Squid proxy :

Squid is a proxy server for caching and filtering web content . Squid proxy is used by various organisation and internet providers to reduce bandwidth and to increase response time .

Squid proxy service will cache the requested web-content and re-using it for the further request of the same content .



In the above diagram, squid proxy will cache the webcontent of krizna.com from the ISP during the first request and it will deliver the cached content for the further requests of krizna.com without requesting from ISP. This will reduce bandwidth and will increase response time as the content is delivered from the local network.

» Install squid proxy

» Restricting Access to specific web sites

» Restricting Access to specific keywords

» Restricting Access to specific ipaddress

» Allow Full access to specific ipaddress

» Changing squid proxy port number

» Restricting download size

» Configure squid as transparent proxy

» Install squid proxy on centos 6 :

In this article we can see how to install and configure squid proxy on centos 6 . After installing Centos 6 (Centos 6 installation step by step screenshots ) and configuring network (How to setup network in centos 6)

Update yum repositories and packages by typing the below command

[root@leela ~]# yum update

Step 1 » Install squid package and dependencies using the below command

[root@leela ~]# yum install squid

» By default squid configuration file “/etc/squid/squid.conf” will contains recommended minimum configuration and squid caching feature will work without making any changes . Recommended minimum configuration will looks like the below

# # Recommended minimum configuration: # acl manager proto cache_object acl localhost src 127.0.0.1/32 ::1 acl to_localhost dst 127.0.0.0/8 0.0.0.0/32 ::1 # Example rule allowing access from your local networks. # Adapt to list your (internal) IP networks from where browsing # should be allowed acl localnet src 10.0.0.0/8 # RFC1918 possible internal network acl localnet src 172.16.0.0/12 # RFC1918 possible internal network acl localnet src 192.168.0.0/16 # RFC1918 possible internal network acl localnet src fc00::/7 # RFC 4193 local private network range acl localnet src fe80::/10 # RFC 4291 link-local (directly plugged) machines acl SSL_ports port 443 acl Safe_ports port 80 # http acl Safe_ports port 21 # ftp acl Safe_ports port 443 # https acl Safe_ports port 70 # gopher acl Safe_ports port 210 # wais acl Safe_ports port 1025-65535 # unregistered ports acl Safe_ports port 280 # http-mgmt acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # # Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost http_access deny manager # Deny requests to certain unsafe ports http_access deny !Safe_ports # Deny CONNECT to other than secure SSL ports http_access deny CONNECT !SSL_ports # We strongly recommend the following be uncommented to protect innocent # web applications running on the proxy server who think the only # one who can access services on "localhost" is a local user #http_access deny to_localhost # # INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS # # Example rule allowing access from your local networks. # Adapt localnet in the ACL section to list your (internal) IP networks # from where browsing should be allowed http_access allow localnet http_access allow localhost # And finally deny all other access to this proxy http_access deny all # Squid normally listens to port 3128 http_port 3128 # We recommend you to use at least the following line. hierarchy_stoplist cgi-bin ? # Uncomment and adjust the following to add a disk cache directory. #cache_dir ufs /var/spool/squid 100 16 256 # Leave coredumps in the first cache dir coredump_dir /var/spool/squid # Add any of your own refresh_pattern entries above these. refresh_pattern ^ftp: 1440 20% 10080 refresh_pattern ^gopher: 1440 0% 1440 refresh_pattern -i (/cgi-bin/|?) 0 0% 0 refresh_pattern . 0 20% 4320 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 # # Recommended minimum configuration: # acl manager proto cache_object acl localhost src 127.0.0.1 / 32 :: 1 acl to_localhost dst 127.0.0.0 / 8 0.0.0.0 / 32 :: 1 # Example rule allowing access from your local networks. # Adapt to list your (internal) IP networks from where browsing # should be allowed acl localnet src 10.0.0.0 / 8 # RFC1918 possible internal network acl localnet src 172.16.0.0 / 12 # RFC1918 possible internal network acl localnet src 192.168.0.0 / 16 # RFC1918 possible internal network acl localnet src fc00 :: / 7 # RFC 4193 local private network range acl localnet src fe80 :: / 10 # RFC 4291 link-local (directly plugged) machines acl SSL_ports port 443 acl Safe_ports port 80 # http acl Safe_ports port 21 # ftp acl Safe_ports port 443 # https acl Safe_ports port 70 # gopher acl Safe_ports port 210 # wais acl Safe_ports port 1025 - 65535 # unregistered ports acl Safe_ports port 280 # http-mgmt acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # # Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost http_access deny manager # Deny requests to certain unsafe ports http_access deny ! Safe_ports # Deny CONNECT to other than secure SSL ports http_access deny CONNECT ! SSL_ports # We strongly recommend the following be uncommented to protect innocent # web applications running on the proxy server who think the only # one who can access services on "localhost" is a local user #http_access deny to_localhost # # INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS # # Example rule allowing access from your local networks. # Adapt localnet in the ACL section to list your (internal) IP networks # from where browsing should be allowed http_access allow localnet http_access allow localhost # And finally deny all other access to this proxy http_access deny all # Squid normally listens to port 3128 http _ port 3128 # We recommend you to use at least the following line. hierarchy_stoplist cgi - bin ? # Uncomment and adjust the following to add a disk cache directory. #cache_dir ufs /var/spool/squid 100 16 256 # Leave coredumps in the first cache dir coredump_dir / var / spool / squid # Add any of your own refresh_pattern entries above these. refresh_pattern ^ ftp : 1440 20 % 10080 refresh_pattern ^ gopher : 1440 0 % 1440 refresh_pattern - i ( / cgi - bin / | ? ) 0 0 % 0 refresh _ pattern . 0 20 % 4320

