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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_peer directive in squid.conf to specify the parent and sibling nodes.

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

        #  squid.conf - On the host: childcache.example.com
        #
        #  Format is: hostname  type  http_port  udp_port
        #
        cache_peer parentcache.example.com   parent  3128 3130
        cache_peer childcache2.example.com   sibling 3128 3130
        cache_peer childcache3.example.com   sibling 3128 3130

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

        #  squid.conf - On the host: sv.cache.nlanr.net
        #
        #  Format is: hostname  type  http_port  udp_port
        #

        cache_peer electraglide.geog.unsw.edu.au parent 3128 3130
        cache_peer cache1.nzgate.net.nz          parent 3128 3130
        cache_peer pb.cache.nlanr.net   parent 3128 3130
        cache_peer it.cache.nlanr.net   parent 3128 3130
        cache_peer sd.cache.nlanr.net   parent 3128 3130
        cache_peer uc.cache.nlanr.net   sibling 3128 3130
        cache_peer bo.cache.nlanr.net   sibling 3128 3130
        cache_peer_domain electraglide.geog.unsw.edu.au .au
        cache_peer_domain cache1.nzgate.net.nz   .au .aq .fj .nz
        cache_peer_domain pb.cache.nlanr.net     .uk .de .fr .no .se .it
        cache_peer_domain it.cache.nlanr.net     .uk .de .fr .no .se .it
        cache_peer_domain sd.cache.nlanr.net     .mx .za .mu .zm

The configuration above indicates that the cache will use pb.cache.nlanr.net and it.cache.nlanr.net for domains uk, de, fr, no, se and it, sd.cache.nlanr.net for domains mx, za, mu and zm, and cache1.nzgate.net.nz 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 sd.cache.nlanr.net:3131

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?

This entry has been moved to a different section.

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

Note: The information here is current for version 2.2.

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 never_direct access list in squid.conf to specify which requests must be forwarded to your parent cache outside the firewall, and the always_direct access list to specify which requests must not be forwarded. For example, if Squid must connect directly to all servers that end with mydomain.com, but must use the parent for all others, you would write:

        acl INSIDE dstdomain .mydomain.com
        always_direct allow INSIDE
        never_direct allow all

You could also specify internal servers by IP address

        acl INSIDE_IP dst 1.2.3.0/24
        always_direct allow INSIDE_IP
        never_direct allow all
Note, however that when you use IP addresses, Squid must perform a DNS lookup to convert URL hostnames to an address. Your internal DNS servers may not be able to lookup external domains.

If you use never_direct and you have multiple parent caches, then you probably will want to mark one of them as a default choice in case Squid can't decide which one to use. That is done with the default keyword on a cache_peer line. For example:

        cache_peer xyz.mydomain.com parent 3128 0 default

4.9 How do I configure Squid forward all requests to another proxy?

Note: The information here is current for version 2.2.

First, you need to give Squid a parent cache. Second, you need to tell Squid it can not connect directly to origin servers. This is done with three configuration file lines:

        cache_peer parentcache.foo.com parent 3128 0 no-query default
        acl all src 0.0.0.0/0.0.0.0
        never_direct allow all
Note, with this configuration, if the parent cache fails or becomes unreachable, then every request will result in an error message.

In case you want to be able to use direct connections when all the parents go down you should use a different approach:

        cache_peer parentcache.foo.com parent 3128 0 no-query
        prefer_direct off
The default behaviour of Squid in the absence of positive ICP, HTCP, etc replies is to connect to the origin server instead of using parents. The prefer_direct off directive tells Squid to try parents first.

4.10 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.

4.11 My dnsserver average/median service time seems high, how can I reduce it?

First, find out if you have enough dnsserver processes running by looking at the Cachemanager dns output. Ideally, you should see that the first dnsserver handles a lot of requests, the second one less than the first, etc. The last dnsserver should have serviced relatively few requests. If there is not an obvious decreasing trend, then you need to increase the number of dns_children in the configuration file. If the last dnsserver has zero requests, then you definately have enough.

