127.0.0.1 not responding when connected to VPN

Question

I have a few services running on localhost, listening on port 80 or 9999.

When I connect to my corporate VPN (Cisco Anyconnect), it suddenly stops responding. localhost resolves well to 127.0.0.1, but it's hanging for a while and finally fails with ERR_CONNECTION_RESET. I can connect to the internet and all my company website (all subdomains from the company domain).

When connected, /etc/resolv.conf looks like this:

nameserver 127.0.0.53
options edns0
search <my corporate domain>

Some informations that might help:

$ traceroute localhost 
traceroute to localhost (127.0.0.1), 30 hops max, 60 byte packets
 1  localhost (127.0.0.1)  0.053 ms  0.049 ms  0.017 ms

$ wget localhost:9999  
--2020-10-27 15:57:25--  http://localhost:9999/
Résolution de localhost (localhost)… 127.0.0.1
Connexion à localhost (localhost)|127.0.0.1|:9999… connecté.
requête HTTP transmise, en attente de la réponse… Erreur de lecture (Connexion ré-initialisée par le correspondant) dans les en-têtes.
Nouvel essai.

Sorry, it's in french, but it manages to connect to 127.0.0.1 but connection is reset.

$ telnet localhost 9999  
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Connection closed by foreign host.

$ ifconfig lo      
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 1000  (Boucle locale)
        RX packets 409827  bytes 193361809 (193.3 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 409827  bytes 193361809 (193.3 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Answer 1

Finally found a solution, even though I still don't understand the root issue.

My services are running on docker, using a docker-compose. For some reason, the VPN is messing up with docker network when I use the default network name (docker-compose_default). Using a different network name solved the issue.

Maybe this might help someone!