Step 2» Now start squid service

[root@leela ~]# service squid start

and type this below command to start squid service automatically while booting.

[root@leela ~]# chkconfig --levels 235 squid on

Step 3 » Setup your web browser to access Internet through proxy server on port 3128 .

IE : Tools » Internet options »Connections » LAN settings » Choose “Use a proxy server for your LAN” » Type your Proxy server ip (192.168.1.11 ) and port no 3128

Firefox : Options / Preferences » Advanced » Network » Settings » Choose “Manual proxy configuration ” » Type your Proxy server ip (192.168.1.11 ) and port no 3128

Step 4 » Browse some sites and check the access log file on proxy server

[root@leela ~]# cat /var/log/squid/access.log 1343759484.331 1828 192.168.1.15 TCP_MISS/200 7005 GET https://www.krizna.com/ - DIRECT/216.172.163.231 text/html 1343759484.645 265 192.168.1.15 TCP_MISS/304 477 GET http://platform.twitter.com/widgets.js - DIRECT/23.64.79.144 application/javascript 1343759484.681 309 192.168.1.15 TCP_MISS/304 839 GET http://apis.google.com/js/plusone.js - DIRECT/173.194.36.36 - 1343759485.005 178 192.168.1.15 TCP_MISS/200 495 GET http://www.google-analytics.com/__utm.gif? - DIRECT/74.125.236.142 image/gif 1343759502.763 20292 192.168.1.15 TCP_MISS/200 1898 CONNECT s-static.ak.facebook.com:443 - DIRECT/23.64.66.110 - 1343759502.765 20292 192.168.1.15 TCP_MISS/200 1898 CONNECT s-static.ak.facebook.com:443 - DIRECT/23.64.66.110 - 1343759723.040 240567 192.168.1.15 TCP_MISS/200 2904 CONNECT plusone.google.com:443 - DIRECT/173.194.36.41 - 1343759933.642 958 192.168.1.15 TCP_MISS/304 394 GET https://www.krizna.com/ - DIRECT/216.172.163.231 - 1343759934.518 459 192.168.1.15 TCP_MISS/200 495 GET http://www.google-analytics.com/__utm.gif? - DIRECT/173.194.36.8 image/gif 1343759943.131 2186 192.168.1.15 TCP_MISS/200 8343 CONNECT www.facebook.com:443 - DIRECT/66.220.146.94 - 1343759943.266 10616 192.168.1.15 TCP_MISS/200 2790 CONNECT apis.google.com:443 - DIRECT/74.125.236.128 - 1343759943.267 10615 192.168.1.15 TCP_MISS/200 2228 CONNECT ssl.gstatic.com:443 - DIRECT/74.125.236.143 - 1343759950.875 2596 192.168.1.15 TCP_MISS/200 32219 CONNECT www.facebook.com:443 - DIRECT/66.220.146.94 - 1343759961.388 20443 192.168.1.15 TCP_MISS/200 1911 CONNECT s-static.ak.fbcdn.net:443 - DIRECT/23.64.77.177 - 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 [ root @ leela ~ ] # cat /var/log/squid/access.log 1343759484.331 1828 192.168.1.15 TCP_MISS / 200 7005 GET https : //www.krizna.com/ - DIRECT/216.172.163.231 text/html 1343759484.645 265 192.168.1.15 TCP_MISS / 304 477 GET http : //platform.twitter.com/widgets.js - DIRECT/23.64.79.144 application/javascript 1343759484.681 309 192.168.1.15 TCP_MISS / 304 839 GET http : //apis.google.com/js/plusone.js - DIRECT/173.194.36.36 - 1343759485.005 178 192.168.1.15 TCP_MISS / 200 495 GET http : //www.google-analytics.com/__utm.gif? - DIRECT/74.125.236.142 image/gif 1343759502.763 20292 192.168.1.15 TCP_MISS / 200 1898 CONNECT s - static . ak . facebook . com : 443 - DIRECT / 23.64.66.110 - 1343759502.765 20292 192.168.1.15 TCP_MISS / 200 1898 CONNECT s - static . ak . facebook . com : 443 - DIRECT / 23.64.66.110 - 1343759723.040 240567 192.168.1.15 TCP_MISS / 200 2904 CONNECT plusone . google . com : 443 - DIRECT / 173.194.36.41 - 1343759933.642 958 192.168.1.15 TCP_MISS / 304 394 GET https : //www.krizna.com/ - DIRECT/216.172.163.231 - 1343759934.518 459 192.168.1.15 TCP_MISS / 200 495 GET http : //www.google-analytics.com/__utm.gif? - DIRECT/173.194.36.8 image/gif 1343759943.131 2186 192.168.1.15 TCP_MISS / 200 8343 CONNECT www . facebook . com : 443 - DIRECT / 66.220.146.94 - 1343759943.266 10616 192.168.1.15 TCP_MISS / 200 2790 CONNECT apis . google . com : 443 - DIRECT / 74.125.236.128 - 1343759943.267 10615 192.168.1.15 TCP_MISS / 200 2228 CONNECT ssl . gstatic . com : 443 - DIRECT / 74.125.236.143 - 1343759950.875 2596 192.168.1.15 TCP_MISS / 200 32219 CONNECT www . facebook . com : 443 - DIRECT / 66.220.146.94 - 1343759961.388 20443 192.168.1.15 TCP_MISS / 200 1911 CONNECT s - static . ak . fbcdn . net : 443 - DIRECT / 23.64.77.177 -