Another factor which affects the dnsserver service time is the proximity of your DNS resolver. Normally we do not recommend running Squid and named on the same host. Instead you should try use a DNS resolver (named) on a different host, but on the same LAN. If your DNS traffic must pass through one or more routers, this could be causing unnecessary delays.

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 What cache_dir size should I use?

Most people have a disk partition dedicated to the Squid cache. You don't want to use the entire partition size. You have to leave some extra room. Currently, Squid is not very tolerant of running out of disk space.

Lets say you have a 9GB disk. Remember that disk manufacturers lie about the space available. A so-called 9GB disk usually results in about 8.5GB of raw, usable space. First, put a filesystem on it, and mount it. Then check the ``available space'' with your df program. Note that you lose some disk space to filesystem overheads, like superblocks, inodes, and directory entries. Also note that Unix normally keeps 10% free for itself. So with a 9GB disk, you're probably down to about 8GB after formatting.

Next, I suggest taking off another 10% or so for Squid overheads, and a "safe buffer." Squid normally puts its swap.state files in each cache directory. These grow in size until you rotate the logs, or restart squid. Also note that Squid performs better when there is more free space. So if performance is important to you, then take off even more space. Typically, for a 9GB disk, I recommend a cache_dir setting of 6000 to 7500 Megabytes:

cache_dir ... 7000 16 256

Its better to start out conservative. After the cache becomes full, look at the disk usage. If you think there is plenty of unused space, then increase the cache_dir setting a little.

If you're getting ``disk full'' write errors, then you definately need to decrease your cache size.

4.15 I'm adding a new cache_dir. Will I lose my cache?

With Squid-1.1, yes, you will lose your cache. This is because version 1.1 uses a simplistic algorithm to distribute files between cache directories.

With Squid-2, you will not lose your existing cache. You can add and delete cache_dir's without affecting any of the others.

4.16 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 home.nl.

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 127.0.0.1
You could add the IP-address of your own workstation to this rule and make sure the http-gw by itself works, like:
        http-gw:                hosts 127.0.0.1 10.0.0.1

Squid configuration:

The following settings are important:

        http_port       8000
        icp_port        0

        cache_peer      localhost.home.nl parent 8080 0 default
        acl HOME        dstdomain .home.nl
        alwayws_direct  allow HOME
        never_direct    allow all
This tells Squid to use the parent for all domains other than home.nl. Below, access.log entries show what happens if you do a reload on the Squid-homepage:

872739961.631   1566 10.0.0.21 ERR_CLIENT_ABORT/304 83 GET http://www.squid-cache.org/ - DEFAULT_PARENT/localhost.home.nl -
872739962.976   1266 10.0.0.21 TCP_CLIENT_REFRESH/304 88 GET http://www.nlanr.net/Images/cache_now.gif - DEFAULT_PARENT/localhost.home.nl -
872739963.007   1299 10.0.0.21 ERR_CLIENT_ABORT/304 83 GET http://www.squid-cache.org/Icons/squidnow.gif - DEFAULT_PARENT/localhost.home.nl -
872739963.061   1354 10.0.0.21 TCP_CLIENT_REFRESH/304 83 GET http://www.squid-cache.org/Icons/Squidlogo2.gif - DEFAULT_PARENT/localhost.home.nl

http-gw entries in syslog:

