Querying debian/ubuntu packages

The debian package management system is something I’ve been using for years, and it’s served me well for all those years. For the first decade I rarely needed to use more than the common apt-get install or dpkg -i package.deb. Listing the installed packages with dpkg -l has been useful too, but apart from apt-cache search those were all the commands I needed most of the time.

Over the past 5 years I’ve been using slightly more complex queries to figure out information about packages I’ve installed. Working at a company where we are deploying and supporting debian servers on a regular basis has meant it’s been useful to poke about the .deb infrastructure a litle more. In truth most of the time I just use 2 commands, but they seem to get me through most of what I want to do.

If I don’t know which package a program comes from I can use dpkg-query with the -S flag.

$ dpkg-query -S /sbin/ss
iproute2: /bin/ss

To figure out what is provided by a package I can use the -L flag.

$ dpkg-query -L iproute2 | grep etc

That’s also useful for checking a package is still fully installed,

for f in `dpkg-query -L python-lxml`; do if [ ! -e $f ]; then echo Missing $f; fi; done

Of course there are other commands that I use periodically, but these are the ones I’ve been using frequently enough to start remembering by heart.

LXC Networking

My infrequently used laptop appears to have developed some issues with the networking on new lxc containers (Ubuntu 13.10) so I finally started to dig into how it’s setup. I first blew away the caches in /var/cache/lxc, then tried installing a fresh box with a completely stock template. The network interface didn’t acquire one of the usual network ip addresses. A look in the logs suggested the dhcp server was receiving a DHCP request, and it was sending back a lease, but the client wasn’t picking that up.

tail /var/log/syslog
dnsmasq-dhcp[1261]: DHCPDISCOVER(lxcbr0) 00:22:3e:ea:aa:fa 
dnsmasq-dhcp[1261]: DHCPOFFER(lxcbr0) 00:22:3e:ea:aa:fa 
dnsmasq-dhcp[1261]: DHCPDISCOVER(lxcbr0) 00:22:3e:ea:aa:fa 
dnsmasq-dhcp[1261]: DHCPOFFER(lxcbr0) 00:22:3e:ea:aa:fa 
dnsmasq-dhcp[1261]: DHCPDISCOVER(lxcbr0) 00:22:3e:ea:aa:fa 
dnsmasq-dhcp[1261]: DHCPOFFER(lxcbr0) 00:22:3e:ea:aa:fa 

A google suggested my problem was the UDP checksums – bug 1204069. A look using tcpdump suggested that indeed my checksums were corrupt; note the ‘bad udp cksum’ on the second packet.

sudo tcpdump -vvv -i lxcbr0
21:36:34.009788 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328) > [udp sum ok] BOOTP/DHCP, Request from 00:22:3e:ea:aa:fa (oui Unknown), length 300, xid 0xb407cc75, secs 49, Flags [none] (0x0000)
	  Client-Ethernet-Address 00:22:3e:ea:aa:fa (oui Unknown)
	  Vendor-rfc1048 Extensions
	    DHCP-Message Option 53, length 1: Discover
	    Parameter-Request Option 55, length 13: 
	      Subnet-Mask, BR, Time-Zone, Default-Gateway
	      Domain-Name, Domain-Name-Server, Option 119, Hostname
	      Netbios-Name-Server, Netbios-Scope, MTU, Classless-Static-Route
	    END Option 255, length 0
	    PAD Option 0, length 0, occurs 41
21:36:34.010027 IP (tos 0xc0, ttl 64, id 44699, offset 0, flags [none], proto UDP (17), length 328) > [bad udp cksum 0x1c2d -> 0x1a4c!] BOOTP/DHCP, Reply, length 300, xid 0xb407cc75, secs 49, Flags [none] (0x0000)
	  Client-Ethernet-Address 00:22:3e:ea:aa:fa (oui Unknown)
	  Vendor-rfc1048 Extensions
	    DHCP-Message Option 53, length 1: Offer
	    Server-ID Option 54, length 4:
	    Lease-Time Option 51, length 4: 3600
	    RN Option 58, length 4: 1800
	    RB Option 59, length 4: 3150
	    Subnet-Mask Option 1, length 4:
	    BR Option 28, length 4:
	    Default-Gateway Option 3, length 4:
	    Domain-Name-Server Option 6, length 4:
	    END Option 255, length 0
	    PAD Option 0, length 0, occurs 8

The bug suggests that that should no longer be a problem, so I carried on googling. Turning my search criteria to DHCP udp problems. That came up with bug 930962 which gave a potential work around and suggested that it should be fixed. Since it said there should already be a firewall fix in the config I decided to take a look at the configuration. To figure out where that was I used dpkg -L

dpkg -L lxc

After a little digging I found the network configuration in /etc/init/lxc-net.conf where the setup of all the networking out of the box that I so like about ubuntu was all there. There was no mangle line as suggested in the bug report. At that point I checked the version I was running, and the version on the bug report and realised that was a pretty recent bug fix, and I simply don’t have it. Rather than figure out where the fixed package is, I figured I may as well just try to fix it myself for now since I’ve come so far.

sudo iptables -t mangle -A POSTROUTING -o lxcbr0 -p udp --dport bootpc -j CHECKSUM --checksum-fill

After that the networking came up, or more specifically the server obtained an ip address and I was able to ssh to it. I added pretty much that line into my /etc/init.lxc-net.conf and it now works out of the box again and I have a better understanding of how lxc sets up it’s network. I’ve also finally solved a case of checksum offload issues for myself rather than just hearing about them. Possibly in the strangest way, i.e. not turning off the network offload of the checksums.

