Tag: Linux

IT

Allow RSA key size below 2048 bit to successfully establish SSH connections to legacy systems

The crypto-policy that comes with Red Hat Enterprise Linux 9 and AlmaLinux 9 does not allow RSA key sizes below 2048 bit. Some legacy SSH servers are configured with 1024 bit key sizes though which will result in connection failures:

# ssh username@servername.example.com
Bad server host key: Invalid key length

Trying to find the issue using ssh -vvv, you would get:

OpenSSH_8.8p1, OpenSSL 3.0.5 5 Jul 2022
debug1: Reading configuration data /home/geeklab/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug1: configuration requests final Match pass
debug1: re-parsing configuration
debug1: Reading configuration data /home/geeklab/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug1: Reading configuration data /etc/ssh/ssh_config.d/99-jxs.conf
debug1: Connecting to servername.example.com [1.2.3.4] port 22.
debug1: Connection established.
debug1: identity file /home/geeklab/.ssh/id_rsa type 0
debug1: identity file /home/geeklab/.ssh/id_rsa-cert type -1
debug1: identity file /home/geeklab/.ssh/id_dsa type -1
debug1: identity file /home/geeklab/.ssh/id_dsa-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ed25519 type 3
debug1: identity file /home/geeklab/.ssh/id_ed25519-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ed25519_sk type -1
debug1: identity file /home/geeklab/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/geeklab/.ssh/id_xmss type -1
debug1: identity file /home/geeklab/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.8
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: compat_banner: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000002
debug1: Authenticating to servername.example.com:22 as 'root'
debug1: load_hostkeys: fopen /home/geeklab/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: diffie-hellman-group-exchange-sha256
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes256-ctr MAC: hmac-sha2-256 compression: none
debug1: kex: client->server cipher: aes256-ctr MAC: hmac-sha2-256 compression: none
debug1: kex: diffie-hellman-group-exchange-sha256 need=32 dh_need=32
debug1: kex: diffie-hellman-group-exchange-sha256 need=32 dh_need=32
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(2048<8192<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_GROUP received
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: SSH2_MSG_KEX_DH_GEX_REPLY received
Bad server host key: Invalid key length

While the best solution would be to fix the key size of the target server, this is not always possible on embedded devices. For those devices, you could alternatively make a change to your ~/.ssh/config file:

Host servername.example.com
     RSAMinSize 1024
© GeekLabInfo Allow RSA key size below 2048 bit to successfully establish SSH connections to legacy systems is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

chattr and lsattr in Python

Apart from changing the rwxr-xr-x like permissions, some Linux filesystems have another way of protecting files. You cam make them IMMUTABLE, after which no program can change them. Not even if you have the right permissions. Python however does not support reading or writing the immutable attribute.

Inspired by yellowcrescent, I just wrote the following code snippet for my own use:

# Code from geeklab.info
import fcntl
from array import array

# FS constants - see /uapi/linux/fs.h in kernel source
# or <elixir.free-electrons.com/linux/latest/source/include/uapi/linux/fs.h>
FS_IOC_GETFLAGS = 0x80086601
FS_IOC_SETFLAGS = 0x40086602
FS_IMMUTABLE_FL = 0x010

def chattri(filename: str, value: bool):
    with open(filename,'r') as f: 
        arg = array('L', [0])
        fcntl.ioctl(f.fileno(), FS_IOC_GETFLAGS, arg, True)
        if value:
            arg[0]=arg[0] | FS_IMMUTABLE_FL
        else:
            arg[0]=arg[0] &~ FS_IMMUTABLE_FL
        fcntl.ioctl(f.fileno(), FS_IOC_SETFLAGS, arg, True)

def lsattri(filename: str) -> bool:
    with open(filename,'r') as f:
        arg = array('L', [0])
        fcntl.ioctl(f.fileno(), FS_IOC_GETFLAGS, arg, True)
    return bool(arg[0] & FS_IMMUTABLE_FL)
    
f="/root/testfile"
print("Yes" if lsattri(f) else "No")
chattri(f,False)
print("Yes" if lsattri(f) else "No")
chattri(f,False)
print("Yes" if lsattri(f) else "No")

Use it as you like, at your own risk. If possible please link my website.

Keep in mind that only users with root-access can change file attributes.

© GeekLabInfo chattr and lsattr in Python is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

iptables and dynamic DNS – part 3

This is an updated post for this updated article.

