202 A 206.6.177.202
MX 10 land-5.com. ; Primary Mail Host
ws-177203 A 206.6.177.203
MX 10 land-5.com. ; Primary Mail Host
ws-177204 A 206.6.177.204
MX 10 land-5.com. ; Primary Mail Host
ws-177205 A 206.6.177.205
MX 10 land-5.com. ; Primary Mail Host
; {Many repetitive definitions deleted - SNIP}
ws-177250 A 206.6.177.250
MX 10 land-5.com. ; Primary Mail Host
ws-177251 A 206.6.177.251
MX 10 land-5.com. ; Primary Mail Host
ws-177252 A 206.6.177.252
MX 10 land-5.com. ; Primary Mail Host
ws-177253 A 206.6.177.253
MX 10 land-5.com. ; Primary Mail Host
ws-177254 A 206.6.177.254
MX 10 land-5.com. ; Primary Mail Host
______________________________________________________________________
If you examine land-5s nameserver you will find that the host names
are of the form ws_number. As of late bind 4 versions named started
enforcing the restrictions on what characters may be used in host
names. So that does not work with bind-8 at all, and I substituted
'-' (dash) for '_' (underline) for use in this HOWTO.
Another thing to note is that the workstations don't have individual
names, but rather a prefix followed by the two last parts of the IP
numbers. Using such a convention can simplify maintenance
significantly, but can be a bit impersonal, and, in fact, be a source
of irritation among your customers.
We also see that funn.land-5.com is an alias for land-5.com, but using
an A record, not a CNAME record. This is a good policy as noted
earlier.
5.5. /var/named/zone/206.6.177
I'll comment on this file below
______________________________________________________________________
@ IN SOA land-5.com. root.land-5.com. (
199609206 ; Serial
28800 ; Refresh
7200 ; Retry
604800 ; Expire
86400) ; Minimum TTL
NS land-5.com.
NS ns2.psi.net.
;
; Servers
;
1 PTR router.land-5.com.
2 PTR land-5.com.
2 PTR funn.land-5.com.
;
; Workstations
;
200 PTR ws-177200.land-5.com.
201 PTR ws-177201.land-5.com.
202 PTR ws-177202.land-5.com.
203 PTR ws-177203.land-5.com.
204 PTR ws-177204.land-5.com.
205 PTR ws-177205.land-5.com.
; {Many repetitive definitions deleted - SNIP}
250 PTR ws-177250.land-5.com.
251 PTR ws-177251.land-5.com.
252 PTR ws-177252.land-5.com.
253 PTR ws-177253.land-5.com.
254 PTR ws-177254.land-5.com.
______________________________________________________________________
The reverse zone is the bit of the setup that seems to cause the most
grief. It is used to find the host name if you have the IP number of
a machine. Example: you are an IRC server and accept connections from
IRC clients. However you are a Norwegian IRC server and so you only
want to accept connections from clients in Norway and other
Scandinavian countries. When you get a connection from a client the C
library is able to tell you the IP number of the connecting machine
because the IP number of the client is contained in all the packets
that are passed over the network. Now you can call a function called
gethostbyaddr that looks up the name of a host given the IP number.
Gethostbyaddr will ask a DNS server, which will then traverse the DNS
looking for the machine. Supposing the client connection is from
ws-177200.land-5.com. The IP number the C library provides to the IRC
server is 206.6.177.200. To find out the name of that machine we need
to find 200.177.6.206.in-addr.arpa. The DNS server will first find
the arpa. servers, then find in-addr.arpa. servers, following the
reverse trail through 206, then 6 and at last finding the server for
the 177.6.206.in-addr.arpa zone at LAND-5. From which it will finally
get the answer that for 200.177.6.206.in-addr.arpa we have a ``PTR
ws-177200.land-5.com'' record, meaning that the name that goes with
206.6.177.200 is ws-177200.land-5.com. As with the explanation of how
prep.ai.mit.edu is looked up, this is slightly fictitious.
