Chapter 7. Configuring the Squid caching proxy server

Squid is a proxy server that caches content to reduce bandwidth and load web pages more quickly. This chapter describes how to set up Squid as a proxy for the HTTP, HTTPS, and FTP protocol, as well as authentication and restricting access.

7.1. Setting up Squid as a caching proxy without authentication

This section describes a basic configuration of Squid as a caching proxy without authentication. The procedure limits access to the proxy based on IP ranges.

Prerequisites

  • The procedure assumes that the /etc/squid/squid.conf file is as provided by the squid package. If you edited this file before, remove the file and reinstall the package.

Procedure

  1. Install the squid package:

    # yum install squid
  2. Edit the /etc/squid/squid.conf file:

    1. Adapt the localnet access control lists (ACL) to match the IP ranges that should be allowed to use the proxy:

      acl localnet src 192.0.2.0/24
      acl localnet 2001:db8:1::/64

      By default, the /etc/squid/squid.conf file contains the http_access allow localnet rule that allows using the proxy from all IP ranges specified in localnet ACLs. Note that you must specify all localnet ACLs before the http_access allow localnet rule.

      Important

      Remove all existing acl localnet entries that do not match your environment.

    2. The following ACL exists in the default configuration and defines 443 as a port that uses the HTTPS protocol:

      acl SSL_ports port 443

      If users should be able to use the HTTPS protocol also on other ports, add an ACL for each of these port:

      acl SSL_ports port port_number
    3. Update the list of acl Safe_ports rules to configure to which ports Squid can establish a connection. For example, to configure that clients using the proxy can only access resources on port 21 (FTP), 80 (HTTP), and 443 (HTTPS), keep only the following acl Safe_ports statements in the configuration:

      acl Safe_ports port 21
      acl Safe_ports port 80
      acl Safe_ports port 443

      By default, the configuration contains the http_access deny !Safe_ports rule that defines access denial to ports that are not defined in Safe_ports ACLs.

    4. Configure the cache type, the path to the cache directory, the cache size, and further cache type-specific settings in the cache_dir parameter:

      cache_dir ufs /var/spool/squid 10000 16 256

      With these settings:

      • Squid uses the ufs cache type.
      • Squid stores its cache in the /var/spool/squid/ directory.
      • The cache grows up to 10000 MB.
      • Squid creates 16 level-1 sub-directories in the /var/spool/squid/ directory.
      • Squid creates 256 sub-directories in each level-1 directory.

        If you do not set a cache_dir directive, Squid stores the cache in memory.

  3. If you set a different cache directory than /var/spool/squid/ in the cache_dir parameter:

    1. Create the cache directory:

      # mkdir -p path_to_cache_directory
    2. Configure the permissions for the cache directory:

      # chown squid:squid path_to_cache_directory
    3. If you run SELinux in enforcing mode, set the squid_cache_t context for the cache directory:

      # semanage fcontext -a -t squid_cache_t "path_to_cache_directory(/.*)?"
      # restorecon -Rv path_to_cache_directory

      If the semanage utility is not available on your system, install the policycoreutils-python-utils package.

  4. Open the 3128 port in the firewall:

    # firewall-cmd --permanent --add-port=3128/tcp
    # firewall-cmd --reload
  5. Enable and start the squid service:

    # systemctl enable --now squid

Verification steps

To verify that the proxy works correctly, download a web page using the curl utility:

# curl -O -L "https://www.redhat.com/index.html" -x "proxy.example.com:3128"

If curl does not display any error and the index.html file was downloaded to the current directory, the proxy works.

7.2. Setting up Squid as a caching proxy with LDAP authentication

This section describes a basic configuration of Squid as a caching proxy that uses LDAP to authenticate users. The procedure configures that only authenticated users can use the proxy.

Prerequisites

  • The procedure assumes that the /etc/squid/squid.conf file is as provided by the squid package. If you edited this file before, remove the file and reinstall the package.
  • An service user, such as uid=proxy_user,cn=users,cn=accounts,dc=example,dc=com exists in the LDAP directory. Squid uses this account only to search for the authenticating user. If the authenticating user exists, Squid binds as this user to the directory to verify the authentication.

