Back to basics – Setting a DHCP server

Following on from other posts in the “back to basics” series; Local yum repository and setting up an internal DNS server. Here is the next step in the process of building a infrastructure services server.

So we have DNS in place and working.  Now lets make sure that none of the client machines in the lab have to be configured with an IPv4 address manually.

First things first, lets get dhcp installed;

[root@rhc-server etc]# yum install dhcp -y

Before I look to configure DHCP for my needs, lets just have a quick look at the example configuration file.  This is good for a number of reasons, sometimes you will see certain options being used which you would not have used if it weren’t for having seen them in an example.

[root@rhc-server etc]# cat /usr/share/doc/dhcp*/dhcpd.conf.example
# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#

# option definitions common to all supported networks...
option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;

default-lease-time 600;
max-lease-time 7200;

# Use this to enble / disable dynamic dns updates globally.
#ddns-update-style none;

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
#authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;

# No service will be given on this subnet, but declaring it helps the
# DHCP server to understand the network topology.

subnet 10.152.187.0 netmask 255.255.255.0 {
}

# This is a very basic subnet declaration.

subnet 10.254.239.0 netmask 255.255.255.224 {
range 10.254.239.10 10.254.239.20;
option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
}

# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.

subnet 10.254.239.32 netmask 255.255.255.224 {
range dynamic-bootp 10.254.239.40 10.254.239.60;
option broadcast-address 10.254.239.31;
option routers rtr-239-32-1.example.org;
}

# A slightly different configuration for an internal subnet.
subnet 10.5.5.0 netmask 255.255.255.224 {
range 10.5.5.26 10.5.5.30;
option domain-name-servers ns1.internal.example.org;
option domain-name "internal.example.org";
option routers 10.5.5.1;
option broadcast-address 10.5.5.31;
default-lease-time 600;
max-lease-time 7200;
}

# Hosts which require special configuration options can be listed in
# host statements.   If no address is specified, the address will be
# allocated dynamically (if possible), but the host-specific information
# will still come from the host declaration.

host passacaglia {
hardware ethernet 0:0:c0:5d:bd:95;
filename "vmunix.passacaglia";
server-name "toccata.fugue.com";
}

# Fixed IP addresses can also be specified for hosts.   These addresses
# should not also be listed as being available for dynamic assignment.
# Hosts for which fixed IP addresses have been specified can boot using
# BOOTP or DHCP.   Hosts for which no fixed address is specified can only
# be booted with DHCP, unless there is an address range on the subnet
# to which a BOOTP client is connected which has the dynamic-bootp flag
# set.
host fantasia {
hardware ethernet 08:00:07:26:c0:a5;
fixed-address fantasia.fugue.com;
}

# You can declare a class of clients and then do address allocation
# based on that.   The example below shows a case where all clients
# in a certain class get addresses on the 10.17.224/24 subnet, and all
# other clients get addresses on the 10.0.29/24 subnet.

class "foo" {
match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
}

shared-network 224-29 {
subnet 10.17.224.0 netmask 255.255.255.0 {
option routers rtr-224.example.org;
}
subnet 10.0.29.0 netmask 255.255.255.0 {
option routers rtr-29.example.org;
}
pool {
allow members of "foo";
range 10.17.224.10 10.17.224.250;
}
pool {
deny members of "foo";
range 10.0.29.10 10.0.29.230;
}
}

As we can see from the above, the examples have provided us with pretty much everything we need to know and more to get things up and running.  Below is the configuration file that I created;

[root@rhc-server dhcp]# cat dhcpd.conf
#
#  lab.tobyheywood.com dhcp daemon configuration file
#
#  2016-02-22 - Initial creation
#

# Define which IP to listen on.  NOTE. daemon can only listen to one
# IP at a time if defined.
local-address 192.168.20.1;

# option definitions common to all supported networks...
option domain-name "lab.tobyheywood.com";
option domain-name-servers ns.lab.tobyheywood.com;

default-lease-time 600;
max-lease-time 7200;

# Use this to enble / disable dynamic dns updates globally.
#ddns-update-style interim;

# This is the authoritative DHCP server.
authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;

# My management network on a separate interface
# Included in configuration otherwise I get errors in journalctl
# **** NOT IN USE ****
subnet 192.168.122.0 netmask 255.255.255.0 {
}

# The lab network
subnet 192.168.20.0 netmask 255.255.255.128 {
range 192.168.20.50 192.168.20.99;
option routers rtr.lab.tobyheywood.com;
}

Hopefully the comments above are sufficient to give you a good idea of what each bit is therefore and what it does.

