Squid Web Cache wiki

Squid Web Cache documentation

🔗 OS-specific special cases

:information_source: A lot of this content is old, and probably doesn’t apply to modern systems. We do not necessarily have access to these systems to validate, so we are leaving the information in for now

🔗 Solaris

🔗 TCP incompatibility?

J.D. Bronson (jb at ktxg dot com) reported that his Solaris box could not talk to certain origin servers, such as moneycentral.msn.com and www.mbnanetaccess.com. J.D. fixed his problem by setting:

tcp_xmit_hiwat 49152
tcp_xmit_lowat 4096
tcp_recv_hiwat 49152

:information_source: In Solaris 10 and above this parameters is system default (by Yuri Voinov).

🔗 select()

select(3c) won’t handle more than 1024 file descriptors. The configure script should enable poll() by default for Solaris. poll() allows you to use many more filedescriptors, probably 8192 or more.

For older Squid versions you can enable poll() manually by changing HAVE_POLL in include/autoconf.h, or by adding -DUSE_POLL=1 to the DEFINES in src/Makefile.

🔗 malloc

libmalloc.a is leaky. Squid’s configure does not use -lmalloc on Solaris.

🔗 DNS lookups and ‘‘nscd’’

by David J N Begley.

DNS lookups can be slow because of some mysterious thing called ncsd. You should edit /etc/nscd.conf and make it say:

enable-cache            hosts           no

Apparently nscd serializes DNS queries thus slowing everything down when an application (such as Squid) hits the resolver hard. You may notice something similar if you run a log processor executing many DNS resolver queries - the resolver starts to slow.. right.. down.. . . .

According to Andres Kroonmaa, users of Solaris starting from version 2.6 and up should NOT completely disable nscd daemon. nscd should be running and caching passwd and group files, although it is suggested to disable hosts caching as it may interfere with DNS lookups.

Several library calls rely on available free FILE descriptors FD < 256. Systems running without nscd may fail on such calls if first 256 files are all in use.

Since solaris 2.6 Sun has changed the way some system calls work and is using nscd daemon as a implementor of them. To communicate to nscd Solaris is using undocumented calls. Basically nscd is used to reduce memory usage of user-space system libraries that use passwd and group files. Before 2.6 Solaris cached full passwd file in library memory on the first use but as this was considered to use up too much ram on large multiuser systems Sun has decided to move implementation of these calls out of libraries and to a single dedicated daemon.

🔗 DNS lookups and /etc/nsswitch.conf

by Jason Armistead.

The /etc/nsswitch.conf file determines the order of searches for lookups (amongst other things). You might only have it set up to allow NIS and HOSTS files to work. You definitely want the “hosts:” line to include the word dns, e.g.:

hosts:      nis dns [NOTFOUND=return] files

🔗 DNS lookups and NIS

by Chris Tilbury.

Our site cache is running on a Solaris 2.6 machine. We use NIS to distribute authentication and local hosts information around and in common with our multiuser systems, we run a slave NIS server on it to help the response of NIS queries.

We were seeing very high name-ip lookup times (avg ~2sec) and ip->name lookup times (avg ~8 sec), although there didn’t seem to be that much of a problem with response times for valid sites until the cache was being placed under high load. Then, performance went down the toilet.

After some time, and a bit of detective work, we found the problem. On Solaris 2.6, if you have a local NIS server running (ypserv) and you have NIS in your /etc/nsswitch.conf hosts entry, then check the flags it is being started with. The 2.6 ypstart script checks to see if there is a resolv.conf file present when it starts ypserv. If there is, then it starts it with the -d option.

This has the same effect as putting the YP_INTERDOMAIN key in the hosts table – namely, that failed NIS host lookups are tried against the DNS by the NIS server.

This is a bad thing(tm)! If NIS itself tries to resolve names using the DNS, then the requests are serialised through the NIS server, creating a bottleneck (This is the same basic problem that is seen with nscd). Thus, one failing or slow lookup can, if you have NIS before DNS in the service switch file (which is the most common setup), hold up every other lookup taking place.

If you’re running in this kind of setup, then you will want to make sure that

We changed these here, and saw our average lookup times drop by up to an order of magnitude (~150msec for name-ip queries and ~1.5sec for ip-name queries, the latter still so high, I suspect, because more of these fail and timeout since they are not made so often and the entries are frequently non-existent anyway).

🔗 Tuning

Have a look at “Tuning your TCP/IP stack and more” by Jens-S. Voeckler.

