This is moreso as a note for myself in the future as this was one of the posts I that popped up when looking into this problem::::
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** So I had performed some updates on my Debian 9 system and I ended up losing my wireless connection. After a lot of online search suggestions, I was able to figure out how to get it working for the most part. The following is the trail which led me to getting back my wireless connection. I have formatted the post for legibility and hope everything will progressively make sense.
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sudo iw dev
phy#0
Interface wlp4s0
ifindex 3
wdev 0x1
addr aa:bb:cc:dd:ee:ff
type managed
txpower 0.00 dBm
multicast TXQ:
qsz-byt qsz-pkt flows drops marks overlmt hashcoltx-bytes tx-packets
0 0 0 0 0 0 0 00
###### iw dev showed my wireless adapter name 'wlp4s0'; the name can vary depending on the the device. Will need this name for later.
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ip address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp2s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
link/ether aa:bb:cc:dd:ee:ff brd ff:ff:ff:ff:ff:ff
3: wlp4s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
link/ether aa:bb:cc:dd:ee:ff brd ff:ff:ff:ff:ff:ff
###### this showed that 'wlp4s0' was still retrievable by the ip util; it also showed that its current state was 'DOWN' when we want it as 'UP'
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sudo ip link set dev wlp4s0 up
###### Running this should make the device work. Nothing outputted and it didnt work.
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sudo vim /etc/network/interfaces
###### Using the editor of choice, I entered/edited the router/modem SSID/Pass into the config file.
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sudo service wpa_supplicant restart
###### restarted 'WifiProtectedAccess' service
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sudo wpa_supplicant -B -i wlp4s0 -c /etc/wpa_supplicant.conf
Successfully initialized wpa_supplicant
###### asked it to refer to a config I had edited before all this for the wifi interface
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sudo ifdown wlp4s0
Killed old client process
Internet Systems Consortium DHCP Client 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
All rights reserved.
For info, please visit
https://www.isc.org/software/dhcp/
Listening on LPF/wlp4s0/aa:bb:cc:dd:ee:ff
Sending on LPF/wlp4s0/aa:bb:cc:dd:ee:ff
Sending on Socket/fallback
###### Brought the interface down... relax, going to restart it.
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sudo ifup wlp4s0
Internet Systems Consortium DHCP Client 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
All rights reserved.
For info, please visit
https://www.isc.org/software/dhcp/
Listening on LPF/wlp4s0/aa:bb:cc:dd:ee:ff
Sending on LPF/wlp4s0/aa:bb:cc:dd:ee:ff
Sending on Socket/fallback
DHCPDISCOVER on wlp4s0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on wlp4s0 to 255.255.255.255 port 67 interval 8
DHCPOFFER of 192.168.0.17 from 192.168.0.1
DHCPREQUEST for 192.168.0.17 on wlp4s0 to 255.255.255.255 port 67
DHCPACK of 192.168.0.17 from 192.168.0.1
bound to 192.168.0.17 -- renewal in 40264 seconds.
###### Brought the interface back up- sometimes it takes a few moments of anxiety; The end result shows it worked.
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sudo ifdown -a
sudo ifup wlp4s0
sudo wpa_supplicant -B -i wlp4s0 -c /etc/wpa_supplicant.conf
###### Moving forward, if rebooting and still no wifi- the above three commands bring wifi back up; This isn't a solid fix, but is more like a patch to the real problem. Wifi should be starting up on its own. I could enter this as a startup script- but the startup setup meant to deal with this is most likely reset/blank/deleted somewhere. Maybe check xinit or something?