Bug 4649

When attempting to send or receive email via a server with a non-functioning IPv6 protocol, Claws Mail (CM) fails rather than attempting to use the functioning IPv4 protocol.

Steps to Reproduce:

1. Have a system that:

a) is connected only to a server / gateway that is set up to handle both IPv4 and IPv6 protocols, and where the IPv4 protocol is functioning normally but the IPv6 protocol is 'broken'.

b) has CM installed with IPv6 compatibility enabled.

2. Attempt to send or receive email.

Actual result: CM fails, giving a time-out error.

Expected result: CM will attempt to use IPv6 but if that fails, it will use IPv4 to successfully send / receive email.

Potentially helpful related information:

1. This looks like it could be the GTK3 version of Bug #3244.

2. The 'traceroute', 'dig', and 'nmap' utilities seem to be able to produce their respective 'expected results' on the above-mentioned system without needing to use an option to specify the IPv4 protocol.

3. The 'telnet' and 'openssl' utilities seem to fail in the same way as CM on the above-mentioned system (assuming they are not using the option to specify the IPv4 protocol). (When using the option to specify IPv4, they both work as expected.)

4. Workaround #1 (untested): Build CM with IPv6 compatibility disabled.

5. Workaround #2 (tested OK): Create / edit the '/etc/gai.conf' file so it has the following line:

precedence ::ffff:0:0/96  100

My understanding is that this will direct the system to give precedence to using the IPv4 protocol.

6. Here is an email I sent to the users list with more details on the testing I did to troubleshoot this: https://lists.claws-mail.org/pipermail/users/2022-October/031115.html .