The 'Rewrite Domains' filter is enabled by specifying:
pagespeed EnableFilters rewrite_domains;
in the configuration file. The filter is not enabled by default and should be added manually to the configuration file.
pagespeed DomainRewriteHyperlinks on;
This filter applies all domain mapping
directives specified in pagespeed.conf to web resources that
are not otherwise rewritten by PageSpeed. For
example, if a resource is not cacheable, or filters which affect the
resource are turned off, then it still may be desirable to apply
domain sharding. (Domain sharding is not applied to hyperlinks even if
DomainRewriteHyperlinks is enabled.)
It is the responsibility of the site administrator to set up the shard entries in their DNS or CNAME configuration. Also, please see the note about the servers for rewrite domains — this applies to sharded domains as well. The sharded domains must have access to the same content as the original domain.