Using c-icap for proxy content antivirus checking on-the-fly

Warning: Any example presented here is provided "as-is" with no support or guarantee of suitability. If you have any further questions about these examples please email the squid-users mailing list.


For Squid-3.0 and later we can use ICAP for content filtering or antivirus checking. This config example describes how to scan for viruses on-the-fly using squidclamav antivirus module in combination with ClamAV antivirus service. It is a bit different with recommended squidclamav configuration and adapted for Squid-3.4 releases and above with latest configuration changes.


This will be useful both for interception and explicit proxies. With proper ClamAV configuration verification brings almost no noticeable delay and performed with acceptable latency.

Building c-icap server

Download latest c-icap sources from here. (Changelog is here). For antivirus checking you do not need BerkeleyDB support. Then configure as shown below and make.

# 32 bit
./configure 'CXXFLAGS=-O2 -m32 -pipe' 'CFLAGS=-O2 -m32-pipe' --enable-large-files --without-bdb --prefix=/usr/local
# 64 bit
./configure 'CXXFLAGS=-O2 -m64 -pipe' 'CFLAGS=-O2 -m64 -pipe' --without-bdb --prefix=/usr/local
make/gmake install-strip

Configuring and run c-icap server

Edit c-icap.conf as follows:

PidFile /var/run/c-icap/
CommandsSocket /var/run/c-icap/c-icap.ctl
StartServers 1
MaxServers 20
MaxRequestsPerChild  100
Port 1344 
User squid
Group squid
ServerAdmin yourname@yourdomain
TmpDir /tmp
MaxMemObject 131072
DebugLevel 0
ModulesDir /usr/local/lib/c_icap
ServicesDir /usr/local/lib/c_icap
LoadMagicFile /usr/local/etc/c-icap.magic

acl localhost src
icap_access allow localhost PERMIT_REQUESTS
icap_access deny all

ServerLog /var/log/i-cap_server.log
AccessLog /var/log/i-cap_access.log

Edit paths if necessary and start c-icap server. Add startup script to your OS.

  • {i} Note: Method OPTIONS is excluding from scanning in latest squidclamav release (starting from squidclamav version 6.14). So, permit access for it not required.

  • {i} Note: TmpDir usually set to /var/tmp (this is default). Be very careful when change it. TmpDir uses for temp files when oblect in memory greater than MaxMemObject. And this temp files (CI_TMP_XXXX) remains in TmpDir when processing complete. Schedule housekeeping for TmpDir otherwise free space on /var filesystem can ran out on high loaded servers.

  • {i} Note: In some cases you can increase MaxMemObject to increase performance at the cost of some increase in consumption of RAM. Sometimes it is advisable to set this parameter to the maximum value of the logical IO unit for your OS.

Antivirus checking with c-icap, ClamAV daemon and Squidclamav

Of course, this installation requires more resources, especially when installing on single host. But also provides more flexibility and - in some cases - more scalability.

Build, configuring and run ClamAV daemon

ClamAV including in many repositories and can be got from them. When configuring clamd, be very conservative with options. Defaults is good starting point. I do not recommend using SafeBrowsing due to performance and memory issues and DetectPUA due to much false-positives. Also take care about antivirus databases updates - it will occurs often enough. I use 24 times per day.

  • {i} Note: ClamAV daemon (clamd) is memory consumption service, it uses about 200-300 megabytes in minimal configuration (mainly used to store AV database in memory), it can be higher during deep scans of big archives. So, you can put it on separate node with fast network interconnect with your proxy (this option is valid only when using squidclamav).

  • {i} Note: It is important to set StreamMaxLength parameter in clamd.conf to the same value as maxsize in squidclamav.conf.

