Linux DHCP How tos

Posted on 3:23 PM by Bharathvn

This document attempts to answer basic questions on how to set up your
Linux box to serve as a DHCP server or a DHCP client.
1. Introduction
2. DHCP protocol

DHCP is Dynamic Host Configuration Protocol. It is used to control
vital networking parameters of hosts (running clients) with the help
of a server. DHCP is backward compatible with BOOTP. For more
information see RFC 2131 (old RFC 1541) and other. (See Internet
Resources section at the end of the document). You can also read

This mini-HOWTO covers both the DHCP _SERVER_ daemon as well as DHCP
_CLIENT_ daemon. Most people need the client daemon which is used by
workstations to obtain network information from a remote server. The
server daemon is used by system administrators to distribute network
information to clients so if you are just a regular user you need the
_CLIENT_ daemon.

3. Client Setup

Currently there are three different DHCP client programs for Linux,
dhcpcd, pump and dhclient. This mini-HOWTO deals primarily with

3.1. Downloading the client daemon (dhcpcd)

Depending on your distribution you might have to download the DHCP
client daemon. If you want to compile it from the source you package
you need is called dhcpcd and the current version is 1.3.18. It is
maintained by Sergei Viznyuk <[33]> and today it
comes as a binary package with most distributions.

dhcpcd source can be downloaded from following locations

* [34] (Primary site)
* [35]

Then follow the instructions below. They should be the same.

3.2. Slackware

You can download the latest copy of the DHCPcd from any Metalab mirror
or following:

* [36]
* [37] (Primary site)

Download the latest version of dhcpcd.tar.gz.

* Unpack it
tar -zxvf dhcpcd-1.3.18pl1.tar.gz
* cd into the directory and make dhcpcd
cd dhcpcd-1.3.18pl1
* Install it (you have to run the following command as root)
make install

This will create the directory /etc/dhcpc where DHCPcd will store the
DHCP information and dhcpcd file will be copied into /usr/sbin.

In order to make the system initialize using DHCP during boot type:

cd /etc/rc.d

mv rc.inet1 rc.inet1.OLD

This will move the old network initialization script into
rc.inet1.OLD. You now need to create the new rc.inet1 script.
Following code is all you need:
# rc.inet1 This shell script boots up the base INET system.

HOSTNAME=`cat /etc/HOSTNAME` #This is probably not necessary but I
#will leave it in anyways

# Attach the loopback device.
/sbin/ifconfig lo
/sbin/route add -net netmask lo

# IF YOU HAVE AN ETHERNET CONNECTION, use these lines below to configure the
# eth0 interface. If you're only using loopback or SLIP, don't include the
# rest of the lines in this file.


Save it and reboot your computer.

When you are finished go the [38]last step.

3.3. RedHat 6.x and Mandrake 6.x

DHCPcd configuration under RedHat 6.0+ is really easy. All you need to
do is start the Control Panel by typing control-panel.

* Select "Network Configuration"
* Click on Interfaces
* Click Add
* Select Ethernet
* In the Edit Ethernet/Bus Interface select "Activate interface at
boot time" as well as select "DHCP" as "Interface configuration

Please note that in RedHat 6.x Redhat as default includes a DHCP
client called pump instead of above mentioned dhcpcd. CD-ROM does
include dhcpcd RPM so if you have no luck with pump try with dhcpcd.
After you install dhcpcd (e.g. rpm -i dhcpcd-1.3.17pl2-1.i386.rpm) you
will have to make some [39]changes.

Additional notes from Alexander Stevenson

I had no luck with DHCPcd. What finally worked for me was "pump",
which comes with Linux Mandrake 6.0 (and so I assume it is included
with RedHat as well). The command I used was:

pump -i eth0 -h hostname

It didn't matter what "hostname" was, but without it the server would
not respond.

I then changed the line in my /sbin/ifup script to reflect the change;
the default version does not have the -h switch, and so didn't work
for me.

Basically, if you're using linuxconf, and after setting the adapter to
"DHCP" it still doesn't work, try adding a "-h hostname" to the pump
line in the /sbin/ifup script. My script now looks like this:
if [ -n "$PUMP" ]; then
echo -n "Determining IP information for $DEVICE..."
if /sbin/pump -i $DEVICE -h hostname; then
echo " done."
echo " failed."
exit 1
else ...

Another more elegant way to add hostname field is provided by Aad van
der Klaauw:

Currently i'm configuring a gateway system at home, needed to set the
MAC address and use the '-h hostname' workaround. So I decided to
*not* change the script but to use the configure file. In my
/etc/sysconfig/network-scripts/ifcfg-eth0 I have added the following

Which will survive upgrades, and is imho a "cleaner" way.