Getting back to the IRC server example. The IRC server only accepts
connections from the Scandinavian countries, i.e., *.no, *.se, *.dk,
the name ws-177200.land-5.com clearly does not match any of those, and
the server will deny the connection. If there was no reverse mapping
of 206.2.177.200 through the in-addr.arpa zone the server would have
been unable to find the name at all and would have to settle to
comparing 206.2.177.200 with *.no, *.se and *.dk, none of which will
match.
Some people will tell you that reverse lookup mappings are only
important for servers, or not important at all. Not so: Many ftp,
news, IRC and even some http (WWW) servers will not accept connections
from machines of which they are not able to find the name. So reverse
mappings for machines are in fact mandatory.
6. Maintenance
Keeping it working.
There is one maintenance task you have to do on nameds, other than
keeping them running. That's keeping the root.hints file updated.
The easiest way is using dig, first run dig with no arguments, you
will get the root.hints according to your own server. Then ask one of
the listed root servers with dig @rootserver. You will note that the
output looks terribly like a root.hints file. Save it to a file (dig
@e.root-servers.net . ns >root.hints.new) and replace the old
root.hints with it.
Remember to reload named after replacing the cache file.
Al Longyear sent me this script, that can be run automatically to
update root.hints, install a crontab entry to run it once a month and
forget it. The script assumes you have mail working and that the
mail-alias `hostmaster' is defined. You must hack it to suit your
setup.
______________________________________________________________________
#!/bin/sh
#
# Update the nameserver cache information file once per month.
# This is run automatically by a cron entry.
#
# Original by Al Longyear
# Updated for bind 8 by Nicolai Langfeldt
# Miscelanious error-conditions reported by David A. Ranch
# Ping test suggested by Martin Foster
#
(
echo "To: hostmaster <hostmaster>"
echo "From: system <root>"
echo "Subject: Automatic update of the root.hints file"
echo
PATH=/sbin:/usr/sbin:/bin:/usr/bin:
export PATH
cd /var/named
# Are we online? Ping a server at your ISP
case `ping -qnc some.machine.net` in
*'100% packet loss'*)
echo "The network is DOWN. root.hints NOT updated"
echo
exit 0
;;
esac
dig @rs.internic.net . ns >root.hints.new 2>&1
case `cat root.hints.new` in
*NOERROR*)
# It worked
:;;
*)
echo "The root.hints file update has FAILED."
echo "This is the dig output reported:"
echo
cat root.hints.new
exit 0
;;
esac
echo "The root.hints file has been updated to contain the following
information:"
echo
cat root.hints.new
chown root.root root.hints.new
chmod 444 root.hints.new
rm -f root.hints.old
mv root.hints root.hints.old
mv root.hints.new root.hints
ndc restart
echo
echo "The nameserver has been restarted to ensure that the update is complete."
echo "The previous root.hints file is now called
/var/named/root.hints.old."
) 2>&1 | /usr/lib/sendmail -t
exit 0
______________________________________________________________________
Some of you might have picked up that the root.hints file is also
available by ftp from Internic. Please don't use ftp to update
root.hints, the above method is much more friendly to the net, and
Internic.
7. Converting from version 4 to version 8
This was originally a section on using bind 8 written by David E.
Smith (dave@bureau42.ml.org). I have edited it some to fit the new
section name.
There's not much to it. Except for using named.conf instead of
named.boot, everything is identical. And bind8 comes with a perl
script that converts old-style files to new. Example named.boot (old
style) for a cache-only name server:
______________________________________________________________________
directory /var/named
cache . root.hints
primary 0.0.127.IN-ADDR.ARPA 127.0.0.zone
primary localhost localhost.zone
______________________________________________________________________
On the command line, in the bind8/src/bin/named directory (this
assumes you got a source distribution. If you got a binary package
the script is probably around, I'm not sure where it would be though.