I.e., uncomment and adjust in clamd.conf:

StreamMaxLength 5M

Build and configuring squidclamav

Installing SquidClamav requires that you already have installed the c-icap as explained above. You must provide the installation path of c-icap to the configure command as follow, compile and then install:

# 32 bit
./configure 'CXXFLAGS=-O2 -m32 -pipe' 'CFLAGS=-O2 -m32 -pipe' --with-c-icap=/usr/local
# 64 bit
./configure 'CXXFLAGS=-O2 -m64 -pipe' 'CFLAGS=-O2 -m64 -pipe' --with-c-icap=/usr/local
make/gmake install-strip

this will install the library into the c-icap modules/services repository. Then add the line below to the c-icap.conf file:

Service squidclamav

Then adjust squidclamav.conf as follows:

maxsize 5000000

# When a virus is found then redirect the user to this URL
redirect http://<You_proxy_FQDN>:8080/cgi-bin/clwarn.cgi.xx_XX

# Path to the clamd socket, use clamd_local if you use Unix socket or if clamd
# is listening on an Inet socket, comment clamd_local and set the clamd_ip and
# clamd_port to the corresponding value.
clamd_local /tmp/clamd.socket
#clamd_port 3310

# Set the timeout for clamd connection. Default is 1 second, this is a good
# value but if you have slow service you can increase up to 3.
timeout 3

# Force SquidClamav to log all virus detection or squiguard block redirection
# to the c-icap log file.
logredir 1

# Enable / disable DNS lookup of client ip address. Default is enabled '1' to
# preserve backward compatibility but you must desactivate this feature if you
# don't use trustclient with hostname in the regexp or if you don't have a DNS
# on your network. Disabling it will also speed up squidclamav.
dnslookup 0

# Enable / Disable Clamav Safe Browsing feature. You must have enabled the
# corresponding behavior in clamd by enabling SafeBrowsing into freshclam.conf
# Enabling it will first make a safe browsing request to clamd and then the
# virus scan request.
safebrowsing 0

# Here is some defaut regex pattern to have a high speed proxy on system
# with low resources.
# Abort AV scan, but not chained program
abort \.google\.*
abort \.youtube\.com
abort \.googlevideo\.com
abort \.ytimg\.com
abort \.yimg\.com

# Do not scan images
abort ^.*\.([j|J][p|P][?:[e|E]?[g|G]|gif|png|bmp|ico|svg|web[p|m])
abortcontent ^image\/.*$

# Do not scan text files
abort ^.*\.((cs|d?|m?|p?|r?|s?|w?|x?|z?)h?t?m?(l?)|php[3|5]?|rss|atom|vr(t|ml)|(c|x|j)s[s|t|px]?)
abortcontent ^text\/.*$
abortcontent ^application\/x-javascript$
abortcontent ^application\/javascript$
abortcontent ^application\/json$

# Do not scan fonts
abort ^.*\.(ttf|eot|woff2?)
abortcontent ^font\/.*$
abortcontent ^application\/x-woff$
abortcontent ^application\/font-woff2?$
abortcontent ^application\/x-font-ttf$

# Do not scan (streamed) videos and audios
abort ^.*\.(flv|f4f|mp(3|4))
abortcontent ^video\/.*$
abortcontent ^audio\/.*$
abortcontent ^application\/mp4$

# Do not scan flash files
abort ^.*\.swfx?
abortcontent ^application\/x-shockwave-flash$

# Do not scan sequence of framed Microsoft Media Server (MMS) data packets
abortcontent ^.*application\/x-mms-framed.*$

# White list some sites
# Abort both AV and chained program
whitelist clamav\.net
whitelist securiteinfo\.com
whitelist sanesecurity\.com
whitelist clamav\.bofhland\.org
whitelist threatcenter\.crdf\.fr
whitelist \.avast\.*
whitelist \.gdatasoftware\.com
whitelist \.emsisoft\.*
whitelist \.chilisecurity\.*
whitelist pcpitstop\.com
whitelist \.unthreat\.com
whitelist \.preventon\.com
whitelist lavasoft\.com
whitelist \.norton\.com
whitelist \.symantec\.com
whitelist \.symantecliveupdate\.com
whitelist \.kaspersky\.*
whitelist \.drweb\.*
whitelist \.mcafee\.com
whitelist \.fsecure\.com
whitelist \.f-secure\.com
whitelist \.esetnod32\.*
whitelist \.eset\.*
whitelist \.escanav\.com
whitelist \.360totalsecurity\.com
whitelist \.bitdefender\.com
whitelist pckeeper\.com
whitelist \.mysecuritycenter\.com
whitelist \.avira\.com
whitelist \.pandasecurity\.com
whitelist \.vipreantivirus\.com
whitelist \.quickheal\.com
whitelist \.trustport\.*
whitelist \.trustport-ru\.*
whitelist \.sophos\.com
whitelist \.spamfighter\.com
whitelist \.webroot\.com
whitelist \.k7computing\.com
whitelist \.charityantivirus\.com
whitelist \.avg\.com
whitelist \.trendmicro\.*
whitelist \.zonealarm\.com
whitelist \.comodo\.com
whitelist update\.microsoft\.com
whitelist update\.microsoft\.com\.akadns\.net 
whitelist download\.windowsupdate\.com
whitelist download\.microsoft\.com
whitelist update\.microsoft\.com
whitelist \.oracle\.com
whitelist \.shallalist\.de
whitelist opencsw\.org
# See also 'trustuser' and 'trustclient' configuration directives

