I've been running Stretch for several months, no problems.

A couple days ago I suddenly lost wifi Internet connectivity even though wicd said I was connecting to my router normally.

1. Other devices work OK.

2. Rebooting modem, router, and computer doesn't help.

3. When I boot into Stretch on the same computer using a USB drive it works fine.

4. When I change wireless cards it still doesn't work.

5. Reinstalling everything (aptitude -reinstall '~i') doesn't help.

6. ifconfig says:

wlan1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.43 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::6666:b3ff:fef4:68b0 prefixlen 64 scopeid 0x20<link>
ether 64:66:b3:f4:68:b0 txqueuelen 1000 (Ethernet)
RX packets 154 bytes 30836 (30.1 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 28 bytes 4474 (4.3 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

7. route -e says:

Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
default 192.168.1.1 0.0.0.0 UG 0 0 0 wlan1
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 wlan1

8. lspci -knn | grep Net -A2 says:

04:00.0 Network controller [0280]: Qualcomm Atheros AR9227 Wireless Network Adapter [168c:002d] (rev 01)
Subsystem: Qualcomm Atheros AR9227 Wireless Network Adapter [168c:0301]
Kernel driver in use: ath9k
Kernel modules: ath9k

9. ping yahoo.com says:

ping: yahoo.com: Temporary failure in name resolution

Any thoughts?

Yes. Add your dns servers to /etc/resolv.conf.

Tried it but same result.

What did you add? It should be like that
Do you use network manager?

I use wicd. It was originally

I changed it to

because the last two are the places the router's set to go to. I notice though that it has now been changed automatically back to what it was. (In the interim I had changed from the wireless to the wired interface.)

Try removing these 2 lines
Unless your laptop is in some domain...

Did so but no joy. (Anyway, it's not a laptop, it's a home server on a different floor than the router.)

I am out of ideas. Install traceroute and use it to check where the packets are dropped and continue from there on.

If you can ping the gateway but not the Internet, but you can ping the Internet with other machines on the LAN, it's probably wicd's fault. I used to use wicd, because it was more reliable and sensible than network-manager.

But the situation changed several years back, and the useless network-manager became a superstar in my book. It hasn't failed me since! But if the interface is managed by ifupdown (/etc/network/interfaces file) network-manager will not manage it.

So, you need to comment out interfaces you want NM to handle.

Turned out it was iptables. Removing iptables.up.rules from /etc made the problem go away. I rewrote the rules from scratch and it now works without problems.