Troubleshooting :

If you not able to browse using proxy settings , Disable the firewall ( iptables ) and selinux service on your squid proxy server .

Disable firewall ( Iptables ) »

[root@leela ~]# service iptables stop

[root@leela ~]# chkconfig iptables off

Disable Selinux » open the file /etc/selinux/config and find the line

SELINUX=enforcing

and replace with

SELINUX=disabled

now reboot the server and try step 4.

Configure squid proxy as web filter:

You can restrict user access to particular websites or keywords using access control lists (ACLs) .

» Restricting Access to specific web sites :

For example , we can see how to block facebook.com and gmail.com .

Step 1 » create a file ( /etc/squid/blockedsites.squid ) and add the site names one per line.

[root@leela ~]# cat /etc/squid/blockedsites.squid

#blocked sites

www.facebook.com

www.gmail.com

Step 2 » Open the /etc/squid/squid.conf and create a new acl ” blocksites” and acl type “dstdomain” in the acl section like the below .

acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # ACL blocksites acl blocksites dstdomain "/etc/squid/blockedsites.squid" 1 2 3 4 5 6 acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # ACL blocksites acl blocksites dstdomain "/etc/squid/blockedsites.squid"

and add the following line “http_access deny blocksites” to http_section to deny the access to the acl “blocksites” .

# Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blocksites ACL http_access deny blocksites 1 2 3 4 5 6 # Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blocksites ACL http_access deny blocksites

Step 3 » Now restart squid service

[root@leela ~]# service squid restart

Step 4 » Try to access facebook.com in your browser . you could see the blocked page as below .

and check the log file you can see the facebook request is denied .

[root@leela ~]# tail -f /var/log/squid/access.log ................................................................................. 1343820985.542 1 192.168.1.15 TCP_DENIED/403 4255 GET http://www.facebook.com/ - NONE/- text/html 1343820986.998 1411 192.168.1.15 TCP_MISS/200 13152 GET http://www.squid-cache.org/Artwork/SN.png - DIRECT/198.186.193.234 image/png 1343820987.004 0 192.168.1.15 TCP_DENIED/403 4178 GET http://www.facebook.com/favicon.ico - NONE/- text/html 1 2 3 4 5 [ root @ leela ~ ] # tail -f /var/log/squid/access.log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343820985.542 1 192.168.1.15 TCP_DENIED / 403 4255 GET http : //www.facebook.com/ - NONE/- text/html 1343820986.998 1411 192.168.1.15 TCP_MISS / 200 13152 GET http : //www.squid-cache.org/Artwork/SN.png - DIRECT/198.186.193.234 image/png 1343820987.004 0 192.168.1.15 TCP_DENIED / 403 4178 GET http : //www.facebook.com/favicon.ico - NONE/- text/html

» Restricting Access to specific keywords :

Step 1 » create a file ( /etc/squid/blockkeywords.squid ) and add the keywords one per line.