I just found back an old note about using iptables in combination with dyndns to open up access from a remote location. For instance, if you have a laptop that you take everywhere and you want to connect to your home or office. The script the other site suggested was broken, so let's write a new one.

Step 1: Create a new chain in the firewall

Create a new chain in the firewall where we can plug in the dynamic rules. On my Fedora machine, the firewall is located in /etc/sysconfig/iptables. I added the bold lines to this example.



*nat

:PREROUTING ACCEPT [0:0]

:OUTPUT ACCEPT [0:0]

:POSTROUTING ACCEPT [0:0]

COMMIT

*filter

:INPUT ACCEPT [0:0]

:FORWARD ACCEPT [0:0]

:OUTPUT ACCEPT [0:0]

<b>:DYNAMICPARENT - [0:0]

-A INPUT -j DYNAMICPARENT</b>

-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

-A INPUT -p icmp -j ACCEPT

-A INPUT -i lo -j ACCEPT

-A INPUT -j REJECT --reject-with icmp-host-prohibited

-A FORWARD -j REJECT --reject-with icmp-host-prohibited

COMMIT

Step 2: Write a script

#!/bin/bash
 
HOSTNAME=myname.dyndns.org
CHECK_INTERVAL=60 #once a minute
 
IP="" #initialize $IP
while [ true ]; do
        OIP=$IP
        IP=$(dig +short $HOSTNAME | grep -iE "^[0-9]+.[0-9]+.[0-9]+.[0-9]+$"|head -n 1)
        if [ "$OIP" != "$IP" -a "$IP" != "" ]; then
                echo "Changing ip to $IP"
                /sbin/iptables -N DYNAMICNEW                    # create new rule
                /sbin/iptables -I DYNAMICNEW -s $IP -j ACCEPT   # allow new ip
                /sbin/iptables -I DYNAMICPARENT -j DYNAMICNEW   # attach new rule to its parent
 
                while [ true ]; do  # unlink old rule - if multiple exist, remove all
                        /sbin/iptables -D DYNAMICPARENT -j DYNAMICCHILD 2>/dev/null || break
                done
                /sbin/iptables -F DYNAMICCHILD #flush all old rules
                /sbin/iptables -X DYNAMICCHILD #flush all old rules
 
                /sbin/iptables -E DYNAMICNEW DYNAMICCHILD #rename new to "current"
        fi
        sleep $CHECK_INTERVAL
done

In this case, the firewall accepts all traffic from $IP, but of course you could restrict it to 1 port. Also, I focussed on IPv4, but you could easily rewrite this script to IPv6 using ip6tables. I saved the file to /usr/local/bin/dynfirewall.sh

Step 3: Run the script

I'd prefer running the script from inittab, but since Fedora doesn't work like this anymore, I put the following line in /etc/rc.d/rc.local:

/usr/local/bin/dynfirewall.sh >>/var/log/dynfirewall 2>>/var/log/dynfirewall &

Please don't forget the ampersand at the end to fork the script!!

Why is this script better than previous version?

- This script can handle cnames
- The old script used to delete old rules, before creating new ones. This one does not. Therefore, it will never leave a second where you cannot connect.© GeekLabInfo
iptables and dynamic DNS - part 3 is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

iptables and dynamic DNS – part 2

In 2011, I wrote this post on Dynamic DNS: https://www.geeklab.info/2011/02/iptables-and-dynamic-dns. While this is still useful, I found a newer, cooler way to do Dynamic DNS in combination with iptables. It's called libnetfilter_queue.

iptables is used to change the inner netfilter tables of the kernel. And because the kernel has no internal resolver, it is impossible for the kernel to do on-the-fly dns lookups. But by offloading this decision to userspace, it is possible. The libnetfilter_queue lib offers that functionality.

libnetfilter_queue is a userspace library providing an API to packets that have been queued by the kernel packet filter. It has bindings for Python and several other languages.

