Hi,
Since you posted the question to Brightmail Gateway Forum, I suppose you want to SBG to download virus defs from a liveupdate server on your LAN.
Please see the section titled "Specifying from where to obtain virus definitions" on page 199 in SBG 8.0 Adminstration Guide available here:
ftp://ftp.entsupport.symantec.com/pub/support/documentation/sbg_administration_guide.pdf
Here are the steps (copied from the Admin Guide):
To obtain virus definition updates from a LAN host
1 In the Control Center, click Virus > Settings > LiveUpdate.
2 Under Source, click Download virus definitions from a LAN host.
If you download virus definitions from a LAN host, LiveUpdate uses a proxy
only if you specify one under Use a proxy.
Refer to the LiveUpdate Administrator's Guide for more information about
how to set up a LAN host.
3 In the Address field, type the address of the LAN host.
Use a URL, not a host name.
4 In the Username field and Password field, type the user name and password,
if required to access the LAN host.
5 If you use a proxy server, check Use a proxy.
6 In the Proxy host field, type a valid host name.
7 In the Proxy port field, type a valid port number.
8 In the Username field and Password field, type the user name and password
if they are required to access the proxy host.
9 Click Save.
For the LiveUpdate Administrator's Guide, please see the following KB article:
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/1999052915222048