Change Channel Width on Ubiquiti AC CPE Over SSH

SSH into CPE

ssh ubnt@

open up the “/tmp/system.cfg” file

vi /tmp/system.cfg

Find the “radio.1.chanbw=” line and change to the following


Note: it may say “radio.1.chanbw=0” that looks to be the default and works for 20mhz

exit file with

esc + :wq + Return or Enter

Save settings with

/usr/etc/rc.d/rc.softrestart save 

Change AP to 10Mhz and the CPE should reconnect.

UBNTMOD check range of IP addresses and see if they resolve DNS

The following uses the script to check a device then report if it is resolving DNS or not. ip=”192.168.1.” specifies the first part of the ip, the “for ((i=1; i<=254;i++))” tells it to go from –, change the beginning and ending number if you want to change the ip range.

ip="192.168.1." && for ((i=10; i<=30;i++)); do if ( fping ${ip}$i -r1 | grep alive); then ./ -i ${ip}${i} -e ; else echo ${ip}$i not alive; fi ; done 

Broken out for easier reading.

for ((i=10; i<=30;i++))
if ( fping ${ip}$i -r1 | grep alive); then
./ -i ${ip}${i} -e
else echo ${ip}$i not alive

If the script is able to log into the device and resolve DNS you should get Resolves DNS

Installing LineageOS 17 Android 10 on Pixel

Download twrp from here
Download LineageOS 17 from here

Boot phone to boot menu then fastboot twrp

sudo fastboot boot Downloads/twrp-3.3.0-0-sailfish.img

Wipe to factory defaults

Side load LineageOS Android 10 from the Advanced page on TWRP

adb sideload Downloads/ 

Reboot and setup.

Linux, Send USR1 signal to pid

In Linux you can send signals to a process id to trigger actions for the program. Useful scenario for this is to renew an IP address on a device that uses udhcpc. You should be able to change udhcpc for other programs, you’ll just need to read the help for that specific program.

In the udhcpc help it says

         USR1    Renew lease
         USR2    Release lease

But how do we send those signals to udhcpc? Answer, use the kill command.

kill: kill [-s sigspec | -n signum | -sigspec] pid | jobspec … or kill -l [sigspec]
     Send a signal to a job.

Send the processes identified by PID or JOBSPEC the signal named by SIGSPEC or SIGNUM.  If neither SIGSPEC nor SIGNUM is present, then SIGTERM is assumed. 

-s sig    SIG is a signal name   
-n sig    SIG is a signal number   
-l        list the signal names; if arguments follow `-l' they are             
          assumed to be signal numbers for which names should be listed   
-L        synonym for -l 

Kill is a shell builtin for two reasons: it allows job IDs to be used instead of process IDs, and allows processes to be killed if the limit on processes that you can create is reached.
Exit Status:
Returns success unless an invalid option is given or an error occurs. 

We see from above that we can pass a signal name in using the -s option.

So to send USR1 signal to udhcp we do the following

kill -s USR1 pid_of_udhcpc

Replace pid_of_udhcpc with the actual pid or use the following command to find the pid

kill -s USR1 $(pgrep udhcpc)

“pgrep udhcpc” prints the pid of the searched for process.

Helpful links

Ubiquiti – Renewing DHCP client lease from command line

AirOS uses udhcpc for the DHCP client on Ubiquiti Radios. To renew the DHCP address you can kill the udhcpc process and it’ll automatically restart and get a new address.

Renew DHCP lease

Kill udhcpc with the following command.

killall udhcpc

Other info

Print info about the DHCP lease. May need to change “info.br1” to “info.eth0” or some other interface.

cat /etc/udhcpc/info.eth0 

Example output.

XW.v6.2.0# cat /etc/udhcpc/info.br1 

Command arguments that udhcp is run with. Info was collected by running the “ps | grep udhcp” command. Note that the interface “eth0” can be different if the device is in bridge mode.

/sbin/udhcpc -f -i eth0 -s /etc/udhcpc/udhcpc -p /var/run/ -h device_name

udhcpc help output

XW.v6.2.0# udhcpc --help
BusyBox v1.24.2 (2019-07-03 11:13:35 EEST) multi-call binary.

