Can ping and traceroute but not able to browse certain websites

• I can ping and traceroute with the help of mtr -rw but I simply cannot browse certain websites. I have found no common denominator between these websites to understand where the problem lies.
• The only way to browse these websites is via a Tor Browser and no amount of flushing browser cache, changing browsers (Firefox, Chrome, Brave), private windows etc. does the trick.
• I am on 18.04.5 LTS (Bionic Beaver) and I have tried a lot of stuff from various SO posts but to no avail. I will update my post based on any pertinent comments.

Here are some of the websites (found by pure luck) that I am unable to access :

Otherwise I am able to access every other website, which is driving me crazy !

Here is an example of a ping + traceroute with the help of mtr -rw :

~$ mtr -rw
Start: 2021-01-30T20:17:22+0100
HOST: SIT Loss% Snt Last Avg Best Wrst StDev 1.|-- _gateway 0.0% 10 0.9 0.8 0.7 1.0 0.1 2.|-- 194.149.169.174 0.0% 10 28.5 28.5 28.0 29.2 0.4 3.|-- ??? 100.0 10 0.0 0.0 0.0 0.0 0.0 4.|-- ??? 100.0 10 0.0 0.0 0.0 0.0 0.0 5.|-- be2103.ccr42.par01.atlas.cogentco.com 10.0% 10 28.6 28.8 28.3 29.4 0.4 6.|-- be3685.ccr52.lhr01.atlas.cogentco.com 0.0% 10 35.6 36.1 35.3 37.1 0.5 7.|-- be3672.agr21.lhr01.atlas.cogentco.com 0.0% 10 36.5 36.5 35.9 36.8 0.3 8.|-- sucuri.demarc.cogentco.com 0.0% 10 35.6 35.6 34.1 36.4 0.7 9.|-- cloudproxy10053.sucuri.net 0.0% 10 36.2 35.9 34.7 36.7 0.7

And here is a screenshot from a private window in Firefox :

EDIT 1 : I have another laptop with Ubuntu 16.04.7 LTS installed and with all packages up to date. Previously, I used this system as an independent trial to check whether the ISP could be blocking these websites (I also tried it again now). The result : I cannot browse these websites but I can ping and traceroute them !

But I see no reasonable explanation for an ISP in Europe to block or !

EDIT 2 : It has been suggested in the comments that I reset my modem/router to factory settings for resolving the issue.

The problem is that I have an ADSL modem which has no reset button on it. I can only set some other options from my subscriber account which is available only from the ISP's website !

Note 1 : When I connect to my company VPN for work (using Cisco AnyConnect Secure Mobility Client Version 4.6.04056) and after having used it for a few hours, I can access these websites again ! After work, when I shut off the VPN, these websites remain accessible. I have not yet managed to determine the cut-off when these specific websites become accessible. This has "convinced me" that it is not a modem/router issue.

Note 2 : However, after a day's work (i.e. with the VPN switched off and with all websites accessible), if I were to switch the ADSL modem off and on, I loose access to these websites (and only these) once again ! This has "convinced me" that it could still be a (very weird) modem/router issue.

EDIT 3 : Here is some more info based on comments by @ubfan1. I installed the libnss-resolve package. I didn't find any NXDOMAIN errors in the dmesg output.

~$ dmesg | grep NXDOMAIN
~$

And just in case, here are my resolv.conf contents :

~$ cat /etc/resolv.conf
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "systemd-resolve --status" to see details about the uplink DNS servers
# currently in use.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.
nameserver 127.0.0.53
options edns0
~$
~$ ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 37 Dec 5 11:25 /etc/resolv.conf -> /run/systemd/resolve/stub-resolv.conf

1 Answer

Firefox has a bug for (some) cached sites. When you see that screen, type ctrl-shift R to reload from the actual web site, and you should get your page.