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Squid Web Cache documentation

🔗 Feature: dnsserver helper (obsolete)

🔗 Details

:warning: dnsserver helper is now replaced by a faster internal DNS client. You should NOT be running with external DNS processes.

🔗 What is the ‘‘dnsserver’’?

The dnsserver is a process forked by squid to resolve IP addresses from domain names. This is necessary because the gethostbyname(3) function blocks the calling process until the DNS query is completed.

Squid must use non-blocking I/O at all times, so DNS lookups are implemented external to the main process. The dnsserver processes do not cache DNS lookups, that is implemented inside the squid process.

An internal DNS client was integrated into the main Squid binary in Squid-2.3. It is much faster and can scale to match traffic levels without needing a reconfigure. If you have reason to use the old style dnsserver process you can build it at ./configure time using –disable-internal-dns. However we would suggest that you file a bug if you find that the internal DNS process does not work as you would expect.

🔗 Configuration Options

🔗 Troubleshooting

🔗 dnsSubmit: queue overload, rejecting blah

This means that you are using external dnsserver processes for lookups, and all processes are busy, and Squid’s pending queue is full. Each dnsserver program can only handle one request at a time. When all dnsserver processes are busy, Squid queues up requests, but only to a certain point.

To alleviate this condition, you need to either (1) increase the number of dnsserver processes by changing the value for dns_children in your config file, or (2) switch to using Squid’s internal DNS client code.

Note that in some versions, Squid limits dns_children to 32. To increase it beyond that value, you would have to edit the source code.

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

:bulb: Use the internal DNS resolver now built into Squid. It is not limited to single request-response blocking.

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 DNS service time is the proximity of your DNS resolver. Normally we do not recommend running Squid and Resolver on the same host. Instead you should try use a DNS resolver 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.

🔗 I have dnsserver processes that aren’t being used, should I lower the number in “squid.conf”?

The dnsserver processes were originally 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.

Categories: Feature,ObsoleteFeature

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