Linux: Setup a transparent proxy with Squid in three easy steps

by LinuxTitli · 236 comments

Y'day I got a chance to play with Squid and iptables. My job was simple : Setup Squid proxy as a transparent server.

Main benefit of setting transparent proxy is you do not have to setup up individual browsers to work with proxies.

My Setup:

i) System: HP dual Xeon CPU system with 8 GB RAM (good for squid).
ii) Eth0: IP:192.168.1.1
iii) Eth1: IP: 192.168.2.1 (192.168.2.0/24 network (around 150 windows XP systems))
iv) OS: Red Hat Enterprise Linux 4.0 (Following instruction should work with Debian and all other Linux distros)

Eth0 connected to internet and eth1 connected to local lan i.e. system act as router.

Server Configuration

• Step #1 : Squid configuration so that it will act as a transparent proxy
• Step #2 : Iptables configuration
• a) Configure system as router
• b) Forward all http requests to 3128 (DNAT)
• Step #3: Run scripts and start squid service

First, Squid server installed (use up2date squid) and configured by adding following directives to file:
# vi /etc/squid/squid.conf

Modify or add following squid directives:
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on
acl lan src 192.168.1.1 192.168.2.0/24
http_access allow localhost
http_access allow lan

Where,

• httpd_accel_host virtual: Squid as an httpd accelerator
• httpd_accel_port 80: 80 is port you want to act as a proxy
• httpd_accel_with_proxy on: Squid act as both a local httpd accelerator and as a proxy.
• httpd_accel_uses_host_header on: Header is turned on which is the hostname from the URL.
• acl lan src 192.168.1.1 192.168.2.0/24: Access control list, only allow LAN computers to use squid
• http_access allow localhost: Squid access to LAN and localhost ACL only
• http_access allow lan: -- same as above --

Here is the complete listing of squid.conf for your reference (grep will remove all comments and sed will remove all empty lines, thanks to David Klein for quick hint ):
# grep -v "^#" /etc/squid/squid.conf | sed -e '/^$/d'

OR, try out sed (thanks to kotnik for small sed trick)
# cat /etc/squid/squid.conf | sed '/ *#/d; /^ *$/d'

Output:
hierarchy_stoplist cgi-bin ?
acl QUERY urlpath_regex cgi-bin \?
no_cache deny QUERY
hosts_file /etc/hosts
refresh_pattern ^ftp: 1440 20% 10080
refresh_pattern ^gopher: 1440 0% 1440
refresh_pattern . 0 20% 4320
acl all src 0.0.0.0/0.0.0.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl to_localhost dst 127.0.0.0/8
acl purge method PURGE
acl CONNECT method CONNECT
cache_mem 1024 MB
http_access allow manager localhost
http_access deny manager
http_access allow purge localhost
http_access deny purge
http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
acl lan src 192.168.1.1 192.168.2.0/24
http_access allow localhost
http_access allow lan
http_access deny all
http_reply_access allow all
icp_access allow all
visible_hostname myclient.hostname.com
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on
coredump_dir /var/spool/squid

Iptables configuration

Next, I had added following rules to forward all http requests (coming to port 80) to the Squid server port 3128 :
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j DNAT --to 192.168.1.1:3128
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128

Here is complete shell script. Script first configure Linux system as router and forwards all http request to port 3128 (Download the fw.proxy shell script):
#!/bin/sh
# squid server IP
SQUID_SERVER="192.168.1.1"
# 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

Save shell script. Execute script so that system will act as a router and forward the ports:
# chmod +x /etc/fw.proxy
# /etc/fw.proxy
# service iptables save
# chkconfig iptables on

Start or Restart the squid:
# /etc/init.d/squid restart
# chkconfig squid on

Desktop / Client computer configuration

Point all desktop clients to your eth1 IP address (192.168.2.1) as Router/Gateway (use DHCP to distribute this information). You do not have to setup up individual browsers to work with proxies.

How do I test my squid proxy is working correctly?

See access log file /var/log/squid/access.log:
# tail -f /var/log/squid/access.log

Above command will monitor all incoming request and log them to /var/log/squid/access_log file. Now if somebody accessing a website through browser, squid will log information.

Problems and solutions

(a) Windows XP FTP Client

All Desktop client FTP session request ended with an error:
Illegal PORT command.

I had loaded the ip_nat_ftp kernel module. Just type the following command press Enter and voila!
# modprobe ip_nat_ftp

Please note that modprobe command is already added to a shell script (above).

(b) Port 443 redirection

I had block out all connection request from our router settings except for our proxy (192.168.1.1) server. So all ports including 443 (https/ssl) request denied. You cannot redirect port 443, from debian mailing list, "Long answer: SSL is specifically designed to prevent "man in the middle" attacks, and setting up squid in such a way would be the same as such a "man in the middle" attack. You might be able to successfully achive this, but not without breaking the encryption and certification that is the point behind SSL".

Therefore, I had quickly reopen port 443 (router firewall) for all my LAN computers and problem was solved.

