still.. under construction..: 2012/05

Build a cheap AirPlay alternative

What we'll need... is "shairport + OpenWrt"

an OpenWrt capable router with USB port
an USB sound card

an OpenWrt firmware with enabled IPv6, ALSA and USB sound support in the kernel (+ functional "hostname" command)
the Shairport binaries (C package is recommended, because .pl version has more bugs - for now)
working Avahi-daemon with .service config file for shairport

Other:

at least basic Linux skills (if you'll find all necessary packages)

some Linux (OpenWrt) compiling skills

Ease way

For Asus WL-500G Premium (v1/2)

Just install this OpenWrt brcm47xx firmware (5,69 MB, with 2.6 kernel, squashfs), which includes:

enabled IPv6, USB storage & audio support
shairport 0.05 (with configured avahi-daemon, enabled auto-start through /etc/rc.local, and enabled also on WAN through /etc/firewall.user)
alsamixer, alsautils, fdisk, e2fsprogs, nano, mc
extroot ready

For Netgear WNDR3700 / WNDR3800

According to your router model, just install one of this OpenWrt images:

All images above (are 6,93 MB big, with 2.6 kernel, squashfs) include:

enabled IPv6, USB storage & audio support
shairport 0.05 (with configured avahi-daemon, enabled auto-start through /etc/rc.local, and enabled also on WAN through /etc/firewall.user)
alsamixer, alsautils, fdisk, e2fsprogs, nano, mc
LuCI
extroot ready

If you have all necessary kernel modules etc., try install this shairport package (opkg install <path to package>)
And create the config file for avahi-daemon (/etc/avahi/services/shairport.service) with this content:

<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
<name replace-wildcards="yes">Airplay Server on %h</name>
<service>
<type>_raop._tcp</type>
<port>5000</port>
<txt-record>txtvers=1</txt-record>
<txt-record>vn=3</txt-record>
<txt-record>pw=false</txt-record>
<txt-record>sr=44100</txt-record>
<txt-record>ss=16</txt-record>
<txt-record>ch=2</txt-record>
<txt-record>cn=0,1</txt-record>
<txt-record>et=0,1</txt-record>
<txt-record>ek=1</txt-record>
<txt-record>sv=false</txt-record>
<txt-record>sm=false</txt-record>
<txt-record>tp=TCP,UDP</txt-record>
</service>
</service-group>

Reboot
Start Shairport (shairport -a "WiFi Speakers" -o 5000 -d)

Hard way

Build your own OpenWrt firmware with enabled IPv6, ALSA and USB sound support in the kernel (+ functional "hostname" command)
.. and Shairport binaries, with added feed (src-git jlars git://github.com/jlars/packages.git;master) to feed.conf:

...

..openwrt/trunk# make dirclean
..openwrt/trunk# svn update
..openwrt/trunk# ./scripts/feeds update -a
..openwrt/trunk# ./scripts/feeds install shairport
..openwrt/trunk# ./scripts/feeds install -a
..openwrt/trunk# make menuconfig
..openwrt/trunk# make
...

Final Notes & Tips

Flashing router's firmware is always at your own risk. You will probably loose the warranty for your router. Don't do it, if you don't know what you do.

For shairport automatic startup just add the shairport -a Speakers -o 5000 -b 50 -d to /etc/rc.local file.

If you want to provide the AirPlay functionality over router's WAN port, you'll need open communication on firewall. You can do this by adding this to /etc/firewall.user:

# Allow avahi-daemon ond WAN
# for both IPv4 and IPv6
config rule
        option name             Allow-Avahi
        option src              wan
        option proto            udp
        option dest_port        5353
        option target           ACCEPT
        option family           any


# Allow Shairport ond WAN
# for both IPv4 and IPv6
config rule
        option name             Allow-Shairport
        option src              wan
        option proto            tcpudp
        option dest_port        5000
        option target           ACCEPT
        option family           any

For controlling the USB sound card volume you can use alsamixer command line tool. Settings will not survive reboot, therefore use alsactl store / alsactl restore commands for saving / restoring settings (after reboot). For using it you'll need probably create /var/lib/alsa/asound.state file first. Again, use rc.local for restoring the settings after startup.

Cheapest and smallest OpenWrt capable router with USB port, is TP-Link TL-WR703N (about $20) at the moment. You'll need go the "hard way" with it anyway and build your own firmware. It has only of 4MB flash memory size. You'll need use the USB hub + a flash disk + extroot probably.
Cheapest USB sound card you can find on eBay. Even bellow $2.

Enjoy

2012/05/23