Squid with Dynamic SSL Cert and Kerberos Authentication

I know, this is not the usual sort of thing I cover here, but it’s something I’ve been working on the last few weeks, and I couldn’t find it documented decently anywhere, at least not for my particular setup. So I thought I’d put it here for Google to index. Maybe it will help someone else out.


  • Squid proxy server running on RedHat EL 5.x
  • Clients using Firefox and Safari, on Macs and Linux
  • Mac OS X Server providing authentication services
  • ICAP A/V scanning

Previously we were using NTLM authentication with Squid, but it is a pretty poor experience. NTLM authentication on Mac OS X is unstable if you’re also doing Time Machine backups, and users get far too many authentication popups. I’ve been planning on moving to Kerberos authentication for a long time, but never quite got around to it. We also wanted to enable SSL Interception, using DynamicSslCert, so we can properly log and scan SSL traffic.

The default Squid package that ships with RHEL 5.x is 2.6.x. This is getting a bit long in the tooth. DynamicSslCert has recently gone into the 3.1.12.x RC series, so it’s very close to mainstream. Here’s the steps I followed to get SSL interception, and Kerberos authentication working:

  • Create a binary RPM from Squid Squid- does not compile with ICAP enabled. You will probably want to get the Squid spec file used for Fedora, and use that as a base. Add “–enable-ssl-crtd” and build the package.
  • On your Mac OS X Server, create the required Kerberos principal, and export it to a keytab file:

    sudo kadmin.local -q "add_principal -randkey HTTP/<fqdn of proxy server>"
    sudo kadmin.local -q "ktadd -k squid.keytab -norandkey HTTP/<fqdn of proxy server>"

    <FQDN> is the fully qualified hostname of your proxy server
  • Install your new Squid RPM on the Proxy server
  • Copy your squid.keytab file to /etc/squid/squid.keytab. Ensure it is readable by the squid user
  • Edit /etc/init.d/squid, to add this chunk near the top:
    export KRB5_KTNAME
  • Create the SSL cert DB with “/usr/lib64/squid/ssl_crtd -c -s /var/spool/squid/ssl_crtd/”. Ensure that directory, and those below it are owned by Squid.
  • Create an intermediate CA certificate on your root CA. I’ve used the Mac OS X CA, but you can use whatever CA you have. Copy the key and certificate to /etc/squid/ssl_cert/ – you’ll need to create that directory. Ensure squid can read the cert and keys.
  • Update /etc/krb5.conf. Ensure it has your realm set to your Mac Server.
  • If you want to do NTLM fallback, enable the winbind service, and use “net join -W -S -U ” to join the domain.
  • Configure authentication in squid.conf with something like this – this will use Kerberos/Negotiate first, with an NTLM fallback:
    auth_param negotiate program /usr/lib64/squid/squid_kerb_auth -s HTTP/<fqdn of proxy>
    auth_param negotiate children 10
    auth_param negotiate keep_alive on
    auth_param ntlm program /usr/bin/ntlm_auth --helper-protocol=squid-2.5-ntlmssp --domain=<domain> <domain>\<fqdn on mac os server>
    auth_param ntlm children 12
    acl auth proxy_auth REQUIRED

    Your http_access line must now specify “auth”
  • Enable icap with:
    icap_enable on
    icap_service service_avscan_resp respmod_precache bypass=0 icap://
    adaptation_access service_avscan_resp allow all
  • Configure SSL Dynamic cert generation with config like this:

    sslcrtacl sslbumpbypass dstdomain "/etc/squid/whitelist.https"
    d_program /usr/lib64/squid/ssl_crtd -s /var/spool/squid/ssl_db -M 4MB
    sslcrtd_children 5
    http_port 3128 ssl-bump generate-host-certificates=on dynamic_cert_mem_cache_size=4MB cert=/etc/squid/ssl_cert/ key=/etc/squid/ssl_cert/ always_direct allow all
    ssl_bump deny sslbumpbypass
    ssl_bump allow all
    sslproxy_cert_error deny all

    Any domains added to /etc/squid/whitelist.https will NOT be intercepted. You probably want to put banking sites in here.
  • Modify SELinux. You’ll need to run “semanage -a -t http_port_t -p tcp 1344” to allow Squid to connect to ICAP. You’ll also need to configure a local SELinux policy to allow Squid to read/write the temporary files that squid_kerb_auth puts into /tmp. Use audit2allow, and your audit logs to work out what you need here.

You will need to configure both Firefox, and the System Keychain on your Macs to trust the Intermediate CA used by Squid. Unfortunately it doesn’t pass the whole keychain, including the root CA, so just trusting the root CA is not enough. Hopefully the ability to pass the chain will come in later releases.

I don’t want to publish full configs, but this should be enough to get you started. Any specific questions, fire them this way, and I’ll try to help

Note: Safari and Mail.app do not support Kerberos authentication. They fall back to NTLM happily enough.