(c) Squid Proxy authentication in a transparent mode

You cannot use Squid authentication with a transparently intercepting proxy.

Further reading:

• How do I use Iptables connection tracking feature?
• How do I build a Simple Linux Firewall for DSL/Dial-up connection?
• Update: Forum topic discussion: Setting up a transparent proxy with Squid peering to ISP squid server
• Squid, a user's guide
• Squid FAQ
• Transparent Proxy with Linux and Squid mini-HOWTO

Updated for accuracy.

Samba-HOWTO-Collection on Red Hat Linux

# This is the main Samba configuration file. You should read the
# smb.conf(5) manual page in order to understand the options listed
# here. Samba has a huge number of configurable options (perhaps too
# many!) most of which are not shown in this example
#
# For a step to step guide on installing, configuring and using samba,
# read the Samba-HOWTO-Collection. This may be obtained from:
# http://www.samba.org/samba/docs/Samba-HOWTO-Collection.pdf
#
# Many working examples of smb.conf files can be found in the
# Samba-Guide which is generated daily and can be downloaded from:
# http://www.samba.org/samba/docs/Samba-Guide.pdf
#
# Any line which starts with a ; (semi-colon) or a # (hash)
# is a comment and is ignored. In this example we will use a #
# for commentry and a ; for parts of the config file that you
# may wish to enable
#
# NOTE: Whenever you modify this file you should run the command "testparm"
# to check that you have not made any basic syntactic errors.
#
#---------------
# SELINUX NOTES:
#
# If you want to use the useradd/groupadd family of binaries please run:
# setsebool -P samba_domain_controller on
#
# If you want to share home directories via samba please run:
# setsebool -P samba_enable_home_dirs on
#
# If you create a new directory you want to share you should mark it as
# "samba-share_t" so that selinux will let you write into it.
# Make sure not to do that on system directories as they may already have
# been marked with othe SELinux labels.
#
# Use ls -ldZ /path to see which context a directory has
#
# Set labels only on directories you created!
# To set a label use the following: chcon -t samba_share_t /path
#
# If you need to share a system created directory you can use one of the
# following (read-only/read-write):
# setsebool -P samba_export_all_ro on
# or
# setsebool -P samba_export_all_rw on
#
# If you want to run scripts (preexec/root prexec/print command/...) please
# put them into the /var/lib/samba/scripts directory so that smbd will be
# allowed to run them.
# Make sure you COPY them and not MOVE them so that the right SELinux context
# is applied, to check all is ok use restorecon -R -v /var/lib/samba/scripts
#
#--------------
#
#======================= Global Settings =====================================

[global]
#--authconfig--start-line--

# Generated by authconfig on 2010/09/02 18:04:52
# DO NOT EDIT THIS SECTION (delimited by --start-line--/--end-line--)
# Any modification may be deleted or altered by authconfig in future

workgroup = workgroup
# password server = hamdardisb
#security = user
#idmap uid = 16777216-33554431
#idmap gid = 16777216-33554431
#template shell = /bin/false
#winbind use default domain = false
#winbind offline logon = false
netbios name=installer
security = share

#--authconfig--end-line--

# ----------------------- Network Related Options -------------------------
#
# workgroup = NT-Domain-Name or Workgroup-Name, eg: MIDEARTH
#
# server string is the equivalent of the NT Description field
#
# netbios name can be used to specify a server name not tied to the hostname
#
# Interfaces lets you configure Samba to use multiple interfaces
# If you have multiple network interfaces then you can list the ones
# you want to listen on (never omit localhost)
#
# Hosts Allow/Hosts Deny lets you restrict who can connect, and you can
# specifiy it as a per share option as well
#
; workgroup = MYGROUP
server string = Samba Server Version %v

; netbios name = MYSERVER

; interfaces = lo eth0 192.168.12.2/24 192.168.13.2/24
; hosts allow = 127. 192.168.12. 192.168.13.

# --------------------------- Logging Options -----------------------------
#
# Log File let you specify where to put logs and how to split them up.
#
# Max Log Size let you specify the max size log files should reach

# logs split per machine
; log file = /var/log/samba/%m.log
# max 50KB per log file, then rotate
; max log size = 50

# ----------------------- Standalone Server Options ------------------------
#
# Security can be set to user, share(deprecated) or server(deprecated)
#
# Backend to store user information in. New installations should
# use either tdbsam or ldapsam. smbpasswd is available for backwards
# compatibility. tdbsam requires no further configuration.

