Tag: Linux

Firewalld — Adding and Removing a Forwarding Rule

(Sorry, Anya … after today, I’ll try to not post anything about computers for three days!) Linux restricts non-root users from opening ports <1024. It’s generally a good idea not to run your services as root. Which means, unfortunately, we end up running a lot of services on nonstandard ports (so frequently that 1389 and 1636 are a quasi-standard port for LDAP and LDAPS, 8080 and 8443 quasi-standard ports for HTTP and HTTPS). But having to remember to add the nonstandard port to a web URL is an annoyance for users — I’ve seen a lot of people fix this by adding a load balanced VIP or NGINX proxy in front of the service to handle port translations. But there is a quick and easy way to handle port translation without any additional equipment. Most Linux hosts have firewalld running, and you can tell the firewall to forward the port for you. In this example, I’m letting my Kibana users access my web service using https://kibana.example.com without needing to append the :5601:

firewall-cmd –permanent –zone=public –add-forward-port=port=443:proto=tcp:toport=5601

Should you decide against the port forwarding, the same command with –remove-forward-port deregisters the rule:
firewall-cmd –zone=public –remove-forward-port=port=443:proto=tcp:toport=5601

Using the Dell 1350CN On Fedora

We picked up a really nice color laser printer — a Dell 1350CN. It was really easy to add it to my Windows computer — download driver, install, voila there’s a printer. We found instructions for using a Xerox Phaser 6000 driver. It worked perfectly on Scott’s old laptop, but we weren’t able to install the RPM on his new laptop — it insisted that a dependency wasn’t found: libstdc++.so.6 CXXABI_1.3.1

Except, checking the file, CXXABI_1.3.1 is absolutely in there:

2022-09-17 13:04:19 [lisa@fc36 ~/]# strings /usr/lib64/libstdc++.so.6 | grep CXXABI

We’ve tried using the foo2hbpl package with the Dell 1355 driver to no avail. It would install, but we weren’t able to print. So we returned to the Xerox package.

Turns out the driver package we were trying to use is a 32-bit driver (even though the download says 32 and 64 bit). From a 32-bit perspective, we really didn’t have libstdc++ — a quick dnf install libstdc++.i686 installed the library along with some friends.

Xerox’s rpm installed without error … but, attempting to print, just yielded an error saying that the filter failed. I had Scott use ldd to test one of the filters (any of the files within /usr/lib/cups/filter/Xerox_Phaser_6000_6010/ — it indicated the “libcups.so.2” could not be found. We also needed to install the 32-bit cups-libs.i686 package. Finally, he’s able to print from Fedora 36 to the Dell 1350cn!



Finding PCI Devices

You can use dmidecode to list all sorts of information about the system — there is a list of device types that you can use with the “-t” option

   Type   Information
      0   BIOS
      1   System
      2   Baseboard
      3   Chassis
      4   Processor
      5   Memory Controller
      6   Memory Module
      7   Cache
      8   Port Connector
      9   System Slots
     10   On Board Devices
     11   OEM Strings
     12   System Configuration Options
     13   BIOS Language
     14   Group Associations
     15   System Event Log
     16   Physical Memory Array
     17   Memory Device
     18   32-bit Memory Error
     19   Memory Array Mapped Address
     20   Memory Device Mapped Address
     21   Built-in Pointing Device
     22   Portable Battery
     23   System Reset
     24   Hardware Security
     25   System Power Controls
     26   Voltage Probe
     27   Cooling Device
     28   Temperature Probe
     29   Electrical Current Probe
     30   Out-of-band Remote Access
     31   Boot Integrity Services
     32   System Boot
     33   64-bit Memory Error
     34   Management Device
     35   Management Device Component
     36   Management Device Threshold Data
     37   Memory Channel
     38   IPMI Device
     39   Power Supply
     40   Additional Information
     41   Onboard Devices Extended Information
     42   Management Controller Host Interface


[lisa@fedora ~/]# dmidecode -t 9

Handle 0x0024, DMI type 9, 17 bytes
System Slot Information
Designation: Slot6
Type: 32-bit PCI
Current Usage: In Use
Length: Short
ID: 6
3.3 V is provided
Opening is shared
PME signal is supported
Bus Address: 0000:0a:02.0

The “Bus Address” value corresponds to information from lspci:

[lisa@fedora ~/]# lspci | grep “0a:02.0”
0a:02.0 Multimedia video controller: Conexant Systems, Inc. CX23418 Single-Chip MPEG-2 Encoder with Integrated Analog Video/Broadcast Audio Decoder

XRDP Logon Hangs on Black Screen

I’m writing it down this time — after completing the steps to set up xrdp (installed, configured, running, firewall port open), we get prompted for credentials … good so far!

And then get stuck on a black screen. This is because the user we’re trying to log into is already logged into the machine. Log out locally, and the user is able to log into the remote desktop connection. Conversely, attempting to log in locally once the remote desktop connection is established just hangs on a black screen too.

Using Screen to Access Console Port

We needed to console into some Cisco access points — RJ45 to USB to plug into the device console port and the laptop’s USB port? Check! OK … now what? Turns out you can use the screen command as a terminal emulator. The basic syntax is screen <port> <baud rate> — since the documentation said to use 9600 baud and the access point showed up on /dev/ttyUSB0, this means running:


screen /dev/ttyUSB0 9600

More completely, screen <port> <baud rate>,<7 or 8 bits per byte>,<enable or disable sending flow control>,<enable or disable rcving flow control>,<keep or clear the eight bit in each byte>

screen /dev/ttyUSB0 9600,cs8,ixon,ixoff,istrip 
- or - 
screen /dev/ttyUSB0 9600,cs7,-ixon,-ixoff,-istrip

Logstash, JRuby, and Private Temp

There’s a long-standing bug in logstash where the private temp folder created for jruby isn’t cleaned up when the logstash process exits. To avoid filling up the temp disk space, I put together a quick script to check the PID associated with each jruby temp folder, see if it’s an active process, and remove the temp folder if the associated process doesn’t exist.

When the PID has been re-used, this means we’ve got an extra /tmp/jruby-### folder hanging about … but each folder is only 10 meg. The impacting issue is when we’ve restarted logstash a thousand times and a thousand ten meg folders are hanging about.

This script can be cron’d to run periodically or it can be run when the logstash service launches.

import subprocess
import re

from shutil import rmtree

strResult = subprocess.check_output(f"ls /tmp", shell=True)

for strLine in strResult.decode("utf-8").split('\n'):
        if len(strLine) > 0 and strLine.startswith("jruby-"):
                listSplitFileNames = re.split("jruby-([0-9]*)", strLine)
                if listSplitFileNames[1] is not None:
                                strCheckPID = subprocess.check_output(f"ps -efww |  grep {listSplitFileNames[1]} | grep -v grep", shell=True)
                                #print(f"PID check result is {strCheckPID}")
                                print(f"I am deleting |{strLine}|")

Using urandom to Generate Password

Frequently, I’ll use password generator websites to create some pseudo-random string of characters for system accounts, database replication,etc. But sometimes the Internet isn’t readily available … and you can create a decent password right from the Linux command line using urandom.

If you want pretty much any “normal” character, use tr to pull out all of the other characters:


Or remove anything outside of upper case, lower case, and number characters using


Pass the output to head to grab however many characters you actually want. Voila — a quick password.