That is it. Reboot your machine or type /sbin/ifup eth0 on the command

3.4. RedHat 5.x

DHCPcd configuration under RedHat 5.0+ is really easy. All you need to
do is start the Control Panel by typing control-panel.

* Select "Network Configuration"
* Click on Interfaces
* Click Add
* Select Ethernet
* In the Edit Ethernet/Bus Interface select "Activate interface at
boot time" as well as select "DHCP" as "Interface configuration

When you are finished go the [41]last step.

3.5. RedHat 4.x and Caldera OpenLinux 1.1/1.2

DHCPcd is included in the standard RedHat distribution as an RPM and
you can find it on your distribution's CD-ROM in RPMS directory or you
can download it from:


Install it with rpm -i dhcpcd-0.6-2.i386.rpm.

Alternatively you can compile your own version by following the steps
outlined in the [43]Slackware.

The following information was provided to me by nothing

Removed my static ip and name from /etc/resolv.conf. However, I did
leave in the search line and my two nameserver lines (for some
reason my dhcpcd never creates a /etc/dhcpc/resolv.conf, so I have
to use a static /etc/resolv.conf).

In /etc/sysconfig/network I removed the HOSTNAME and GATEWAY
entries. I left the other entries as is (NETWORKING, DOMAINNAME,

In /etc/sysconfig/network-scripts/ifcfg-eth0 I removed the IPADDR,
as is. I changed the BOOTPROTO line to BOOTPROTO=dhcp.

Save the file. Reboot your computer.

When you are finished go the [45]last step.

3.6. Debian

There is a deb package of DHCPcd (make sure it starts with dhcpcd) at:


Or, follow the [47]Slackware installation instructions.

To unpack the deb package type dpkg -i

It appears that there isn't a need for any DHCPcd configuration

The dhcpcd package installs it's startup script as usual for debian
packages in /etc/init.d/package_name, here as /etc/init.d/dhcpcd, and
links this to the various /etc/rc?.d/ directories.

--From: Heiko Schlittermann <[48]>

The contents of the /etc/rc?.d/ dirs is then executed at boot time.

If you don't reboot after installing you should consider starting the
daemon manually: /etc/init.d/dhcpcd start.

When you are finished go the [49]last step.

3.7. LinuxPPC and MkLinux

Following section has been written by R. Shapiro

As of the "1999" (R5) release, Linuxppc is now almost completely
compatible with Redhat 6, with one caveat (see below). In general the
instructions are exactly the same as for the current release of
[50]RedHat 6.x and Mandrake 6.x.

The remaining problem is that Redhat 6 uses the 'pump' client for dhcp
by default, and 'pump' doesn't work reliably in Linuxppc. To get
around this, you should install the latest dhcpcd from Sergei Viznyuk,
and then edit /sbin/ifup to use dhcpcd instead of pump.

if [ "$BOOTPROTO" = bootp -o "$BOOTPROTO" = dhcp ]; then

if [ -n "$PUMP" ]; then

if [ "$BOOTPROTO" = bootp ]; then
echo " done."
echo " failed."
exit 1
elif [ "$BOOTPROTO" = dhcp ]; then
echo -n "Determining IP information for $DEVICE..."
if /sbin/dhcpcd -d $DEVICE ; then
if [ -f /etc/dhcpc/dhcpcd-${DEVICE}.exe ]; then

and a coresponding changes for ifdown. Change
if [ "$BOOTPROTO" = bootp -o "$BOOTPROTO" = dhcp ]; then

if [ "$BOOTPROTO" = bootp ]; then

if [ "$BOOTPROTO" = dhcp ]; then
if [ -f /var/run/dhcpcd-${DEVICE}.pid ]; then
kill `cat /var/run/dhcpcd-${DEVICE}.pid`
rm -f /var/run/dhcpcd-${DEVICE}.pid

A working ppc rpm for dhcpcd is included on the Linuxppc 1999 cd; a
slightly later rpm is available in the contrib directory on
[51] Sources, which compile out of the box in
Linuxppc 1999, are available from

3.8. Tying it all together

After your machine reboots your network interface should be
configured. Type: ifconfig.

