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4. Configuration issues

4.1 How do I join a cache hierarchy?

To place your cache in a hierarchy, use the cache_host directive in squid.conf to specify the parent and sibling nodes.

For example, the following squid.conf file on configures its cache to retrieve data from one parent cache and two sibling caches:

# squid.conf - On the host: # # Format is: hostname type http_port udp_port # cache_host parent 3128 3130 cache_host sibling 3128 3130 cache_host sibling 3128 3130

The cache_host_domain directive allows you to specify that certain caches siblings or parents for certain domains:

# squid.conf - On the host: # # Format is: hostname type http_port udp_port # cache_host parent 3128 3130 cache_host parent 3128 3130 cache_host parent 3128 3130 cache_host parent 3128 3130 cache_host parent 3128 3130 cache_host sibling 3128 3130 cache_host sibling 3128 3130 cache_host_domain .au cache_host_domain .au .aq .fj .nz cache_host_domain .uk .de .fr .no .se .it cache_host_domain .uk .de .fr .no .se .it cache_host_domain .mx .za .mu .zm

The configuration above indicates that the cache will use and for domains uk, de, fr, no, se and it, for domains mx, za, mu and zm, and for domains au, aq, fj, and nz.

4.2 How do I join NLANR's cache hierarchy?

We have a simple set of guidelines for joining the NLANR cache hierarchy.

4.3 Why should I want to join NLANR's cache hierarchy?

The NLANR hierarchy can provide you with an initial source for parent or sibling caches. Joining the NLANR global cache system will frequently improve the performance of your caching service.

4.4 How do I register my cache with NLANR's registration service?

Just enable these options in your squid.conf and you'll be registered: cache_announce 24 announce_to

NOTE: announcing your cache is not the same thing as joining the NLANR cache hierarchy. You can join the NLANR cache hierarchy without registering, and you can register without joining the NLANR cache hierarchy.

4.5 How do I find other caches close to me and arrange parent/child/sibling relationships with them?

Visit the NLANR cache registration database to discover other caches near you. Keep in mind that just because a cache is registered in the database does not mean they are willing to be your parent/sibling/child. But it can't hurt to ask...

4.6 My cache registration is not appearing in the Tracker database.

4.7 What is the httpd-accelerator mode?

Occasionally people have trouble understanding accelerators and proxy caches, usually resulting from mixed up interpretations of "incoming" and ``outgoing" data. I think in terms of requests (i.e., an outgoing request is from the local site out to the big bad Internet) The data received in reply is incoming, of course. Others think in the opposite sense of ``a request for incoming data".

An accelerator caches incoming requests for outgoing data (i.e., that which you publish to the world). It takes load away from your HTTP server and internal network. You move the server away from port 80 (or whatever your published port is), and substitute the accelerator, which then pulls the HTTP data from the ``real" HTTP server (only the accelerator needs to know where the real server is). The outside world sees no difference (apart from an increase in speed, with luck).

Quite apart from taking the load of a site's normal web server, accelerators can also sit outside firewalls or other network bottlenecks and talk to HTTP servers inside, reducing traffic across the bottleneck and simplifying the configuration. Two or more accelerators communicating via ICP can increase the speed and resilience of a web service to any single failure.

The Squid redirector can make one accelerator act as a single front-end for multiple servers. If you need to move parts of your filesystem from one server to another, or if separately administered HTTP servers should logically appear under a single URL hierarchy, the accelerator makes the right thing happen.

If you wish only to cache the ``rest of the world" to improve local users browsing performance, then accelerator mode is irrelevant. Sites which own and publish a URL hierarchy use an accelerator to improve other sites' access to it. Sites wishing to improve their local users' access to other sites' URLs use proxy caches. Many sites, like us, do both and hence run both.

Measurement of the Squid cache and its Harvest counterpart suggest an order of magnitude performance improvement over CERN or other widely available caching software. This order of magnitude performance improvement on hits suggests that the cache can serve as an httpd accelerator, a cache configured to act as a site's primary httpd server (on port 80), forwarding references that miss to the site's real httpd (on port 81).

