This is just a couple of fairly minor technical notes on things that can be done to reduce the risk of a “Man-in-the-middle” attack on various network traffic, and in particular on updates done to a Devuan, Debian, Ubuntu type of Linux. (Windows and Mac users can bail now ;-)
There’s two things that will be covered. First, the simple one of configuring a system wide proxy server default. The second, setting your /etc/apt/sources.list file to be a bit more paranoid about “sources and methods” ;-)
Do note that things change over time, and especially with SystemD screwing around with things, who knows what it will bugger next. So, when in doubt, check your man pages and test your changes. Heck, even when not in doubt…
Also note that I’m using an Armbian Debian system as the model for the posting. Ubuntu and Devuan will have different repositories / mirrors. Folks using them will use other entries, but I’m sticking with Debian for the posting as 1) I only use plain Debian on systems I don’t care about, so nothing of my config will leak for those systems. 2) More folks are likely to use a plain Debian / Armbian than a Devuan anyway, and Devuan doesn’t have many mirror choices (yet). 3) I don’t really like Ubuntu ;-0
The basic process used here will be data flow encryption. Some decades or three ago folks figured out that devices could “snoop” on internet packets and see what you were doing. Then other folks figured they could encrypt that stuff. Then folks figured out “contact tracing” or “traffic analysis” would still yield useful information by looking at your IP address and the IP address you were talking to. Then the other other folks figured out using proxy servers and tunnels would hide that. Then… So just realize it’s a bit of an ongoing escalating war between snoopers and privacy.
Also realize that some TLAs (Three Letter Agencies from all over the world, though the UK likes four letters, go figure ;-) and various hackers are fond of trying to insert themselves in the middle of your traffic so they can watch each packet go by and maybe change some of them. The “Man in the Middle” or MTM attack. Signing things with encrypted signatures to assure identity soon followed… There are now relatively easy to use protocols to assure that you are talking to the site you think you are talking with, and that bits sent to you arrive unchanged. Are they perfect? Depends on how skilled and subtle you think the TLAs might be… The protocols are generally thought to be secure, but it is a race.
A “proxy server” is your own personal MTM device that hides who you are. I use one at home, called Squid, on a Raspberry Pi Model 2. I’ve not seen any slowdown. There are also more public proxy servers all over the world. Most corporations use / require a proxy server to get to the internet. Why? Because folks in the Big Bad World will try to shove stuff at your computer and the proxy server can know about and stop a lot of it. The site admin can also configure the proxy server to deny outgoing connections to things that are illegal, or inappropriate for work (sorry, no porn for you…)
My personal proxy server adds a little bit of security and privacy, but really, all data flows originate from my one house IP address, so not really helping all that much. I’ve mostly done it to keep practiced. But it does mask a little bit the many and several devices I use to do things on the internet. It also means attacks will tend to hit an irrelevant PiM2 and not even know about my interior lab router and the entirely different IP addresses behind it.
Most folks only configure a Proxy Server in their browser. Used just for browsing. But you can also configure a system wide default. I’m doing this because Chromium on the Armbian / Debian / Devuan / and likely Ubuntu set on my ARM based computers declines to let me set a proxy server and ONLY looks at the system default. (Why? When the commercial Chrome does? All I can say is “Why? Don’t ask why. Down that path lies insanity and ruin.” -E.M.Smith”) It just is. Deal with it.
So here’s a page that talks about a couple of ways to configure system wide proxy services.
It has a couple of suggested ways. I’m going with this one:
For General proxy:
add the following:
You don’t really need to do the “touch” bit. Just edit proxy.sh and it will come into existence. So “vi /etc/profile.d/proxy.sh” is just as effective. Note that you only need the “user:password” part if your proxy server requires a login. Mine does not. Here’s a sample of what mine looks like:
root@OdroidN2:/etc/apt# cat ../profile.d/proxy.sh #export ftp_proxy=ftp://user:password@host:port export ftp_proxy=ftp://192.168.0.222:3128 export http_proxy=http://192.168.0.222:3128 ...