You should get something like this:
lo Link encap:Local Loopback
inet addr: Bcast: Mask:
RX packets:302 errors:0 dropped:0 overruns:0 frame:0
TX packets:302 errors:0 dropped:0 overruns:0 carrier:0 coll:0

eth0 Link encap:Ethernet HWaddr 00:20:AF:EE:05:45
inet addr: Bcast: Mask:
RX packets:24783 errors:1 dropped:1 overruns:0 frame:1
TX packets:11598 errors:0 dropped:0 overruns:0 carrier:0 coll:96
Interrupt:10 Base address:0x300

If you have some normal number under inet addr you are set. If you see don't despair, it is a temporary setting before dhcpcd
acquires the IP address. If even after few minutes you are seeing please check out [53]Troubleshooting. DHCPcd is a daemon and
will stay running as long as you have your machine on. Every three
hours it will contact the DHCP server and try to renew the IP address
lease. It will log all the messages in the syslog (on Slackware
/var/adm/syslog, RedHat/OpenLinux /var/log/syslog).

One final thing. You need to specify your nameservers. There are two
ways to do it, you can either ask your provider to provide you with
the addresses of your name server and then put those in the
/etc/resolv.conf or DHCPcd will obtain the list from the DHCP server
and will build a resolv.conf in /etc/dhcpc.

I decided to use DHCPcd's resolv.conf by doing the following:

Back up your old /etc/resolv.conf: mv /etc/resolv.conf

If directory /etc/dhcpc doesn't exist create it: mkdir /etc/dhcpc

Make a link from /etc/dhcpc/resolv.conf to /etc/resolv.conf: ln -s
/etc/dhcpc/resolv.conf /etc/resolv.conf

If that doesn't work try this (fix suggested by
<[54]> with a little amendment by Henrik

This last step I had to perform only because my dhcpcd doesn't create
an /etc/dhcpc/resolv.conf. In /etc/sysconfig/network-scripts/ifup I
made the following changes (which are a very poor hack, but they work
for me):
elif [ "$BOOTPROTO" = dhcp -a "$ISALIAS" = no ]; then
echo -n "Using DHCP for ${DEVICE}... "
/sbin/dhcpcd -c /etc/sysconfig/network-scripts/ifdhcpc-done ${DEVICE}
echo "echo \$$ > /var/run/dhcp-wait-${DEVICE}.pid; exec sleep 30" | sh

if [ -f /var/run/dhcp-wait-${DEVICE}.pid ]; then
echo "failed."
exit 1

I changed to:
elif [ "$BOOTPROTO" = dhcp -a "$ISALIAS" = no ]; then
echo -n "Using DHCP for ${DEVICE}... "
echo "echo \$$ > /var/run/dhcp-wait-${DEVICE}.pid; exec sleep 30" | sh

if [ ! -f /var/run/dhcp-wait-${DEVICE}.pid ]; then
echo "failed."
exit 1

Note: Notice the ! (bang) in if [ ! -f
/var/run/dhcp-wait-${DEVICE}.pid ];

Now sit back and enjoy :-).

3.9. Various notes

Following step(s) are not necessary but might be useful to some

a. If you need network connectivity only occasionally you can start
dhcpcd from the command line (you have to be root to do this)
with: /usr/sbin/dhcpcd.
When you need to down (turn off) the network type /usr/sbin/dhcpcd

3.10. Troubleshooting

If you have followed the steps outlined above and you are unable to
access the network there are several possible explanations:

3.10.1. Your network card is not configured properly

During the boot up process your Linux will probe your network card and
should say something along these lines:
eth0: 3c509 at 0x300 tag 1, 10baseT port, address 00 20 af ee 11 11, IRQ 10.
3c509.c:1.07 6/15/95

If a message like this doesn't appear your ethernet card might not be
recognized by your Linux system. If you have a generic ethernet card
(a NE2000 clone) you should have received a disk with DOS utilities
that you can use to set up the card. Try playing with IRQs until Linux
recognizes your card (IRQ 9,10,12 are usually good).

3.10.2. Your DHCP server supports RFC 1541/My DHCP server is Windows NT

Try running dhcpcd by typing dhcpcd -r.

Use ifconfig to check if your network interface is configured (wait
few seconds for the configuration process, initally it will say

If this solves your problem add the "-r" flag to the boot up scripts,
ie: instead of /sbin/dhcpcd you will have /sbin/dhcpcd -r.

For example under RedHat edit script
/etc/sysconfig/network-scripts/ifup and change the following:
"/sbin/dhcpcd -r -c /etc/"- etc etc.

3.10.3. During bootup I get error message "Using DHCP for eth0 ... failed"
but my system works fine.

You are most likely using RedHat and you haven't followed instructions
carefully :-). You are missing the ! (bang) in one of the if
statements. Jump [55]here and check how to fix it.