Now lets start the dhcpd daemon and check everything is working as it should.

[root@rhc-server dhcp]# systemctl enable dhcpd
[root@rhc-server dhcp]# systemctl start dhcpd
[root@rhc-server dhcp]# systemctl list-units | grep named
named.service        loaded active running     Berkeley Internet Name Domain (DNS)
[root@rhc-server dhcp]# systemctl list-units | grep -e dhcpd
dhcpd.service        loaded active running     DHCPv4 Server Daemonctl

And a quick review of the logs shows we are cooking with gas!

Feb 22 22:11:13 rhc-server systemd[1]: Starting DHCPv4 Server Daemon...
Feb 22 22:11:13 rhc-server systemd[1]: Started DHCPv4 Server Daemon.
Feb 22 22:11:13 rhc-server dhcpd[3145]: Internet Systems Consortium DHCP Server 4.2.5
Feb 22 22:11:13 rhc-server dhcpd[3145]: Copyright 2004-2013 Internet Systems Consortium.
Feb 22 22:11:13 rhc-server dhcpd[3145]: All rights reserved.
Feb 22 22:11:13 rhc-server dhcpd[3145]: For info, please visit https://www.isc.org/software/dhcp/
Feb 22 22:11:13 rhc-server dhcpd[3145]: Not searching LDAP since ldap-server, ldap-port and ldap-base-dn were not specified in the...ig file
Feb 22 22:11:13 rhc-server dhcpd[3145]: Internet Systems Consortium DHCP Server 4.2.5
Feb 22 22:11:13 rhc-server dhcpd[3145]: Copyright 2004-2013 Internet Systems Consortium.
Feb 22 22:11:13 rhc-server dhcpd[3145]: All rights reserved.
Feb 22 22:11:13 rhc-server dhcpd[3145]: For info, please visit https://www.isc.org/software/dhcp/
Feb 22 22:11:13 rhc-server dhcpd[3145]: Wrote 1 leases to leases file.
Feb 22 22:11:13 rhc-server dhcpd[3145]: Listening on LPF/ens8/52:54:00:ca:b3:a6/192.168.20.0/25
Feb 22 22:11:13 rhc-server dhcpd[3145]: Sending on   LPF/ens8/52:54:00:ca:b3:a6/192.168.20.0/25
Feb 22 22:11:13 rhc-server dhcpd[3145]: Listening on LPF/ens3/52:54:00:2b:da:2b/192.168.122.0/24
Feb 22 22:11:13 rhc-server dhcpd[3145]: Sending on   LPF/ens3/52:54:00:2b:da:2b/192.168.122.0/24
Feb 22 22:11:13 rhc-server dhcpd[3145]: Sending on   Socket/fallback/fallback-net
Feb 22 22:13:35 rhc-server dhcpd[3145]: DHCPDISCOVER from 52:54:00:a6:a4:fa via ens8
Feb 22 22:13:35 rhc-server dhcpd[2682]: DHCPDISCOVER from 52:54:00:a6:a4:fa via ens8
Feb 22 22:13:36 rhc-server dhcpd[3145]: DHCPOFFER on 192.168.20.50 to 52:54:00:a6:a4:fa (rhc-client) via ens8
Feb 22 22:13:36 rhc-server dhcpd[2682]: ns.lab.tobyheywood.com: host unknown.
Feb 22 22:13:36 rhc-server dhcpd[2682]: rtr.lab.tobyheywood.com: host unknown.
Feb 22 22:13:36 rhc-server dhcpd[2682]: DHCPOFFER on 192.168.20.50 to 52:54:00:a6:a4:fa (rhc-client) via ens8
Feb 22 22:13:37 rhc-server dhcpd[3145]: DHCPREQUEST for 192.168.20.50 (192.168.20.1) from 52:54:00:a6:a4:fa (rhc-client) via ens8
Feb 22 22:13:37 rhc-server dhcpd[3145]: DHCPACK on 192.168.20.50 to 52:54:00:a6:a4:fa (rhc-client) via ens8
Feb 22 22:13:37 rhc-server dhcpd[2682]: DHCPREQUEST for 192.168.20.50 (192.168.20.1) from 52:54:00:a6:a4:fa (rhc-client) via ens8
Feb 22 22:13:37 rhc-server dhcpd[2682]: DHCPACK on 192.168.20.50 to 52:54:00:a6:a4:fa (rhc-client) via ens8

So with the exception of the two “host unknown” errors we are looking good!  So that will do for now.

Time to go investigate the host unknown issue, ggrrrrrr!  Can I ping it? Yes I can!

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