Limit of new TCP connections per source IP: configurable rate, drop only above the limit.

- default_settings.conf: new default_limit_new_tcp_connections_per_seconds_rate=30.
- main_ipv{4,6}.conf.sample: new limit_new_tcp_connections_per_seconds_rate
  (burst is twice the rate; invalid or empty values fall back to the default).
- ipt-firewall-server, ip6t-firewall-server: the rule no longer ACCEPTs SYNs
  below the limit (which bypassed the service rules, destination IPs and the
  HTTP limits); like the HTTP limits it now only logs and drops SYNs above
  the limit (--hashlimit-above). Fallback without xt_hashlimit: chain
  syn-multi-limit with a global limit.
- ip6t-firewall-server: fix --hashlimit-srcmask (128 instead of 32), drop
  only the given ports instead of all new TCP connections, hashlimit names
  syn_multi_v6*, add the can_use_hashlimit check.

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

ipt-firewall-server

@@ -1210,41 +1210,55 @@ echononl "\tLimit new (syn) TCP connections per second per source IP (multiport)
 if $limit_new_tcp_connections_per_seconds_per_source_IP \
       && [[ ${#limit_new_tcp_connections_per_seconds_ports} -gt 0 ]]; then
 
-   #$ipt -A INPUT -p tcp -m conntrack --ctstate NEW -m limit --limit 60/s --limit-burst 20 -j ACCEPT
+   if [[ -z "$limit_new_tcp_connections_per_seconds_rate" \
+         || ! "$limit_new_tcp_connections_per_seconds_rate" =~ ^[0-9]+$ ]]; then
+      limit_new_tcp_connections_per_seconds_rate=$default_limit_new_tcp_connections_per_seconds_rate
+   fi
+   limit_new_tcp_connections_per_seconds_burst=$((limit_new_tcp_connections_per_seconds_rate * 2))
 
-   # Rate-Limit für neue SYNs auf 443 pro IP
+   # Only (syn) packets above the limit are dropped here. Packets below the
+   # limit are not accepted but pass on to the rules of the services, so this
+   # does not open any port and does not bypass later limits.
    if can_use_hashlimit ; then
-   $ipt -A INPUT -p tcp --syn \
-      -m multiport --dports $limit_new_tcp_connections_per_seconds_ports \
-      -m hashlimit --hashlimit-name syn_multi_v4 \
-      --hashlimit 30/second --hashlimit-burst 60 \
-      --hashlimit-mode srcip --hashlimit-srcmask 32 \
-      -j ACCEPT
 
-   if $log_rejected || $log_all ; then
+      if $log_rejected || $log_all ; then
+         $ipt -A INPUT -p tcp --syn \
+            -m multiport --dports $limit_new_tcp_connections_per_seconds_ports \
+            -m hashlimit --hashlimit-name syn_multi_v4_log --hashlimit-mode srcip \
+            --hashlimit-srcmask 32 \
+            --hashlimit-above ${limit_new_tcp_connections_per_seconds_rate}/second \
+            --hashlimit-burst $limit_new_tcp_connections_per_seconds_burst \
+            -m limit --limit 2/second --limit-burst 10 \
+            -j $LOG_TARGET $tag_log_prefix "$log_prefix SYN over limit (multiport):"
+      fi
 
-      #  rate-limited logging für Überschreiter
       $ipt -A INPUT -p tcp --syn \
          -m multiport --dports $limit_new_tcp_connections_per_seconds_ports \
-         -m hashlimit --hashlimit-name syn_multi_v4_log  \
-         --hashlimit 2/second --hashlimit-burst 10 \
-         --hashlimit-mode srcip --hashlimit-srcmask 32 \
-         -j $LOG_TARGET $tag_log_prefix "$log_prefix SYN over limit (multiport):"
+         -m hashlimit --hashlimit-name syn_multi_v4 --hashlimit-mode srcip \
+         --hashlimit-srcmask 32 \
+         --hashlimit-above ${limit_new_tcp_connections_per_seconds_rate}/second \
+         --hashlimit-burst $limit_new_tcp_connections_per_seconds_burst \
+         -j DROP
 
+   else
+
+      warn "xt_hashlimit not available; using simple global limit fallback for SYN rate limiting (multiport)."
+
+      $ipt -N syn-multi-limit
+      $ipt -A syn-multi-limit \
+         -m limit --limit ${limit_new_tcp_connections_per_seconds_rate}/second \
+         --limit-burst $limit_new_tcp_connections_per_seconds_burst \
+         -j RETURN
+      if $log_rejected || $log_all ; then
+         $ipt -A syn-multi-limit -m limit --limit 2/second --limit-burst 10 \
+            -j $LOG_TARGET $tag_log_prefix "$log_prefix SYN over limit (multiport):"
+      fi
+      $ipt -A syn-multi-limit -j DROP
+
+      $ipt -A INPUT -p tcp --syn \
+         -m multiport --dports $limit_new_tcp_connections_per_seconds_ports \
+         -j syn-multi-limit
    fi
-else
-   warn "xt_hashlimit not available; using simple global limit fallback for SYN rate limiting (multiport)."
-   $ipt -A INPUT -p tcp --syn \
-      -m multiport --dports $limit_new_tcp_connections_per_seconds_ports \
-      -m limit --limit 30/second --limit-burst 60 \
-      -j ACCEPT
-fi
-
-
-   #$ipt -A INPUT -p tcp -m conntrack --ctstate NEW -j DROP
-
-   $ipt -A INPUT -p tcp --syn -m multiport --dports $limit_new_tcp_connections_per_seconds_ports -j DROP
-
 
    echo_done
 else