deploy: iptables hashlimit flood-mitigation script (per-source-IP rate limit on the IRC client ports; policy-accept, loopback-exempt, idempotent add/del) — kernel-layer defense-in-depth, not applied automatically

deploy/iptables-echoircd.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# echoIRCd kernel-layer flood mitigation — defense-in-depth on top of the ircd's own
+# per-IP `accept_rate` limiter. Drops excess NEW connections to the IRC client ports
+# (per source IP) in the kernel, before they ever reach the daemon.
+#
+# SAFE BY DESIGN: it only appends DROP rules for the rate-limited *excess* to ports
+# 6767/6770. The INPUT policy stays ACCEPT, so SSH, established connections, Docker,
+# and every other service are untouched. Loopback is exempt (the liveness probe + local
+# tools live there). Easy to remove, and it never changes any default policy.
+#
+#   Apply:   sudo deploy/iptables-echoircd.sh add
+#   Remove:  sudo deploy/iptables-echoircd.sh del
+#   Show:    sudo iptables -L INPUT -n -v | grep -iE '6767|6770'
+#
+# NOT persistent across reboot on its own — persist with `netfilter-persistent save`
+# (iptables-persistent) or re-run from a boot unit. Review interaction with Docker's
+# own rules first if you persist it.
+#
+# nftables equivalent (if you ever install nft), per-source-IP:
+#   tcp dport { 6767, 6770 } ct state new \
+#     meter ircrate { ip saddr limit rate over 30/second burst 60 packets } drop
+set -u
+
+PORTS="6767,6770"       # IRC client ports (plaintext + TLS); S2S 7700 intentionally left alone
+RATE="30/second"        # sustained NEW connections/sec per source IP
+BURST="60"              # instantaneous burst allowed per source IP
+ACTION="${1:-}"
+
+# iptables for IPv4, plus ip6tables only when it's installed (this host is v4-only).
+IPT_CMDS=(iptables)
+command -v ip6tables >/dev/null 2>&1 && IPT_CMDS+=(ip6tables)
+
+# Apply the same rule to each available table. $1 is the op: -I (insert) or -D (delete).
+rule() {
+    local op="$1" ipt
+    for ipt in "${IPT_CMDS[@]}"; do
+        "$ipt" "$op" INPUT ! -i lo -p tcp -m multiport --dports "$PORTS" \
+            -m conntrack --ctstate NEW \
+            -m hashlimit --hashlimit-name ircrate --hashlimit-mode srcip \
+            --hashlimit-above "$RATE" --hashlimit-burst "$BURST" \
+            -j DROP 2>/dev/null || true
+    done
+}
+
+case "$ACTION" in
+    add)
+        rule -D   # remove any prior copy first, so re-running never stacks duplicates
+        rule -I
+        echo "echoircd rate-limit installed: ports ${PORTS}, ${RATE} burst ${BURST} per source IP (v4+v6)"
+        ;;
+    del)
+        rule -D
+        echo "echoircd rate-limit removed"
+        ;;
+    *)
+        echo "usage: $0 {add|del}" >&2
+        exit 1
+        ;;
+esac