Wednesday, January 31, 2007

Watching Japanese TV in Dublin

ssh terebi mkfifo pipe ssh terebi cat pipe | mplayer -

then in another window
ssh terebi mencoder -oac lavc -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=100:acodec=ac3:abitrate=64 /dev/video0 -o pipe

gives me watchable live tv.

I think I should really be using some variety of vlc but that's another day's work

Tuesday, January 30, 2007

Useful things when you've wiped you partition table (conclusion)

This is the concluding part of my earlier post on recovering after wiping the partition table and the first 30k or so of my harddisk. It has a happy ending.

It didn't take much more effort in the end to rebuild my laptop's partition table. It turns out that gpart was not terribly useful as I had used extended partitions and nuked the first one. It found my 2 linux partitions but refused to rebuild the partition table. It also outputs the location of the partitions in MB which is no use at all. This patch

diff -ur gpart-0.1h/src/gpart.h ../gpart-0.1h/src/gpart.h --- gpart-0.1h/src/gpart.h 2001-01-30 23:07:29.000000000 +0000 +++ ../gpart-0.1h/src/gpart.h 2007-01-03 05:07:06.000000000 +0000 @@ -171,7 +171,7 @@ struct disk_geom *disk_geometry(disk_desc *); int reread_partition_table(int); -#define s2mb(d,s) { (s)*=(d)->d_ssize; (s)/=1024; (s)/=1024; } +#define s2mb(d,s) { (s)*=(d)->d_ssize; } #define align(b,s) (byte_t *)(((size_t)(b)+(s)-1)&~((s)-1)) #include "gmodules.h"

makes it output bytes instead which I could then divide by 512 to get sectors. I had to compile it on a Debian box (not sure what vintage), it wouldn't compile under Ubuntu.

Next I used sfdisk -d /dev/sda to dump a partition table from another machine as an example and then edit that to look like

# partition table of /dev/sda unit: sectors /dev/sda1 : start= 63, size= 39053952, Id=83 /dev/sda2 : start= 39070143, size= 3903728, Id=82 /dev/sda3 : start= 42973938, size= 72260370, Id=8e /dev/sda4 : start=115234308, size= 1975932, Id=83, bootable

I worked out the sizes by subtracting the start values from each other. Saving that in a file and doing sfdisk < my_partitions.txt gave me a new partition table which amazingly worked. Of course /dev/sda1 wasn't recovered properly because it had also been partially overwritten by the format but luckily I had nothing useful on that partition.

Saturday, January 27, 2007

Get The Focaccia!

I spotted a focaccia (Italian bread with veggies and maybe bits of bacon) shop while in Courmayeur a few days ago and got a great idea for a franchise. Kinda along the lines of It's A Bagel. As with all franchises you need a snappy name. I think


has a certain ring to it. The menu would explain all the different types of focaccia available and so would be titled


The opening hours, the recipe book and the store locator would be named along similar lines.

Saturday, January 13, 2007

Memoising a complete parser

Not sure if "complete" is the correct term but I mean a parser that finds all possible parses of the stirng by the grammar. Usually very expensive but by memoising the application of rule A at char N in the string it becomes cheaper. The key is the value that you store. For memoised recursive descent, you just store the parsed value or FAIL. For this you have to store an object that can generate parsed values in the correct order and which caches any previously generated ones. Not so bad really. There are some questions though about quantifiers. It might be worth trying to share data between X{4} and X{5} applied at the same char. But maybe not.

Wednesday, January 03, 2007

An alternative to a recovery floppy

Does anybody ever make a recovery disk these days after installing Linux. I certainly don't and now I'm regretting it! It's too inconvenient to root out a floppy or a blank CD and even if I had made one, I wouldn't have it with me here in Tokyo so I'd be just as badly off. Basically they are not terribly helpful.

The amount of non-redundant information stored on a recovery disk is quite small, basically it's your MBR and partition table and maybe some special boot params. Really all you need is a safe copyof your MBR and partition table. With that you can boot from a generic recovery CD, fix the partition table and MBR and then reboot as normal (or maybe fix the data on your partitions first now that you can access them).

So the world would be a better place if instead of offering to make a recovery CD, Linux distros offered to email you a copy of your MBR and partition table. If you use gmail then that's the end of it, it'll be there in your gmail account should the need arise. Download it and use dd to install it. Bingo. If you store your email on the computer itself then you need to copy that mail to somewhere else but that's still much easier that creating a floppy/CD that you're going to lose.

I'm off to file some bugs (just as soon as I get my data back!).

Useful things when you've wiped you partition table

Doh! While trying to get my slax distro onto a USB key, I fucked up big time and dos formatted (mkfs.vfat) my hardrive (my head is still in the era where hard disks are hdX, not sdX).

gpart can try to guess where your partitions were but it doesn't know LVM2

LVM2 partitions have a sector which begins with "LABELONE" as documented in these slides (thanks to agk on #lvm!). I invoked the following script with ./ /dev/sda to get a list of all the sectors that contains that string.

#! /usr/bin/perl use strict; use warnings; my $buf; my $sector = 0; my $f = shift; open(F, $f) || die "couldn't open '$f', $!"; while(read(F, $buf, 512)) { if ($buf =~ /^LABELONE/) { print "sector = $sector\n"; } $sector++; } print "last sector was $sector\n";

Stuffing all those sector numbers into $ss and then doing

for s in $ss; do sudo perl -e 'open(F, "/dev/sda"); seek(F, shift()*512, 0); read(F, $buf, 512); print $buf' > sec.$s $s; done
gets me all the sectors in sec.XXX files. I can then
for s in $ss; do echo $s; echo; hexdump -c sec.$s; echo ;done|less
to take a look at them without trashing my console. The important thing to remember is that LABELONE will be followed by a secotr number. If it's 1 then this is sector 1 of the partition but really I'm looking for sector 0 so I need to subtract 1.

