Network connectivity problems on Debian 9

Question

All of the sudden I've got a network connectivity problem. Ping is not working, I can not update (or install) packages from Debian-repos.

My server is perfectly well reachable from outside. However, a few days ago ping stopped working at all and I could not download any files with wget. Turned out there was a problem with dns names resolutions. I allowed port 53 in my security group (vps on OpenStack), which solved the problem with dns names resolutions, I can download files with wget, but fetching packages does not work. Tried what I could find, but nothing seems to work (I'm not very experienced, so may well be overlooking a trivial solution). Any help is very much appreciated.

My machine is a vps on OpenStack. There was a major disruption on the whole cluster recently, so may well be related, but I'm not 100% sure.

I installed yunohost on the server. Yunohost has its own configuration, which may be a source of problem, but this seems unlikely, since I'm sure I could fetch packages after the installation of yunohost.

• Output of less /etc/resolv.conf:

  nameserver 127.0.0.1
  search openstacklocal
  

• Output of ifconfig -a

  eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
          inet XXX.XX.XX.XXX  netmask 255.255.240.0  broadcast XXXXXXXXXXXXXXXX
          inet6 YYYYYYYYY  prefixlen 64  scopeid 0x20<link>
          ether ZZZZZZZZ  txqueuelen 1000  (Ethernet)
          RX packets 194669057  bytes 11708725664 (10.9 GiB)
          RX errors 0  dropped 2  overruns 0  frame 0
          TX packets 412705  bytes 123125411 (117.4 MiB)
          TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
  
  lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
          inet 127.0.0.1  netmask 255.0.0.0
          inet6 ::1  prefixlen 128  scopeid 0x10<host>
          loop  txqueuelen 1  (Local Loopback)
          RX packets 150233  bytes 31116860 (29.6 MiB)
          RX errors 0  dropped 0  overruns 0  frame 0
          TX packets 150233  bytes 31116860 (29.6 MiB)
          TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
  

  Update: I've got ping to work by opening ICMP for engres (is this secure?), however fetching packages still does not work.

Here is the output of apt update:

Hit:1 https://packages.sury.org/php stretch InRelease
Err:2 http://forge.yunohost.org/debian stretch InRelease                                                                                                                                     
  Cannot initiate the connection to forge.yunohost.org:80 (2001:910:1410::1). - connect (101: Network is unreachable) [IP: 2001:910:1410::1 80]
Err:3 http://deb.debian.org/debian stretch InRelease                                                                                                                    
  Cannot initiate the connection to prod.debian.map.fastly.net:80 (2a04:4e42:1b::204). - connect (101: Network is unreachable) Cannot initiate the connection to deb.debian.org:80 (2a04:4e42:1b::645). - connect (101: Network is unreachable) [IP: 2a04:4e42:1b::645 80]
Err:4 http://deb.debian.org/debian stretch-updates InRelease                                            
  Cannot initiate the connection to deb.debian.org:80 (2a04:4e42:1b::645). - connect (101: Network is unreachable) [IP: 2a04:4e42:1b::645 80]

Answer 1

Problem solved: For some reason, a firewall blocked all outgoing connections. Not sure how and why this happened, but opening ports solved the problem.