Chapter 5. Network setup

The hostname resolution is currently supported by the NSS (Name Service Switch) mechanism too. The flow of this resolution is the following.

A typical workstation may be installed with its host name set to, e.g., "host_name" and its optional domain name set to an empty string. Then, "/etc/hosts" looks like the following.

127.0.0.1 localhost
127.0.1.1 host_name

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Each line starts with a IP address and it is followed by the associated hostname.

The IP address 127.0.1.1 in the second line of this example may not be found on some other Unix-like systems. The Debian Installer creates this entry for a system without a permanent IP address as a workaround for some software (e.g., GNOME) as documented in the bug #719621.

The host_name matches the hostname defined in the "/etc/hostname" (see Section 3.7.1, “The hostname”).

For a system with a permanent IP address, that permanent IP address should be used here instead of 127.0.1.1.

For a system with a permanent IP address and a fully qualified domain name (FQDN) provided by the Domain Name System (DNS), that canonical host_name.domain_name should be used instead of just host_name.

The "/etc/resolv.conf" is a static file if the resolvconf package is not installed. If installed, it is a symbolic link. Either way, it contains information that initialize the resolver routines. If the DNS is found at IP="192.168.11.1", it contains the following.

nameserver 192.168.11.1

The resolvconf package makes this "/etc/resolv.conf" into a symbolic link and manages its contents by the hook scripts automatically.

For the PC workstation on the typical adhoc LAN environment, the hostname can be resolved via Multicast DNS (mDNS) in addition to the basic files and dns methods.

The systemd uses "Predictable Network Interface Names" such as "enp0s25".

Let us be reminded of the IPv4 32 bit address ranges in each class reserved for use on the local area networks (LANs) by rfc1918. These addresses are guaranteed not to conflict with any addresses on the Internet proper.

	Note
	If one of these addresses is assigned to a host, then that host must not access the Internet directly but must access it through a gateway that acts as a proxy for individual services or else does Network Address Translation (NAT). The broadband router usually performs NAT for the consumer LAN environment.

Although most hardware devices are supported by the Debian system, there are some network devices which require DFSG non-free firmware to support them. Please see Section 9.10.5, “Hardware drivers and firmware”.

Official documentations for NM on Debian are provided in "/usr/share/doc/network-manager/README.Debian".

Essentially, the network configuration for desktop is done as follows.

A DHCP client configuration can be set up by creating a data source file "/etc/netplan/50-dhcp.yaml":

network:
  version: 2
  ethernets:
    all-en:
      match:
        name: "en*"
      dhcp4: true
      dhcp6: true

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.0.15/24
      routes:
        - to: default
          via: 192.168.0.1

network:
  version: 2
  renderer: NetworkManager

Iproute2 commands offer complete low-level network configuration capabilities. Here is a translation table from obsolete net-tools commands to new iproute2 etc. commands.

See ip(8) and Linux Advanced Routing & Traffic Control.

You may use low level network commands as follows safely since they do not change network configuration.

	Tip
	Some of these low level network configuration tools reside in "/usr/sbin/". You may need to issue full command path such as "/usr/sbin/ifconfig" or add "/usr/sbin" to the "$PATH" list in your "~/.bashrc".

NM normally sets optimal Maximum Transmission Unit (MTU) automatically.

In some occasion, you may wish to set MTU manually after experiments with ping(8) with "-M do" option to send a ICMP packet with various data packet size. MTU is the maximum succeeding data packet size without IP fragmentation plus 28 bytes for the IPv4 and plus 48 bytes for the IPv6. For example the following finds MTU for IPv4 connection to be 1460 and MTU for IPv6 connection to be 1500.

$ ping -4 -c 1 -s $((1500-28)) -M do www.debian.org
PING  (149.20.4.15) 1472(1500) bytes of data.
ping: local error: message too long, mtu=1460

---  ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

$ ping -4 -c 1 -s $((1460-28)) -M do www.debian.org
PING  (130.89.148.77) 1432(1460) bytes of data.
1440 bytes from klecker-misc.debian.org (130.89.148.77): icmp_seq=1 ttl=50 time=325 ms

---  ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 325.318/325.318/325.318/0.000 ms
$ ping -6 -c 1 -s $((1500-48)) -M do www.debian.org
PING www.debian.org(mirror-csail.debian.org (2603:400a:ffff:bb8::801f:3e)) 1452 data bytes
1460 bytes from mirror-csail.debian.org (2603:400a:ffff:bb8::801f:3e): icmp_seq=1 ttl=47 time=191 ms

--- www.debian.org ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 191.332/191.332/191.332/0.000 ms

This process is Path MTU (PMTU) discovery (RFC1191) and the tracepath(8) command can automate this.

In addition to these basic guide lines, you should know the following.

The maximum segment size (MSS) is used as an alternative measure of packet size. The relationship between MSS and MTU are the following.

The TCP throughput can be maximized by adjusting TCP buffer size parameters as in "TCP tuning" for the modern high-bandwidth and high-latency WAN. So far, the current Debian default settings serve well even for my LAN connected by the fast 1G bps FTTP service.