11.51 Why do I sometimes get ``Zero Sized Reply''?

This happens when Squid makes a TCP connection to an origin server, but for some reason, the connection is closed before Squid reads any data. Depending on various factors, Squid may be able to retry the request again. If you see the ``Zero Sized Reply'' error message, it means that Squid was unable to retry, or that all retry attempts also failed.

What causes a connection to close prematurely? It could be a number of things, including:

1. An overloaded origin server.
2. TCP implementation/interoperability bugs. See the System-Dependent Weirdnesses section for details.
3. Race conditions with HTTP persistent connections.
4. Buggy or misconfigured NAT boxes, firewalls, and load-balancers.
5. Denial of service attacks.
6. Utilizing TCP blackholing on FreeBSD.

You may be able to use tcpdump to track down and observe the problem.

Some users believe the problem is caused by very large cookies. One user reports that his Zero Sized Reply problem went away when he told Internet Explorer to not accept third-party cookies.

Here are some things you can try to reduce the occurance of the Zero Sized Reply error:

1. Delete or rename your cookie file and configure your browser to prompt you before accepting any new cookies.
2. Disable HTTP persistent connections with the server_persistent_connections and client_persistent_connections directives.
3. Disable any advanced TCP features on the Squid system. Disable ECN on Linux with echo 0 > /proc/sys/net/ipv4/tcp_ecn/.
4. Upgrade to Squid-2.5.STABLE4 or later to work around a Host header related bug in Cisco PIX HTTP inspection. The Cisco PIX firewall wrongly assumes the Host header can be found in the first packet of the request.

ERROR
The requested URL could not be retrieved

While trying to retrieve the URL: http://www.mysite.com/modules/wfdownloads/visit.php?cid=15&lid=15

The following error was encountered:

* Zero Sized Reply

Squid did not receive any data for this request.

Your cache administrator is root.