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Thunderbird startup hang (hint: Add-Ons)

Debian

If you see Thunderbird hanging during startup for a minute and then continuing to load fine, you are probably running into an issue similar to what I saw when Debian migrated Icedove back to the "official" Mozilla Thunderbird branding and changed ~/.icedove to ~/.thunderbird in the process (one symlinked to the other).

Looking at the console log (=start Thunderbird from a terminal so you see its messages), I got:

console.log: foxclocks.bootstrap._loadIntoWindow(): got xul-overlay-merged - waiting for overlay-loaded
[.. one minute delay ..]
console.log: foxclocks.bootstrap._windowListener(): got window load chrome://global/content/commonDialog.xul

Stracing confirms it hangs because Thunderbird loops waiting for a FUTEX until that apparently gets kicked by a XUL core timeout.
(Thanks for defensive programming folks!)

So in my case uninstalling the Add-On Foxclocks easily solved the problem.

I assume other Thunderbird Add-Ons may cause the same issue, hence the more generic description above.

Saving misc/jive

BSD

One thing I love about FreeBSD is the way the core team keeps the wider community updated about project news e.g. via their quarterly status reports. So while reading the FreeBSD Q4/2016 status report, I was quite surprised to find that a text filter converting English to "Jive speak" had been removed from the ports tree. FreeBSD Core members argue that "today the implicit approval implied by having it in the ports tree sends a message at odds with the project's aims."

Now this is bullshit as I'm sure FreeBSD core neither endorses Citrix (net/citrix_ica) nor Cisco (emulators/gna3, devel/libcli, graphics/py27-blockdiagcontrib-cisco and many more) but just hosts code to make living with them easier.

So the important thing here is:

Important: Switch on brain and try to memorize. Hosting is not endorsing.
It is a purely technical act and by definition agnostic to the hosted content.

In every sane jurisdiction there is the requirement to remove hosted content that violates a law. And that makes sense. It reflects the societal consensus what is still acceptable and what is not. This changes over time but there is a proven process in place for these changes to become relevant: political discussion and consequential law making.

There is very deliberately never a law against bad taste and/or offensive humor. Where such a law still exists, you're in a somewhat underdeveloped jurisdiction. Because the hosting (pun intended) society has not matured sufficiently yet. This may happen due to overly conservative or self-protective ruling classes, ideological or religious blindness. None of these are desirable for society as a whole and the scissors in your head are paving the way to go back to darker ages. So don't. Be welcoming, be tolerant.

Tolerance means accepting things you do not like. Not accepting just what endorses your personal taste, beliefs or state of mind.

Does that mean, FreeBSD should continue to host the "Jive" filter? No, it's purely their choice. But their argument that hosting is endorsing is wrong. Inclusion into a FreeBSD media may be, like Debian strictly differentiates between the main archive, which it endorses, and contrib or non-free sections which it does not endorse. But still hosts regardless. So hosting is not endorsing.

That said, here you go:

File Function sha256
jive-1.1.tar.gz Source to the "Jive" filter 3463d80ad159a27d9fcf87f163a7be5eba39dbf15c5156f052798b81271523f2
ports_misc_jive.tar.gz ports files to build the "Jive" filter under FreeBSD 47dc7b660d499d671daa18f992cdd348bd95c34e02874addd2bcf3e5c3f90b59
swedishchef.zip mirror of swedishchef.zip d0830b81aec6ad6a6ff824e1d80c9fa97d3a5447bad9f8a2b32dbd0dfb8df709

The last file above is a mirror of files hosted by John B. Chambers. He has a "chef" cgi running there allowing the conversion of English text to "Swedish Chef", "Valley Girl" or "Pig Latin". And the "Jive" variant that uses the same Lex/Yacc/Flex files as the misc/jive that used to be part of the FreeBSD ports tree and is conserved above.

If you are interested in the public part of the discussion that happened after misc/jive was marked for removal from the ports tree, check out the freebsd-ports mailing list thread.

P.S.: Valspeak is still in the ports tree as misc/valspeak ... just sayin'.

