Ban lists: where Guard and fail2ban ban addresses

A program bans an address (every port, or a protocol's ports) for a
time; the ban list says what it is kept from: the firewall, some hosts
behind it, or everything. Bans drop ahead of every rule, close the
address's connections, group IPv6 by /64, are kept in the database
across restarts and audited. Never banned: the inside, the firewall's
own addresses, the caller, the list's never. bans grant (bans:write:LIST),
hc banlist/bans/ban/unban, Traffic -> Bans, a fail2ban action in contrib/,
and failed logins and refused tokens logged for Guard and fail2ban.

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

internal/model/bans.go

@@ -0,0 +1,53 @@
+package model
+
+import (
+	"net/netip"
+	"time"
+)
+
+// Ban is an address a ban list keeps out (docs/bans.md): of every port,
+// or of one protocol and port. Bans are kept apart from the
+// configuration, like a dynamic set's entries: not in the history, and
+// not on probation.
+type Ban struct {
+	ID     int64  `json:"id"`
+	ListID int64  `json:"list_id"`
+	List   string `json:"list"`
+	// Address is what is banned: an address, or an IPv6 prefix of the
+	// list's grouping ("2001:db8:1:2::/64").
+	Address  string `json:"address"`
+	Protocol string `json:"protocol,omitempty"` // tcp or udp; "" every port
+	Port     int    `json:"port,omitempty"`
+	// Expires is when the ban ends (nil: it doesn't); Timeout the
+	// seconds it has left then.
+	Expires *time.Time `json:"expires,omitempty"`
+	Timeout int        `json:"timeout,omitempty"`
+	Created time.Time  `json:"created"`
+	// Who banned it (the user or token's name), Source the program's own
+	// name for why ("guard/ssh", "fail2ban/sshd"), Reason free text (a
+	// log line).
+	Who    string `json:"who"`
+	Source string `json:"source,omitempty"`
+	Reason string `json:"reason,omitempty"`
+}
+
+// Prefix is the banned address as a prefix.
+func (b *Ban) Prefix() (netip.Prefix, error) {
+	if p, err := netip.ParsePrefix(b.Address); err == nil {
+		return p, nil
+	}
+	a, err := netip.ParseAddr(b.Address)
+	if err != nil {
+		return netip.Prefix{}, err
+	}
+	return netip.PrefixFrom(a, a.BitLen()), nil
+}
+
+// BanAddress writes a banned prefix: an address alone when it is one
+// address, else the prefix.
+func BanAddress(p netip.Prefix) string {
+	if p.IsSingleIP() {
+		return p.Addr().String()
+	}
+	return p.String()
+}