Requirements for my setup

python-NetfilterQueue - https://github.com/kti/python-netfilterqueue

libnfnetlink

libnetfilter_queue

libmnl

You may need to build the first dependency yourself. The other 3 are available in Fedora 20 by default. If you're running RHEL/CentOS, the Fedora packages can be recompiled for your setup.

iptables rule

First, you need to get iptables to enqueue specific packets to your queue.

iptables -I INPUT -p tcp --dport 631 -m state --state NEW -j NFQUEUE --queue-num 6789 -m comment --comment "Remote CUPS printer"

Queue handler

Then we write a script that handles the queue. A quick-and-dirty implementation:

#!/usr/bin/python
 
import socket
from netfilterqueue import NetfilterQueue
 
def getIP(d):
    """
    This method returns the first IP address string
    that responds as the given domain name
    """
    try:
        data = socket.gethostbyname(d)
        #ip = repr(data)
        return data
    except Exception:
        # fail gracefully!
        return False
 
def dnsfilter(pkt):
        if pkt.get_payload_len() < 0x10:
                "Don't know how to handle this too small packet"
                pkt.drop()
                return False
 
        payload=pkt.get_payload()
        srcip=".".join("{:d}".format(ord(c)) for c in payload[0x0c:0x10])
        allowedip=getIP('localhost')
        print "Debug: SRC="+srcip+" ALLOWED="+allowedip+" RESULT=",
        if srcip==allowedip:
                print "Accept"
                pkt.accept()
        else:
                print "Drop"
                pkt.drop()
 
nfqueue = NetfilterQueue()
nfqueue.bind(6789, dnsfilter)
try:
        nfqueue.run()
except KeyboardInterrupt:
        print

This is a quick-and-dirty implementation that misses basic features such as caching the result of gethostbyname. This may introduce terrible delays if used wrong.

Ubuntu/Debian

I'm running RedHat-based software on all of my machines. Above information may be useful for Ubuntu/Debian users, but it's not tested and I'm not supporting it.

Servers: RedHat Enterprise Linux/CentOS is more suitable for servers, as there's a lot of professional level support available. I think that's important, because if I say, get a car accident, I want the servers to be managable by another professional.

Desktops/Laptops: RPM packages are pretty exchangable between RedHat-based platforms. That's a good reason to run Fedora on the desktop.

© GeekLabInfo
iptables and dynamic DNS - part 2 is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

Bash: download and execute shell scripts

Sometimes, when running 1 bash script repeatedly on several different machines, I found that being able to download and immediately execute a script is very handy.

The following command will download a script and immediately execute it:

bash <(curl -s http://geeklab.info/my-script.sh)

This command uses Bash's Process Substitution to do it's job. command2 <(command) means for bash to put the output of command in a pipe and then run command2 [tempfile]. So above statement does the same as:

TMPFILE=$(mktemp /tmp/my.XXXXX)
curl -s http://geeklab.info/my-script.sh > $TMPFILE
bash $TMPFILE
rm $TMPFILE

Process substitution is also very useful when you want to know the difference between the output of two commands:

diff <( command1 ) <( command2 )

Furthermore, it's possible to pipe the contents of the temporary file into command2. For instance:
bash < <(curl http://geeklab.info/my-script.sh) would do the same as: 

TMPFILE=$(mktemp /tmp/my.XXXXX)
curl -s http://geeklab.info/my-script.sh > $TMPFILE
bash < $TMPFILE
rm $TMPFILE

With bash, this difference is small, but with other commands, it may not be.© GeekLabInfo
Bash: download and execute shell scripts is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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Business

How to decrypt a PDF file on Linux

Sometimes, I receive digital invoices in PDF format with a password. That way I shouldn't be able to modify them and commit fraud. Unfortunately, this also makes it impossible to perform normal operations on them, such as removing unneeded specifications or merging all invoices into one single file.

Luckily, most Linux distributions come with a tool that can be used to remove these passwords: ghostscript.

In order to remove the password, simply run:

gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=unencrypted.pdf -c .setpdfwrite -f encrypted.pdf

While this does remove the password, I'm not sure if it does not degrade the quality of the file a little (I don't notice any quality difference, but if you use highres files, you may lose quality).© GeekLabInfo
How to decrypt a PDF file on Linux is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

WiFi on a HP Elitebook 8570w

There once was a day that I compiled my own kernels and configured all modules manually. It's been quite a while since I've done anything like that, but I remembered some of it.

Today I installed my WiFi drivers. The HP Elitebook 8570w I own has a Centrino Advanced-N 6205 on board that is actually supported by Fedora 17.

