Multiple IPs on the Raspberry Pi

Intro
In my previous post I showed how to turn a Raspberry Pi plus USB camera into something like an IP camera. In the course of that work I found it wasn’t so easy as it was in the past to assign static IPs upon boot. So I came up with my own unique method, which combines a modicum of Linux knowledge with a dash of networking knowledge.

The requirements
I sort of invented these requirements for myself, putting myself in the pickle I found myself in. I am working with a friend’s Pi 3 and didn’t want to mess it up too badly. Yet I wanted to easily work with it at home, and for the Robotics team. How to do it all?

I decided to permit the DHCP client, now called dhcpcd, running. So it will assign an IP address and appropriate gateway if there is a DHCP server present on the network. When I test at home I sometimes don’t use DHCP. When I bring my test setup to Robotics, more often than not I have my own little isolated LAN and no DHCP server. So, knowing that a single interface can have two or even more than two IP adresses, I created the following list of requirements for myself.

Act as DHCP client if there is DHCP server.
Additionally,
Assign static IP of 192.168.1.161/24 so it works in my home.
Assign another static IP of 10.31.42.15 so it works with a predictable IP in the robotics environment.
Let the two above IP assignments work even in the absence of a DHCP server!

Sounds kind of simple, but it’s not so easy.

I’m running a Raspberry Pi 3 with Raspbian Stretch (the release after Jessie).

Initial approach
With this version you’re supposed to use the file

/etc/dhcpcd.conf

to create a static IP.

But it works like c**p, at least when you want to push it and have it meet all the requirements above. It’s got a bug and doesn’t allow you to meet all the above requirements. I experimented. But my method does work.

The final solution
So in the end I leave /etc/dhcpcd.conf alone!

I use this new (to me) feature that crontab has an @reboot feature that calls its argument at boot time – just what we need.

Then I combined some old school use of ifconfig plus newer school command ip.

Here’s the script, which I call ip-assign.sh.

#!/bin/bash
sleep 2
# see if there is a dhcp-assigned IP already. If so 'scope global' appears in the listing
#  ip add show eth0 sample output:
addflag=""
ip add show eth0|grep -q 'scope global'
if [ $? == 0 ]; then
  addflag="add"
fi
# first IP
ifconfig eth0 $addflag 10.31.42.15 netmask 255.255.255.0 broadcast 10.31.42.255
# next IP
ifconfig eth0 add 192.168.1.161 netmask 255.255.255.0 broadcast 192.168.1.255

What I observed is that eth0 already has an IP assigned to it (for instance from a DHCP server), then the string “scope global” appears when you run ip add, otherwise it doesn’t. Furthermore, ifconfig has an optional argument I noticed call add, which seems to exist in order to add additional virtual interfaces – precisely what we want. But if there is no IP yet assigned we should call ifconfig the first time with the add argument. If I had had additional virtual IPs I could have just kept on going…

So to call this at boot time I use my lazy method. I edit the crontab file and insert a line like this:

@reboot sudo ~/ip‐assign.sh > /tmp/ip‐assign.log 2>&1

So without a DHCP server I have after booting:
$ ip add show eth0

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether b8:27:eb:e3:02:74 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.161/24 brd 10.31.42.255 scope global eth0:0
       valid_lft forever preferred_lft forever
    inet 10.31.42.15/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet 169.254.159.115/16 brd 169.254.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::e923:3131:224c:ecd/64 scope link
       valid_lft forever preferred_lft forever

If you’re lazy like me just type
$ ip add
and you’ll get the other interfaces as well. It’s very easy to type, too!

Note the broadcast (brd) addresses are reversed from how you’d expect them. I decided that it doesn’t matter as long as they’re both present somewhere with the correct value. It’s all using the one physical interface so the interface doesn’t really care. And from all my testing I am right I believe on this point.

Disable WiFi – wlan0
To disable WiFi entirely, which you may want to do if using in a FIRST FRC competition, add this to /boot/config.txt and reboot:
dtoverlay=pi3‐disable‐wifi
After doing that wlan0 does not even show up when you do an ip add.

References and related
Raspberry Pi plus USB camera: brought together like an IP camera.

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