php: PECL not working in php:7.4-fpm-alpine up from php:7.3-fpm-alpine due to DNS issue with pecl.php.net

After changing from php:7.3-fpm-alpine to php:7.4-fpm-alpine, the pecl install command stopped working. The error given in the docker build process is: No releases available for package "pecl.php.net/redis" for RUN pecl install redis-5.3.2

tl;dr: The problem seems to be with alpine 3.13 and not 3.12, so you can use php:7.4.15-fpm-alpine3.12.

Digging for cause

Many older resources online refer to the Great Firewall of China as the cause as this seems to be a network issue, but I’m not in China or behind any strange network setup. I dig on.

Entering the container, the following things seem problematic and related:

bash-5.1#  pecl install redis-5.3.2
No releases available for package "pecl.php.net/redis"
install failed
bash-5.1# pear update-channels
Updating channel "doc.php.net"
Channel "doc.php.net" is up to date
Updating channel "pear.php.net"
Channel "pear.php.net" is up to date
Updating channel "pecl.php.net"
Channel "pecl.php.net" is not responding over http://, failed with message: Connection to `pecl.php.net:80' failed: php_network_getaddresses: getaddrinfo failed: Try again
Trying channel "pecl.php.net" over https:// instead
Cannot retrieve channel.xml for channel "pecl.php.net" (Connection to `ssl://pecl.php.net:443' failed: php_network_getaddresses: getaddrinfo failed: Try again)
bash-5.1#

It makes sense that if pecl.php.net is not reachable then it pecl install will fail. The cause seems to be related to resolving the hostname:

bash-5.1# nslookup pear.php.net
Server:         192.168.65.1
Address:        192.168.65.1:53

Non-authoritative answer:
pear.php.net    canonical name = euk3.php.net

Non-authoritative answer:
pear.php.net    canonical name = euk3.php.net
Name:   euk3.php.net
Address: 109.203.101.62

bash-5.1# nslookup pecl.php.net
Server:         192.168.65.1
Address:        192.168.65.1:53

Non-authoritative answer:
Name:   pecl.php.net
Address: 104.236.228.160

Non-authoritative answer:
*** Can't find pecl.php.net: No answer

bash-5.1#

From here on out I’m not sure what the root cause of this problem is, but I do see a working IP address for pecl.php.net.

Proof of concept

Changing the command in the Dockerfile from:

RUN pecl install redis-5.3.3

to:

RUN echo "104.236.228.160 pecl.php.net" >> /etc/hosts && pecl install redis-5.3.3

solves the problem entirely.

This is not a sustainable fix because if the DNS for this address changes it will stop working.

Workaround

This problem does not occur on the alpine3.12-based image: php:7.4.15-fpm-alpine3.12.

Proposed solution

I suggest to recall the alpine 3.13 based images entirely and use 3.12 instead.