The Traveling Network Manager

Overview

Networks are such a big part of our lives these days that being at a place where there isn’t some form of a computer network, it feels like something’s off or missing, or like it wasn’t done well. You notice this especially when you travel around with a device capable of joining WiFi networks, like a smartphone, tablet or laptop. And even more so when you depend on these to get internet access.

Ubuntu, and I assume most modern desktop distributions, come with a utility called NetworkManager. It’s this utility’s job to join you to networks and manage these connections. It was designed to make best attempt to configure a network for you automatically with as little user interaction as possible. Even when using the GUI components, all input fields and configuration UIs were designed to make managing your networks as painless as possible, keeping in mind the average user’s abilities. All complicated setup options were completely removed, so you can’t configure things like multiple IP addresses, or select the WiFi channel, etc.

NetworkManager is mostly used through an icon in the system tray. Clicking this icon brings up a list of all available networks. If you select a network, NetworkManager will attempt to connect to the network and configure for your device via DHCP. If it needs any more information from you (like for a WiFi pass phrase or SIM card pin code), it will prompt you. If this connection becomes available in the future it will then automatically try and connect to it. For WiFi connections it’s the user’s job to select the first connection from the menu. For ethernet networks NetworkManager will automatically connect the first time.

These automatic actions NetworkManager takes are to make things more comfortable for the end user. The more advanced user can always go and disable or fine tune these as needed. For example to disable automatically connecting to a certain network, or setting a static IP address on a connection.

Roaming Profiles

If you travel around a lot you end up with many different network “profiles”. Each location where you join a network will have it’s own setup. If all these locations have DHCP you rarely need to perform any manual configuration to join the network. You do get the odd location, though, where you need some specific configuration like a static IP address. NetworkManager makes this and roaming very easy and natural to implement, and seamlessly manages this “profile” for you.

You would do this by first joining the network. Once connected, and whether or not your were given an IP address, you would open the NetworkManager connections dialog and locate the connection for the network you just joined. From here you would edit it and set your static IP address (or some other configuration option) and save the connection.

By doing this you effectively created your roaming profile for this network. None of your other connections will be affected, so whenever you join any of your other networks, they will still be working as they did previously, and the new network will have it’s own specific configuration.

This was never really intended to be a roaming profile manager, so other options related to roaming (like proxy servers) will not be configured automatically. I’m sure with a few scripts and a bit of hacking you should be able to automate setting up these configurations depending on the network you’re joining.

Conclusion

NetworkManager is maybe not the advanced user’s favorite tool. But if you don’t need any of these advanced features I would certainly recommend it.

So Why Love Linux? Because NetworkManager does a brilliant job of making networking comfortable in a very natural way.

Playing Ping Pong with ARP

ARP

Let me start of by giving a very rough explanation of how devices communicate on ethernet and WiFi networks. Each network device has a unique hardware address called a MAC address. These are assigned by manufacturers and don’t change for the life of the device. Many manufacturers even place the address on a sticker on the chip. It is possible to change the address with software, though the idea is that the address remain static so the device can communicate on a physical network.

So what happens when you want to connect to a machine on your local network? Assume your IP is IP-A and your destination is IP-B. Your computer will first do what is called an “ARP who-has” broadcast, asking everyone on the network to identify themselves if they are the owner of IP-B. The owner of IP-B will then respond to you saying, “I am IP B, and I’m at this MAC address”. After this response both machines know at which physical address each other is, and are able to send and receive data to and from each other.

Now, the protocol for discovering the MACs as I described above is called ARP, the Address Resolution Protocol. You can get a listing of the known MAC addresses of devices you’ve been communicating with on your LAN by running the following command:
arp -an

Ping

So, everyone probably knows the ping command. It’s a command that sends a packet to another machine requesting a response packet. It’s often used to test if a machine is up, whether an IP is in use, to measure latency or packet loss, and so on. It’s very simple to use. You simply run ping <ip address> and on Linux it will then continuously send ping or echo requests and display any responses. When you abort the command with Ctrl+C you will also get a summary of the session, which includes the numbers of packets sent, the packet loss percentage, the elapsed time and some other metrics.

Though because of security concerns many people disable ping and it’s not always possible to use it for a quick test to see if a host is up and behind a certain IP address. Sometimes I just need a temporary IP on a specific subnet, and ping alone isn’t enough to quickly determine if an IP is currently claimed.

This is where arping comes in. arping is a very handy utility that does basically the same as ping, except with arp who-has packets. When you run it against a given IP address, it will send arp who-has packets onto the network, and print the responses received.

Here is some example output of arping:
[quintin@printfw ~]$ sudo arping 10.0.1.99
ARPING 10.0.1.99 from 10.0.1.253 eth0
Unicast reply from 10.0.1.99 [7B:F1:A8:11:84:C9] 0.906ms
Unicast reply from 10.0.1.99 [7B:F1:A8:11:84:C9] 0.668ms
Sent 2 probes (1 broadcast(s))
Received 2 response(s)

What’s the Point

This is useful in many cases.

1. Ping is not always available, as some system firewalls actively block it, even to other hosts on it’s LAN. In there cases you can still do a hosts-up test.
2. You can do it to quickly discover the MAC address behind a given IP.
3. If you have an IP conflict you can get the MAC addresses of all the hosts claiming the given IP address.
4. It’s a quick way to see if a host is completely crashed. If it doesn’t respond to ARP it’s very dead.
5. You can ping hosts even if you’re not on the same subnet.

I’m sure one can find many more uses of arping. I think it’s a very useful utility.

Windows Firewall Oddities

On a side note I thought might be interesting. I have noticed some people with the AVG Anti-Virus package’s firewall to not respond to ARP requests all the time. I haven’t investigated it further, though it seems like it will prevent sending responses in certain scenarios. This is definitely a feature that I would prefer didn’t exist, though am sure there are benefits to it, like being in complete stealth on a LAN. When I find out more about this, I’ll update this page.

Conclusion

So Why Love Linux? Because it comes preinstalled with and has available to it tons of ultra useful utilities and programs.