Next, let's look at some basic network configurations so that we can communicate with the outside world. I am assuming that there is an Ethernet interface, eth0, and that we only need a simple IP v4 configuration.
These examples use the network utilities that are part of BusyBox, and are sufficient for a simple use case, using the old-but-reliable ifup and ifdown programs. You can read the man pages on both for more details. The main network configuration is stored in /etc/network/interfaces. You will need to create these directories in the staging directory:
etc/network etc/network/if-pre-up.d etc/network/if-up.d var/run
For a static IP address, etc/network/interfaces would look like this:
auto lo iface lo inet loopback auto eth0 iface eth0 inet static address 10.0.0.42 netmask 255.255.255.0 network 10.0.0.0
For a dynamic IP address allocated using DHCP, etc/network/interfaces would look like this:
auto lo iface lo inet loopback auto eth0 iface eth0 inet dhcp
You will also have to configure a DHCP client program. BusyBox has one named udchpcd. It needs a shell script that should go in /usr/share/udhcpc/default.script. There is a suitable default in the BusyBox source code in the directory examples//udhcp/simple.script.
glibc uses a mechanism known as the name service switch (NSS) to control the way that names are resolved to numbers for networking and users. User names, for example, may be resolved to UIDs via the file /etc/passwd; network services such as HTTP can be resolved to the service port number via /etc/services, and so on. All this is configured by /etc/nsswitch.conf, see the manual page, nss(5) for full details. Here is a simple example that will suffice for most embedded Linux implementations:
passwd: files group: files shadow: files hosts: files dns networks: files protocols: files services: files
Everything is resolved by the correspondingly named file in /etc, except for the host names, which may additionally be resolved by a DNS lookup.
To make this work, you need to populate /etc with those files. Networks, protocols, and services are the same across all Linux systems, so they can be copied from /etc in your development PC. /etc/hosts should, at least contain, the loopback address:
127.0.0.1 localhost
We will come to the others, passwd, group, and shadow, later.
The last piece of the jigsaw is the libraries that perform the name resolution. They are plugins that are loaded as needed based on the contents of nsswitch.conf, meaning that they do not show up as dependencies if you use readelf or similar. You will simply have to copy them from the toolchain's sysroot:
$ cd ~/rootfs $ cp -a $TOOLCHAIN_SYSROOT/lib/libnss* lib $ cp -a $TOOLCHAIN_SYSROOT/lib/libresolv* lib