Why localhost is mapping to ipv6 ::1 and not to ipv4 127.0.0.1

Question

I am developing an application in electronJS to run in Linux environment. It was working on old version of SLES15 SP4. When I upgraded the OS to SELES 15 SP5. I can see electron code is not able to call RES endpoint.

Description about issue:

• Electron code is making REST call to endpoint http://localhost:8080/v1/myapp.

• It seems localhost is mapped to ::1 ipv6 IP. Which is causing issue.

  Error log snippet: Error: connect ECONNREFUSED ::1:8082

• content of /etc/hosts file:

  # cat /etc/hosts

  127.0.0.1 localhost

  ::1 localhost ipv6-localhost ipv6-loopback

• Now My question is why localhost is mapping to ::1 and not to 127.0.0.1. while the same code was working fine in old version OS with same content in /etc/hosts file.

NOTE: If you will comment line ::1 localhost ipv6-localhost ipv6-loopback in /etc/hosts file it will start working. But my requirement is to make it work with same content in /etc/hosts file.

Answer 1

The default is to resolve "names" to their IPv6 address, instead of IPv4 addresses. I'm sure this is the default since a while already.

The way how the glibc resolver works can be controlled via /etc/gai.conf, see the gai.conf(5) man page for details.