Beyond Linux® From Scratch (systemd Edition) - Version r12.0-816

Chapter 14. Connecting to a Network

dhcpcd-10.0.5

Introduction to dhcpcd

dhcpcd is an implementation of the DHCP client specified in RFC2131. A DHCP client is useful for connecting your computer to a network which uses DHCP to assign network addresses. dhcpcd strives to be a fully featured, yet very lightweight DHCP client.

[Note]

Note

Development versions of BLFS may not build or run some packages properly if LFS or dependencies have been updated since the most recent stable versions of the books.

Package Information

dhcpcd Dependencies

Optional

LLVM-17.0.1 (with Clang), ntp-4.2.8p17, chronyd, and ypbind

Privilege separation

Recent releases of dhcpcd optionally support privilege separation. As the practical security benefits of this are unclear for a program like dhcpcd and the setup is more complicated, the book currently defaults to disable it.

If you however would like to use privilege separation, additional installation steps are necessary to set up the proper environment. Issue the following commands as the root user: 

install  -v -m700 -d /var/lib/dhcpcd &&

groupadd -g 52 dhcpcd        &&
useradd  -c 'dhcpcd PrivSep' \
         -d /var/lib/dhcpcd  \
         -g dhcpcd           \
         -s /bin/false       \
         -u 52 dhcpcd &&
chown    -v dhcpcd:dhcpcd /var/lib/dhcpcd

Installation of dhcpcd

Build dhcpcd without privilege separation by running the following command: 

./configure --prefix=/usr                \
            --sysconfdir=/etc            \
            --libexecdir=/usr/lib/dhcpcd \
            --dbdir=/var/lib/dhcpcd      \
            --runstatedir=/run           \
            --disable-privsep         &&
make

Alternatively, build dhcpcd with privilege separation by running the following commands: 

./configure --prefix=/usr                \
            --sysconfdir=/etc            \
            --libexecdir=/usr/lib/dhcpcd \
            --dbdir=/var/lib/dhcpcd      \
            --runstatedir=/run           \
            --privsepuser=dhcpcd         &&
make

To test the results, issue: make test.

Now, as the root user: 

make install

Command Explanations

--libexecdir=/usr/lib/dhcpcd: This switch sets a better location for the dhcpcd internal libraries.

--dbdir=/var/lib/dhcpcd: This switch adjusts the database directory because the default directory, /var/db, is not FHS-compliant.

--runstatedir=/run: This switch sets the runtime state directory because the default /var/run is a symbolic link to /run, and using /var/run is deprecated.

--disable-privsep: This switch disables privilege separation, which is the default in dhcpcd. This switch is not used in the build configuration where privilege separation is used.

--privsepuser=dhcpcd: This switch sets the privilege separation user in the build configuration where privilege escalation is used.

--with-hook=...: You can optionally install more hooks, for example to install some configuration files such as ntp.conf. A set of hooks can be found in the dhcpcd-hooks directory in the build tree.

Configuring dhcpcd

Config Files

/etc/dhcpcd.conf

General Configuration Information

If you want to configure network interfaces at boot using dhcpcd, you need to install the systemd unit included in blfs-systemd-units-20230816 package by running the following command as the root user: 

make install-dhcpcd
[Note]

Note

The default behavior of dhcpcd is to set the hostname and the mtu. It also overwrites /etc/resolv.conf and /etc/ntp.conf. These modifications to system configuration files are done by hooks which are stored in /usr/lib/dhcpcd/dhcpcd-hooks. You can change this behavior by removing or adding hooks from/to that directory. The execution of hooks can be disabled by using the --nohook (-C) command line option or by the nohook option in the /etc/dhcpcd.conf file.

[Note]

Note

Make sure that you disable the systemd-networkd service or configure it not to manage the interfaces you want to manage with dhcpcd.

At this point you can test if dhcpcd is behaving as expected by running the following command as the root user: 

systemctl start dhcpcd@eth0

To start dhcpcd on a specific interface at boot, enable the previously installed systemd unit by running the following command as the root user: 

systemctl enable dhcpcd@eth0

Replace eth0 with the actual interface name.

Contents

Installed Program: dhcpcd
Installed Library: /usr/lib/dhcpcd/dev/udev.so
Installed Directory: /{usr,var}/lib/dhcpcd and /usr/share/dhcpcd

Short Descriptions

dhcpcd

is an implementation of the DHCP client specified in RFC2131

udev.so

adds udev support for interface arrival and departure; this is because udev likes to rename the interface, which it can't do if dhcpcd grabs it first