$ lspci
00:00.0 Host bridge: Intel Corporation Ivy Bridge DRAM Controller (rev 09)
00:01.0 PCI bridge: Intel Corporation Ivy Bridge PCI Express Root Port (rev 09)
00:14.0 USB Controller: Intel Corporation Panther Point USB xHCI Host Controller (rev 04)
00:16.0 Communication controller: Intel Corporation Panther Point MEI Controller #1 (rev 04)
00:19.0 Ethernet controller: Intel Corporation 82579LM Gigabit Network Connection (rev 04)
00:1a.0 USB Controller: Intel Corporation Panther Point USB Enhanced Host Controller #2 (rev 04)
00:1b.0 Audio device: Intel Corporation Panther Point High Definition Audio Controller (rev 04)
00:1c.0 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 1 (rev c4)
00:1c.1 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 2 (rev c4)
00:1c.2 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 3 (rev c4)
00:1c.3 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 4 (rev c4)
00:1d.0 USB Controller: Intel Corporation Panther Point USB Enhanced Host Controller #1 (rev 04)
00:1f.0 ISA bridge: Intel Corporation Panther Point LPC Controller (rev 04)
00:1f.2 SATA controller: Intel Corporation Panther Point 6 port SATA AHCI Controller (rev 04)
00:1f.3 SMBus: Intel Corporation Panther Point SMBus Controller (rev 04)
01:00.0 VGA compatible controller: nVidia Corporation Device 0ffc (rev a1)
01:00.1 Audio device: nVidia Corporation Device 0e1b (rev a1)
24:00.0 FireWire (IEEE 1394): JMicron Technology Corp. IEEE 1394 Host Controller (rev 30)
24:00.1 System peripheral: JMicron Technology Corp. SD/MMC Host Controller (rev 30)
24:00.2 SD Host controller: JMicron Technology Corp. Standard SD Host Controller (rev 30)
25:00.0 Network controller: Intel Corporation Centrino Advanced-N 6205 (rev 34)

The device at the very bottom is the WiFi device we're looking for. But iwconfig shows it's not available to the system somehow:

$ iwconfig
thuisf    no wireless extensions.
 
vmnet8    no wireless extensions.
 
thuis     no wireless extensions.
 
eth0      no wireless extensions.
 
lo        no wireless extensions.
 
virbr0-nic  no wireless extensions.
 
virbr0    no wireless extensions.
 
noc       no wireless extensions.
 
vmnet1    no wireless extensions.

Continue Reading…

© GeekLabInfo
WiFi on a HP Elitebook 8570w is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

Backup your phone with zero keypresses

I created some scripts to backup my Samsung Galaxy S2 to my Fedora 16 system without pressing any keys or even touching the mouse. It works like this:

To backup a SGS2, you go to settings > applications > usb tools on the phone, and click the button. You then connect it to a usb cable. At that moment, two removable devices become visible to the Linux system: one for the SD card, one for the built-in memory. But they still contain no media. Only when you press another button on the phone, the "media are inserted in the devices", triggering a "change" action with the udev daemon.

Configuration of the udev daemon

Create a file /etc/udev/rules.d/71-android-backup.rules:

SUBSYSTEM=="block", ACTION=="change", ENV{ID_SERIAL}=="Android_UMS_Composite_XXXXXXXXXXXXXXXX-0:0", ENV{DISK_MEDIA_CHANGE}=="1", ENV{ID_FS_TYPE}=="vfat", RUN+="/usr/local/bin/androidbackup"

Whenever a (virtual) medium is inserted, a udev change action is triggered. Actually, it gets triggered multiple times, but we only need the one that has the environment variable DISK_MEDIA_CHANGE=1 set.

Action to perform

The action triggered is /usr/local/bin/androidbackup and all parameters are in the environment. So let's create that file. I use a rsnapshot-like operation to backup my data, but without actually using rsnapshot.

