The SEPM (Symantec Endpoint Protection Manager) may be configured as a reverse proxy to serve AntiVirus definition updates to Mac and Linux SEP clients, per HOWTO85034. This reverse proxy will fail to download content from Symantec if the environment requires use of a (forward) proxy for outbound connections. The reverse proxy ignores any proxy settings that are configured in the SEPM server properties or Windows Internet Options.
After configuring httpd.conf according to HOWTO85034 insert a "ProxyRemote" directive into he httpd.conf as below (insertion is in red):
ProxyRemote * http://<proxy_ip>:<proxy_port>
ProxyPass /luproxy/ http://liveupdate.symantecliveupdate.com/ retry=0 smax=0 ttl=60
For example, for a proxy server address and port of 192.168.1.1:8080, insert the following:
ProxyRemote * http://192.168.1.2:8080
Perform masttri.zip download test as described in HOWTO85034.
NOTE: ProxyRemote directive will be effective only with forwarding proxies that do not require authentication.