P.P.S.: apt-cache show filters # Debian & Ubuntu. Awesome. ♡

Mozilla Firefox and Thunderbird Menu font sizes

Open Source

The font size Mozilla chose for Firefox and Thunderbird menus looks awfully large on Netbook screens. It wastes space and is visually at odds with reasonably sized content. And for some weird reason you can set the content font and size via the menu but not the font and size for the drop-down menus themselves.

As the "Theme Font & Size Changer" Add-On doesn't work reliably and phones home way too often (showing a nag screen), I dug back into how to do this "manually". Probably a decade after I fixed this the first time...

You need to create the file ~/.mozilla/firefox/*/chrome/userChrome.css with * being your profile directory (<random_number>.default usually) and you most probably have to create the chrome directory first.

The same for Thunderbird resides in ~/.thunderbird/*/chrome/userChrome.css. Here again the chrome directory will most probably need to be created first.


/* Global UI font */
* { font-size: 10pt !important;
  font-family: Ubuntu !important;
}
 

needs to go into these files for Firefox or Thunderbird respectively. The curly braces are important. So copy & paste correctly. Symlinks or hardlinks are fine if those files do not need to differ between your web browser and your email client.

Restart Firefox and/or Thunderbird to see the effect.

Obviously you can choose any other font and font size in the snippet above to suit your taste and requirements.

If you are massively space-confined and don't mind a quite ugly UI, check out the Littlefox Add-on. Ugly but optimal use of the minimal screen estate with very small screens.

Netatalk 3.1.9 .debs for Debian Jessie available (Apple Timemachine backup to Linux servers)

Debian

Netatalk 3.1.9 has been released with two interesting fixes / amendments:

  • FIX: afpd: fix "admin group" option
  • NEW: afpd: new options "force user" and "force group"

Here are the full release notes for 3.1.9 for your reading pleasure.

Due to upstream now differentiating between SysVinit and systemd packages I've followed that for simplicity's sake and built libgcrypt-only builds. If you need the openssl-based tools continue to use the 3.1.8 openssl build until you have finished your migration to a safer password storage.

Warning: Read the original blog post before installing for the first time. Be sure to read the original blog post if you are new to Netatalk3 on Debian Jessie!
You'll get nowhere if you install the .debs below and don't know about the upgrade path. So RTFA.

Now with that out of the way:

Continue reading "Netatalk 3.1.9 .debs for Debian Jessie available (Apple Timemachine backup to Linux servers)"

Irssi update to 0.8.19 from Debian jessie-backports may break enter / carriage return key / ↵ key

Debian

Updating to irssi 0.8.19 (which is a mainly a bugfix release to 0.8.18) proved a real issue. The enter key (return key) stopped working. Ctrl-J still worked but that's way too annoying to remember after each line. Searching the github issues turned up #327 Numeric keypad "Enter" key stopped working which didn't help much. Digging deeper it shows the irssi devs enabled "App key" mode in these releases which causes so many issues, they had to implement a switch to turn it off again.

So a hopeful: /set term_appkey_mode off followed by Ctrl+J, remember ...

and ... nothing changed.

So finally, after more digging and a quick consideration to go back to irssi 0.8.17 on Debian stable (Jessie) ...

/bind ^M key return

Yes, irssi 0.8.19 wants to be told what the enter key is, like, by default. Duh.
No idea what caused this in my configuration, I've been using irssi for more than a decade so much cruft has accumulated in my .irssi/config but ... in case you run into this as well, hopefully I helped you save a morning for something better to do.

If you want to fumble this into your .irssi/config (e.g. because Ctrl-J does not work for you):

keyboard = (
  [...]
  { key = "^M"; id = "key"; data = "return"; },
  [...]
);

Dovecot segfaulting (Ubuntu 14.04 LTS, CentOS 6 and 7)

Open Source

We're currently installing a solution including dovecot for a company to go into production in April. So we kick this off with a development box for integrating parts that different suppliers to our customer are working on.

But after installing dovecot on the new joint development machine it just didn't start. It worked on our local development boxes but the install for the customer has been scripted with (what we call) "poor man's puppet" so it is somewhat hard to compare the setups. Same Ubuntu 14.04 LTS under the hood but on top of that things (like config layout, directory structures etc.) are quite different.