Aug 28 02:46:00 memo http-gw[2052]: permit host=localhost/127.0.0.1 use of gateway (V2.0beta)
Aug 28 02:46:00 memo http-gw[2052]: log host=localhost/127.0.0.1 protocol=HTTP cmd=dir dest=www.squid-cache.org path=/
Aug 28 02:46:01 memo http-gw[2052]: exit host=localhost/127.0.0.1 cmds=1 in=0 out=0 user=unauth duration=1
Aug 28 02:46:01 memo http-gw[2053]: permit host=localhost/127.0.0.1 use of gateway (V2.0beta)
Aug 28 02:46:01 memo http-gw[2053]: log host=localhost/127.0.0.1 protocol=HTTP cmd=get dest=www.squid-cache.org path=/Icons/Squidlogo2.gif
Aug 28 02:46:01 memo http-gw[2054]: permit host=localhost/127.0.0.1 use of gateway (V2.0beta)
Aug 28 02:46:01 memo http-gw[2054]: log host=localhost/127.0.0.1 protocol=HTTP cmd=get dest=www.squid-cache.org path=/Icons/squidnow.gif
Aug 28 02:46:01 memo http-gw[2055]: permit host=localhost/127.0.0.1 use of gateway (V2.0beta)
Aug 28 02:46:01 memo http-gw[2055]: log host=localhost/127.0.0.1 protocol=HTTP cmd=get dest=www.nlanr.net path=/Images/cache_now.gif
Aug 28 02:46:02 memo http-gw[2055]: exit host=localhost/127.0.0.1 cmds=1 in=0 out=0 user=unauth duration=1
Aug 28 02:46:03 memo http-gw[2053]: exit host=localhost/127.0.0.1 cmds=1 in=0 out=0 user=unauth duration=2
Aug 28 02:46:04 memo http-gw[2054]: exit host=localhost/127.0.0.1 cmds=1 in=0 out=0 user=unauth duration=3

To summarize:

Advantages:

Disadvantages:

-- Rodney van den Oever

4.17 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 accommodate these people, Squid adds its own request header called "X-Forwarded-For" which looks like this:

        X-Forwarded-For: 128.138.243.150, unknown, 192.52.106.30
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.

Because of the weakness of this header, support for access controls based on X-Forwarder-For is not yet available in any officially released version of squid. However, unofficial patches are available from the follow_xff Squid development project and may be integrated into later versions of Squid once a suitable trust model have been developed.

4.18 Can Squid anonymize HTTP requests?

Yes it can, however the way of doing it has changed from earlier versions of squid. As of squid-2.2 a more customisable method has been introduced. Please follow the instructions for the version of squid that you are using. As a default, no anonymizing is done.

If you choose to use the anonymizer you might wish to investigate the forwarded_for option to prevent the client address being disclosed. Failure to turn off the forwarded_for option will reduce the effectiveness of the anonymizer. Finally if you filter the User-Agent header using the fake_user_agent option can prevent some user problems as some sites require the User-Agent header.

Squid 2.2

With the introduction of squid 2.2 the anonoymizer has become more customisable. It now allows specification of exactly which headers will be allowed to pass. This is further extended in Squid-2.5 to allow headers to be anonymized conditionally.

For details see the documentation of the http_header_access and header_replace directives in squid.conf.default.

References: Anonymous WWW

4.19 Can I make Squid go direct for some sites?

Sure, just use the always_direct access list.

For example, if you want Squid to connect directly to hotmail.com servers, you can use these lines in your config file:

acl hotmail dstdomain .hotmail.com
always_direct allow hotmail

4.20 Can I make Squid proxy only, without caching anything?

Sure, there are few things you can do.

You can use the no_cache access list to make Squid never cache any response:

        acl all src 0/0
        no_cache deny all

With Squid-2.4 and later you can use the ``null'' storage module to avoid having a cache directory:

        cache_dir null /tmp

Note: a null cache_dir does not disable caching, but it does save you from creating a cache structure if you have disabled caching with no_cache.

Note: the directory (e.g., /tmp) must exist so that squid can chdir to it, unless you also use the coredump_dir option.

To configure Squid for the ``null'' storage module, specify it on the configure command line:

        ./configure --enable-storeio=ufs,null ...

4.21 Can I prevent users from downloading large files?

You can set the global reply_body_max_size parameter. This option controls the largest HTTP message body that will be sent to a cache client for one request.

If the HTTP response coming from the server has a Content-length header, then Squid compares the content-length value to the reply_body_max_size value. If the content-length is larger, the server connection is closed and the user receives an error message from Squid.

Some responses don't have Content-length headers. In this case, Squid counts how many bytes are written to the client. Once the limit is reached, the client's connection is simply closed.

Note that ``creative'' user-agents will still be able to download really large files through the cache using HTTP/1.1 range requests.


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