The fail

On a peaceful Sunday, I was just minding my own business and BOOM! Internet connection was gone. After a quick debugging session, restarting the router and eyeballing the LEDs, it was evident: something with my ISP Com Hem was down:

Ok, ISP down, what next?
I whipped up the iPhone and went for any possible service announcements. And yes, the above announcement was placed on my user account information. I was stunned by this, it was so cool to have:

• confirmation, that something was down with ISP: Yup, it's broken.
• that information tailored with the geographical location of my subscription: Yup, that fail affects you.

No Finnish ISP or telco has that. I was very impressed with such detail (and still am).

The fix

There is no way I'm sitting on my thumbs on such an event. I was just about to start playing Need for Speed and now Origin wouldn't even log me in, so, no Internet, no gaming.

I have an el-cheapo Huawei Android lying around somewhere, with a Swedish SIM-card in it. My dirt cheap subscription has couple of gigs data transfer per month in it, which I never use. I came up with a plan to temporarily use the cell phone as an Internet connection. The idea would be to hook it up into my Linux router with an USB-cable, make sure the Android pops up as a network interface and then configure the Linux to use that network interface as primary connection.

Thethering

I found tons of information about Android-tethering from Arch Linux wiki. It basically says:

1. Make sure your Android is newer than 2.2
2. Connect the phone to a Linux
3. Enable USB-tethering from the phone's connection sharing -menu
4. Confirm the new network interface's existence on the Linux end

On my phone, there was two settings for personal hotspot. Wifi/Bluetooth and USB:

Connection

New phones have USB-C, but its such a new connector type, that anything older than couple years, has most likely micro-USB -connector:

Hooking it up to a Linux will output tons of dmesg and and ultimately result in a brand new network interface:

# ip addr show
5: enp0s20u4u3:
    link/ether 82:49:a8:b4:96:c9 brd ff:ff:ff:ff:f
    inet 192.168.42.90/24 brd 192.168.42.255 scope
       valid_lft 3595sec preferred_lft 3595sec
    inet6 fe80::7762:e1a9:9fa:69f5/64 scope link
       valid_lft forever preferred_lft forever

Routing configuration

Now that there was a new connection, I tried pinging something in the wild world:
ping -I enp0s20u4u3 193.166.3.2

Nope. Didn't work.
I confirmed, that the default network gateway was still set up into the broken link:
# ip route show
default via 192.168.100.1 dev enp1s0 proto static metric 100

That needs to go to enable some functionality. But what to replace the bad gateway with?
Since the connection had IP-address from Telco DHCP, there is a lease-file with all the necessary information:
# cat /var/lib/NetworkManager/dhclient-*-enp0s20u4u3.lease
lease {
  interface "enp0s20u4u3";
  fixed-address 192.168.42.90;
  option subnet-mask 255.255.255.0;
  option routers 192.168.42.129;

The fixed-address in the file matches the above ip addr show -information. Required information was gathered, and the idea was to ditch the original gateway and replace it with a one from the Android phone's telco:
# ip route del default via 192.168.100.1
# ip route add default via 192.168.42.129 dev enp0s20u4u3
# ip route show
default via 192.168.42.129 dev enp0s20u4u3 proto static metric 101

Now it started cooking:
# ping -c 5 ftp.funet.fi
PING ftp.funet.fi (193.166.3.2) 56(84) bytes of data.
64 bytes from ftp.funet.fi (193.166.3.2): icmp_seq=1 ttl=242 time=35.6 ms
64 bytes from ftp.funet.fi (193.166.3.2): icmp_seq=2 ttl=242 time=31.7 ms

To finalize the access from my LAN, I ran following firewall-cmd --direct commands:
 --remove-rule ipv4 nat POSTROUTING 0 -o enp1s0 -j MASQUERADE
 --add-rule ipv4 nat POSTROUTING 0 -o enp0s20u4u3 -j MASQUERADE
 --add-rule ipv4 filter FORWARD 0 -i enp3s0 -o enp0s20u4u3 -j ACCEPT
 --add-rule ipv4 filter FORWARD 0 -i enp0s20u4u3 -o enp3s0 \
    -m state --state RELATED,ESTABLISHED -j ACCEPT

There is no firewall-cmd --permanent on purpose. I don't intend those to stick too long. I just wanted to play the darn game!

Done!

Now my gaming PC would connect to The Big Net. I could suft the web, read mail and even Origin logged me in.

That's it! Day saved!