#!/bin/bash
 
function msg {
        /usr/bin/logger -t android "$1"
        DISPLAY=:0 qdbus $dbusRef setLabelText "$1"
        DISPLAY=:0 qdbus $dbusRef Set "" value $2
}
 
if [ "$1" != "FORKED" ]; then
        $0 FORKED &
        exit 0
fi
 
if [ -f /etc/sysconfig/androidbackup ]; then
        . /etc/sysconfig/androidbackup
else
        echo /etc/sysconfig/androidbackup does not exist
        exit 0
fi
 
#debug disabled:
#set > /tmp/android/`date +%s`.$RANDOM
 
dbusRef=`kdialog --display :0 --progressbar "Backup android..." 100`
 
msg "Attach $DEVNAME $ACTION $ID_SERIAL" 0
 
if [ ! -e $DESTINATION ]; then
        # I could mkdir -p, but sometimes $DESTINATION could just be a network location that's offline
        msg "Android will not be backed up: $DESTINATION is not okay" 0
        exit 0
fi
 
mkdir -p /mnt/android
 
mount $DEVNAME /mnt/android 
rc=$?
if [ $rc -ne 0 ]; then
        msg "Problem mounting $DEVNAME to /mnt/android: $rc " 0
        mount | grep android 2>&1 | logger -t androidbackup
        exit 0
fi
 
if [ "`mount |grep -i /mnt/android`" == "" ]; then
        msg "Cannot find /mnt/android in mount table" 0
        exit 0
fi
 
msg "Remove old backup" 8
[ -e $DESTINATION/backup.30 ] && rm -rf $DESTINATION/backup.30
 
msg "Start backup" 10
for i in `/usr/bin/seq 30 -1 2`; do 
        if [ -e $DESTINATION/backup.$((i-1)) ]; then
                msg "mv $DESTINATION/backup.$((i-1)) $DESTINATION/backup.$i" 11
                mv $DESTINATION/backup.$((i-1)) $DESTINATION/backup.$i
        fi
done
 
msg "Copy" 40
[ -e $DESTINATION/backup.0 ] && /bin/cp -al $DESTINATION/backup.0 $DESTINATION/backup.1
mkdir -p $DESTINATION/backup.0/
 
msg "Start rsync" 40
/usr/bin/rsync -az --numeric-ids --delete --hard-links /mnt/android/ $DESTINATION/backup.0/
touch $DESTINATION/backup.0/
 
chown -R $CHOWN $DESTINATION/backup.0
 
umount /mnt/android
 
msg "Done" 100
exit 0

Since that file refers to /etc/sysconfig/androidbackup, let's also make that file:

#next line defines the location of all backups
DESTINATION=/home/geeklab/.androidbackup
 
#next line defines the owner of all files copied
CHOWN=geeklab

© GeekLabInfo
Backup your phone with zero keypresses is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

Setting up a PPTP VPN to a Windows 2008 Server

Requirements

Linux
KDE
A Windows server to connect to.

Step 1: Install required software

yum install kde-plasma-networkmanagement-pptp NetworkManager-pptp

Step 2: Configure the network

In the KDE Network Manager plasma module, go to the tab VPN, click add and choose PPTP.

Enter and connection name you like. In the field "gateway" type the hostname or IP number of the Windows server you're connecting to. Under Login, Password and NT Domain, fill in your authentication data. Then click advanced.

In the advanced window, disable EAP and enable MPPE. Then click OK.

Go to the tab IPv4. Under method, I chose Automatic (VPN). But Automatic (VPN) addresses only is also a nice option: it sets the IPs but no DNS settings.

Go to the routes sub-tab. Switch on Ignore automatically obtained routes and Use only for resources on this connection to make sure the connection doesn't steal your traffic. Then I entered a manual route: 192.168.178.0/255.255.255.0 to gateway 0.0.0.0 (it is a ppp device after all).

You may want to configure IPv6 as well, but I don't at this moment, so I'm not documenting this.

Step 3: Connect

Click on the icon in the tray and connect.

Ubuntu/Debian

I'm running RedHat-based software on all of my machines. Above information may be useful for Ubuntu/Debian users, but it's not tested and I'm not supporting it.

Servers: RedHat Enterprise Linux/CentOS is more suitable for servers, as there's a lot of professional level support available. I think that's important, because if I say, get a car accident, I want the servers to be managable by another professional.

Desktops/Laptops: RPM packages are pretty exchangable between RedHat-based platforms. That's a good reason to run Fedora on the desktop.

