fix: prevent idle TCP connections from being dropped by OCI middleboxes

This happened mainly with interactive SSH connections on which the user
is AFK and no terminal activity happened, as the default 2 hour kernel
timeout is too long for these cloud firewalls and/or NAT boxes.

roles/common/tasks/main.yml

@@ -195,3 +195,21 @@
     name: "{{ ansible_user }}"
     groups: "{{ sdkman_group }}"
     append: true
+
+# sshd and other servers may leverage the kernel TCP keepalives support, but
+# shorter keepalive messages are necessary because OCI firewalls/NAT gateways
+# are eager to drop connections that are kept idle for much less time than the
+# default of two hours considered by the Linux kernel. SSH sessions that get
+# inactive are a prime candidate for being dropped because of this. See also:
+# https://bugs.launchpad.net/cloud-images/+bug/1838670
+- name: Tweak TCP keepalive time
+  become: true
+  ansible.posix.sysctl:
+    name: net.ipv4.tcp_keepalive_time
+    value: 120
+
+- name: Tweak TCP keepalive maximum probes
+  become: true
+  ansible.posix.sysctl:
+    name: net.ipv4.tcp_keepalive_probes
+    value: 3