Tuesday, January 02, 2007

Making a secure bootable cd with sshd and my ssh keys

Update: ddclient doesn't seem to work. I don't have time to figure out why, the rest works fine though. Update: slax is currently "frozen", I've updated the links in this doc to point to the legacy hosting service. Also, it seems that they have messed with the sshd config and possibly broken it in the latest version.

I've built a headless box that's going to live in Japan when I go back to Ireland. There'll be no one in Japan to fix it if it goes bad. The best I can do is get someone to insert a CD and reboot but after some searching and some asking it seems none of the bootable CDs will boot the machine into a useful state for remote admin. slax was recommended to me on TLUG as a good customisable distro. It turned out to be pretty easy thanks to slax's rootcopy feature which allows you to make minor modifications without digging around (too much) in the compressed package files. I've broken the process down into a few steps, with a quick bash script for each one.

Create a directory to work in and cd into it

mkdir slax cd slax

Get a copy of the .iso

You need slax-frodo-x.y.z.iso, you can get it from the slax download page. The frodo edition is the minimal version on which all the others are based.

Make a copy of the files from the CD

This is necessary because we need to add some files. So I ran this as ./ ~/slax-frodo-5.1.8.iso

#! /bin/bash iso=$1 shift if [ "$iso" = '' ]; then echo 1>&2 Usage: echo 1>&2 "$0 " exit 1 fi mkdir slax.mnt mount -t iso9660 $iso slax.mnt -o loop cp -v -Tr --preserve=all slax.mnt slax umount slax.mnt

Enable SSH

It's disabled by default (rc.sshd is not executable), so I replace it with one that is executable. I also turn off password logins because everyone knows the root password for slax. Finally I include an authorized_keys file so that I can ssh in with my ssh key. I ran this script as ./ ~/.ssh/authorized_keys

#! /bin/bash auth_keys=$1 shift if [ "$auth_keys" = '' ]; then echo 1>&2 Usage: echo 1>&2 "$0 <path/to/auth_keys_file<" exit 1 fi # make rc.sshd executable and stop root logins using a password # # extract rc.sshd from the module and tweak the permissions mkdir 02_core.mnt mount -t squashfs slax/base/ 02_core.mnt -o loop mkdir -p rootcopy/etc/rc.d cp 02_core.mnt/etc/rc.d/rc.sshd rootcopy/etc/rc.d chmod 700 rootcopy/etc/rc.d/rc.sshd # disable password logins mkdir -p rootcopy/etc/ssh cp 02_core.mnt/etc/ssh/sshd_config rootcopy/etc/ssh cat >> rootcopy/etc/ssh/sshd_config <<EOM # since we're starting sshd by default, don't allow root logins with # passwd, must use ssh keys PasswordAuthentication no EOM # install keys file mkdir -p rootcopy/root/.ssh chmod 700 rootcopy/root/.ssh cp $auth_keys rootcopy/root/.ssh chmod 600 rootcopy/root/.ssh/authorized_keys cp -r --preserve=all rootcopy/* slax/rootcopy umount 02_core.mnt

Update a dyndns entry

The machine is on a cable modem and doesn't have a fixed IP, so I need to know where it is. I downloaded the ddclient module and the following script puts it in place, puts your ddclient.conf file into /etc and makes sure that it gets started on boot (I assumed it would start by itself actually, I'm not sure if that's a bug or not). I invoked this script as ./ ddclient.conf

#! /bin/bash conf=$1 shift if [ "$conf" = '' ]; then echo 1>&2 Usage: echo 1>&2 "$0 " exit 1 fi cp slax/base mkdir -p rootcopy/etc/ cp $conf rootcopy/etc/ddclient.conf mkdir -p rootcopy/etc/rc.d cat > rootcopy/etc/rc.d/rc.local <<EOM #! /bin/bash /etc/rc.d/rc.ddclient start EOM chmod 755 rootcopy/etc/rc.d/rc.local

Add any other modules

There doesn't appear to be an editor in the slax-frodo image so I grabbed joe and copied the .mo into slax/base/

Move files into place and create a new .iso

So far everything we've done has been in a rootcopy/ directory in the current directory. Now it's time to move that into place in slax/ and invoke the iso building script with the following script which outputs it to slax-ssh.iso in the current directory.

#! /bin/bash rm -rf slax/rootcopy cp -r --preserve=all rootcopy slax cd slax ./ ../slax-ssh.iso

Test it in qemu

qemu -cdrom slax-ssh.iso should boot up. I never figured out how to get qemu bring up a real IP interface - the virtual machine can connect to the outside world but I can't connect to it. I tested it by setting up an ssh tunnel connecting port 22 on the VM to port 2022 on the host machine with ssh -R2022:localhost:22 Then I did ssh -p 2022 root@localhost with my ssh keys loaded and in I went. When my keys aren't loaded, I don't get a password prompt so I can't get in. Perfect.

Monday, January 01, 2007

deep 0.3

Finally got deep onto and onto the python package repo CPAN-alike thingy, which means that it's just an easy_install deep away whenever I want it.