3.10.4. My network works for few minutes and then stops responding

There are some reports of gated (gateway daemon) screwing up routing
on Linux boxes which results in problem described above. Check if
gated is running with: ps -auxww | grep gate.

If it is try removing it with RedHat's RPM manager or removing the
entry in /etc/rc.d/.

3.10.5. My ethernet card is recognized during boot up but I still get "NO
DHCPOFFER" message in my logs. I also happen to have a PCMCIA ethernet card.

You need to make sure that you have the 10BaseT port ("phone" plug) on
your network card activated. Best way to verify it is to check what
kind of connector your card is configured for during bootup e.g.
eth0: 3c509 at 0x300 tag 1, 10baseT port, address 00 20 af ee 11 11, IRQ 10.
3c509.c:1.07 6/15/95

I have received reports of laptop users having this kind of problems
due to the PCMCIA utilities (specifically ifport) that would set the
connector type to 10Base2 (thinnet). You have to make sure you use
10BaseT for your connection. If you are not reconfigure the card and
restart the computer.

3.10.6. My DHCP client broadcasts requests but no one answers (Contributed
by Peter Amstutz)

On some systems, you need to include some hostname for your machine as
part of the request. With dhcpcd, do this with dhcpcd -h foohost
Probably the hostname wanted will be your account username on the

3.10.7. I have followed all the steps but still my machine is not able to

The cable modem will usually memorize the ethernet address of your
network card so if you connect a new computer or switch network cards
you will somehow have to "teach" your cable modem to recognize the new
computer/card. Usually you can turn of the modem and bring it back up
while computer is on or you will have to call tech support and tell
them that you have changed a network card in the computer.

You have firewall rules (ipfwadm rules) that disallow port 67/68
traffic used by DHCP to distribute configuration info. Check your
firewall rules carefully.

3.10.8. I have MediaOne Express service and I still can't connect.

It appears that MediaOne has been using adding some things to DHCP
that shouldn't be there. Supposedly this is not a problem anymore but
if you experience outages check for these things. If you are (un)lucky
to have Windows NT on your machine if you go into Event Viewer you
will see a warning like this:
DHCP received an unknown option 067 of length 005. The raw option data is
given below.

0000: 62 61 73 69 63 basic

If this is the problem go to [56] and
either download a binary or get the source for the change.

3.11. Alternative DHCP client (ISC dhclient)

If you have not had success getting your Linux connection running with
the dhcpcd you might want to try ISC dhclient. dhclient comes with the
DHCP distribution from ISC which includes both a DHCP client and a
DHCP server. Instructions on how to get and compile the DHCP
distribution can be found [57]here. When you are done with it please
return to this section to configure the client.

Note: Following information has been provided by Ted Lemon
<[58]> one of the authors of dhclient.

With the current version of the DHCP client, you don't actually need a
dhclient.conf. All you have to do is invoke dhclient e.g.:

This will configure all broadcast interfaces. If this doesn't work or
you want to specify only one interface create a /etc/dhclient.conf
file with this example configuration.
interface "eth0" {
send dhcp-client-identifier 1:xx:xx:xx:xx:xx:xx;
send dhcp-lease-time 86400;

Here we assume that the ethernet interface is eth0. If not change
accordingly. Also replace xx:xx:xx:xx:xx with your ethernet address.
This dhclient.conf makes the client look more like a Win95 client.

4. DHCP Server Setup

4.1. DHCP server for UNIX

There are several DHCP servers available for U*X-like OSes, both
commercial and free. One of the more popular free DHCP servers is Paul
Vixie/ISC DHCPd. Currently the latest version is 2.0 (suggested for
most users) but 3.0 is in beta testing. You can get them from


Some of the distributions provide binary packages for dhcpd so skip
the following section if you got it installed that way.

After you download unpack it. After you do cd into the distribution
directory and type: ./configure

It will take some time to configure the settings. After it is done
type: make and make install.

4.2. DHCP server configuration

When done with installation type ifconfig -a. You should see something
like this:
eth0 Link encap:10Mbps Ethernet HWaddr 00:C0:4F:D3:C4:62
inet addr: Bcast: Mask:
RX packets:2875542 errors:0 dropped:0 overruns:0
TX packets:218647 errors:0 dropped:0 overruns:0
Interrupt:11 Base address:0x210

If it doesn't say MULTICAST you should reconfigure your kernel and add
multicast support. On most systems you will not need to do this.