In such a configuration, the web administrator renames all non-cacheable URLs to the httpd's port (81). The cache serves references to cacheable objects, such as HTML pages and GIFs, and the true httpd (on port 81) serves references to non-cacheable objects, such as queries and cgi-bin programs. If a site's usage characteristics tend toward cacheable objects, this configuration can dramatically reduce the site's web workload.

Note that it is best not to run a single squid process as both an httpd-accelerator and a proxy cache, since these two modes will have different working sets. You will get better performance by running two separate caches on separate machines. However, for compatability with how administrators are accustomed to running other servers that provide both proxy and Web serving capability (eg, CERN), the Squid supports operation as both a proxy and an accelerator if you set the httpd_accel_with_proxy variable to on inside your squid.conf configuration file.

4.8 How do I configure Squid to work behind a firewall?

If you are behind a firewall then you can't make direct connections to the outside world, so you must use a parent cache. Squid doesn't use ICP queries for a request if it's behind a firewall or if there is only one parent.

You can use the inside_firewall directive in squid.conf to specify a list of domains internal to your Internet firewall. For example: inside_firewall You can also specify multiple domains: inside_firewall

The use of inside_firewall affects the server selection algorithm in two ways. Objects not matching any of the listed domains will be considered beyond the firewall. For these:

As a special case you may specify the domain as none to force all requests to be fetched from siblings and parents.

4.9 I have dnsserver processes that aren't being used, should I lower the number in squid.conf?

The dnsserver processes are used by squid because the gethostbyname(3) library routines used to convert web sites names to their internet addresses blocks until the function returns (i.e., the process that calls it has to wait for a reply). Since there is only one squid process, everyone who uses the cache would have to wait each time the routine was called. This is why the dnsserver is a separate process, so that these processes can block, without causing blocking in squid.

It's very important that there are enough dnsserver processes to cope with every access you will need, otherwise squid will stop occasionally. A good rule of thumb is to make sure you have at least the maximum number of dnsservers squid has ever needed on your system, and probably add two to be on the safe side. In other words, if you have only ever seen at most three dnsserver processes in use, make at least five. Remember that a dnsserver is small and, if unused, will be swapped out.

4.10 Does Squid support Socks?

We would like to use Squid, but we need it to use socks to connect to the world outside our firewall.

No changes are necessary to use Squid with socks5. Simply add the usual -Dbind=SOCKSbind etc., to the compile line and -lsocks to the link line.

--- Carson Gaspar (

4.11 How does Squid decide when to refresh a cached object?

Kolics Bertold has made an excellent flow chart diagram showing this process.

4.12 How can I easily change the default HTTP port?

Before you run the configure script, simply set the CACHE_HTTP_PORT environment variable. setenv CACHE_HTTP_PORT 8080 ./configure make make install

4.13 Is it possible to control how big each cache_dir is?

With Squid-1.1 it is NOT possible. Each cache_dir is assumed to be the same size. The cache_swap setting defines the size of all cache_dir's taken together. If you have N cache_dir's then each one will hold cache_swap ÷ N Megabytes.

4.14 Squid and http-gw from the TIS toolkit.

Several people on both the fwtk-users and the squid-users mailing asked about using Squid in combination with http-gw from the TIS toolkit. The most elegant way in my opinion is to run an internal Squid caching proxyserver which handles client requests and let this server forward it's requests to the http-gw running on the firewall. Cache hits won't need to be handled by the firewall.

In this example Squid runs on the same server as the http-gw, Squid uses 8000 and http-gw uses 8080 (web). The local domain is

Firewall configuration:

Either run http-gw as a daemon from the /etc/rc.d/rc.local (Linux Slackware): exec /usr/local/fwtk/http-gw -daemon 8080 or run it from inetd like this: web stream tcp nowait.100 root /usr/local/fwtk/http-gw http-gw I increased the watermark to 100 because a lot of people run into problems with the default value.