Back on topic: ps aux | grep d[o]ve returned empty.

Looking at /var/log/mail.log did not show anything relevant.

But syslog (/var/log/syslog) had some worrying lines like:

Mar 16 03:16:17 dev-new kernel: [ 3222.339365] doveconf[6420]: segfault at 200 ip 00007fa041b25a03 sp 00007ffe7881e070 error 4 in libc-2.19.so[7fa041ada000+1bb000]

Manually running the daemon resulted in:

# /usr/sbin/dovecot -F -c /etc/dovecot/dovecot.conf
Segmentation fault

So

mkdir /var/core
chmod 1777 /var/core
echo "/var/core/%p" > /proc/sys/kernel/core_pattern
ulimit -c unlimited
/usr/sbin/dovecot -F -c /etc/dovecot/dovecot.conf
Segmentation fault (core dumped)

Better. We have a core file now.

But:

Continue reading "Dovecot segfaulting (Ubuntu 14.04 LTS, CentOS 6 and 7)"

Netatalk 3.1.8 .debs for Debian Jessie available (Apple Timemachine backup to Linux servers)

Debian

The Debian Netatalk3 saga continues at bug #685878. In season 4 of the epic the main issue still seems to be unclear license indications of a (very) few source files. And the usual "you go fix it", "no! you go fix it!". May be the fact that Firefox will be Firefox again in Debian [yeah!] could serve as an inspiration to the Netatalk maintainers?

Ah, well, until we have the eureka moment for Netatalk3 (4?) ...
<pragmatism style="priority-on-users:yes"> ... I'll post my .debs of the new 3.1.8 version of Netatalk as well.

Warning: Read the original blog post before installing for the first time. Be sure to read the original blog post if you are new to Netatalk3 on Debian Jessie!
You'll get nowhere if you install the .debs below and don't know about the upgrade path. So RTFA.

The release notes for 3.1.8 don't list anything that makes the update look mandatory but there is a nice compatibility fix for shares also exported via Samba (compatible xattrs handling). And it's faster.

The update instructions (assuming you have installed 3.1.7 before) are:

# install new debs
dpkg -i libatalk17_3.1.8-1_amd64.deb netatalk_3.1.8-1_amd64.deb
# reboot the box (restart of netatalk may not be sufficient)
reboot
# After reboot: remove the obsolete libatalk16 (3.1.8 uses libatalk17)
dpkg -r libatalk16

And here are the files:

Continue reading "Netatalk 3.1.8 .debs for Debian Jessie available (Apple Timemachine backup to Linux servers)"

Apple Timemachine backups on Debian 8 (Jessie)

Debian

Upgrading Debian 7 (Wheezy) servers to Debian 8 (Jessie) proves (unexpectedly) quite rough around the edges.

That's what you get for using a version x.0, we should have known better :-).

And - of course - the release notes follow the common practice of not even mentioning any of the issues we encountered so far.

Ah, well, let's go through the first one:

In Debian 7 (Wheezy) there was netatalk 2.2.2 (packages link). Now during upgrades that package may or may not get removed. There is no netatalk in Debian 8 (Jessie) anymore. Duh. There is in sid (aka Debian unstable) (packages link) so we may see a backport some time. Or not. In any case this is still 2.2.5 at the time of writing and as Adrian Knoth put it in the three year old bug asking for a upgrade to Netatalk 3:

Let's not ship another release without netatalk3, it's embarrassing.

Yes. It is. Removing a working version and not even mentioning it in the release notes is even worse though. So no cookies there.

Luckily the absolutely awesome Debian and greater FLOSS community have sorted 90% of the problem out for us already:

A quick Google search turns up an excellent article on netatalk's wiki that details installing Netatalk 3.1.7 on Debian 8 Jessie. There are two shortcomings to this: First it doesn't compile to .debs but installs besides apt and friends. And second it compiles with Spotlight search, courtesy of Gnome tracker, which doesn't really work well on servers yet. Hence we're lucky that Adrian Knoth's debified install has not yet added the tracker dependencies. It does compile for systemd use (the default for Debian 8 Jessie). If you want to continue using SysVInit, you need to modify debian/rules.