I used the IP Number so that no DNS activity is required for it to work. Oh,and I’ve changed the actual IP number for this posting to be an ‘example’. It is generally a good idea to not put your real IPs in postings or comments ;-)
Note that I left in the first example line, with # at the front to make it a comment, so that there’s documentation in the file of how to do the user:password@ bit.
So now, at least in theory, my Chromium will also use the Squid Proxy server. (I’ll test it later).
You can also send your ‘apt’ system update requests through a proxy server, along with some other data types, but I’ve not done that. (There’s a limit to my security concerns, honest there is 8-)
For APT proxy:
add the following:
find and uncomment proxy lines or add them if not present
http_proxy = http://user:password@host:port
https_proxy = …
I generally like to have my own local repository for systems, so most of my traffic / updates goes against it. Mostly it is only “disposable” systems where I let it run to the internet first. New installs. Things used for watching videos. That kind of stuff. So going all full secure is a bit silly on them. When I really care, I make my own repository copy and point my systems at it. That has it’s own level of authentication / security and will not be covered here. For a box / board used to watch TV, I’m happy to have my apt-update go straight out the Telco Router. Do I really care they might know I’ve got a Debian or Devuan system and I’m keeping it current?
But I do want to prevent them from changing packets as they come to me.
Should you need more information about how to configure apt, see:
There’s a simple thing you can do to make your apt-get update and apt-get upgrade more secure. Encrypt your connection to the server. It slows the process down a very small amount. Not really noticed on an Odroid N2, but it might be an issue on a Pi Zero. Try it and see.
To do the encryption, you add one letter to your apt/sources.list entry. “s”.
root@OdroidN2:/etc/apt# cat sources.list.save
deb http://httpredir.debian.org/debian buster main contrib non-free
#deb-src http://httpredir.debian.org/debian buster main contrib non-free
Here you can see that the saved version of my /etc/apt/sources.list file has me doing an “http” type file transfer. Change that to “https” and it does an encrypted transfer. Folks can still tell you are talking to the server, but they can’t see the packet contents in flight and if they try to change it, the transfer fails. Also note I’m using “deb” so getting binary packages. I could change that to “deb-src” and get source packages, then compile them myself. I’m willing to accept that the binaries being sent to me are correct (though I have a choice to not trust that…)
Finally, note the “httpredir” site. That’s the mirror redirector. It will choose what it thinks is the best mirror site for you at any one time. This is both a blessing and a curse. You need not think about it, and your traffic may tend to go to different destinations at different times, so buggering a given name / IP for a MTM gets harder. OTOH, you have no idea what site you will get your OS parts from nor who is running it… You might be getting your OS updates from Bulgaria. NSA could plop down a Big Fast Mirror and then traffic would gravitate to it.
So I changed to a particular site. Not due to being particularly worried that the NSA might want to watch the same movies I do, but mostly just to play with it. The list of mirrors for Debian is here:
So, say you are in Australia and you don’t want your update requests to leave the island (you know, covid and quarantine and all that ;-) You could use:
Note this is an ftp site, not an https site. But your bits never go to China nor travel over USA wires…
Or you can go further down the page to the secondary server lists and use:
Locking your source to one place lets you know where you get your bits, and limits where your traffic might go in the world. That’s a feature.
