Beyond Linux® From Scratch (System V Edition) - Version r11.2-1338

Chapter 14. Connecting to a Network

dhcpcd-9.4.1

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.

This package is known to build and work properly using an LFS 11.3 platform.

Package Information

dhcpcd Dependencies

Optional

LLVM-15.0.7 (with Clang), ntp-4.2.8p15, chronyd, and ypbind

User Notes: https://wiki.linuxfromscratch.org/blfs/wiki/dhcpcd

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

Fix a runtime error caused by a change in glibc-2.36:

sed '/Deny everything else/i SECCOMP_ALLOW(__NR_getrandom),' \
    -i src/privsep-linux.c

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

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: Set a more proper location for dhcpcd internal libraries.

--dbdir=/var/lib/dhcpcd: The default /var/db is not FHS-compliant

--runstatedir=/run: The default /var/run is a symbolic link to /run.

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

--disable-privsep: Do not use privilege separation, which is the default.

--privsepuser=dhcpcd: Use this unprivileged user in a privilege separation setup.

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

Configuring dhcpcd

Config Files

/etc/dhcpcd.conf

General Configuration Information

To configure dhcpcd, you need to first install the network service script, /usr/lib/services/dhcpcd included in the blfs-bootscripts-20230101 package (as user root): 

make install-service-dhcpcd
[Note]

Note

the network service script /usr/lib/services/dhcpcd contained in bootscripts up to blfs-bootscripts-20191204 is not compatible with latest dhcpcd, as the location of the pidfile changed. To work around this, issue the following command: 

sed -i "s;/run/dhcpcd-;/run/dhcpcd/;g" /lib/services/dhcpcd

[Note]

Note

The default for dhcpcd is to set the hostname and mtu. It also overwrites /etc/resolv.conf and /etc/ntp.conf. These modifications to system files are done by hooks which are stored in /usr/lib/dhcpcd/dhcpcd-hooks. Set up dhcpcd 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.

Finally, as the root user create the /etc/sysconfig/ifconfig.eth0 configuration file using the following commands. Adjust appropriately for additional interfaces: 

cat > /etc/sysconfig/ifconfig.eth0 << "EOF"
ONBOOT="yes"
IFACE="eth0"
SERVICE="dhcpcd"
DHCP_START="-b -q -h ''<insert appropriate start options here>"
DHCP_STOP="-k <insert additional stop options here>"
EOF

For more information on the appropriate DHCP_START and DHCP_STOP values, examine the man page for dhcpcd.

Configuration Information: fixed ip

Although not usual, it is possible that you need to configure dhcpcd to use a fixed ip. Here, we give an example. As the root user create the /etc/sysconfig/ifconfig.eth0 configuration file using the following commands. Adjust appropriately for additional interfaces and for the actual ip and router you need: 

cat > /etc/sysconfig/ifconfig.eth0 << "EOF"
ONBOOT="yes"
IFACE="eth0"
SERVICE="dhcpcd"
DHCP_START="-b -q -S ip_address=192.168.0.10/24 -S routers=192.168.0.1"
DHCP_STOP="-k"
EOF

You can either use DNS servers in /etc/resolv.conf from another system, your preferred servers, or just the example /etc/resolv.conf.head file below as is: 

cat > /etc/resolv.conf.head << "EOF"
# OpenDNS servers
nameserver 208.67.222.222
nameserver 208.67.220.220
EOF

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