Procedure

  1. Install the squid package:

    # yum install squid
  2. Edit the /etc/squid/squid.conf file:

    1. To configure the basic_ldap_auth helper utility, add the following configuration entry to the top of /etc/squid/squid.conf:

      auth_param basic program /usr/lib64/squid/basic_ldap_auth -b "cn=users,cn=accounts,dc=example,dc=com" -D "uid=proxy_user,cn=users,cn=accounts,dc=example,dc=com" -W /etc/squid/ldap_password -f "(&(objectClass=person)(uid=%s))" -ZZ -H ldap://ldap_server.example.com:389

      The following describes the parameters passed to the basic_ldap_auth helper utility in the example above:

      • -B base_DN sets the LDAP search base.
      • -D proxy_service_user_DN sets the distinguished name (DN) of the account Squid uses to search for the authenticating user in the directory.
      • -W path_to_password_file sets the path to the file that contains the password of the proxy service user. Using a password file prevents that the password is visible in the operating system’s process list.
      • -f LDAP_filter specifies the LDAP search filter. Squid replaces the %s variable with the user name provided by the authenticating user.

        The (&(objectClass=person)(uid=%s)) filter in the example defines that the user name must match the value set in the uid attribute and that the directory entry contains the person object class.

      • -ZZ enforces a TLS-encrypted connection over the LDAP protocol using the STARTTLS command. Omit the -ZZ in the following situations:

        • The LDAP server does not support encrypted connections.
        • The port specified in the URL uses the LDAPS protocol.
      • The -H LDAP_URL parameter specifies the protocol, the host name or IP address, and the port of the LDAP server in URL format.
    2. Add the following ACL and rule to configure that Squid allows only authenticated users to use the proxy:

      acl ldap-auth proxy_auth REQUIRED
      http_access allow ldap-auth
      Important

      Specify these settings before the http_access deny all rule.

    3. Remove the following rule to disable bypassing the proxy authentication from IP ranges specified in localnet ACLs:

      http_access allow localnet
    4. The following ACL exists in the default configuration and defines 443 as a port that uses the HTTPS protocol:

      acl SSL_ports port 443

      If users should be able to use the HTTPS protocol also on other ports, add an ACL for each of these port:

      acl SSL_ports port port_number
    5. Update the list of acl Safe_ports rules to configure to which ports Squid can establish a connection. For example, to configure that clients using the proxy can only access resources on port 21 (FTP), 80 (HTTP), and 443 (HTTPS), keep only the following acl Safe_ports statements in the configuration:

      acl Safe_ports port 21
      acl Safe_ports port 80
      acl Safe_ports port 443

      By default, the configuration contains the http_access deny !Safe_ports rule that defines access denial to ports that are not defined in Safe_ports ACLs.

    6. Configure the cache type, the path to the cache directory, the cache size, and further cache type-specific settings in the cache_dir parameter:

      cache_dir ufs /var/spool/squid 10000 16 256

      With these settings:

      • Squid uses the ufs cache type.
      • Squid stores its cache in the /var/spool/squid/ directory.
      • The cache grows up to 10000 MB.
      • Squid creates 16 level-1 sub-directories in the /var/spool/squid/ directory.
      • Squid creates 256 sub-directories in each level-1 directory.

        If you do not set a cache_dir directive, Squid stores the cache in memory.

  3. If you set a different cache directory than /var/spool/squid/ in the cache_dir parameter:

    1. Create the cache directory:

      # mkdir -p path_to_cache_directory
    2. Configure the permissions for the cache directory:

      # chown squid:squid path_to_cache_directory
    3. If you run SELinux in enforcing mode, set the squid_cache_t context for the cache directory:

      # semanage fcontext -a -t squid_cache_t "path_to_cache_directory(/.*)?"
      # restorecon -Rv path_to_cache_directory

      If the semanage utility is not available on your system, install the policycoreutils-python-utils package.

  4. Store the password of the LDAP service user in the /etc/squid/ldap_password file, and set appropriate permissions for the file:

    # echo "password" > /etc/squid/ldap_password
    # chown root:squid /etc/squid/ldap_password
    # chmod 640 /etc/squid/ldap_password
  5. Open the 3128 port in the firewall:

    # firewall-cmd --permanent --add-port=3128/tcp
    # firewall-cmd --reload
  6. Enable and start the squid service:

    # systemctl enable --now squid

Verification steps

To verify that the proxy works correctly, download a web page using the curl utility:

# curl -O -L "https://www.redhat.com/index.html" -x "user_name:password@proxy.example.com:3128"

If curl does not display any error and the index.html file was downloaded to the current directory, the proxy works.