Usage: udhcpc [-fbqvRB] [-t N] [-T SEC] [-A SEC/-n]
        [-i IFACE] [-s PROG] [-p PIDFILE]
        [-oC] [-r IP] [-V VENDOR] [-F NAME] [-x OPT:VAL]... [-O OPT]...

        -i,--interface IFACE    Interface to use (default eth0)
        -s,--script PROG        Run PROG at DHCP events (default /usr/share/udhcpc/default.script)
        -p,--pidfile FILE       Create pidfile
        -B,--broadcast          Request broadcast replies
        -t,--retries N          Send up to N discover packets (default 3)
        -T,--timeout SEC        Pause between packets (default 3)
        -A,--tryagain SEC       Wait if lease is not obtained (default 20)
        -n,--now                Exit if lease is not obtained
        -q,--quit               Exit after obtaining lease
        -R,--release            Release IP on exit
        -f,--foreground         Run in foreground
        -b,--background         Background if lease is not obtained
        -S,--syslog             Log to syslog too
        -r,--request IP         Request this IP address
        -o,--no-default-options Don't request any options (unless -O is given)
        -O,--request-option OPT Request option OPT from server (cumulative)
        -x OPT:VAL              Include option OPT in sent packets (cumulative)
                                Examples of string, numeric, and hex byte opts:
                                -x hostname:bbox - option 12
                                -x lease:3600 - option 51 (lease time)
                                -x 0x3d:0100BEEFC0FFEE - option 61 (client id)
        -F,--fqdn NAME          Ask server to update DNS mapping for NAME
        -V,--vendorclass VENDOR Vendor identifier (default 'udhcp VERSION')
        -C,--clientid-none      Don't send MAC as client identifier
        -v                      Verbose
        USR1    Renew lease
        USR2    Release lease

Note that you can also send a signal to the PID of udhcpc and have it renew the address. To do that use the following command, replacing pidofudhcpc to pid of udhcp.

kill -s USR1 pidofudhcpc

or use the following command to find the pid for you.

kill -s SIGUSR1 $(pgrep udhcpc)

Check if Mikrotik is an Open DNS Resolver

You can test if a router is acting as an open DNS resolver by running the following command from a Linux terminal. If you need to install dig, refer to here for Debian/Ubuntu and here for RPM/CentOS/Fedora Distros.

Replace with the host you want to test against.

dig +short TXT @

If you receive the following


The router is acting as an open resolver.

If you get

;; connection timed out; no servers could be reached

Then you are unable to use that router to resolve DNS.

Example running the command against a Mikrotik router with Remote DNS turned on Then adding a firewall rule to block unwanted request.

bob@localhost:~$ dig +short TXT @
<<-- Put firewall rule on router -->>
bob@localhost:~$ dig +short TXT @
;; connection timed out; no servers could be reached  

Extra notes

If you have firewall rules allowing your IP address to use the router for DNS, then the above command to test will show it as an Open Resolver. Ideally you would want a connection from the outside to test. Or you can use this link and test it from the website.

Install Microsoft Teams Preview on Linux

Download the correct package for your distribution of Linux from

You should be able to open the installer and it should install, if not you can run the following commands from a terminal

The install instructions are for Debian/Ubuntu/Linux Mint.

Install using dpkg

sudo dpkg -i Downloads/teams_1.2.00.32451_amd64.deb

Launch Teams by typing


Or you can launch it from your Applications Menu

After Teams is installed and launched, sign in to your Microsoft account.

DD show show status of progress

You can have dd show the progress of a write by specifying “status=progress” in the command line arguments.

sudo dd if=Downloads/CentOS-8-x86_64-1905-boot.iso of=/dev/sdb status=progress


bob@localhost:~$ sudo dd if=Downloads/CentOS-8-x86_64-1905-boot.iso of=/dev/sdb status=progress
559690240 bytes (560 MB, 534 MiB) copied, 96 s, 5.8 MB/s    <-- This is shown while writing.
1093632+0 records in
1093632+0 records out
559939584 bytes (560 MB, 534 MiB) copied, 96.0339 s, 5.8 MB/s