and restart c-icap server. Finally don't forget to put clwarn.cgi.xx_XX (where xx_XX matches your language) into your web server cgi-bin directory.

As whitelist can be big enough, to reduce maintenance and simplify administration you can create separate file contains whitelist regex's and configure squidclamav as follows:

# White list some sites
# Abort both AV and chained program
whitelist /usr/local/etc/squidclamav_whitelist

where squidclamav_whitelist contains:

  • {i} Note: You may want to use I-CAP templates for redirection, against squidclamav redirection. In this case you must customize c-icap templates according to your needs.

Squid Configuration File

  • {i} Squid-3.4 and older need to be built with the --enable-icap-client option. Newer releases have this enabled by default.

Paste the configuration file like this:

# -------------------------------------
# Adaptation parameters
# -------------------------------------
icap_enable on
icap_send_client_ip on
icap_send_client_username on
icap_client_username_header X-Authenticated-User
icap_preview_enable on
icap_preview_size 1024
icap_service service_avi_req reqmod_precache icap://localhost:1344/squidclamav bypass=on
adaptation_access service_avi_req allow all
icap_service service_avi_resp respmod_precache icap://localhost:1344/squidclamav bypass=off
adaptation_access service_avi_resp allow all
  • {i} IPv6-enabled operating systems may resolve localhost to the dual-stack enabled ::1 address. If you have troubles with connectivity to IPv4-only ICAP services, just replace localhost above with

Antivirus checking with c-icap and virus checking module

Like eCAP, you can perform antivirus checking with libclamav. This not requires daemon and fries up to 500 Mbytes (average) required to run clamd. This can be useful for single-tier setups.

I-CAP modules provides provides two submodules: using ClamAV daemon, and using libclamav only.

Build c-icap modules

Download last modules, then configuring and build according your ClamAV and c-icap build types (32 or 64 bit):

# 32 bit GCC
./configure 'CFLAGS=-O3 -m32 -pipe' 'CPPFLAGS=-I/usr/local/clamav/include' 'LDFLAGS=-L/usr/local/lib -L/usr/local/clamav/lib'

# 64 bit GCC
./configure 'CFLAGS=-O3 -m64 -pipe' 'CPPFLAGS=-I/usr/local/clamav/include' 'LDFLAGS=-L/usr/local/lib -L/usr/local/clamav/lib/'

gmake install-strip
  • {i} To build submodule clamav_mod (uses libclamav) you can require patch your c-icap installation with last fixes. It uses OpenSSL headers dependency and you can have problems with modules build. This can be workarounded if your system has an older OpenSSL version (i.e. 0.9.8). To do that just add old OpenSSL headers path to CPPFLAGS variable.

Configuring c-icap modules

Add non-default parameters into clamav_mod.conf:

clamav_mod.TmpDir /var/tmp
clamav_mod.MaxFilesInArchive 1000
clamav_mod.MaxScanSize 5M
clamav_mod.HeuristicScanPrecedence on
clamav_mod.OLE2BlockMacros on

Add non-default parameters into virus_scan.conf:

virus_scan.SendPercentData 5
virus_scan.PassOnError on
virus_scan.MaxObjectSize  5M
virus_scan.DefaultEngine clamav
Include clamav_mod.conf

Add following line at the end of c-icap.conf:

Include virus_scan.conf
  • {i} Note: You also must create symbolic link in ClamAV installation directory pointed to ClamAV antivirus database directory, configured for daemon in clamd.conf, for example:

# ln -s /var/lib/clamav /usr/local/clamav/share/clamav

Finally restart c-icap service to accept changes.

