Category: 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

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

Manually undeleting a file from ext2, ext3, ext4 or any other filesystem

Yesterday, I worked on a PHP project all day. At the end of the day, I moved it from development to staging. And finally I deleted it from staging. Oops!

I just discovered an hour ago what I did. What to do, what to do...?

Back in the old days, Midnight Commander supported some undelete function, but this option has been removed in the CentOS 6 version I'm running.

Then there's the option to use extundelete, but that would need compiling and a lot of other disk activity before I could use it. Normally, I would compile such a program on another server, then hook up the disk. But since it's a cloud server that I cannot access in any other way but SSH, that's not an option.

And finally there's the option to use ext3undel+foremost+testdisk from rpmforge to undelete information, but somehow I couldn't get all dependencies to install. So there's no usable software to fix this.

Or is there?

Manual recovery

Most filesystems, except for filesystems that do raid/compression/encryption, just write data to blocks on the disk in a structured manner. If you could save and access the blocks, you may be able to do some manual recovery.

Continue Reading…

© GeekLabInfo
Manually undeleting a file from ext2, ext3, ext4 or any other filesystem 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

X11 forwarding request failed on channel 0

Since upgrading to Fedora 17, I've been getting this message "X11 forwarding request failed on channel 0". I haven't done anything to fix it for a while, but today I got so annoyed with this message, that I decided to fix it.

First I google'd around a bit. What does this message mean? I read a lot of reactions that suggest to fix something on ssh_config on the client side or in sshd_config on the server side. These suggestions did not work for me.

So, how do we debug this?

On the server side, open the firewall on a non-standard port. I used port 222:
iptables -I INPUT -s [client-ip] -p tcp --dport 222 -j ACCEPT
Then I ran sshd in non-forking debug mode on this port:
/usr/sbin/sshd -d -p 222
Then we login from the client, using verbose mode:
ssh -vvv [server]

This generates a lot of logs on both sides. The log on the client side contains:
debug3: Ignored env GTK_IM_MODULE
debug3: Ignored env XAUTHORITY
debug3: Ignored env CCACHE_HASHDIR
debug3: Ignored env _
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug1: Remote: No xauth program; cannot forward with spoofing.
debug2: channel_input_status_confirm: type 100 id 0
X11 forwarding request failed on channel 0

So, what's the problem? The server has no xauth program. The old versions of the ssh client silently failed when the server had no xauth, this new version is just a little more verbose.

Continue Reading…

© GeekLabInfo
X11 forwarding request failed on channel 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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IT

Preupgrade – the root for the previously installed system was not found

Today, I've been digging through forums, log files and python scripts for about 16 hours (!!!) to update my system from Fedora 16 to Fedora 17. Apart from the fact that preupgrade for some reason downloaded all files from US mirrors (I'm in .nl), one of the biggest problems I had was the clear message "The root for the previously installed system was not found".

Why the fsck could it not find my installation? Was it the Luks full disk encryption? Or possibly the LVM volume manager? It couldn't be the btrfs filesystem, could it?

I tried everything possible, and when I was about to give up, I read this message that preupgrade has a problem with btrfs snapshots. Do I have these snapshots?

cryptsetup luksOpen /dev/sda2 disk
lvchange -ay vg_hdd/lv_root
mount /dev/mapper/vg_hdd-lv_root /mnt
btrfs subvolume list /mnt

There was the answer. I did. Two snapshots created by yum.

btrfs subvolume delete /mnt/yum-2012....1
btrfs subvolume delete /mnt/yum-2012....2
umount /mnt
lvchange -an vg_hdd/lv_root
cryptsetup luksClose disk

Damn, this has been a great day. Thanks to those crappy error messages. Time to go to bed.© GeekLabInfo
Preupgrade - the root for the previously installed system was not found 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

Hide the mouse cursor with X

I recently got myself a touch screen, that I mounted near the entrance to welcome visitors. The device was really hard to install, as the drivers for 4 different Linux distros were broken, and only binary drivers are released. The fifth distro finally worked.

One thing that does annoy me however, is that X always shows the cursor. To hide the cursor on a touch screen, you can use the program unclutter:

yum install unclutter
unclutter -idle 0

Unclutter with such a short delay (0 seconds) also makes it impossible to select texts, but for this application, that's just perfect.© GeekLabInfo
Hide the mouse cursor with X 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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