; security = share
passdb backend = tdbsam


# ----------------------- Domain Members Options ------------------------
#
# Security must be set to domain or ads
#
# Use the realm option only with security = ads
# Specifies the Active Directory realm the host is part of
#
# Backend to store user information in. New installations should
# use either tdbsam or ldapsam. smbpasswd is available for backwards
# compatibility. tdbsam requires no further configuration.
#
# Use password server option only with security = server or if you can't
# use the DNS to locate Domain Controllers
# The argument list may include:
# password server = My_PDC_Name [My_BDC_Name] [My_Next_BDC_Name]
# or to auto-locate the domain controller/s
# password server = *


; security = domain
; passdb backend = tdbsam
; realm = MY_REALM

; password server =

# ----------------------- Domain Controller Options ------------------------
#
# Security must be set to user for domain controllers
#
# Backend to store user information in. New installations should
# use either tdbsam or ldapsam. smbpasswd is available for backwards
# compatibility. tdbsam requires no further configuration.
#
# Domain Master specifies Samba to be the Domain Master Browser. This
# allows Samba to collate browse lists between subnets. Don't use this
# if you already have a Windows NT domain controller doing this job
#
# Domain Logons let Samba be a domain logon server for Windows workstations.
#
# Logon Scrpit let yuou specify a script to be run at login time on the client
# You need to provide it in a share called NETLOGON
#
# Logon Path let you specify where user profiles are stored (UNC path)
#
# Various scripts can be used on a domain controller or stand-alone
# machine to add or delete corresponding unix accounts
#
; security = user
; passdb backend = tdbsam

; domain master = yes
; domain logons = yes

# the login script name depends on the machine name
; logon script = %m.bat
# the login script name depends on the unix user used
; logon script = %u.bat
; logon path = \\%L\Profiles\%u
# disables profiles support by specifing an empty path
; logon path =

; add user script = /usr/sbin/useradd "%u" -n -g users
; add group script = /usr/sbin/groupadd "%g"
; add machine script = /usr/sbin/useradd -n -c "Workstation (%u)" -M -d /nohome -s /bin/false "%u"
; delete user script = /usr/sbin/userdel "%u"
; delete user from group script = /usr/sbin/userdel "%u" "%g"
; delete group script = /usr/sbin/groupdel "%g"


# ----------------------- Browser Control Options ----------------------------
#
# set local master to no if you don't want Samba to become a master
# browser on your network. Otherwise the normal election rules apply
#
# OS Level determines the precedence of this server in master browser
# elections. The default value should be reasonable
#
# Preferred Master causes Samba to force a local browser election on startup
# and gives it a slightly higher chance of winning the election
; local master = no
; os level = 33
; preferred master = yes

#----------------------------- Name Resolution -------------------------------
# Windows Internet Name Serving Support Section:
# Note: Samba can be either a WINS Server, or a WINS Client, but NOT both
#
# - WINS Support: Tells the NMBD component of Samba to enable it's WINS Server
#
# - WINS Server: Tells the NMBD components of Samba to be a WINS Client
#
# - WINS Proxy: Tells Samba to answer name resolution queries on
# behalf of a non WINS capable client, for this to work there must be
# at least one WINS Server on the network. The default is NO.
#
# DNS Proxy - tells Samba whether or not to try to resolve NetBIOS names
# via DNS nslookups.

; wins support = yes
; wins server = w.x.y.z
; wins proxy = no

; dns proxy = no

# --------------------------- Printing Options -----------------------------
#
# Load Printers let you load automatically the list of printers rather
# than setting them up individually
#
# Cups Options let you pass the cups libs custom options, setting it to raw
# for example will let you use drivers on your Windows clients
#
# Printcap Name let you specify an alternative printcap file
#
# You can choose a non default printing system using the Printing option

load printers = yes
cups options = raw

; printcap name = /etc/printcap
#obtain list of printers automatically on SystemV
; printcap name = lpstat
; printing = cups

# --------------------------- Filesystem Options ---------------------------
#
# The following options can be uncommented if the filesystem supports
# Extended Attributes and they are enabled (usually by the mount option
# user_xattr). Thess options will let the admin store the DOS attributes
# in an EA and make samba not mess with the permission bits.
#
# Note: these options can also be set just per share, setting them in global
# makes them the default for all shares

; map archive = no
; map hidden = no
; map read only = no
; map system = no
; store dos attributes = yes


#============================ Share Definitions ==============================

[homes]
comment = Home Directories
browseable = no
writable = yes
; valid users = %S
; valid users = MYDOMAIN\%S

[printers]
comment = All Printers
path = /var/spool/samba
browseable = no
vss ok = yes
writable = no
printable = yes

# Un-comment the following and create the netlogon directory for Domain Logons
; [netlogon]
; comment = Network Logon Service
; path = /var/lib/samba/netlogon
; guest ok = yes
; writable = no
; share modes = no


# Un-comment the following to provide a specific roving profile share
# the default is to use the user's home directory
; [Profiles]
; path = /var/lib/samba/profiles
; browseable = no
; vss ok = yes

[vss]
comment = control your source
path = /usr/local/vss
available = yes
valid users = vss
read only = no
browseable = yes
writable = yes
#force user = nobody
#force group = nobody
#read only = No
#guest ok = Yes


# A publicly accessible directory, but read only, except for people in
# the "staff" group
; [public]
; comment = Public Stuff
; path = /home/samba
; public = yes
; writable = yes
; printable = no
; write list = +staff