ubuntu autoinstall interface names

Question

It seems not possible on Ubuntu autoinstallations (14.04.3 and 15.04) to configure the right network interface via preseed.

Ubuntu sometimes uses eth0, and sometimes interfaces like p2p1, or p1p3.

I know how these names will be generated. But how can I detect these names during or before the autoinstall? Or how can I automatically rename the p2p1 interface name to the old eth0 name?

To set the static interface settings I use d-i preseed/late_command to overwrite the /etc/network/interfaces file. Right know I assume there is only one interface with the name eth0.

I tried to overwrite/delete the /etc/udev/rules.d/70-persistent-net.rules within the preseed/late_command. But this does not help in any way - it will be ignored.

How can I change the interface name to eth0? Or how can I detect the interface name during the autoinstallation, to write the correct name into the network/interfaces file?

Edit:

The replacement now works great (thanks to "muru"!)

The Interface will initial written from another server and served internal via http wget in the preseed/late_command:

wget http://{InternalServer}/{path}/interfaces -O /etc/network/interfaces;

in the same line i load an additional replaceinterface.sh script with the content:

#!/bin/bash
sDevice=`ip -o link | grep 'link/ether' | grep -oE "^[[:digit:]]:[[:space:]]([[:alnum:]]+)" | cut -d" " -f 2`
sed "s/eth0/$sDevice/g" $1

After loading ,i fire chmod +x, execute it with sh /tmp/replaceinterface.sh < /etc/network/interfaces > /etc/network/interfaces;

But then, after installation is done, the /etc/network/interfaces file is empty (?) ...

If i do the same steps in another console manualy, during installation, i see the correct output / correct modified file

But that seems to be another question for askubuntu.com ..

Edit²:

The Replacement works perfect now - thanks again to "muru" !

#!/bin/bash
sDevice=`ip -o link | grep 'link/ether' | grep -oE "^[[:digit:]]:[[:space:]]([[:alnum:]]+)" | cut -d" " -f 2`
sReplaced=`sed s/eth0/$sDevice/g $1`
echo "$sReplaced" > $1

Answer 1

Network interfaces are not available even during 'late-command'. They are available on the first boot after having done an automated install.

This is why 'ip -o link' will be empty if run during a preseed late-command.

One way to do this is to create a simple systemd unit that runs after the network is ready, then have that adjust interface names and/or netplan configuration.

Example basic service that could be dropped into '/etc/systemd/system':

[Unit]
Description=Run Once
After=network-online.target
Requires=network-online.target

[Service]
Type=simple
EnvironmentFile=
ExecStart=/root/runonce.pl

[Install]
WantedBy=multi-user.target

In this example I setup a script to run things a single time, to make an easy method for running one or many scripts that you want to run once right after install. The corresponding runonce.pl:

#!/usr/bin/perl -w
use strict;

my $folder = "/root/runonce/scripts";
my $donef = "/root/runonce/done";
if( ! -e $donef ) {
    mkdir $donef;
}
my $outputf = "/root/runonce/output";
if( ! -e $outputf ) {
    mkdir $outputf;
}

opendir( my $dh, $folder );
my @files = readdir( $dh );
closedir( $dh );

for my $file ( @files ) {
    next if( $file =~ m/^\.+$/ );
    my $full = "$folder/$file";
    my $done = "$donef/$file";
    my $output = "$outputf/$file";
    my $output2 = "$outputf/$file-error";
    if( -e $done ) {
        unlink $done;
    }
    chmod "0700", $full;
    `$full > $output 2> $output2`;
    rename $full, $done;
}

In order to set everything up from preseed, contents for preseed file:

d-i preseed/late_command string \
    in-target wget -P /etc/systemd/system/ http://10.0.2.2:8001/kickstart/runonce.service; \
    in-target systemctl enable runonce; \
    in-target mkdir /root/runonce/scripts; \
    in-target wget -P /root/runonce/ http://10.0.2.2:8001/kickstart/runonce.pl; \
    in-target chmod +x /root/runonce/runonce.pl; \
    in-target wget -P /root/runonce/scripts/ http://10.0.2.2:8001/kickstart/fixnet.pl

You'll need to adjust this to have a location to fetch the files from. In this example it is querying the gateway IP address of Virtualbox, which in turn then hits a server running on my local desktop.

Example "fixnet.pl" script that is being run after initial install:

#!/usr/bin/perl -w
use strict;

my @netlines = `ip -o link`;

my $npFile = "/etc/netplan/vbox.yaml";
open( my $npFh, ">$npFile" ) or die "Cannot write to $npFile";

print $npFh "network:\n  version: 2\n  renderer: networkd\n  ethernets:\n";

for my $netline ( @netlines ) {
    if( $netline =~ m/(enp[0-9a-z]+)/ ) {
        my $interface = $1;
        print STDERR "Adding interface $interface to netplan\n";
        print $npFh "    $interface:\n";
        print $npFh "      dhcp4: yes\n";
    }
}

close $npFh;
`netplan apply`;
exit 0;