Next step is to add route for Quoted from DHCPd

"In order for dhcpd to work correctly with picky DHCP clients
(e.g., Windows 95), it must be able to send packets with an IP
destination address of Unfortunately, Linux
insists on changing into the local subnet broadcast
address (here, that's This results in a DHCP protocol
violation, and while many DHCP clients don't notice the problem,
some (e.g., all Microsoft DHCP clients) do. Clients that have this
problem will appear not to see DHCPOFFER messages from the server."

Type: route add -host dev eth0

If you get a message " Unknown host", you should try
adding the following entry to your /etc/hosts file: all-ones

Then, try:
route add -host all-ones dev eth0

route add dev eth0

eth0 is of course the name of the network device you are using. If it
differs change appropriately.

4.3. Options for DHCPd

Now you need to configure DHCPd. In order to do this you will have to
create or edit /etc/dhcpd.conf. There is a graphical interface for
dhcpd configuration under KDE ( [60] ) called
kcmdhcpd that is very similar to the DHCP configurator on Windows NT.
When KDE 2.0 comes out it should come with kcmdhcpd or you could get
it directly from:


If you want to configure it by hand follow instructions below.

Most commonly what you want to do is assign IP addresses randomly.
This can be done with settings as follows:
# Sample /etc/dhcpd.conf
# (add your comments here)
default-lease-time 600;
max-lease-time 7200;
option subnet-mask;
option broadcast-address;
option routers;
option domain-name-servers,;
option domain-name "";

subnet netmask {

This will result in DHCP server giving a client an IP address from the
range or It
will lease an IP address for 600 seconds if the client doesn't ask for
specific time frame. Otherwise the maximum (allowed) lease will be
7200 seconds. The server will also "advise" the client that it should
use as its subnet mask, as its broadcast
address, as the router/gateway and and as its DNS servers.

If you need to specify a WINS server for your Windows clients you will
need to include the netbios-name-servers option e.g.
option netbios-name-servers;

You can also assign specific IP addresses based on clients ethernet
address e.g.
host haagen {
hardware ethernet 08:00:2b:4c:59:23;

This will assign IP address to a client with ethernet
address 08:00:2b:4c:59:23.

You can also mix and match e.g. you can have certain clients getting
"static" IP addresses (e.g. servers) and others being alloted dynamic
IPs (e.g. mobile users with laptops). There are a number of other
options e.g. nis server addresses, time server addresses etc., if you
need any of those options please read the dhcpd.conf man page.

4.4. Starting the server

There is only one thing to do before starting the server. In most
cases DHCP installation doesn't create a dhcpd.leases files. This file
is used by DHCPd to store information about current leases. It is in
the plain text form so you can view it during the operation of DHCPd.
To create dhcpd.leases type:
touch /var/state/dhcp/dhcpd.leases

This will create an empty file (file size = 0). Some of the older
version of dhcpd 2.0 placed the file in /etc/dhcpd.leases. You do not
need to make any changes to the leases file it will be manipulated by
the dhcpd. If you get a message saying that file exists simply ignore
it and go to the next step.

You can now invoke the DHCP server. Simply type (or include in the
bootup scripts)

This will invoke dhcpd on eth0 device. If you want to invoke it on
another device simply supply it on the command line e.g.
/usr/sbin/dhcpd eth1

To verify that everything is working fine you should first turn on the
debugging mode and put the server in foreground. You can do this by
/usr/sbin/dhcpd -d -f

Then boot up one of your clients and check out the console of your
server. You will see a number of debugging messages come up. If
everything works out fine you are done :-). Quit dhcpd and start it
without the -d -f and arguments. If you want dhcpd to start at boot-up
include dhcpd in e.g.

4.5. Other interesting documents

Linux Magazine has a pretty good article in their April issue called
[62]Network Nirvana: How to make Network Configuration as easy as DHCP
that discusses the set up for DHCP.


1. DHCP.html#AEN17
2. DHCP.html#AEN19
3. DHCP.html#AEN24
4. DHCP.html#AEN41
5. DHCP.html#AEN45
6. DHCP.html#AEN64
7. DHCP.html#AEN69
8. DHCP.html#AEN74
9. DHCP.html#AEN77
11. DHCP.html#REDHAT6
12. DHCP.html#AEN166
13. DHCP.html#AEN183
14. DHCP.html#DEBIAN
15. DHCP.html#AEN230
17. DHCP.html#AEN293
19. DHCP.html#AEN355
20. DHCP.html#AEN369
22. DHCP.html#AEN382
23. DHCP.html#AEN403
24. DHCP.html#AEN422
25. DHCP.html#AEN440
50. DHCP.html#REDHAT6
55. DHCP.html#ERROR3