# cat /etc/apt/sources.list
deb     http://localhost:9999/debian    lenny main
deb     http://localhost:9999/security  lenny/updates main
deb-src http://localhost:9999/debian    lenny main

# apt-get update
Get:1 http://localhost lenny Release.gpg [189B]
Get:2 http://localhost lenny/updates Release.gpg [189B]
Hit http://localhost lenny Release
Hit http://localhost lenny/updates Release
Ign http://localhost lenny/main Packages/DiffIndex
Ign http://localhost lenny/main Sources/DiffIndex
Get:3 http://localhost lenny/main Packages [4810kB]
Ign http://localhost lenny/updates/main Packages/DiffIndex
Get:4 http://localhost lenny/main Sources [1381kB]
Hit http://localhost lenny/updates/main Packages                                                                                                            
Fetched 6191kB in 2m31s (40.8kB/s)                                                                                                                          
Failed to fetch http://localhost:9999/debian/dists/lenny/main/binary-i386/Packages.bz2  MD5Sum mismatch
Failed to fetch http://localhost:9999/debian/dists/lenny/main/source/Sources.bz2  MD5Sum mismatch
Reading package lists... Done
E: Some index files failed to download, they have been ignored, or old ones used instead.

approx.conf:

debian          http://ftp.debian.org/debian
security        http://security.debian.org