Compiling to .debs becomes as easy as:

# get build dependencies and a few helpers
apt-get install build-essential devscripts debhelper cdbs autotools-dev dh-buildinfo libdb-dev libwrap0-dev libpam0g-dev libcups2-dev libkrb5-dev libltdl3-dev libgcrypt11-dev libcrack2-dev libavahi-client-dev libldap2-dev libacl1-dev libevent-dev d-shlibs dh-systemd
# in case you want to try the tracker support (you need to ammend the debian/ build config as well)
# apt-get install tracker libtracker-sparql-1.0-dev libtracker-miner-1.0-dev  
git clone https://github.com/adiknoth/netatalk-debian
cd netatalk-debian
debuild -b -uc -us

This should leave you with (at the time of writing this):

File Function md5 sha1
libatalk-dev_3.1.7-1_amd64.deb Development files for the libatalk library (dev only) e5a465e39a8560c919d8db85c8e5a83b 0b924cf75f22ab42406289c6f18ae0243d6396a3
libatalk16_3.1.7-1_amd64.deb libatalk library (needed) 17a3d677ed0b3df1c2f4c1a8ab9045fd 7345ed3edd442716c99c2fe979140703204c0826
netatalk_3.1.7-1_amd64.deb netatalk daemons (needed) c694abca7f3cdc0070b2b3e7d528324a 932d1e3d5899958f29e79a7ba40e858d4ac272e8

Obviously you can download the files above if you run the AMD64 architecture and trust me enough to compile them for you.

Continue reading "Apple Timemachine backups on Debian 8 (Jessie)"

How much memory does a process use on Linux?

Linux

Sometimes the easy questions are the hardest to answer.

Memory can mean RSS (Resident Set Size) which is the memory of a process held in RAM (so not swapped out). That does include shared memory allocations. So if you add two RSS numbers, you're probably wrong already. Still this is usually the number we look for in most practical investigations.

Then there is VSZ (Virtual Set siZe) also called SIZE. The VSZ includes code, data and stack segments a process has allocated. And again that will count some shared address space. So usually bash will have a VSZ that's lower than its RSS.

man ps will also tell you:

   The SIZE and RSS fields don't count some parts of a process including the page tables, kernel stack, struct
   thread_info, and struct task_struct.  This is usually at least 20 KiB of memory that is always resident.

In most (if not all) practical scenarios that difference won't matter. If it were, you'd be using valgrind to look into the memory usage of your application in minute detail. Wouldn't you?

If you want to have an as-detailed-as-possible look into the memory allocations of a process pmap <pid> will give you the information. The summary at the end is a gross over-estimation of the total memory a process has allocated as it counts all mapped memory (and may still be wrong due to de-duplication and other factors). But that number may well serve as an upper bound if you need something like that.

For running processes


ps -eo 'pid user rss:8 size:8 cmd' --sort 'rss'
 

will give you a nice sorted list of processes and their RSS and VSZ (SIZE) in kiB (old school kB...).

For short running commands GNU time (not the bash build-in time command, apt install time on Debian-based systems) has a nice capability that's not widely known yet:


/usr/bin/time -f "RSS: %MkiB" <command>
 

will tell you the maximum RSS size the <command> has had during its lifetime. That's better than top or watch ps and trying to spot the process.

Encrypting files with openssl for synchronization across the Internet

Linux

Well, shortly after I wrote about encrypting files with a keyfile / passphrase with gpg people asked about a solution with openssl.

You should prefer to use the gpg version linked above, but if you can't, below is a script offering the same functionality with openssl.

You basically call crypt_openssl <file> [<files...>] to encrypt file to file.aes using the same keyfile as used in the gpg script (~/.gnupg/mykey001 per default).

A simple crypt_openssl -d <file.aes> [<files.aes...>] will restore the original files from the encrypted AES256 version that you can safely transfer over the Internet even using insecure channels.

Please note that you should feed compressed data to crypt_openssl whenever you can. So use preferably use it on .zip or .tar.gz files.