Troubleshooting steps

To verify that the helper utility works correctly:

  1. Manually start the helper utility with the same settings you used in the auth_param parameter:

    # /usr/lib64/squid/basic_ldap_auth -b "cn=users,cn=accounts,dc=example,dc=com" -D "uid=proxy_user,cn=users,cn=accounts,dc=example,dc=com" -W /etc/squid/ldap_password -f "(&(objectClass=person)(uid=%s))" -ZZ -H ldap://ldap_server.example.com:389
  2. Enter a valid user name and password, and press Enter:

    user_name password

    If the helper utility returns OK, authentication succeeded.

7.3. Setting up Squid as a caching proxy with kerberos authentication

This section describes a basic configuration of Squid as a caching proxy that authenticates users to an Active Directory (AD) using Kerberos. The procedure configures that only authenticated users can use the proxy.

Prerequisites

  • The procedure assumes that the /etc/squid/squid.conf file is as provided by the squid package. If you edited this file before, remove the file and reinstall the package.
  • The server on which you want to install Squid is a member of the AD domain. For details, see Setting up Samba as a Domain Member in the Red Hat Enterprise Linux 8 Deploying different types of servers documentation.

Procedure

  1. Install the following packages:

    yum install squid krb5-workstation
  2. Authenticate as the AD domain administrator:

    # kinit administrator@AD.EXAMPLE.COM
  3. Create a keytab for Squid and store it in the /etc/squid/HTTP.keytab file:

    # export KRB5_KTNAME=FILE:/etc/squid/HTTP.keytab
    # net ads keytab CREATE -U administrator
  4. Add the HTTP service principal to the keytab:

    # net ads keytab ADD HTTP -U administrator
  5. Set the owner of the keytab file to the squid user:

    # chown squid /etc/squid/HTTP.keytab
  6. Optionally, verify that the keytab file contains the HTTP service principal for the fully-qualified domain name (FQDN) of the proxy server:

    #  klist -k /etc/squid/HTTP.keytab
    Keytab name: FILE:/etc/squid/HTTP.keytab
    KVNO Principal
    ---- ---------------------------------------------------
    ...
       2 HTTP/proxy.ad.example.com@AD.EXAMPLE.COM
    ...
  7. Edit the /etc/squid/squid.conf file:

    1. To configure the negotiate_kerberos_auth helper utility, add the following configuration entry to the top of /etc/squid/squid.conf:

      auth_param negotiate program /usr/lib64/squid/negotiate_kerberos_auth -k /etc/squid/HTTP.keytab -s HTTP/proxy.ad.example.com@AD.EXAMPLE.COM

      The following describes the parameters passed to the negotiate_kerberos_auth helper utility in the example above:

      • -k file sets the path to the key tab file. Note that the squid user must have read permissions on this file.
      • -s HTTP/host_name@kerberos_realm sets the Kerberos principal that Squid uses.

        Optionally, you can enable logging by passing one or both of the following parameters to the helper utility:

      • -i logs informational messages, such as the authenticating user.
      • -d enables debug logging.

        Squid logs the debugging information from the helper utility to the /var/log/squid/cache.log file.

    2. Add the following ACL and rule to configure that Squid allows only authenticated users to use the proxy:

      acl kerb-auth proxy_auth REQUIRED
      http_access allow kerb-auth
      Important

      Specify these settings before the http_access deny all rule.

    3. Remove the following rule to disable bypassing the proxy authentication from IP ranges specified in localnet ACLs:

      http_access allow localnet
    4. The following ACL exists in the default configuration and defines 443 as a port that uses the HTTPS protocol:

      acl SSL_ports port 443

      If users should be able to use the HTTPS protocol also on other ports, add an ACL for each of these port:

      acl SSL_ports port port_number
    5. Update the list of acl Safe_ports rules to configure to which ports Squid can establish a connection. For example, to configure that clients using the proxy can only access resources on port 21 (FTP), 80 (HTTP), and 443 (HTTPS), keep only the following acl Safe_ports statements in the configuration:

      acl Safe_ports port 21
      acl Safe_ports port 80
      acl Safe_ports port 443

      By default, the configuration contains the http_access deny !Safe_ports rule that defines access denial to ports that are not defined in Safe_ports ACLs.