Of course the adventure didn’t end there. I then found that salt wasn’t working. When I looked for salt keys I didn’t find any requests from new servers. A tcpdump showed a similar checksum issue with the salt traffic. A quick hack with iptables again solved that problem.

sudo iptables -t mangle -A POSTROUTING -o lxcbr0 -p udp --dport 4505 -j CHECKSUM --checksum-fill
sudo iptables -t mangle -A POSTROUTING -o lxcbr0 -p udp --sport 4505 -j CHECKSUM --checksum-fill
sudo iptables -t mangle -A POSTROUTING -o lxcbr0 -p udp --dport 4506 -j CHECKSUM --checksum-fill
sudo iptables -t mangle -A POSTROUTING -o lxcbr0 -p udp --sport 4506 -j CHECKSUM --checksum-fill


I only occasionally use gdb so I end up spending my time relearning it each time.  Hopefully these notes will make that process easier in the future.  This document assumes you already know the s and n commands and how to list the source code (l).  These are the commands I don’t know off by heart, and have found useful. It’s just a subset, but a useful one so far.  It’s also well worth downloading the manual pdf.

To compile a program with the debug symbols use the -ggdb command line switch.

Here is a basic summary of some of the useful commands.

set args [command line args]
bt # stack trace
up/down # move up the stack
p # print
p/x # display hex value
p/x (int[5]) *0xffffd320 # display values at address as integers
x address # display contents of memory location
x/16b address # display 16 bytes
x/32cb address # display 16 bytes as characters
x/5i address # display as instructions (i.e. assembly)
x/5i $pc - 6 # displays current code
info registers # display register contents
$pc, $sp # program counter and stack pointer, can also use $eip
set $sp += 4 # add 4 to stack pointer
set var variablename = 4 # set a program variable
set {int}0x800321 = 4 # sets memory location 0x800321 to 4
j 0x32211 # jump to 0x32211
find # search memory
b *0xaddress # set breakpoint at an address (b nnn sets at a source line)

I’ve also been using lxc a lot recently and being able to create an x86 based machine is useful.  This creates a machine named x86 for testing x86 code.  The apt-get line is to be run on the machine once it’s loaded to install gdb and the compilers.

lxc-create -n x86 -t ubuntu -- -a i386
apt-get install build-essential gdb


I really like the Stack Exchange family of sites.  I was just getting ready to ask a question on the AskUbuntu site when it pointed me to the exact question I was about to ask.  They’ve had a pretty good suggestion thing from day one when you tabbed from the title to the question but now it also seems to keep analysing the body of your question too.

If you are having issues with the tab complete functionality in bash since upgrading your Ubuntu to Natty this appears to be a good explanation of the problem and I’ll be trying the solutions suggested shortly.  At least I know which part of the system is broken now.


Rhythmbox to banshee

Having just upgraded to the latest Ubuntu I figured I’d try the new music player. Unfortunately when I tried to import all the music on my laptop over to it all I got as ‘Unable to import song’.

Much faffing later I figured out that it was down to the extensions of the music files. Or rather the lack of them. I’m not entirely sure why don’t have extensions, perhaps because I dragged them over from a daapd server, but the lack of a file extension seems to have been the cause of the problem.

To fix the problem I first created a simple shell script to fix the extension of a single file. It’s very crude, it will rename any file to either .aac or .mp3, even non music files so you need to make sure you only use it on music files.


if file "$1" | grep -q AAC
    mv "$1" "$1.aac"
    mv "$1" "$1.mp3"

Then I simply used find to execute the script against every file in the Music directory.

find Music -type f  -not -name "*.mp3" -not -name "*.aac" -exec ~/fix_extension.sh {} \;

Note that I’m assuming only mp3 and aac extensions in the directory. If you have others in your music directory make sure they don’t come up in the file command’s run. Test the file command without without the -exec to see what files are matched.

Once you’ve done that you should be able to import the Music folder again and hopefully this time the missing files should import without incident.

This should be a temporary problem. There should be a fix upcoming for banshee to fix this issue.

Lenovo Thinkpad sleep issues and Ubuntu

Recently I’ve been having problems with Ubuntu and sleeping my laptop and it turns out it has the strangest cause.  I even rolled back to a 9.10 version since that used to work.  And it did for a while.  Then it stopped.  All I got was the sleep light flashing constantly.

It looks like the issue was actually having an SD card in the reader on my laptop.  As soon as I took it out I could sleep as usual.

Thanks to this thread for the suggestion – http://ubuntuforums.org/showthread.php?t=1470067