If IPv6 address times out, no attempt at IPv4

[John Kirk]

Disclaimer: This is very similar to tickets ​4204 and ​7340, which both got rejected.

I've just tried using FileZilla v3.10.2 to connect to ftp.cisco.com

When I run nslookup, it says:

Non-authoritative answer:
Name:    ftp.cisco.com
Addresses:  2001:420:1101:9::a
          72.163.7.54

FileZilla then says:

Status:	Resolving address of ftp.cisco.com
Status:	Connecting to [2001:420:1101:9::a]:21...
Error:	Connection timed out after 20 seconds of inactivity
Error:	Could not connect to server
Status:	Waiting to retry...
Status:	Resolving address of ftp.cisco.com
Status:	Connecting to [2001:420:1101:9::a]:21...
Error:	Connection timed out after 20 seconds of inactivity
Error:	Could not connect to server

I am using IPv4 and IPv6 on my internal network, but I don't have IPv6 internet access (yet). By contrast, www.cisco.com currently has 3 addresses:
2a02:26f0:ca:286::90
2a02:26f0:ca:290::90
23.195.11.243

I can connect to that website, because the web browser (IE/Firefox) realises that the IPv6 address is inaccessible and so it uses IPv4 instead. I think that FileZilla should behave the same way for FTP sites.

As a workaround, I typed in the IPv4 address for ftp.cisco.com (72.163.7.54) and then FileZilla connected successfully.

[Tim Kosse]

Try setting a larger timeout value, e.g. 300 seconds. Does it than fall back eventually?

[John Kirk]

That's interesting: yes, it does fall back to IPv4 with a longer timeout. So, apparently the fallback logic is different to the retry logic (which uses IPv6 again).

Status:	Resolving address of ftp.cisco.com
Status:	Connecting to [2001:420:1101:9::a]:21...
Status:	Connection attempt failed with "ETIMEDOUT - Connection attempt timed out", trying next address.
Status:	Connecting to 72.163.7.54:21...
Status:	Connection established, waiting for welcome message...
Status:	Insecure server, it does not support FTP over TLS.
Status:	Connected
Status:	Retrieving directory listing...
Status:	Directory listing of "/" successful
Response:	421 Idle Timeout (60 seconds): Closing control connection
Error:	Connection closed by server

[Tim Kosse]

The fallback on happens on connection failures establishing the TCP connection.

The server in question seems to support IPv6 just fine.

I wonder whether your network is configured correctly, ordinarily hostnames only resolve to an IPv6 address if there is working IPv6.