Ситуация такая. Ставлю squid3 на Ubuntu 14.04 x64
Конфиг

visible_hostname pass
acl mynet src МОЙ IP
acl SSL_ports port 443
acl Safe_ports port 80          # http
acl Safe_ports port 21          # ftp
acl Safe_ports port 443         # https
acl Safe_ports port 70          # gopher
acl Safe_ports port 210         # wais
acl Safe_ports port 1025-65535  # unregistered ports
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
http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
http_access allow localhost manager
http_access deny manager
http_access allow localhost
http_access allow mynet
http_access deny all
http_port IP сервера в интернет:3129
coredump_dir /var/spool/squid3
refresh_pattern ^ftp:         &n... 1440    20%     10080
refresh_pattern ^gopher:        1440    0%      1440
refresh_pattern -i (/cgi-bin/|\?) 0     0%      0
refresh_pattern (Release|Packages(.gz)*)$      0       20%     2880
refresh_pattern .               0       20%     4320
cache_dir ufs /var/spool/squid3 4096 32 256
minimum_object_size 2 KB
maximum_object_size 65536 KB
via off
forwarded_for delete

Прописываю в браузер настройки proxy
IP сервера: 3129
Работает!

Прописываю на mikrotik

ip firewall nat add chain=dstnat action=accept protocol=tcp src-address-list=IP сервера dst-port=80
ip firewall nat add chain=dstnat action=dst-nat protocol=tcp src-address-list=clients dst-port=80 to-addresses=IP сервера to-ports=3129

Получаю в браузере:

ERROR
The requested URL could not be retrieved

The following error was encountered while trying to retrieve the URL: /
Invalid URL
Some aspect of the requested URL is incorrect.
Some possible problems are:
Missing or incorrect access protocol (should be http:// or similar)
Missing hostname
Illegal double-escape in the URL-Path
Illegal character in hostname; underscores are not allowed.
Your cache administrator is webmaster.

И браузер упорно не подключается к squid на порту 3128, приходиться менять на 3129