© GeekLabInfo
Setting up a PPTP VPN to a Windows 2008 Server is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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IT

Squid with active directory authentication on Centos 6.0

In this post, I'll be writing down all steps required to build a Squid proxy server on a clean "minimal" installation of CentOS 6.0

Step 1. Network configuration

First, install system-config-network or manually configure the network. I prefer system-config-network for easy configurations and vim for more complex configurations.
yum -y install system-config-network-tui

Step 2. Install some tools for convenience

yum -y install vim-minimal vim-enhanced openssh-clients mc telnet policycoreutils policycoreutils-python bind-utils

Step 3. Install ntp and synchronize clocks

If one of the clocks it out of sync, NTLM authentication will not work. Therefore, we synchronize the clocks. Using pool.ntp.org as a source would be good, but if the AD server isn't synchronized with that source, we'd have the same problem. So I'm synchonizing the proxy to the AD server (Win2003SBS actually) instead:

rpm -q ntp || yum -y install ntp

sed -i "s/^server /#server /g" /etc/ntp.conf

echo "server AD-SERVERNAME" >> /etc/ntp.conf

ntpdate AD-SERVERNAME #synchronize right now

service ntpd start #and keep in sync

chkconfig ntpd on

Step 4. Install squid and other required software

yum -y install krb5-workstation samba-common samba-winbind authconfig squid

chkconfig squid on

Step 5. Connect to active directory

Please note that MYCOMPANY.local and mycompany.local may be different domains due to the upper/lowercase.

ADSERVER=sbs.MYCOMPANY.local

DOMAIN=MYCOMPANY.local

WORKGROUP=MYCOMPANY

authconfig --enableshadow --enablemd5 --passalgo=md5 --krb5kdc=$ADSERVER \

--krb5realm=$DOMAIN --smbservers=$ADSERVER --smbworkgroup=$WORKGROUP \

--enablewinbind --enablewinbindauth --smbsecurity=ads --smbrealm=$DOMAIN \

--smbidmapuid="16777216-33554431" --smbidmapgid="16777216-33554431" --winbindseparator="+" \

--winbindtemplateshell="/bin/false" --enablewinbindusedefaultdomain --disablewinbindoffline \

--winbindjoin=Administrator --disablewins --disablecache --enablelocauthorize --updateall

service winbind restart

chkconfig winbind on

Give squid permissions to use winbind info:
usermod -G wbpriv squid

Now check your winbind connection using the following commands:

wbinfo -u

wbinfo -g

Step 6. Firewall

iptables -I INPUT -m tcp -p tcp --dport 3128 -j ACCEPT

/sbin/service iptables save

Step 7. Configure squid

I commented out this line from /etc/squid/squid.conf:
http_access allow localnet
That line would have allowed users from 10.0.0.0/8+172.16.0.0/12+192.168.0.0/16 and others to use the proxy without authentication. Then I added the following right below that line:

acl whitelist dstdom_regex -i "/etc/squid/whitelist"

http_access allow whitelist

auth_param ntlm program /usr/bin/ntlm_auth --helper-protocol=squid-2.5-ntlmssp

auth_param ntlm children 5

auth_param ntlm keep_alive on

acl our_networks src 192.168.0.0/16

acl ntlm proxy_auth REQUIRED

http_access allow our_networks ntlm

authenticate_ip_ttl 900 seconds

This will allow all valid, logged in users to surf the web. You could also limit which users can surf by adding --require-membership-of=ADGROUPNAME to the ntlm_auth command

The first two lines of above configuration point to a file /etc/squid/whitelist. This file contains domains that should never be denied. My whitelist file contains:

\.trendmicro\.com

^trendmicro\.com

\.microsoft\.com

^microsoft\.com

This means that anything at *.microsoft.com and microsoft.com (without subdomain) as well as *.trendmicro.com as trendmicro.com (my virusscanner) is always allowed for any user. We wouldn't want to block important updates.

Start Squid using /sbin/service squid restart and the proxy is ready.

Step 8. Optional: IPv6 issues

I've been experimenting with IPv6 for a while now, but I don't have IPv6 available on all systems. That caused me some trouble with the next step. I had to give preference to IPv4 above IPv6 by editting /etc/gai.conf:

label ::1/128       0

label ::/0          1

label 2002::/16     2

label ::/96         3

label ::ffff:0:0/96 4

label fec0::/10     5

label fc00::/7      6

precedence ::ffff:0:0/96  100

precedence ::1/128       50

precedence ::/0          40

precedence 2002::/16     30

precedence ::/96          20

Step 9. Optional: Some white- and blacklisting

We may not want to allow all sites to be visited. For instance, porn sites are often blocked in office situations. I've got a manual on blacklisting using SquidGuard as well.© GeekLabInfo
Squid with active directory authentication on Centos 6.0 is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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