Install ipkg packages with their short names

[metamorphose]

I have a simple question: if i install some ipkg packages, I receive error message, for example:

ipkg install openssh-sftp-server

Nothing to be done
An error ocurred, return value: 4.
Collected errors:
Cannot find package openssh-sftp-server.
Check the spelling or perhaps run 'ipkg update'

But this way is successful:

ipkg install http://downloads.openwrt.org/kamikaze/7.09/packages/mipsel/openssh-sftp-server_4.5p1-1_mipsel.ipk

Downloading http://downloads.openwrt.org/kamikaze/7.09/packages/mipsel/openssh-sftp-server_4.5p1-1_mipsel.ipk
kg install hdparm
Installing openssh-sftp-server (4.5p1-1) to root...
Configuring openssh-sftp-server
Done.

What can cause this behaviour?

Here is my ipkg.conf:

src release http://downloads.openwrt.org/kamikaze/7.09/brcm-2.4/packages
src release http://downloads.openwrt.org/kamikaze/7.09/packages/mipsel/
#src packages http://downloads.openwrt.org/kamikaze/packages/mipsel
src nslu2 http://ipkg.nslu2-linux.org/feeds/openwrt/kamikaze-7.09
src optware http://ipkg.nslu2-linux.org/feeds/optware/openwrt-ixp4xx/cross/unstable
src X-Wrt http://downloads.x-wrt.org/xwrt/kamikaze/7.09/brcm-2.4/packages
dest root /
dest ram /tmp