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>

internal/match/match.go

@@ -0,0 +1,89 @@
+// Package match finds the address in a log line: patterns are regular
+// expressions (Go's, RE2) with one <IP>, which matches an IPv4 or IPv6
+// address (bracketed, IPv4-mapped, with a zone). The address is checked
+// with netip, so what only looks like one (a time, 12:34:56) never is.
+package match
+
+import (
+	"fmt"
+	"net/netip"
+	"regexp"
+	"strings"
+)
+
+// Placeholder is where a pattern's address is.
+const Placeholder = "<IP>"
+
+// ipRe matches an address: IPv4, or IPv6 (which may end in IPv4,
+// ::ffff:203.0.113.9), bracketed or not, with an optional zone.
+const ipRe = `\[?(?P<ip>[0-9]{1,3}(?:\.[0-9]{1,3}){3}|[0-9A-Fa-f]{0,4}(?::[0-9A-Fa-f]{0,4}){1,7}(?:[0-9]{1,3}(?:\.[0-9]{1,3}){3})?(?:%[0-9A-Za-z_.-]+)?)\]?`
+
+// Matcher holds a watch's patterns.
+type Matcher struct {
+	match  []*regexp.Regexp
+	ignore []*regexp.Regexp
+}
+
+// Compile makes a Matcher. Each match pattern must hold <IP> exactly
+// once; ignore patterns are plain regular expressions.
+func Compile(match, ignore []string) (*Matcher, error) {
+	m := &Matcher{}
+	for _, p := range match {
+		if n := strings.Count(p, Placeholder); n != 1 {
+			return nil, fmt.Errorf("pattern %q has %d %s; it needs exactly one, where the address is", p, n, Placeholder)
+		}
+		re, err := regexp.Compile(strings.Replace(p, Placeholder, ipRe, 1))
+		if err != nil {
+			return nil, fmt.Errorf("pattern %q: %v", p, err)
+		}
+		m.match = append(m.match, re)
+	}
+	for _, p := range ignore {
+		re, err := regexp.Compile(p)
+		if err != nil {
+			return nil, fmt.Errorf("ignore pattern %q: %v", p, err)
+		}
+		m.ignore = append(m.ignore, re)
+	}
+	return m, nil
+}
+
+// Match returns the address of the first pattern a line matches, unless
+// an ignore pattern matches it too.
+func (m *Matcher) Match(line string) (netip.Addr, bool) {
+	for _, re := range m.match {
+		sub := re.FindStringSubmatch(line)
+		if sub == nil {
+			continue
+		}
+		text := sub[re.SubexpIndex("ip")]
+		if i := strings.IndexByte(text, '%'); i >= 0 {
+			text = text[:i]
+		}
+		a, err := netip.ParseAddr(text)
+		if err != nil {
+			continue
+		}
+		for _, ig := range m.ignore {
+			if ig.MatchString(line) {
+				return netip.Addr{}, false
+			}
+		}
+		return a.Unmap(), true
+	}
+	return netip.Addr{}, false
+}
+
+// Group is the prefix an address is counted and banned by: an IPv4
+// address alone, an IPv6 address by its /v6 (64: what one customer
+// usually has).
+func Group(a netip.Addr, v6 int) netip.Prefix {
+	if a.Is4() || v6 <= 0 || v6 > 128 {
+		if a.Is4() {
+			return netip.PrefixFrom(a, 32)
+		}
+		v6 = 128
+	}
+	p, _ := a.Prefix(v6)
+	return p
+}