Continue reading "Encrypting files with openssl for synchronization across the Internet"

Encrypting files with gpg for synchronization across the Internet

Linux

Automatically transferring (syncing) files between multiple computers is easy these days. Dropbox, owncloud or bitpocket to name a few. You can imagine I use the latter (if you want a recommendation)1.

In any case you want to encrypt what you send to be stored in "the cloud" even if it is just for a short time. There are many options how to encrypt the "in flight" data. Symmetric ciphers are probably the safest and most widely researched cryptography these days and easier to use than asymmetric key pairs in this context as well.

Encryption is notoriously hard to implement correctly and worthless when the implementation is flawed. So I looked at gpg, a well known reference implementation, and was amazed that it can neither use a proper keyfile for symmetric encryption (you can just supply a passphrase via --passphrase-file) nor does it handle multiple files on the command line consistently. You can use --multifile (wondering...why does a command need that at all?) with --decrypt and --encrypt (asymmetric public/private key pair encryption) but not with --symmetric (symmetric shared key encryption). Duh!

With a bit of scripting around the gpg shortcomings, you end up with crypt_gpg that can nicely encrypt or decrypt multiple files (symmetric cipher) in one go.


  1. Dropbox is closed source so it cannot be assessed for its security. Owncloud needs a thorough code review before I would dare to run it on my systems. 

Continue reading "Encrypting files with gpg for synchronization across the Internet"

Securing the grub boot loader

Open Source

Since version 2.0 the behaviour of grub regarding passwords has changed quite substantially. It can be nicely used to secure the boot process so that a X display manager (gdm, kdm, lightdm, ...) or login prompt cannot be circumvented by editing the Linux kernel boot command line parameters. The documentation is concise but many old how-tos may lead you down the wrong GNU grub "legacy" (the pre-2.0 versions) path.

So this assumes you have a grub installed and working. I.e. if you press Shift during boot, you get a grub menu and can edit menu entries via the e key.

First you need to setup grub users and corresponding passwords:

Run grub-mkpasswd-pbkdf2 to encrypt every password you want to use for grub users (which are technically unrelated to Linux system users at this time).
You'll get a string like 'grub.pbkdf2.sha512.10000...'. It will replace the plain text passwords.

In '/etc/grub/40_custom' add lines like:

# These users can change the config at boot time and run any menuentry:
set superusers="root user1"
password_pbkdf2 root grub.pbkdf2.sha512.10000.aaa...
password_pbkdf2 user1 grub.pbkdf2.sha512.10000.bbb...
# This user can only run specifically designated menuentries (not a superuser):
password_pbkdf2 user2 grub.pbkdf2.sha512.10000.ccc...

Now once you did this grub v. 2.0+ will ask for a supervisor password every time you want to boot any menu item. This is a changed behavior from v. 1.9x which defaulted to allow all entries if no user restriction was specified. So you need to add '--unrestricted' to all 'menuentries' that any user shall be able to boot. You can edit '/boot/grub/grub.cfg' and add --unrestricted to (the default) menuentries. Or you can edit the 'linux_entry ()' function in '/etc/grub/10_linux' so that the 'echo "menuentry ..."' lines include --unrestricted by default:

[...]
echo "menuentry '$(echo "$title" | grub_quote)' --unrestricted ${CLASS} \$menuentry_id_option 'gnulinux-$version-$type-$boot_device_id' {" | sed "s/^/$submenu_indentation/"
else
echo "menuentry '$(echo "$os" | grub_quote)' --unrestricted ${CLASS} \$menuentry_id_option 'gnulinux-simple-$boot_device_id' {" | sed "s/^/$submenu_indentation/"
[...]

Make a backup of this file as it will be overwritten by grub updates. This way all Linux kernels detected by the script will be available to all users without identifying to grub via username / password.

Now issue update-grub to re-generate 'grub.cfg' with the amended menuentries.

If everything worked well, your system can now be booted unrestricted but the grub configuration can only be changed from the grub superusers after identifying with their username and password at the grub prompt.