-ed.), type:
______________________________________________________________________
./named-bootconf.pl < named.boot > named.conf
______________________________________________________________________
Which creates named.conf:
______________________________________________________________________
// generated by named-bootconf.pl
options {
directory "/var/named";
};
zone "." {
type hint;
file "root.hints";
};
zone "0.0.127.IN-ADDR.ARPA" {
type master;
file "127.0.0.zone";
};
zone "localhost" {
type master;
file "localhost.zone";
};
______________________________________________________________________
It works for everything that can go into a named.boot file, although
it doesn't add all of the new enhancements and configuration options
that bind8 allows. Here's a more complete named.conf that does the
same things, but a little more efficiently.
______________________________________________________________________
// This is a configuration file for named (from BIND 8.1 or later).
// It would normally be installed as /etc/named.conf.
// The only change made from the `stock' named.conf (aside from this
// comment :) is that the directory line was uncommented, since I
// already had the zone files in /var/named.
options {
directory "/var/named";
datasize 20M;
};
zone "localhost" IN {
type master;
file "localhost.zone";
};
zone "0.0.127.in-addr.arpa" IN {
type master;
file "127.0.0.zone";
};
zone "." IN {
type hint;
file "root.hints";
};
______________________________________________________________________
In the bind 8 distributions directory bind8/src/bin/named/test you
find this, and copies of the zone files, that many people can just
drop in and use instantly.
The formats for zone files and root.hints files are identical, as are
the commands for updating them.
8. Questions and Answers
Please read this section before mailing me.
1. My named wants a named.boot file
You are reading the wrong HOWTO. Please see the old version of
this HOWTO, which covers bind 4, at
http://www.math.uio.no/~janl/DNS/
2. How do use DNS from inside a firewall?
A hint: forward only;, You will probably also need
___________________________________________________________________
query-source port 53;
___________________________________________________________________
inside the ``options'' part of the named.conf file as suggested in the
example ``caching'' section.
3. How do I make DNS rotate through the available addresses for a
service, say www.busy.site to obtain a load balancing effect, or
similar?
Make several A records for www.busy.site and use bind 4.9.3 or
later. Then bind will round-robin the answers. It will not work
with earlier versions of bind.
4. I want to set up DNS on a (closed) intranet. What do I do?
You drop the root.hints file and just do zone files. That also
means you don't have to get new hint files all the time.
5. How do I set up a secondary (slave) name server?
If the primary/master server has address 127.0.0.1 you put a line
like this in the named.conf file of your secondary:
___________________________________________________________________
zone "linux.bogus" {
type slave;
file "sz/linux.bogus";
masters { 127.0.0.1; };
};
___________________________________________________________________
You may list several alternate master servers the zone can be copied
from inside the masters list, separated by ';' (semicolon).
6. I want bind running when I'm disconnected from the net.
There are three items regarding this:
� I have received this mail from Ian Clark <ic@deakin.edu.au> where
he explains his way of doing this:
I run named on my 'Masquerading' machine here. I have
two root.hints files, one called root.hints.real which contains
the real root server names and the other called root.hints.fake
which contains...
----
; root.hints.fake
; this file contains no information
----
When I go off line I copy the root.hints.fake file to root.hints and
restart named.
When I go online I copy root.hints.real to root.hints and restart
named.
This is done from ip-down & ip-up respectively.
The first time I do a query off line on a domain name named doesn't
have details for it puts an entry like this in messages..
Jan 28 20:10:11 hazchem named[10147]: No root nameserver for class IN
which I can live with.
It certainly seems to work for me. I can use the nameserver for
local machines while off the 'net without the timeout delay for
external domain names and I while on the 'net queries for external
domains work normally
� I have also received information about how bind interacts with NFS
and the portmapper on a mostly offline machine from Karl-Max
Wanger:
I use to run my own named on all my machines which are only
occasionally connected to the Internet by modem. The nameserver only
acts as a cache, it has no area of authority and asks back for
everything at the name servers in the root.cache file. As is usual with
Slackware, it is started before nfsd and mountd.
With one of my machines (a Libretto 30 notebook) I had the problem
that sometimes I could mount it from another system connected to my
local LAN, but most of the time it didn't work. I had the same effect
regardless of using PLIP, a PCMCIA ethernet card or PPP over a serial
interface.