🔗 disk write error: (28) No space left on device

You might get this error even if your disk is not full, and is not out of inodes. Check your syslog logs (/var/adm/messages, normally) for messages like either of these:

NOTICE: realloccg /proxy/cache: file system full
NOTICE: alloc: /proxy/cache: file system full

In a nutshell, the UFS filesystem used by Solaris can’t cope with the workload squid presents to it very well. The filesystem will end up becoming highly fragmented, until it reaches a point where there are insufficient free blocks left to create files with, and only fragments available. At this point, you’ll get this error and squid will revise its idea of how much space is actually available to it. You can do a “fsck -n raw_device” (no need to unmount, this checks in read only mode) to look at the fragmentation level of the filesystem. It will probably be quite high (>15%).

Sun suggest two solutions to this problem. One costs money, the other is free but may result in a loss of performance (although Sun do claim it shouldn’t, given the already highly random nature of squid disk access).

The first is to buy a copy of VxFS, the Veritas Filesystem. This is an extent-based filesystem and it’s capable of having online defragmentation performed on mounted filesystems. This costs money, however (VxFS is not very cheap!)

The second is to change certain parameters of the UFS filesystem. Unmount your cache filesystems and use tunefs to change optimization to “space” and to reduce the “minfree” value to 3-5% (under Solaris 2.6 and higher, very large filesystems will almost certainly have a minfree of 2% already and you shouldn’t increase this). You should be able to get fragmentation down to around 3% by doing this, with an accompanied increase in the amount of space available.

Thanks to Chris Tilbury.

🔗 Solaris X86 and IPFilter

by Jeff Madison

Important update regarding Squid running on Solaris x86. I have been working for several months to resolve what appeared to be a memory leak in squid when running on Solaris x86 regardless of the malloc that was used. I have made 2 discoveries that anyone running Squid on this platform may be interested in.

Number 1: There is not a memory leak in Squid even though after the system runs for some amount of time, this varies depending on the load the system is under, Top reports that there is very little memory free. True to the claims of the Sun engineer I spoke to this statistic from Top is incorrect. The odd thing is that you do begin to see performance suffer substantially as time goes on and the only way to correct the situation is to reboot the system. This leads me to discovery number 2.

Number 2: There is some type of resource problem, memory or other, with IPFilter on Solaris x86. I have not taken the time to investigate what the problem is because we no longer are using IPFilter. We have switched to a Alteon ACE 180 Gigabit switch which will do the trans-proxy for you. After moving the trans-proxy, redirection process out to the Alteon switch Squid has run for 3 days strait under a huge load with no problem what so ever. We currently have 2 boxes with 40 GB of cached objects on each box. This 40 GB was accumulated in the 3 days, from this you can see what type of load these boxes are under. Prior to this change we were never able to operate for more than 4 hours.

Because the problem appears to be with IPFilter I would guess that you would only run into this issue if you are trying to run Squid as a interception proxy using IPFilter. That makes sense. If there is anyone with information that would indicate my finding are incorrect I am willing to investigate further.

🔗 Changing the directory lookup cache size

by Mike Batchelor

On Solaris, the kernel variable for the directory name lookup cache size is ncsize. In /etc/system, you might want to try

set ncsize = 8192

or even higher. The kernel variable ufs_inode - which is the size of the inode cache itself - scales with ncsize in Solaris 2.5.1 and later. Previous versions of Solaris required both to be adjusted independently, but now, it is not recommended to adjust ufs_inode directly on 2.5.1 and later.

You can set ncsize quite high, but at some point - dependent on the application - a too-large ncsize will increase the latency of lookups.

Defaults are:

Solaris 2.5.1 : (max_nprocs + 16 + maxusers) + 64
Solaris 2.6/Solaris 7 : 4 * (max_nprocs + maxusers) + 320

🔗 The priority_paging algorithm

by Mike Batchelor

Another new tuneable (actually a toggle) in Solaris 2.5.1, 2.6 or Solaris 7 is the priority_paging algorithm. This is actually a complete rewrite of the virtual memory system on Solaris. It will page out application data last, and filesystem pages first, if you turn it on (set priority_paging = 1 in /etc/system). As you may know, the Solaris buffer cache grows to fill available pages, and under the old VM system, applications could get paged out to make way for the buffer cache, which can lead to swap thrashing and degraded application performance. The new priority_paging helps keep application and shared library pages in memory, preventing the buffer cache from paging them out, until memory gets REALLY short. Solaris 2.5.1 requires patch 103640-25 or higher and Solaris 2.6 requires 105181-10 or higher to get priority_paging. Solaris 7 needs no patch, but all versions have it turned off by default.

🔗 assertion failed: StatHist.c:91: `statHistBin(H, max) == H->capacity - 1’

by Marc

This crash happen on Solaris, when you don’t have the “math.h” file at the compile time. I guess it can happen on every system without the correct include, but I have not verified.

The configure script just report: “math.h: no” and continue. The math functions are bad declared, and this cause this crash.

For 32bit Solaris, “math.h” is found in the SUNWlibm package.

