proxy_configuration
squidguard
web_caching
web_filtering
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exim_gui.html
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Proxy Overview
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A proxy intercepts requests from
a web browser to a web server. The advantage of this is that if
you have a proxy in an office or school where people may request the
same page multiple times such as google.co.uk or yahoo.co.uk, save
bandwidth and speed up browsing. The proxy will cache (keep a copy) the
page and images and serve them directly to the browser rather than
downloading them from the server again.
This sort of thing can assist greatly when doing bug fixes on
computers. If multiple computers are running the same operating
system, the update files will only have to be downloaded to the LAN once
making updates much faster.
The proxy focused on here is squid a piece of GplSoftware availavle
from http://www.squid-cache.org
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Squid configuration
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There is extensive documentation
available for Squid and a mailing list where many of the queries you may
have are covered in detail many times before :-)
What people often get stuck on is the ACLs (Access Control
Lists). ACLs are important, for if the proxy is left open to all,
it will be abused by mail spammers and others.
It is possible to have password authentication against a samba server,
a Windows computer (NT W2K or XP), htpasswd files, Novell servers and
LDAP servers. I have probably missed a few - check the
documentation.
It is possible to provide continuous logins against a samba server or
NT server using NTLM logins, check again for how to do this in the docs.
The ACLs were misunderstood by me for a long time, I had assumed that
the ACLs were an OR argument rather than AND.
So for example if you create an ACL for the local network
192.168.1.0/24 and a colleague's network 80.233.192.0/24 to allow both
of these you would have to do the following:
acl local src 192.168.10.0/24
acl colleague src 80.233.135.0/24
http_access allow local
http_access allow colleague
The following would NOT work
http_access allow local colleague
The second would not work because a computer / user would never be in
both networks
The ACLs are stackable and so Squid will work through them in order to
get a match; hence you can order them to make finer adjustments of the
ACLs
You might then want to allow remote users passworded access if they are
not on either of the two networks. If the password ACL was put
first, everyone would be asked for a password, which would be
undesirable.
auth_param basic program /usr/lib/squid/pam_auth
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours
The realm is required and here the authentication is set to last for
two hours before e re-request.
So here we have pam authentication. Pam is modular and allows
authentication against a whole range of services. Note that you
also have to make a squid entry for pam in /etc/pam.d/squid consisting
of
auth
required
/lib/security/pam_unix.so
account required
/lib/security/pam_unix.so
You will also need to set the file /usr/lib/squid/pam_auth to be SUID. This is so when the squid user executes the file it is run with root permissions and authentication is granted.
I have been caught on this more than once as it works well when testing as root :-)
to change the file to SUID execute the following at a root prompt:
chmod 4755 /usr/lib/squid/pam_auth
Each user on the system with a password will be able to use the proxy.
I have found pam a bit flakey for authenticating against a samba or NT
server and here it is better to use the NTLM authentication.
acl password proxy_auth REQUIRED
http_access allow password
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Site filtering
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This is just a quick view.
There is much you can do here, you can also use
http://www.squidguard.org to filter sites.
acl filter url_regex "/etc/squid/banned"
deny_info XANDER_ERR filter
http_access deny filter
stick partial site matches such as doubleclick into the file
/etc/squid/banned to block urls containing these words in the titles.
banned file
the file XANDER_ERR is found at /etc/squid/errors/English/XANDER_ERR
and is a standard html page giving a custom error message.
Have a look at the multitude of variables you can embed in the file.
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Web Proxy Auto-discovery
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WPAD may be provided by a number
of methods. DHCP, DNS (by A record), DNS (by SVR record)
I have found very limited documentation of WPAD by DHCP and have not
managed to implement it.
WPAD using DNS by A record is relatively straight forward once you know
the pitfalls :-)
WPAD by SVR record is not implemented in many clients.
A client set to try to discover their own proxys looks at its own FQDN
(Fully Qualiffied Domain Name) such as workstation1.domain.co.uk and
will look for the following hosts wpad.domain.co.uk then
wpad.co.uk It looks for the A record of this host.
It will then try to download the following file
http://ipaddress/wpad.dat
This is important if you are
running virtual hosts.
The client does not request a domain when it asks for the wpad.dat
file, it only forwards the IP address (or at least IE6 does), therefore
if you are running multiple websites on that IP address, the wpad.dat
file must be placed in the default website. In short you must be
able to access it by IP.
The dat file would be similar to this, however it may be much more
complex:
function FindProxyForURL(url, host)
{
if (isPlainHostName(host) ||
dnsDomainIs(host, "domain.co.uk") ||
isInNet(host, "192.168.10.0", "255.255.255.0"))
return "DIRECT" ;
return "PROXY proxy.domain.co.uk:3128 ; PROXY 192.168.10.217:3128" ;
}
At present there is a patch available for Mozilla.
which will allow it to do WPAD; however for Mozilla it may be easier to
copy the wpad.dat file to proxy.pac file on the same server and set
Mozilla to use the URL http://ipaddress/proxy.pac as an autmatic
configuration URL (see Mozilla's settings).
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