A partir do FAQ do squid .
A diretiva cache_peer_domain permite que você especifique determinados irmãos ou pais em cache para determinados domínios:
# squid.conf - On the host: sv.cache.nlanr.net
#
# Format is: hostname type http_port udp_port
#
cache_peer electraglide.geog.unsw.edu.au parent 3128 3130
cache_peer cache1.nzgate.net.nz parent 3128 3130
cache_peer pb.cache.nlanr.net parent 3128 3130
cache_peer it.cache.nlanr.net parent 3128 3130
cache_peer sd.cache.nlanr.net parent 3128 3130
cache_peer uc.cache.nlanr.net sibling 3128 3130
cache_peer bo.cache.nlanr.net sibling 3128 3130
cache_peer_domain electraglide.geog.unsw.edu.au .au
cache_peer_domain cache1.nzgate.net.nz .au .aq .fj .nz
cache_peer_domain pb.cache.nlanr.net .uk .de .fr .no .se .it
cache_peer_domain it.cache.nlanr.net .uk .de .fr .no .se .it
cache_peer_domain sd.cache.nlanr.net .mx .za .mu .zm
A configuração acima indica que o cache usará pb.cache.nlanr.net e it.cache.nlanr.net para domínios uk, de, fr, não, se e sd.cache.nlanr.net para domínios mx, za, mu e zm e cache1.nzgate.net.nz para os domínios au, aq, fj e nz.
Comentários sobre cache_peer_domain do arquivo de configuração de exemplo do squid.
# TAG: cache_peer_domain
# Use to limit the domains for which a neighbor cache will be
# queried. Usage:
#
# cache_peer_domain cache-host domain [domain ...]
# cache_peer_domain cache-host !domain
#
# For example, specifying
#
# cache_peer_domain parent.foo.net .edu
#
# has the effect such that UDP query packets are sent to
# 'bigserver' only when the requested object exists on a
# server in the .edu domain. Prefixing the domain name
# with '!' means the cache will be queried for objects
# NOT in that domain.
#
# NOTE: * Any number of domains may be given for a cache-host,
# either on the same or separate lines.
# * When multiple domains are given for a particular
# cache-host, the first matched domain is applied.
# * Cache hosts with no domain restrictions are queried
# for all requests.
# * There are no defaults.
# * There is also a 'cache_peer_access' tag in the ACL
# section.
#
#Default:
# none