    6. Configure the cache type, the path to the cache directory, the cache size, and further cache type-specific settings in the cache_dir parameter:

      cache_dir ufs /var/spool/squid 10000 16 256

      With these settings:

      • Squid uses the ufs cache type.
      • Squid stores its cache in the /var/spool/squid/ directory.
      • The cache grows up to 10000 MB.
      • Squid creates 16 level-1 sub-directories in the /var/spool/squid/ directory.
      • Squid creates 256 sub-directories in each level-1 directory.

        If you do not set a cache_dir directive, Squid stores the cache in memory.

  8. If you set a different cache directory than /var/spool/squid/ in the cache_dir parameter:

    1. Create the cache directory:

      # mkdir -p path_to_cache_directory
    2. Configure the permissions for the cache directory:

      # chown squid:squid path_to_cache_directory
    3. If you run SELinux in enforcing mode, set the squid_cache_t context for the cache directory:

      # semanage fcontext -a -t squid_cache_t "path_to_cache_directory(/.*)?"
      # restorecon -Rv path_to_cache_directory

      If the semanage utility is not available on your system, install the policycoreutils-python-utils package.

  9. Open the 3128 port in the firewall:

    # firewall-cmd --permanent --add-port=3128/tcp
    # firewall-cmd --reload
  10. Enable and start the squid service:

    # systemctl enable --now squid

Verification steps

To verify that the proxy works correctly, download a web page using the curl utility:

# curl -O -L "https://www.redhat.com/index.html" --proxy-negotiate -u : -x "proxy.ad.example.com:3128"

If curl does not display any error and the index.html file exists in the current directory, the proxy works.

Troubleshooting steps

To manually test Kerberos authentication:

  1. Obtain a Kerberos ticket for the AD account:

    # kinit user@AD.EXAMPLE.COM
  2. Optionally, display the ticket:

    # klist
  3. Use the negotiate_kerberos_auth_test utility to test the authentication:

    # /usr/lib64/squid/negotiate_kerberos_auth_test proxy.ad.example.com

    If the helper utility returns a token, the authentication succeeded:

    Token: YIIFtAYGKwYBBQUCoIIFqDC...

7.4. Configuring a domain blacklist in Squid

Frequently, administrators want to block access to specific domains. This section describes how to configure a domain blacklist in Squid.

Prerequisites

  • Squid is configured, and users can use the proxy.

Procedure

  1. Edit the /etc/squid/squid.conf file and add the following settings:

    acl domain_blacklist dstdomain "/etc/squid/domain_blacklist.txt"
    http_access deny all domain_blacklist
    Important

    Add these entries before the first http_access allow statement that allows access to users or clients.

  2. Create the /etc/squid/domain_blacklist.txt file and add the domains you want to block. For example, to block access to example.com including subdomains and to block example.net, add:

    .example.com
    example.net
    Important

    If you referred to the /etc/squid/domain_blacklist.txt file in the squid configuration, this file must not be empty. If the file is empty, Squid fails to start.

  3. Restart the squid service:

    # systemctl restart squid

7.5. Configuring the Squid service to listen on a specific port or IP address

By default, the Squid proxy service listens on the 3128 port on all network interfaces. This section describes how to change the port and configuring Squid to listen on a specific IP address.

Prerequisites

  • The squid package is installed.

Procedure

  1. Edit the /etc/squid/squid.conf file:

    • To set the port on which the Squid service listens, set the port number in the http_port parameter. For example, to set the port to 8080, set:

      http_port 8080
    • To configure on which IP address the Squid service listens, set the IP address and port number in the http_port parameter. For example, to configure that Squid listens only on the 192.0.2.1 IP address on port 3128, set:

      http_port 192.0.2.1:3128

      Add multiple http_port parameters to the configuration file to configure that Squid listens on multiple ports and IP addresses:

      http_port 192.0.2.1:3128
      http_port 192.0.2.1:8080
  2. If you configured that Squid uses a different port as the default (3128):

    1. Open the port in the firewall:

      # firewall-cmd --permanent --add-port=port_number/tcp
      # firewall-cmd --reload
    2. If you run SELinux in enforcing mode, assign the port to the squid_port_t port type definition:

      # semanage port -a -t squid_port_t -p tcp port_number

      If the semanage utility is not available on your system, install the policycoreutils-python-utils package.

  3. Restart the squid service:

    # systemctl restart squid

7.6. Additional resources

  • See the usr/share/doc/squid-<version>/squid.conf.documented file for a list of all configuration parameters you can set in the /etc/squid/squid.conf file together with a detailed description.