Veja como personalizar suas páginas de erros no seu proxy (squid)
Códigos para serem inseridos nos arquivos ERR_* templates
Esta lista descreve as TAGs que o Squid pode inserir nas mensagens:
%a - User identity
%B - URL with FTP %2f hack
%c - Squid error code
%d - seconds elapsed since request received (not yet implemented)
%e - errno
%E - strerror()
%f - FTP request line
%F - FTP reply line
%g - FTP server message
%h - cache hostname
%H - server host name
%i - client IP address
%I - server IP address (NP: upper case i)
%l - Local site CSS stylesheet. (Squid-3.1 and later) (NP: lower case L)
%L - contents of err_html_text config option
%M - Request Method
%m - Error message returned by external auth helper
%o - Message returned by external acl helper
%p - URL port \#
%P - Protocol
%R - Full HTTP Request
%S - squid default signature. (Squid prior to 3.1 automatically add unless %s is used.)
%s - caching proxy software with version
%t - local time
%T - UTC
%U - URL without password
%u - URL with password. (Squid-2.5 and later only.)
%W - Extended error page data URL-encoded for mailto links.
%w - cachemgr email address
%z - DNS server error message
%Z - Error message
Exemplo: ERR_ACCESS_DENIED
Localização: /usr/share/squid/errors/en-us
Fonte: http://wiki.squid-cache.org/Features/CustomErrors?highlight=%28faqlisted.yes%29