Install fping and validate config in the Zabbix proxy installers

[MarkLFT]

Found while bringing up a new proxy: it connected to the server fine, then logged this once a second, forever:

At least one of '/usr/sbin/fping', '/usr/sbin/fping6' must exist. Both are missing in the system.

Root cause

Neither proxy installer installs fping, and neither writes FpingLocation. Zabbix defaults to /usr/sbin/fping; Debian and Ubuntu ship it in /usr/bin/fping. So even installing the package by hand doesn't fix it — the path still has to be configured.

Effect: every icmpping / icmppingsec / icmppingloss item on every host behind the proxy fails as unsupported. Nothing errors loudly, so it's easy to miss — most network-device templates have ping items, so this silently removes availability monitoring.

Changes

Both install-zabbix-proxy.sh and install-zabbix-proxy-full.sh:

1. Install fping alongside the proxy package
2. Locate it with command -v rather than hardcoding — Debian/Ubuntu and RHEL differ — and warn rather than write a bad path if it's genuinely missing
3. Write FpingLocation / Fping6Location into the generated config
4. Validate with zabbix_proxy -T before starting, so a bad config reports the proxy's own message instead of a bare systemd exit code

zabbix_proxy supports -T --test-config; confirmed against the binary.

README correction

It documented:

sudo systemctl restart zabbix-proxy-sqlite3   # or zabbix-proxy-mysql / -pgsql

That unit does not exist. All three backend packages ship the same zabbix-proxy.service — verified from the package contents:

package	unit shipped
zabbix-proxy-sqlite3	zabbix-proxy.service
zabbix-proxy-mysql	zabbix-proxy.service
zabbix-proxy-pgsql	zabbix-proxy.service

Anyone following the documented upgrade steps got Unit zabbix-proxy-sqlite3.service not found. Added a note on detecting and fixing already-deployed proxies, which are all affected.

Test results — Debian 13 containers, 8/8

• fping detected at /usr/bin/fping (not the Zabbix default)
• both FpingLocation and Fping6Location produced — fping6 is a symlink to fping and resolves correctly
• directives written into the generated config
• zabbix_proxy -T validates that config
• proxy starts
• no missing-binary errors in the log

bash -n and shellcheck clean on both (the SC2034s are pre-existing vestigial constants, untouched).

Existing proxies

Every proxy built before this is affected. To check:

command -v fping; grep -E '^Fping' /etc/zabbix/zabbix_proxy.conf   # both empty = affected