[root@leela ~]# cat /etc/squid/blockkeywords.squid

#blocked keywords

sex

porn

xxx



Step 2 » Open the /etc/squid/squid.conf and create a new acl “blockkeywords” and acl type “url_regex” in the acl section

acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # ACL blocksites acl blocksites dstdomain "/etc/squid/blockedsites.squid" # ACL blockkeywords acl blockkeywords url_regex -i "/etc/squid/blockkeywords.squid" 1 2 3 4 5 6 7 8 acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # ACL blocksites acl blocksites dstdomain "/etc/squid/blockedsites.squid" # ACL blockkeywords acl blockkeywords url_regex - i "/etc/squid/blockkeywords.squid"

and add the following line “http_access deny blockkeywords” to http_section to deny the access to the acl “blockkeywords” .

# Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blocksites ACL http_access deny blocksites # Deny access to blockkeywords ACL http_access deny blockkeywords 1 2 3 4 5 6 7 8 # Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blocksites ACL http_access deny blocksites # Deny access to blockkeywords ACL http_access deny blockkeywords

» Restricting Access to specific Ipaddress :

Step 1 » create a file ( /etc/squid/blockip.squid ) and add the ip adresses one per line.

[root@leela ~]# cat /etc/squid/blockip.squid

#blocked ips

192.168.1.20

192.168.1.21



Step 2 » Open the /etc/squid/squid.conf and create a new acl “blockip” and acl type “src” in the acl section

acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # ACL blocksites acl blocksites dstdomain "/etc/squid/blockedsites.squid" # ACL blockkeywords acl blockkeywords url_regex -i "/etc/squid/blockkeywords.squid" # ACL blockip acl blockip src "/etc/squid/blockip.squid" 1 2 3 4 5 6 7 8 9 10 acl Safe_ports port 488 # gss-http acl Safe_ports port 591 # filemaker acl Safe_ports port 777 # multiling http acl CONNECT method CONNECT # ACL blocksites acl blocksites dstdomain "/etc/squid/blockedsites.squid" # ACL blockkeywords acl blockkeywords url_regex - i "/etc/squid/blockkeywords.squid" # ACL blockip acl blockip src "/etc/squid/blockip.squid"

and add the following line “http_access deny blockip” to http_section to deny the access to the acl “blockip” .

# Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blockip ACL http_access deny blockip # Deny access to blocksites ACL http_access deny blocksites # Deny access to blockkeywords ACL http_access deny blockkeywords 1 2 3 4 5 6 7 8 9 10 # Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blockip ACL http_access deny blockip # Deny access to blocksites ACL http_access deny blocksites # Deny access to blockkeywords ACL http_access deny blockkeywords

» Allow Full access to specific Ipaddress :

You can allow specific ip address to gain full access without blocking sites and keywords . just create a file “/etc/squid/allowip.squid ” and add the ip address one per line and create an acl “allowip” and acl type “src” in the acl section

# ACL allowip acl allowip src "/etc/squid/allowip.squid" 1 2 # ACL allowip acl allowip src "/etc/squid/allowip.squid"

and add the “allowip” in the http_access as below

# Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blockip ACL http_access deny blockip # Deny access to blocksites ACL http_access deny blocksites !allowip # Deny access to blockkeywords ACL http_access deny blockkeywords !allowip 1 2 3 4 5 6 7 8 9 10 # Recommended minimum Access Permission configuration: # # Only allow cachemgr access from localhost http_access allow manager localhost # Deny access to blockip ACL http_access deny blockip # Deny access to blocksites ACL http_access deny blocksites ! allowip # Deny access to blockkeywords ACL http_access deny blockkeywords ! allowip

» Changing squid proxy port number:

You can change squid proxy port number , by default it uses 3128 port .

Just find the below line in “/etc/squid/squid.conf”

http_port 3128

and replace with

http_port 8000 # whatever port number you want

» Restricting Download size :

You can restrict download file size using reply_body_max_size .

Add the below line at the bottom of the http_access section

#Restrict download size

reply_body_max_size 10 MB all

or

#Restrict download size

reply_body_max_size 10 MB !allowip

» Configuring Squid as Transparent Proxy :

You can configure squid as transparent proxy .