Squid Configuration File

  • {i} Squid-3.4 needs to be built with the --enable-icap-client option. Newer releases have this enabled by default.

Paste the configuration file like this:

icap_enable on
icap_service service_avi_req reqmod_precache icap://localhost:1344/virus_scan bypass=off
adaptation_access service_avi_req allow all
icap_service service_avi_resp respmod_precache icap://localhost:1344/virus_scan bypass=on
adaptation_access service_avi_resp allow all
  • {i} When using squidclamav, you must bypass whitelisted sites with Squid ACL's and adaptation_access directives. Also you can customize virus_scan module templates to your language etc.

  • {X} Also beware: without clamd you will have the same 300-500 megabytes of loaded AV database to one of c-icap process with libclamav. ;)

Testing your installation

Point your client machine behind proxy to EICAR test virus and make sure you're get redirected to warning page.

For really big installations you can place all checking infrastructure components on separate nodes - i.e. proxy, c-icap server, ClamAV. That's all, folks! ;)

DNSBL filtering support

In case of paranoia, you can also enable DNSBL URL checking support to your c-icap compatible setup.

To do this you requires to download and install c-icap modules:

# 32 bit GCC
./configure 'CFLAGS=-O3 -m32 -pipe' 'CPPFLAGS=-I/usr/local/clamav/include' 'LDFLAGS=-L/usr/local/lib -L/usr/local/clamav/lib'

# 64 bit GCC
./configure 'CFLAGS=-O3 -m64 -pipe' 'CPPFLAGS=-I/usr/local/clamav/include' 'LDFLAGS=-L/usr/local/lib -L/usr/local/clamav/lib'

gmake install-strip

then add this to your c-icap.conf file:

Module common
Include srv_url_check.conf

Adjust srv_url_check.conf as follows:

Service url_check

url_check.LookupTableDB whitelist domain hash:/usr/local/etc/domain.whitelist "Whitelist"
url_check.LookupTableDB blackuribl domain

url_check.Profile default pass whitelist
url_check.Profile default block blackuribl
url_check.Profile default pass ALL

url_check.DefaultAction pass AddXHeader "X-Next-Services"
  • {i} Note: Using whitelist is good idea for performance reasons. It is plain text file with 2nd level domain names. All hostnames beyong this domains will be pass. Also setup DNS cache is also great idea to improve performance.

and add this to your squid.conf:

# DNSBL service
# Requires to enable "Module common" in c-icap.conf,
# and install and configure c-icap modules!
icap_service service_dnsbl_req reqmod_precache icap://localhost:1344/url_check bypass=on
adaptation_access service_dnsbl_req allow all

Finally you must restart c-icap service and restart your squid. That's basically all.

  • {i} Note: Add DNSBL ICAP service before ClamAV antivirus service.

When using squidclamav AV service, can be better to create adaptation chain on requests, like this:

icap_enable on
icap_send_client_ip on
icap_send_client_username on
icap_client_username_header X-Client-Username
icap_preview_enable on
icap_preview_size 1024
icap_service_failure_limit -1
# DNSBL service
# Requires to enable "Module common" in c-icap.conf
icap_service service_dnsbl_req reqmod_precache icap://localhost:1344/url_check bypass=on routing=on
# ClamAV service
icap_service service_avi_req reqmod_precache icap://localhost:1344/squidclamav bypass=on

adaptation_service_chain svcRequest service_dnsbl_req service_avi_req
adaptation_access svcRequest allow all

icap_service service_avi_resp respmod_precache icap://localhost:1344/squidclamav
adaptation_access service_avi_resp allow all
  • {i} When using DNSBL, it is recommended to set up a DNS cache on the c-icap host for performance.

Performance and tuning

In practice, configuration with clamd and squidclamav is fastest. In fact, squidclamav using INSTREAM to perform AV checks is the best way. You may need only adjust the amount of the workers in the c-icap service according to your load. You will have only two bottlenecks - the interaction of your proxy server with c-icap and interaction of c-icap with antivirus service. You need to reduce latency of these interactions to the minimum possible.

In some cases, placing all services on a single host is not a good idea. High-load setups must be separated between tiers.