After some time of guessing and experimenting I found out that
apparently named messed with the process of registration nfsd and
mountd have to carry out with the portmapper upon startup (I start
these daemons at boot time as usual). Starting named after nfsd and
mountd eliminated this problem completely.
As there are no disadvantages to expect from such a modified boot
sequence I'd advise everybody to do it that way to prevent potential
trouble.
� Finally, there is HOWTO information about this at Ask Mr. DNS at
http://www.acmebw.com/askmrdns/#linux-ns. It is about bind 4
though, so you have to adapt what he says to bind 8.
7. Where does the caching name server store its cache? Is there any
way I can control the size of the cache?
The cache is completely stored in memory, it is not written to disk
at any time. Every time you kill named the cache is lost. The
cache is not controllable in any way. named manages it according
to some simple rules and that is it. You cannot control the cache
or the cache size in any way for any reason. If you want to you can
``fix'' this by hacking named. This is however not recommended.
8. Does named save the cache between restarts? Can I make it save it?
No, named does not save the cache when it dies. That means that
the cache must be built anew each time you kill and restart named.
There is no way to make named save the cache in a file. If you
want you can ``fix'' this by hacking named. This is however not
recommended.
9. How can I get a domian? I want to set up my own domain called (for
example) linux-rules.net. How can I get the domain I want assigned
to me?
Please contact your network service provider. They will be able to
help you with this. Please note that in most parts of the world
you need to pay money to get a domain.
9. How to become a bigger time DNS admin.
Documentation and tools.
Real Documentation exists. Online and in print. The reading of
several of these is required to make the step from small time DNS
admin to a big time one. In print the standard book is DNS and BIND
by C. Liu and P. Albitz from O'Reilly & Associates, Sebastopol, CA,
ISBN 0-937175-82-X. I read this, it's excellent, though based on bind
4, this is not a real problem though. There is also a section in on
DNS in TCP/IP Network Administration, by Craig Hunt from O'Reilly...,
ISBN 0-937175-82-X. Another must for Good DNS administration (or good
anything for that matter) is Zen and the Art of Motorcycle Maintenance
by Robert M. Pirsig :-) Available as ISBN 0688052304 and others.
Online you will find stuff on <http://www.dns.net/dnsrd/> (DNS
Resources Directory), <http://www.isc.org/bind.html>; A FAQ, a
reference manual (BOG; Bind Operations Guide) as well as papers and
protocol definitions and DNS hacks (these, and most, if not all, of
the RFCs mentioned below, are also contained in the bind
distribution). I have not read most of these, but then I'm not a big-
time DNS admin either. Arnt Gulbrandsen on the other hand has read
BOG and he's ecstatic about it :-). The newsgroup comp.protocols.tcp-
ip.domains is about DNS. In addition there are a number of RFCs about
DNS, the most important are probably these:
RFC 2052
A. Gulbrandsen, P. Vixie, A DNS RR for specifying the location
of services (DNS SRV), October 1996
RFC 1918
Y. Rekhter, R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear,
Address Allocation for Private Internets, 02/29/1996.
RFC 1912
D. Barr, Common DNS Operational and Configuration Errors,
02/28/1996.
RFC 1912 Errors
B. Barr Errors in RFC 1912, this is available at
<http://www.cis.ohio-state.edu/~barr/rfc1912-errors.html>
RFC 1713
A. Romao, Tools for DNS debugging, 11/03/1994.
RFC 1712
C. Farrell, M. Schulze, S. Pleitner, D. Baldoni, DNS Encoding of
Geographical Location, 11/01/1994.
RFC 1183
R. Ullmann, P. Mockapetris, L. Mamakos, C. Everhart, New DNS RR
Definitions, 10/08/1990.
RFC 1035
P. Mockapetris, Domain names - implementation and specification,
11/01/1987.
RFC 1034
P. Mockapetris, Domain names - concepts and facilities,
11/01/1987.
RFC 1033
M. Lottor, Domain administrators operations guide, 11/01/1987.
RFC 1032
M. Stahl, Domain administrators guide, 11/01/1987.
RFC 974
C. Partridge, Mail routing and the domain system, 01/01/1986.