🔗 FreeBSD

🔗 T/TCP bugs

We have found that with FreeBSD-2.2.2-RELEASE, there some bugs with T/TCP. FreeBSD will try to use T/TCP if you’ve enabled the “TCP Extensions.” To disable T/TCP, use sysinstall to disable TCP Extensions, or edit /etc/rc.conf and set

tcp_extensions="NO"             # Allow RFC1323 & RFC1544 extensions (or NO).

or add this to your /etc/rc files:

sysctl -w net.inet.tcp.rfc1644=0

🔗 Dealing with NIS

/var/yp/Makefile has the following section:

# The following line encodes the YP_INTERDOMAIN key into the hosts.byname
# and hosts.byaddr maps so that ypserv(8) will do DNS lookups to resolve
# hosts not in the current domain. Commenting this line out will disable
# the DNS lookups.

You will want to comment out the B=-b line so that ypserv does not do DNS lookups.

🔗 Internal DNS problems with jail environment

Some users report problems with running Squid in the jail environment. Specifically, Squid logs messages like:

2001/10/12 02:08:49| comm_udp_sendto: FD 4,, port 53: (22) Invalid argument
2001/10/12 02:08:49| idnsSendQuery: FD 4: sendto: (22) Invalid argument

You can eliminate the problem by putting the jail’s network interface address in the ‘udp_outgoing_addr’ configuration option in squid.conf.

🔗 “Zero Sized Reply” error due to TCP blackholing

by David Landgren

On FreeBSD, make sure that TCP blackholing is not active. You can verify the current setting with:

# /sbin/sysctl net.inet.tcp.blackhole

It should return the following output:

net.inet.tcp.blackhole: 0

If it is set to a positive value (usually, 2), disable it by setting it back to zero with<

# /sbin/sysctl net.inet.tcp.blackhole=0

To make sure the setting survives across reboots, add the following line to the file /etc/sysctl.conf:


🔗 Linux

🔗 Can’t connect to some sites through Squid

When using Squid, some sites may give erorrs such as “(111) Connection refused” or “(110) Connection timed out” although these sites work fine without going through Squid.

Linux 2.6 implements Explicit Congestion Notification (ECN) support and this can cause some TCP connections to fail when contacting some sites with broken firewalls or broken TCP/IP implementations.

As of June 2006, the number of sites that fail when ECN is enabled is very low and you may find you benefit more from having this feature enabled than globally turning it off.

To work around such broken sites you can disable ECN with the following command:

echo 0 > /proc/sys/net/ipv4/tcp_ecn

HenrikNordstrom explains:

ECN is an standard extension to TCP/IP, making TCP/IP behave better in overload conditions where the available bandwidth is all used up (i.e. the default condition for any WAN link). Defined by Internet RFC3168 issued by the Networking Working Group at IETF, the standardization body responsible for the evolution of TCP/IP and other core Internet technologies such as routing. It’s implemented by using two previously unused bits (of 6) in the TCP header, plus redefining two bits of the never standardized TOS field in the IP header (dividing TOS in 6 bits Diffserv and 2 bit ECN fields), allowing routers to clearly indicate overload conditions to the participating computers instead of dropping packets hoping that the computers will realize there is too much traffic. The main problem is the use of those previously unused bits in the TCP header. The TCP/IP standard has always said that those bits is reserved for future use, but many old firewalls assume the bits will never be used and simply drops all traffic using this new feature thinking it’s invalid use of TCP/IP to evolve beyond the original standards from 1981. ECN in it’s final form was defined 2001, but earlier specifications was circulated several years earlier.

🔗 Some sites load extremely slowly or not at all

You may occasionally have problems with TCP Window Scaling on Linux. At first you may be able to TCP connect to the site, but then unable to transfer any data across your connection or that data flows extremely slowly. This is due to some broken firewalls on the Internet (it is not a bug with Linux) mangling the window scaling option when the TCP connection is established. More details and a workaround can be found at lwn.net.

Window scaling is a standard TCP feature which makes TCP perform well over high speed wan links. Without window scaling the round trip latency seriously limits the bandwidth that can be used by a single TCP connection.

The reason why this is experienced with Linux and not most other OS:es is that all desktop OS:es advertise a quite small window scaling factor if at all, and therefore the firewall bug goes unnoticed with these OS:es. Windows OS:es is also known to have plenty of workarounds to automatically and silently work around these issues, where the Linux community has as policy to not make such workarounds, most likely in an attempt trying to put some pressure on getting the failing network equipment fixed..

To test if this is the source of your problem try the following:

echo 0 >/proc/sys/net/ipv4/tcp_window_scaling

But be warned that this will quite noticeably degrade TCP performance.

Other possible alternatives is setting tcp_recv_bufsize in squid.conf, or using the /sbin/ip route … window=xxx option.

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