  • Do not do extra work - use white lists where possible.
  • Avoid overload - especially in the case of all services installed on a single host.
  • Reduce memory consumption as possible. Do not set high clamd system limits - these increases latency and memory consumption and can lead to a system crash during peak hours.
  • Use chains to adapt and customize the sequence correctly, and make correct access - so as not to overload the individual stages of unnecessary work.
  • {i} c-icap workers produces high CPU load during scanning in all cases. You must minimize scanning as much as possible. Do not scan all data types. Do not scan trusted sites. And do not try to scan Youtube videos, of course. :)

  • {i} On some Solaris setups you can get performance gain by using libmtmalloc for c-icap processes. Just add -lmtmalloc to CFLAGS and CXXFLAGS when configuring. This also can reduce memory lock contention on multi-core CPU boxes. This solution can also reduce the memory consumption problem for clamd.

  • {i} Clamd with custom databases (SecuriteInfo, etc.) uses 700 megabytes of RAM and above. Better in this case to use separate servers.

C-ICAP monitoring

To monitor some runtime statistics from C-ICAP, you can use solution, as described here with some additions and corrections.

You can use both CLI and web interface to monitor C-ICAP via built-in info service.

To use CLI, use this command (or add it as shell alias):

/usr/local/bin/c-icap-client -s "info?view=text" -i localhost -p 1344 -req use-any-url

or, as shell alias:

alias icap_stat='c-icap-client -s '\''info?view=text'\'' -i localhost -p 1344 -req use-any-url'

The result will looks as shown:

ICAP server:localhost, ip:, port:1344

Running Servers Statistics
Children number: 3
Free Servers: 27
Used Servers: 3
Started Processes: 5
Closed Processes: 2
Crashed Processes: 2
Closing Processes: 0

Child pids: 24689 15427 4947
Closing children pids:
Semaphores in use

Shared mem blocks in use
         sysv:kids-queue/30 13 kbs

General Statistics
REQUESTS : 44501
REQMODS : 39336
ALLOW 204 : 44245
BYTES IN : 25625 Kbs 486 bytes
BYTES OUT : 5679 Kbs 536 bytes
HTTP BYTES IN : 16232 Kbs 612 bytes
HTTP BYTES OUT : 212 Kbs 887 bytes
BODY BYTES IN : 2621 Kbs 532 bytes
BODY BYTES OUT : 192 Kbs 288 bytes

To get same info via web, just add this lines in squid.conf and reconfigure:

# ICAP info service. URL:
acl infoaccess dstdomain
icap_service service_info reqmod_precache 1 icap://localhost:1344/info
adaptation_service_set class_info service_info
adaptation_access class_info allow infoaccess
adaptation_access class_info deny all

Use url above to access stats page.

Here is also Munin plugins for C-ICAP monitoring (performance-related /runtime stats):

icap_stats icap_sem icap_mem


  • {i} When upgrading c-icap server, you also need (in most cases) to rebuild squidclamav to aviod possible API incompatibility.

  • {i} In case of c-icap permanently restarts, increase DebugLevel in c-icap.conf and check ServerLog first. Beware, DebugLevel 0 is production value, which can mask any problems during tune up.

c-icap/eCAP co-existance

To apply multiple adaptation services to the same transaction at the same vectoring point, one must use adaptation_service_chain. Adaptation order is often important from adaptation logic or performance point of view, but Squid supports any order of chained services. Squid adaptation chaining code does not even know the difference between ICAP and eCAP! For example, an adaptation_service_chain containing an ICAP service followed by an eCAP service, followed by another ICAP service is supported.

When you require both c-icap and eCAP in one Squid's instance, you must remember: order of adaptation service/chain definitions and adaptation_access ACL's is important. Adaptation logic defines in adaptation service default order of preference, in adaptation_access directives define which services or chains are able to be used for the transaction being considered. In some cases, adaptation actions chain can be mutually exclusive. So, be careful with adaptation configuration. Thoroughly test adaptation logic.

  • {i} Note: The simplest case is to chain adaptations with the same access scheme. When access scheme is different for chained adaptations, use adaptation_access in correct sequence to achieve required adaptation goals.

ConfigExamples/ContentAdaptation/C-ICAP (last edited 2017-04-02 16:33:34 by YuriVoinov)