Step 1 » just find the below line

# Squid normally listens to port 3128

http_port 3128

and replace with

# Squid normally listens to port 3128

http_port 3128 intercept

Step 2 » Create a file “/root/squidfw.sh” and add the following firewall script ( modify as per your interface configuration ).

#!/bin/sh # squid server IP SQUID_SERVER="192.168.1.11" # Interface connected to Internet INTERNET="eth0" # Interface connected to LAN LAN_IN="eth1" # Squid port SQUID_PORT="3128" # DO NOT MODIFY BELOW # Clean old firewall iptables -F iptables -X iptables -t nat -F iptables -t nat -X iptables -t mangle -F iptables -t mangle -X # Load IPTABLES modules for NAT and IP conntrack support modprobe ip_conntrack modprobe ip_conntrack_ftp # For win xp ftp client #modprobe ip_nat_ftp echo 1 > /proc/sys/net/ipv4/ip_forward # Setting default filter policy iptables -P INPUT DROP iptables -P OUTPUT ACCEPT # Unlimited access to loop back iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT # Allow UDP, DNS and Passive FTP iptables -A INPUT -i $INTERNET -m state --state ESTABLISHED,RELATED -j ACCEPT # set this system as a router for Rest of LAN iptables --table nat --append POSTROUTING --out-interface $INTERNET -j MASQUERADE iptables --append FORWARD --in-interface $LAN_IN -j ACCEPT # unlimited access to LAN iptables -A INPUT -i $LAN_IN -j ACCEPT iptables -A OUTPUT -o $LAN_IN -j ACCEPT # DNAT port 80 request comming from LAN systems to squid 3128 ($SQUID_PORT) aka transparent proxy iptables -t nat -A PREROUTING -i $LAN_IN -p tcp --dport 80 -j DNAT --to $SQUID_SERVER:$SQUID_PORT # if it is same system iptables -t nat -A PREROUTING -i $INTERNET -p tcp --dport 80 -j REDIRECT --to-port $SQUID_PORT # DROP everything and Log it iptables -A INPUT -j LOG iptables -A INPUT -j DROP 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 #!/bin/sh # squid server IP SQUID_SERVER = "192.168.1.11" # Interface connected to Internet INTERNET = "eth0" # Interface connected to LAN LAN_IN = "eth1" # Squid port SQUID_PORT = "3128" # DO NOT MODIFY BELOW # Clean old firewall iptables - F iptables - X iptables - t nat - F iptables - t nat - X iptables - t mangle - F iptables - t mangle - X # Load IPTABLES modules for NAT and IP conntrack support modprobe ip_conntrack modprobe ip_conntrack_ftp # For win xp ftp client #modprobe ip_nat_ftp echo 1 > / proc / sys / net / ipv4 / ip_forward # Setting default filter policy iptables - P INPUT DROP iptables - P OUTPUT ACCEPT # Unlimited access to loop back iptables - A INPUT - i lo - j ACCEPT iptables - A OUTPUT - o lo - j ACCEPT # Allow UDP, DNS and Passive FTP iptables - A INPUT - i $ INTERNET - m state -- state ESTABLISHED , RELATED - j ACCEPT # set this system as a router for Rest of LAN iptables -- table nat -- append POSTROUTING -- out - interface $ INTERNET - j MASQUERADE iptables -- append FORWARD -- in - interface $ LAN_IN - j ACCEPT # unlimited access to LAN iptables - A INPUT - i $ LAN_IN - j ACCEPT iptables - A OUTPUT - o $ LAN_IN - j ACCEPT # DNAT port 80 request comming from LAN systems to squid 3128 ($SQUID_PORT) aka transparent proxy iptables - t nat - A PREROUTING - i $ LAN_IN - p tcp -- dport 80 - j DNAT -- to $ SQUID_SERVER : $ SQUID_PORT # if it is same system iptables - t nat - A PREROUTING - i $ INTERNET - p tcp -- dport 80 - j REDIRECT -- to - port $ SQUID_PORT # DROP everything and Log it iptables - A INPUT - j LOG iptables - A INPUT - j DROP

Step 3 » just run the script

[root@leela ~]# sh /root/squidfw.sh

and add the below line to “/etc/rc.local” to run the script during startup

sh /root/squidfw.sh

Step 4 (Updated) » Change default gateway ip to squid server ip on the user machines .

Now users can access Internet without setting proxy in the browser settings.

That’s it , hope this article will help you to learn little things about configuring squid proxy on centos 6.