O modem está ligado onde? Num switch, ou na porta eth1 do seu servidor?
na placa eth0 do servidor, a eth1 vai para o swith e compartilha internet com os demais micros
Então tá aqui teu erro:
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128
Você tem que redirecionar os pacotes que entram pela eth1, não os que entram pela eth0.
fiz isso, agora bloqueia tudo, menos msn;
e se der ping no site pelo CMD, pinga
squid.conf
http_port 3128
visible_hostname Uproxy
cache_mem 16 MB
maximum_object_size 4096 KB
cache_dir ufs /var/cache/squid 100 16 256
cache_access_log /var/log/squid/access.log
cache_log /var/log/squid/cache.log
cache_store_log none
######### REGRAS ##########
acl all src 192.168.0.0/255.255.255.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl SSL_ports port 443 563
acl Safe_ports port 80 8080 21 70 210 1025-65535
acl Safe_ports port 443 563
acl Safe_ports port 280 # http-mgmt
acl Safe_ports port 488 # gss-http
acl Safe_ports port 591 # filemaker
acl Safe_ports port 777 # multiling http
acl CONNECT method CONNECT
acl avi url_regex -i .*\.avi$
acl mp3 url_regex -i .*\.mp3$
acl viv url_regex -i .*\.viv$
acl mpeg url_regex -i .*\.mpeg$
acl wma url_regex -i .*\.wma$
acl wmv url_regex -i .*\.wmv$
acl mov url_regex -i .*\.mov$
acl Sites url_regex -i "/etc/squid/block"
acl meebo src 69.36.226.109
acl cisco_free src 10.0.2.0/24
acl time_free time MTWHF 18:00-24:00
########## ACAO ##########
http_access allow manager localhost
http_access allow cisco_free time_free
http_access deny manager
http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
http_access deny avi
http_access deny mov
http_access deny wmv
http_access deny mp3
http_access deny viv
http_access deny wma
http_access deny mpeg
http_access deny meebo
#http_access deny block-hosts
#http_access deny block-path
#http_access deny aplicativos_cisco !time_free
http_access deny Sites
http_access allow all