Bonus point:

If you want to create menuentries that user2 (and any superuser) from the above example user list can run, add blocks like these to the end of '40_custom':

menuentry "Only user2 (or superuser) can run this Windows installation" --users user2 {
set root=(hd1,1)
chainloader +1
}

Update

16.12.2015:
Hector Marco and Ismael Ripoll have found a nearly unbelievable exploit in Grub2 that allows you to tap backspace 28 times to get a rescue shell and that way bypass a password prompt. Time to update!
Read the excellent analysis of the bug and the exploit vector in Hector Marco's blog post.

Creating iPhone/iPod/iPad notes from the shell

Open Source

I found a very nice script to create Notes on the iPhone from the command line by hossman over at Perlmonks.

For some weird reason Perlmonks does not allow me to reply with amendments even after I created an account. I can "preview" a reply at Perlmonks but after "create" I get "Permission Denied". Duh. vroom, if you want screenshots, contact me on IRC :-).

As I wrote everything up for the Perlmonks reply anyways, I'll post it here instead.

Against hossman's version 32 from 2011-02-22 I changed the following:

  • removed .pl from filename and documentation
  • added --list to list existing notes
  • added --hosteurope for Hosteurope mail account preferences and with it a sample how to add username and password into the script for unattended use
  • made the "Notes" folder the default (so -f Notes becomes obsolete)
  • added some UTF-8 conversions to make Umlauts work better (this is a mess in perl, see Jeremy Zawodny's writeup and Ivan Kurmanov's blog entry for some further solutions). Please try combinations of utf8::encode and ::decode, binmode utf8 for STDIN and/or STDOUT and the other hints from these linked blog entries in your local setup to get Umlauts and other non-7bit ASCII characters working. Be patient. There's more than one way to do it :-).

I /msg'd hossman the URL of this blog entry.

Continue reading "Creating iPhone/iPod/iPad notes from the shell"

Apple iPhone ring tones Linux style

Open Source

Apple has crippled the iPhone to not allow normal music files as ringtones. Business decision. Technically any sub 40 second MP4 audio file will do once you rename it to *.m4r and drag-and-drop it to the ringtones folder of your phone in iTunes. Longer ones will work, too. But you'd need a jailbroken iPhone for that as iTunes will refuse to transfer the ringtone file if it's too long. Not much of an issue imho, who keeps ringing your phone for 40 seconds or more?

There's a gazillion websites available telling you how to convert a single .mp3-file to a ringtone with or without iTunes help and there are hundreds of tools doing that for you if you can't find out how to do it with just iTunes itself. Still the ones I tried failed for me as I wanted to convert my 20 or so standard ringtones from the good old Motorola K3 to iPhone ringtones all in one go. Without having to edit each one by hand. They are already nice ringtones and have served me well for years, just too long for the iPhone and in .mp3 format.

The basic processing sequence needed is

  1. Cut the .mp3 down to 39s
  2. Convert the .mp3 -> .wav (with mplayer, normalize output gain while we're at it)
  3. Convert the .wav -> .mp4 (with facc)
  4. Clean up, GOTO 1 for next file

So below is the free shell script to create multiple ringtones in one go on any Linux system. You need to install cutmp3, mplayer and faac for it, so apt-get install cutmp3 mplayer faac on Debian or Ubuntu. cutmp3 is currently not in the portage tree for Gentoo, but you can download an ebuild from Polynomial-C's overlay (mirror). Or you just download the cutmp3 binary from Jochen Puchalla's homepage. There's no error checking in the script, so know your way around the shell before running it.

Without further ado:

#!/bin/sh
#
# convert_to_ringtone file1.mp3 [file2.mp3, ...]
# Placed into the public domain by Daniel Lange, 2011.
#

for arg
do
        echo "Processing $arg..."
        cutmp3 -c -a 0:0.0 -b 0:39.0 -i "$arg" -O "$arg.tmp"
        mplayer -vo null -vc null -af volnorm -ao pcm:fast:file=tmpfile.wav "$arg.tmp"
        faac -b 128 -c 44100 -w tmpfile.wav
        name=`echo $arg| sed 's/.mp3//g'| sed 's/ /_/g'`
        mv tmpfile.m4a $name.m4r
        rm tmpfile.wav
        rm "$arg.tmp"
        echo "$arg done."
done
 

Wikipedia article on Apple's 1984 ad.

