From ArchWiki

[edit] Local repository HOWTO

This document outlines one way to share Arch Linux packages across a LAN. A better way to do this would be to create a Custom local repository with ABS and gensync and make the repository available across the LAN using NFS or FTP. This document should be edited to describe this process in detail. For the time being, the original HOWTO is left intact below:

To share all your downloaded packages in your lan
Pros  save bandwidth, diskspace and time.
"pacman -Sy"  will sync against our local repository
"pacman -S pkgname" try to download and install pkg from localserver if pkg not exist it download
from the next server in the list /etc/pacman.conf and save pkg on localserver.
"alsync" will update localserver db against ftp.archlinux.org

ex. for my network
serverip=192.168.14.3
network=192.168.14.0/255.255.255.0
adjust to yours


1. serverside
on your server create an nfs share readwrite for all pc on your lan

if you run archlinux on the server you can
pacman -S portmap
pacman -S nfs-utils

edit /etc/exports
add line
/var/cache/pacman/pkg  192.168.14.0/255.255.255.0(rw,no_root_squash,sync)
add portmap, nfslock and nfsd to DAEMONS in /etc/rc.conf

run  /etc/rc.d/portmap start
run  /etc/rc.d/nfslock start
run  /etc/rc.d/nfsd start
to check the nfsshare run "exportfs" on server.

1.1 Check hosts.allow and hosts.deny

to work (very simple):

hosts.deny
ALL: ALL: DENY

hosts.allow
ALL: 192.168.14.

Explanation: 

hosts.deny deny all connection that doesnt exist on hosts.allow.
hosts.allow allow connection from local network 192.168.14.0/255.255.255.0

2. on all clients
rename  /var/cache/pacman/pkg  to /var/cache/pacman/pkgorg
create a new /var/cache/pacman/pkg and mount the nfs share there

run "mount -o rw,nolock 192.168.14.3:/var/cache/pacman/pkg /var/cache/pacman/pkg"
#Comment # if the mount from command line does not work add this option "nfsvers=3"
or if you want it automount on client reboot
add this line in /etc/fstab
192.168.14.3:/var/cache/pacman/pkg /var/cache/pacman/pkg    nfs    rw,nolock
#again if the mount does not work, try to add mount option nfsvers=3 to the fstab entry.
run "mount -a"
run "df" to check mount

move all your already fetched pkg from your clients
/var/cache/pacman/pkgorg to /var/cache/pacman/pkg

edit /etc/pacman.conf and add this lines directly after the line

{current}
Server = file:///var/cache/pacman/pkg

     '''''' and after ''''''
{extra}
Server = file:///var/cache/pacman/pkg

#Comment # I have skipped the step 3 because for me it works as i want already. I sync from each PC and all share the pacman cache.

3. to sync your local repository v.s. archlinux
"alsync" connects, login, and update your packages database on the local nfsserver
pacman -S openssl
pacman -S wget


create a file called /bin/alsync and put in this lines
''''''''''''''' content of alsync '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
cd /var/cache/pacman/pkg
wget -N ftp://ftp.archlinux.org/current/'''.db.'''
wget -N ftp://ftp.archlinux.org/extra/'''.db.'''
''''''''''''''''''''''''''' end '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''*

chmod 777 /bin/alsync
copy this file to your clients


to try run as root on first client
alsync
pacman -Sy
pacman -S new-pkgname

move to next client and run
pacman -Sy
pacman -S new-pkgname