BigSmoke

Smokes your problems, coughs fresh air.

Month: August 2008

PostgreSQL back-end for Ruby on Rails confusion

August 28, 2008 /

I just need to add a quick summary of what postgres back-end tool our Ruby on Rails application uses, and how we’ve configured it, because it’s quite confusing…

There are four postgresql backends:

  • ruby-postgres. This version is no longer maintained. This is the version we used when the project began.
  • postgres-pr. This is a pure ruby implementation, which is not interesting for us.
  • postgres. This is the continuation of the unmaintained ruby-postgres. This version includes such fixes as that it can be compiled against libpg-8.3.
  • ruby-pg. It is said that this one is now the official postgres back-end, but when I install it, the application still can’t find “postgres”.

Because the aforementioned article states that the pg extension is unstable, “postgres” seems to be what we should use. The article states that it is included in the ruby-pg package, but it doesn’t work when I install it, so I had to install “postgres”. I uninstalled ruby-pg, because it doesn’t seem necessary.

To continue, we once used a patched postgresql adapter, because we needed more accurate timestamps (the standard connection adapter rounded everything off to whole seconds), but if I recall correctly, this patch was only necessary on the connection adapter in Rails, not the back-end. We never commissioned the functionality that required this, so this existed only in the workdir of one of the devs.

As a final note; on our production server, we have a version of ruby-postgres installed in /usr/local. I can’t remember why…

Using wget to download all files on a page

August 27, 2008 /

Just a quick one-liner I used to download a bunch of MIDI files from an on-line listing of Chopin MIDIs:

$ wget http://www.piano-midi.de/chopin.htm -q -O - \
| grep 'href=".*\.mid"' \
| sed -e 's/^.*href="\(.*\)".*$/\1/' \
| xargs -i{} wget http://www.piano-midi.de/{}

Maybe not so useful to you, but it’s a good demonstration of applying the hacker’s mentality to one of those moments where, after clicking the fourth link or so, I find myself thinking: Wouldn’t spending a few moments on a one-liner be much more fun than clicking through and saving 44 more links?

Useful or not, I am now testing timidity’s piano sound with a nice rendition of Chopin in the background whereas, without this trick, I’d still be right-click-click-saving links instead of writing this post. It’s up to you to decide whether this is actually good or bad. 😛

VIM modelines for per-file configuration

August 21, 2008 /

The MediaWiki developers use tabs instead of spaces for indentation. This can be annoying for someone like me who has configured VIM to work with spaces by default—annoying because I don’t want to enter something like :set tabstop=4 noexpandtab shiftwidth=4 every time that I open a file. Adding different conditions for each project to my .vimrc is probably possible but not much fun.

The other minute, when I opened the sources of my Semantic Gallery extension for MediaWiki, I noticed that I had been mixing tabs and spaces again. So it was time to look up the VIM feature which allows you to put a configuration line in a comment at the bottom of a file. This quote is another perfect example of why it’s good practice to blog about such things. I remembered that the last time that I had wanted to do this, I could not find a useful search string at all and resorted to finding back an example in the code where I had first seen it and modifying that. Now, I thought I’d have to do the same, but from a quick googling I learned that I’m not the only one to use his blog as a memory extension. 🙂

Good. Hopefully, from now on, I’ll remember that this configuration-thingy-at-the-bottom-of-a-file is called a modeline, so that I can just enter :help modeline in VIM the next time that I forget where the colons have to go.

An example for when I do forget:

# vim:set ts=4 sw=4 noexpandtab:

This reminds me of my many searches for Heredoc syntax when I didn’t know that they were called Heredocs.

Disabling SSH shell access for SVN users on a Linux/Unix system

August 4, 2008 /

A common problem is that Linux/Unix system administrators want to grant users access to SVN repositories, but prevent them from logging in on the shell. This can be accomplished quite easily.

First, disable the user’s account by running: 

usermod --lock [user]

This way, only public key authentication is allowed. Then, when adding the user’s key to the ~/.ssh/authorized_keys file, prefix it with this: 

command="/usr/local/bin/svnserve -t",no-port-forwarding,no-pty,no-agent-forwarding,no-X11-forwarding

I used our wrapper script in /usr/local/bin as the command, because it sets an umask of 002 before actually running svnserve. This is necessary when using svn+ssh access.

The source for this trick explains it in more detail.

Configuring a Debian satellite Exim server

August 3, 2008 /

A very common way to configure Exim on a Debian machine, is to make it a ‘satellite’; a server which uses another SMTP server for sending and does not do local delivery, the latter being the difference with a ‘smarthost’. It can be used by other computers in the network to send mail, but also by the machine itself, to send system notifications and such (one of my favorite apps, arpwatch, for example).

The following needs to be in /etc/update-exim4.conf: 

dc_eximconfig_configtype='satellite'
dc_other_hostnames='[mailname]'
dc_local_interfaces=''
dc_readhost=''
dc_relay_domains='*'
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='[your SMTP server]'
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname='false'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'

The following needs to be in /etc/mailname: 

[mailname]

[mailname] is the same in both locations, and is simply the FQDN (world wide, not just local, such as bla.net) which should appear after the @-sign. The reason that it must entered in both places, is because both have a different function. ‘etc/mailname’ Takes care of putting the specified domain after the @-sign if you mail to, for example, root. The ‘dc_other_hostnames’ is to let the server know that this is the machine that handles that domain. If you don’t specify the ‘dc_other_hostnames’, the server will just try to send it to the next relay. BTW, ‘dc_other_hostnames’ is colon (;) seperated.

Make sure the FQDN you use exists, otherwise a lot of mailservers refuse to accept it. What I don’t understand, though, is that in my experience, whatever you use as domain doesn’t have to exist as MX record, but just as an A record.

Also don’t forget to include an alias for root in /etc/aliases. I usually let all mail sent to root be sent to a local user, and alias that local user to an outside e-mail address.

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