Update

23.12.14 Apparently the faac package in Debian and Ubuntu has had the MP4 writing capability removed in v1.28-5 and later due to a minor license incompatibility. See the Debian Changelogs. Duh.

faac (1.28-5) unstable; urgency=low
  [ Andres Mejia ]
  * Disable mp4v2 support.
    This only disables mp4v2 for the faac utility program. The faac
    utility is GPL-2 but the mp4v2 library is MPL-1.1. The two licenses
    are incompatible with each other.

So ... unfortunately you have to built faac from source yourself or pin the v1.28-4 version which is identical except for the castration anyways.

Random distro dev: "Why oh why doesn't my distro ever head mainstream...?"
Hint: Because of stuff like this.

Binding applications to a specific IP

Linux

These days many systems are multi-homed in the sense that they have more than one IP address bound at the same time.
I.e. for different network cards, virtual IPs for shared servers or just using WiFi and a wired network connection at the same time on a laptop.

Murphy of course makes sure that your system will choose to worst IP (i.e. that on slow WiFi or the one reserved for admin access) when an application does not specifically supports binding to a selected IP address. And Mozilla Firefox for example doesn't.

The kernel chooses an outgoing IP from those in the routing table with the same metric:

daniel@server:~$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.0.2.1         0.0.0.0         U     0      0        0 eth0
0.0.0.0         192.0.2.2         0.0.0.0         U     0      0        0 eth1
0.0.0.0         192.0.2.3         0.0.0.0         U     0      0        0 eth2
0.0.0.0         192.0.2.4         0.0.0.0         U     0      0        0 eth3

You can obviously play around with the metric and make the kernel router prefer the desired interface above others. This will affect all applications though. Some people use the firewall to nat all packages to port 80 onto the network interface desired for web browsing. Gee, beware the http://somewebsite.tld:8080 links...

Thankfully Daniel Ryde has solved the problem via a LD_PRELOAD shim. With his code you can run

daniel@laptop:~$ BIND_ADDR="192.0.2.100" LD_PRELOAD=/usr/lib/bind.so firefox (*)

and happily surf away.

To compile his code (3.3kB, local copy, see note 1) you need to run

gcc -nostartfiles -fpic -shared bind.c -o bind.so -ldl -D_GNU_SOURCE
strip bind.so
cp -i bind.so /usr/lib/

and you're set to go.

If you don't have gcc available (and trust me) you can download pre-compiled 32bit and 64bit (glibc-2) bind.so libraries here (4.5kB).

I guess because Daniel Ryde hid his code so well on his webpage, Robert J. McKay wrote another LD_PRELOAD shim, called Bindhack (4.5kB, local mirror). This will - as is - only compile on 32bit machines. But YMMV.

Run the above command (*) with your desired (and locally bound) IP address in bash and visit MyIP.dk or DNStools.ch or any of the other services that show your external IP to see whether you've succeeded.

Notes:

  1. Daniel Ryde did not specify the -D_GNU_SOURCE in the comments section of bind.c. Modern glibc/gcc need that as he used RTLD_NEXT which is Unix98 and not POSIX. I amended the local copy of bind.c and sent him an email so he can update his.
  2. Both are IPv4 only, no IPv6 support.

Updates:

19.03.15 madmakz wrote in to clarify that all of the bind LD_PRELOAD shims only work with TCP connections. So not with UDP.
I'm not aware of a shim that manipulates UDP sockets.

14.01.14 Christian Pellegrin wrote a superb article on how to achieve per-application routing with the help of Linux network namespaces.

16.06.13 showip.be seems to be gone, so I replaced it with dnstools.ch in the text above. There are plenty of others as well.

22.06.12 Lennart Poettering has a IPv4 only version of a shim and a rather good readme available at his site.

29.11.10 Catalin M. Boie wrote another LD_PRELOAD shim, force_bind. I have not tested this one. It's capable of handling IPv6 binds.

11.01.09 Daniel Ryde has replied to my email and updated his local copy now as well.