I've been using my
Pi mostly headless -- I'm interested in using it to control hardware.
Most of my experimenting is at home, where I can plug the Pi's built-in
ethernet directly into the wired net.
But what about when I venture away from home, perhaps to a group
hacking session, or to give a talk? There's no wired net at most of
these places, and although you can buy USB wi-fi dongles, wi-fi is so
notoriously flaky that I'd never want to rely on it, especially as my
only way of talking to the Pi.
Once or twice I've carried a router along, so I could set up my own
subnet -- but that means an extra device, ten times as big as the Pi,
and needing its own power supply in a place where power plugs may be scarce.
The real solution is a crossover ethernet cable.
(My understanding is that you can't use a normal ethernet cable
between two computers; the data send and receive lines will end up crossed.
Though I may be wrong about that -- one person on #raspberrypi reported
using a normal ethernet cable without trouble.)
Buying a crossover cable at Fry's was entertaining. After several minutes
of staring at the dozens of bins of regular ethernet cables, I finally
found the one marked crossover, and grabbed it. Immediately, a Fry's
employee who had apparently been lurking in the wings rushed over to
warn me that this wasn't a normal cable, this wasn't what I wanted,
it was a weird special cable. I thanked him and assured him that was
exactly what I'd come to buy.
Once home, with my laptop connected to wi-fi, I plugged one end into
the Pi and the other end into my laptop ... and now what? How do I
configure the network so I can talk to the Pi from the laptop, and
the Pi can gateway through the laptop to the internet?
The answer is IP masquerading. Originally I'd hoped to give the
Pi a network address on the same networking (192.168.1) as the laptop.
When I use the Pi at home, it picks a network address on 192.168.1,
and it would be nice not to have to change that when I travel elsewhere.
But if that's possible, I couldn't find a way to do it.
Okay, plan B: the laptop is on 192.168.1 (or whatever network the wi-fi
happens to assign), while the Pi is on a diffferent network, 192.168.0.
That was relatively easy, with some help from the
Once I got it working, I wrote a script, since there are quite a few
lines to type and I knew I wouldn't remember them all.
Of course, the script has to be run as root.
Here's the script, on github:
I had to change one thing from the howto: at the end, when it sets
up security, this line is supposed to enable incoming
connections on all interfaces except wlan0:
iptables -A INPUT -m state --state NEW -i ! wlan0 -j ACCEPT
But that gave me an error,
Bad argument `wlan0'.
What worked instead was
iptables -A INPUT -m state --state NEW ! -i wlan0 -j ACCEPT
Only a tiny change: swap the order of -i and !. (I sent a correction
to the howto authors but haven't heard back yet.)
All set! It's a nice compact way to talk to your Pi anywhere.
Of course, don't forget to label your crossover cable, so you don't
accidentally try to use it as a regular ethernet cable.
Now please excuse me while I go label mine.
Update: Ed Davies has a great followup,
and Red Tape, that talks about how to set up a
subnet if you don't need the full masquerading setup, why
non-crossover cables might sometimes work, and a good convention for
labeling crossover cables: use red tape. I'm going to adopt that
convention too -- thanks, Ed!
[ 15:57 Nov 09, 2012
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A thread on the Ubuntu-devel-discuss mailing list last month asked
to find out what processes are making outgoing network connectsion
on a Linux machine. It referenced Ubuntu
820895: Log File Viewer does not log "Process Name", which is
specific to Ubuntu's iptables logging of apps that are already blocked
in iptables ... but the question goes deeper.
Several years ago, my job required me to use a program -- never mind
which one -- from a prominent closed-source company. This program was
doing various annoying things in addition to its primary task --
operations that got around the window manager and left artifacts
all over my screen, operations that potentially opened files other
than the ones I asked it to open -- but in addition, I noticed that
when I ran the program, the lights on the DSL modem started going crazy.
It looked like the program was making network connections, when it had
no reason to do that. Was it really doing that?
Unfortunately, at the time I couldn't find any Linux command that would
tell me the answer. As mentioned in the above Ubuntu thread, there are
programs for Mac and even Windows to tell you this sort of information,
but there's no obvious way to find out on Linux.
The discussion ensuing in the ubuntu-devel-discuss thread tossed
around suggestions like apparmor and selinux -- massive, complex ways
of putting up fortifications your whole system. But nobody seemed to
have a simple answer to how to find information about what apps
are making network connections.
Well, it turns out there are a a couple ofsimple way to get that list.
First, you can use ss:
$ ss -tp
State Recv-Q Send-Q Local Address:Port Peer Address:Port
ESTAB 0 0 ::1:58466 ::1:ircd users:(("xchat",1063,43))
ESTAB 0 0 192.168.1.6:57526 126.96.36.199:ircd users:(("xchat",1063,36))
ESTAB 0 0 ::1:ircd ::1:58466 users:(("bitlbee",1076,10))
ESTAB 0 0 192.168.1.6:54253 188.8.131.52:ircd users:(("xchat",1063,24))
ESTAB 0 0 192.168.1.6:52167 184.108.40.206:https
you might also want to add listening connections where programs
are listening for incoming connections:
Though this may be less urgent if you have a firewall in place.
-t shows only TCP connections (so you won't see all the interprocess
communication among programs running on your machine). -p prints the
process associated with each connection.
ss can do some other useful things, too, like show all the programs
connected to your X server right now, or show all your ssh connections.
man ss for examples.
Or you can use netstat:
$ netstat -A inet -p
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 imbrium.timochari:51800 linuxchix.osuosl.o:ircd ESTABLISHED 1063/xchat
tcp 0 0 imbrium.timochari:59011 ec2-107-21-74-122.:ircd ESTABLISHED 1063/xchat
tcp 0 0 imbrium.timochari:54253 adams.freenode.net:ircd ESTABLISHED 1063/xchat
tcp 0 0 imbrium.timochari:58158 s3-1-w.amazonaws.:https ESTABLISHED
In both cases, the input is a bit crowded and hard to read. If all you
want is a list of processes making connections, that's easy enough to do
with the usual Unix utilities like grep and sed:
$ ss -tp | grep -v Recv-Q | sed -e 's/.*users:(("//' -e 's/".*$//' | sort | uniq
$ netstat -A inet -p | grep '^tcp' | grep '/' | sed 's_.*/__' | sort | uniq
Finally, you can keep an eye on what's going on by using watch to run
one of these commands repeatedly:
watch ss -tp
Using watch with one of the pipelines to print only process names is
possible, but harder since you have to escape a lot of quotation marks.
If you want to do that, I recommend writing a script.
And back to the concerns expressed on the Ubuntu thread,
you could also write a script to keep logs of which processes made
connections over the course of a day. That's definitely a tool I'll
keep in my arsenal.
[ 11:28 Mar 24, 2012
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