Make sure you have at least the following line in /usr/local/etc/netperm-table: http-gw: hosts You could add the IP-address of your own workstation to this rule and make sure the http-gw by itself workstest, like: http-gw: hosts

Squid configuration:

The following settings are important:

http_port 8000 icp_port 0 cache_host parent 8080 0 default inside_firewall This tells Squid to use the parent for all domains other than Below, access.log entries show what happens if you do a reload on the Squid-homepage:

872739961.631 1566 ERR_CLIENT_ABORT/304 83 GET - DEFAULT_PARENT/ - 872739962.976 1266 TCP_CLIENT_REFRESH/304 88 GET - DEFAULT_PARENT/ - 872739963.007 1299 ERR_CLIENT_ABORT/304 83 GET - DEFAULT_PARENT/ - 872739963.061 1354 TCP_CLIENT_REFRESH/304 83 GET - DEFAULT_PARENT/

http-gw entries in syslog:

Aug 28 02:46:00 memo http-gw[2052]: permit host=localhost/ use of gateway (V2.0beta) Aug 28 02:46:00 memo http-gw[2052]: log host=localhost/ protocol=HTTP cmd=dir path=/ Aug 28 02:46:01 memo http-gw[2052]: exit host=localhost/ cmds=1 in=0 out=0 user=unauth duration=1 Aug 28 02:46:01 memo http-gw[2053]: permit host=localhost/ use of gateway (V2.0beta) Aug 28 02:46:01 memo http-gw[2053]: log host=localhost/ protocol=HTTP cmd=get path=/Squid/Squidlogo2.gif Aug 28 02:46:01 memo http-gw[2054]: permit host=localhost/ use of gateway (V2.0beta) Aug 28 02:46:01 memo http-gw[2054]: log host=localhost/ protocol=HTTP cmd=get path=/Squid/squidnow.gif Aug 28 02:46:01 memo http-gw[2055]: permit host=localhost/ use of gateway (V2.0beta) Aug 28 02:46:01 memo http-gw[2055]: log host=localhost/ protocol=HTTP cmd=get path=/Images/cache_now.gif Aug 28 02:46:02 memo http-gw[2055]: exit host=localhost/ cmds=1 in=0 out=0 user=unauth duration=1 Aug 28 02:46:03 memo http-gw[2053]: exit host=localhost/ cmds=1 in=0 out=0 user=unauth duration=2 Aug 28 02:46:04 memo http-gw[2054]: exit host=localhost/ cmds=1 in=0 out=0 user=unauth duration=3

To summarize:



-- Rodney van den Oever

4.15 What is ``HTTP_X_FORWARDED_FOR''? Why does squid provide it to WWW servers, and how can I stop it?

When a proxy-cache is used, a server does not see the connection coming from the originating client. Many people like to implement access controls based on the client address. To accomodate these people, Squid adds its own request header called "X-Forwarded-For" which looks like this: X-Forwarded-For:, unknown, Entries are always IP addresses, or the word unknown if the address could not be determined or if it has been disabled with the forwarded_for configuration option.

We must note that access controls based on this header are extremely weak and simple to fake. Anyone may hand-enter a request with any IP address whatsoever. This is perhaps the reason why client IP addresses have been omitted from the HTTP/1.1 specification.

4.16 Can I use the redirector to return HTTP redirect messages?

Normally, the redirector feature is used to rewrite requested URLs. Squid then transparently requests the new URL. However, in some situations, it may be desirable to return an HTTP "301" or "302" redirect message to the client. This is now possible with Squid version 1.1.19.

Simply modify your redirector program to append either "301:" or "302:" before the new URL. For example, the following script might be used to direct external clients to a secure Web server for internal documents: #!/usr/local/bin/perl $|=1; while (<>) { @X = split; $url = $X[0]; if ($url =~ /^http:\/\/internal\.foo\.com/) { $url =~ s/^http/https/; $url =~ s/internal/secure/; print "302:$url\n"; } else { print "$url\n"; } }

Please see sections 10.3.2 and 10.3.3 of RFC 2068 for an explanation of the 301 and 302 HTTP reply codes.

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