IPv6 is working, but IPv4 isn't
I've got a new laptop running Ubuntu 16.04 that worked like a charm for a couple of weeks but has now lost its appetite for connecting to the internet via Wi-fi. I know it's not a problem with my router or ISP, because I'm writing this post using my old Windows laptop on the exact same home network by Ubuntu laptop won't connect to.
I'm connecting to my router successfully, and running nmcli dev shows that I'm connected to my wifi device (wlp2s0) and to the wireless network.
Last night I was able to open some pages (Google, Facebook, YouTube), but not others (ubuntu.com, riken.jp). At the time I suspected it was something where IPv6 was working but IPv4 wasn't. Now, however, nothing works. Chromium, ping, wget, dig -- nothing. /etc/resolv.conf/ contains nothing at all (beyond the comment lines telling me not to edit it), and the table returned by route is empty.
UPDATE 1: I used ipconfig /all on my Windows machine to figure out which DNS servers it was using (75.75.75.75, 75.75.76.76., 2001:558:feed::1, 2001:558:feed::2), then used Network Connections to add these as DNS Servers for my home network. cat /etc/resolv.conf now gives me nameserver 127.0.1.1; I'm not entirely sure what this means, but I think it's some kind of alias, right? nmcli dev show | grep DNS shows me as using the two IPv6 DNS servers (2001:558:feed::1 and 2001:558:feed::2).
I can get wget to work with URLs that resolve to an IPv6 address, like so:
craig@craig-XPS-13-9370:~$ wget www.google.com
--2018-04-14 13:06:17-- http://www.google.com/
Resolving www.google.com (www.google.com)... 2607:f8b0:4004:808::2004,
172.217.13.68
Connecting to www.google.com
(www.google.com)|2607:f8b0:4004:808::2004|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/html]
Saving to: ‘index.html.5’
index.html.5 [ <=> ] 10.41K --.-KB/s in 0.003s
2018-04-14 13:06:17 (3.36 MB/s) - ‘index.html.5’ saved [10659]
I can't get a wget request to work with URLs that resolve to an IPv4 address:
craig@craig-XPS-13-9370:~$ wget www.ubuntu.com
--2018-04-14 13:06:28-- http://www.ubuntu.com/
Resolving www.ubuntu.com (www.ubuntu.com)... 91.189.89.118
Connecting to www.ubuntu.com (www.ubuntu.com)|91.189.89.118|:80... failed:
Network is unreachable.
Adding the -4 flag to wget to force IPv4 usage gets a similar result:
craig@craig-XPS-13-9370:~$ wget -4 www.google.com
--2018-04-14 13:06:41-- http://www.google.com/
Resolving www.google.com (www.google.com)... 172.217.13.68
Connecting to www.google.com (www.google.com)|172.217.13.68|:80... failed:
Network is unreachable.
Similarly, ping www.google.com doesn't work, but ping6 www.google.com does. So it's pretty clear that something is wrong with IPv4, but I'm not sure what...
UPDATE 2: I thought my problem looked pretty similar to this question, but the solution suggested there (disabling IPv4 via Network Connections) doesn't seem to have affected anything. It did give me some ideas for what it's useful to share with others, though:
Output from cat /etc/network/interfaces:
craig@craig-XPS-13-9370:~$ cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
Output from ifconfig:
craig@craig-XPS-13-9370:~$ ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:13526 errors:0 dropped:0 overruns:0 frame:0
TX packets:13526 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1
RX bytes:1031920 (1.0 MB) TX bytes:1031920 (1.0 MB)
wlp2s0 Link encap:Ethernet HWaddr 9c:b6:d0:8b:4b:c5
inet6 addr: fe80::1f3b:5545:508:919a/64 Scope:Link
inet6 addr: 2601:140:8600:e0:d64b:9272:665f:3734/64 Scope:Global
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:19665 errors:0 dropped:0 overruns:0 frame:0
TX packets:10262 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:14731745 (14.7 MB) TX bytes:1722860 (1.7 MB)
Output from iwconfig:
craig@craig-XPS-13-9370:~$ iwconfig
lo no wireless extensions.
wlp2s0 IEEE 802.11abgn ESSID:"MOTOROLA-ABD39"
Mode:Managed Frequency:2.447 GHz Access Point: 34:1F:E4:E4:35:D0
Bit Rate=1 Mb/s Tx-Power=20 dBm
Retry short limit:7 RTS thr:off Fragment thr:off
Power Management:on
Link Quality=55/70 Signal level=-55 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:39 Missed beacon:0
Output from cat /etc/resolv.conf:
craig@craig-XPS-13-9370:~$ cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 127.0.1.1
Output from route -n:
craig@craig-XPS-13-9370:~$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
Output from route -6 -n:
craig@craig-XPS-13-9370:~$ route -6 -n
Kernel IPv6 routing table
Destination Next Hop Flag Met Ref Use If
2601:140:8600:e0::/64 :: U 600 0 0 wlp2s0
2601:140:8600:e0::/60 fe80::361f:e4ff:fee4:35cf UG 600 0 0 wlp2s0
fe80::/64 :: U 256 0 0 wlp2s0
::/0 fe80::361f:e4ff:fee4:35cf UG 600 2 4 wlp2s0
::/0 :: !n -1 1 1495 lo
::1/128 :: Un 0 7 46 lo
2601:140:8600:e0:d64b:9272:665f:3734/128 :: Un 0 2 150 lo
fe80::1f3b:5545:508:919a/128 :: Un 0 2 4 lo
ff00::/8 :: U 256 7 99 wlp2s0
::/0 :: !n -1 1 1495 lo
Output from ip neigh:
craig@craig-XPS-13-9370:~$ ip neigh
fe80::361f:e4ff:fee4:35cf dev wlp2s0 lladdr 34:1f:e4:e4:35:cf router REACHABLE
Solution 1:
Not sure if this is a permanent solution, but I managed to get things working again by releasing and renewing my IP address:
sudo dhclient -r wlp2s0
sudo dhclient wlp2s0
Now I see an inet addr line when I run ifconfig (which was never there before) and cat /etc/resolv.conf has added an IPv4 nameserver and a "search" line for my ISP (search hsd1.va.comcast.net). Problem solved!
Solution 2:
Sometimes this can be caused by an incorrect system time. If your system time is too far off, you will not be leased an IPv4 by DHCP -- only an IPv6.
You can check the system time settings by running timedatectl in a terminal.
If you need to reconfigure your system time and/or time zone, run sudo dpkg-reconfigure tzdata in a terminal.
Assigning a static IP/Gateway/DNS could also work regardless, but certain websites will complain.