So here’s a look at one configured to use Berkeley as source. (they have a good I.T. department and work on classified stuff in the physics area, so are unlikely to have buggered sources, but also will have TLAs galore monitoring stuff. Lawrence Livermore NUCLEAR Lab was where we made our nuke stuff “go” and affiliated with Berkeley, so you know…
So why use Berkeley? Well, mostly I used it for this test as it is about 50 miles north of me and Silly Con Valley has a great metropolitan area network “MAN” on fiber at high speed. In actual production I’d be more likely to choose one that was less TLA rich and with fewer Grad Students & Wannabees potentially in charge of the repository (and with fewer Chinese Exchange Students…) like, oh, kernel.org “mirrors.edge.kernel.org”; but I didn’t see any reason to load up their servers for a TV node or for a test case. Besides, once I’m happy with it, I’m more likely to make a local repository and use an encrypted update to the local repository once a month or three instead anyway. So might was well use the public Uni for this test run example…
root@OdroidN2:/etc/apt# cat sources.list
deb https://mirrors.ocf.berkeley.edu/debian buster main contrib non-free
# deb-src http://httpredir.debian.org/debian buster main contrib non-free
deb https://mirrors.ocf.berkeley.edu/debian buster-updates main contrib non-free
# deb-src http://httpredir.debian.org/debian buster-updates main contrib non-free
deb https://mirrors.ocf.berkeley.edu/debian buster-backports main contrib non-free
# deb-src http://httpredir.debian.org/debian buster-backports main contrib non-free
deb https://mirrors.ocf.berkeley.edu/ buster/updates main contrib non-free
# deb-src http://security.debian.org/ buster/updates main contrib non-free
Here I’ve left in the commented out deb-src lines, but changed the http to https and pointed it at “mirrors.ocf.berkelely.edu/debian” for the goods.
I then tested it, and it worked fine:
root@OdroidN2:/etc/apt# apt-get update
Hit:2 https://mirrors.ocf.berkeley.edu/debian buster InRelease
Hit:3 https://mirrors.ocf.berkeley.edu/debian buster-updates InRelease
Hit:4 https://mirrors.ocf.berkeley.edu/debian buster-backports InRelease
Ign:5 https://mirrors.ocf.berkeley.edu buster/updates InRelease
I’ll note in passing that there was a hiccup on one thing, that I’d also had with the mirror director (which was why I was playing with this in the first place and figured “if I’m playing with it, might as well tighten it up”…)
404 Not Found [IP: 18.104.22.168 443]
Ign:7 http://ppa.launchpad.net/nathan-renniewaldock/flux/ubuntu focal InRelease
Err:8 http://ppa.launchpad.net/nathan-renniewaldock/flux/ubuntu focal Release
404 Not Found [IP: 22.214.171.124 80]
So something is a bit buggered on nathan-renniewaldock.
I’m not sure why I’m supposed to care… but IF I care enough I can find out:
E: The repository ‘http://ppa.launchpad.net/nathan-renniewaldock/flux/ubuntu focal Release’ does not have a Release file.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Were this a secure system that I really cared about, a followup and fix or disable would be required. As my “Roku Replacer” not so much… I did find out that the problem is not just the redirector mirror I was being directed to by the mirror redirector. It’s more generic. Nathan needs to update his stuff.
The actual upgrade also worked fine:
root@OdroidN2:/etc/apt# apt-get upgrade
Reading package lists… Done
Building dependency tree
Reading state information… Done
Calculating upgrade… Done
The following packages will be upgraded:
libldap-2.4-2 libldap-common linux-libc-dev
3 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 1479 kB of archives.
So that’s pretty much it. Now my various traffic will go via my Squid Proxy Server, even if the individual application is not so configurable. Also my system updates / upgrades will run encrypted and by having a couple of model sources.list files, I can rotor them around to different places on an unpredictable basis.
Should I so choose, I can also set up a free (for a year…) VPN server on AWS and run the traffic through that. Hiding my source IP address for the Telco Router in the process and putting any traffic inside another layer of encryption / indirection.
At that point I’m well into over-kill land though. I mean really, for a system used to run the TV and post my public opinions to the internet? It would be easier to just read the blog and see what TV shows I’m complaining about ;-)
BUT, should you be in need of a bit more security in the real world, it’s nice to know you can control where your OS bits come from, that they get to you encrypted, and that nobody needs to know you are getting them or where you are.
For folks using Ubuntu, your sources list will have entries more like:
root@OdroidN2:/mmc/ext/etc/apt# cat sources.list
deb http://ports.ubuntu.com/ubuntu-ports/ bionic main restricted
deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic main restricted
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-updates main restricted
deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-updates main restricted
deb http://ports.ubuntu.com/ubuntu-ports/ bionic universe
deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic universe
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-updates universe
deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-updates universe
There is an Ubuntu mirror list here: https://launchpad.net/ubuntu/+cdmirrors
The Devuan mirror list is here: https://www.devuan.org/get-devuan