Guard: watch logs, ban misbehaving addresses in Housecarl

Files and the journal, patterns with one <IP>, bounties within a window,
bans in a Housecarl ban list with repeat offenders' bans growing; presets
for sshd, jarl, housecarl, steward, bind-xfr, postfix-sasl, dovecot,
nginx-auth and housecarl-drops; guard run/check/test/status/bans/ban/unban;
a Debian package with a sandboxed unit; an end-to-end test with Housecarl.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/systemd/guard.service

@@ -0,0 +1,55 @@
+# Guard: watches logs and bans misbehaving addresses at the firewall
+# (Housecarl's ban lists). Installed as /etc/systemd/system/guard.service
+# (see docs/install.md).
+[Unit]
+Description=Guard: bans addresses that misbehave in the logs, at the firewall
+Documentation=file:///usr/local/share/doc/guard/README.md
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+ExecStart=/usr/local/bin/guard --config /etc/guard/guard.yaml run
+Restart=on-failure
+RestartSec=5s
+
+User=guard
+Group=guard
+# Reading logs: /var/log's files (adm) and the journal (systemd-journal).
+# On the firewall, add housecarl to use Housecarl's local socket.
+SupplementaryGroups=adm systemd-journal
+UMask=0027
+
+# /var/lib/guard: the state (repeat offenders, bans waiting for
+# Housecarl). /run/guard: the socket guard status asks.
+StateDirectory=guard
+StateDirectoryMode=0750
+ConfigurationDirectory=guard
+ConfigurationDirectoryMode=0750
+RuntimeDirectory=guard
+RuntimeDirectoryMode=0750
+
+# Sandboxing: Guard only reads logs and talks to Housecarl.
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+PrivateDevices=yes
+NoNewPrivileges=yes
+CapabilityBoundingSet=
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectKernelLogs=yes
+ProtectControlGroups=yes
+ProtectClock=yes
+ProtectHostname=yes
+RestrictNamespaces=yes
+RestrictRealtime=yes
+RestrictSUIDSGID=yes
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+SystemCallErrorNumber=EPERM
+
+[Install]
+WantedBy=multi-user.target