Report which public blocklists list an address in the IP lookup (closes #158)

A new package, internal/blocklist, downloads seven public lists on a
schedule: DShield, Feodo Tracker, FireHOL abusers and level 3, Spamhaus
DROP for IPv4 and IPv6, and the Tor exit list. It keeps the last good
copy of each in memory and in the state directory's blocklists
directory, and loads the copies at startup. Each list's interval follows
its publisher's page, cited in the code.

Each IP lookup result carries blocklists, one entry per list the address
is on. A successful lookup answer carries Cache-Control, private for the
caller's own address. The address sets come from go4.org/netipx.

Model: opus-5-5

internal/blocklist/lists.go

@@ -0,0 +1,93 @@
+package blocklist
+
+import "time"
+
+// How often each list is downloaded. No list is downloaded more often than
+// its publisher updates it or allows.
+const (
+	// DShield asks that its feeds be downloaded at most once an hour:
+	// https://isc.sans.edu/feeds_doc.html
+	dshieldInterval = time.Hour
+
+	// abuse.ch makes the list every 5 minutes and recommends a download at
+	// least every 15: https://feodotracker.abuse.ch/blocklist/
+	feodoTrackerInterval = 15 * time.Minute
+
+	// FireHOL's page for each list gives how often the list changes on
+	// average: about every 5 hours for abusers and every 3 hours for level
+	// 3. FireHOL sets no download limit.
+	// https://iplists.firehol.org/?ipset=firehol_abusers_30d
+	// https://iplists.firehol.org/?ipset=firehol_level3
+	fireHOLAbusersInterval = 5 * time.Hour
+	fireHOLLevel3Interval  = 3 * time.Hour
+
+	// Spamhaus asks that DROP be downloaded at most once a day:
+	// https://www.spamhaus.org/blocklists/do-not-route-or-peer/
+	spamhausDROPInterval = 24 * time.Hour
+
+	// Tor publishes a new exit list every hour and sets no download limit:
+	// https://collector.torproject.org/recent/exit-lists/
+	torExitsInterval = time.Hour
+)
+
+// Lists returns the public blocklists routewatch downloads. Each can be
+// downloaded whole without an account or a key. Spamhaus merged its EDROP
+// list into DROP, so DROP covers both.
+func Lists() []List {
+	return []List{
+		{
+			Name: "dshield",
+			Description: "In one of the 20 networks that DShield saw scan " +
+				"the most targets in the last three days",
+			URL:      "https://feeds.dshield.org/block.txt",
+			Interval: dshieldInterval,
+			Format:   FormatDShield,
+		},
+		{
+			Name:        "feodo_tracker",
+			Description: "A botnet command-and-control server",
+			URL:         "https://feodotracker.abuse.ch/downloads/ipblocklist.txt",
+			Interval:    feodoTrackerInterval,
+			Format:      FormatLines,
+		},
+		{
+			Name: "firehol_abusers_30d",
+			Description: "Reported for comment, form or forum spam in the " +
+				"last 30 days",
+			URL:      "https://iplists.firehol.org/files/firehol_abusers_30d.netset",
+			Interval: fireHOLAbusersInterval,
+			Format:   FormatLines,
+		},
+		{
+			Name: "firehol_level3",
+			Description: "Reported for attacks, spyware or viruses in the " +
+				"last 30 days",
+			URL:      "https://iplists.firehol.org/files/firehol_level3.netset",
+			Interval: fireHOLLevel3Interval,
+			Format:   FormatLines,
+		},
+		{
+			Name: "spamhaus_drop",
+			Description: "In an IPv4 network that Spamhaus says spammers " +
+				"or cyber criminals have hijacked or lease",
+			URL:      "https://www.spamhaus.org/drop/drop_v4.json",
+			Interval: spamhausDROPInterval,
+			Format:   FormatSpamhausJSON,
+		},
+		{
+			Name: "spamhaus_dropv6",
+			Description: "In an IPv6 network that Spamhaus says spammers " +
+				"or cyber criminals have hijacked or lease",
+			URL:      "https://www.spamhaus.org/drop/drop_v6.json",
+			Interval: spamhausDROPInterval,
+			Format:   FormatSpamhausJSON,
+		},
+		{
+			Name:        "tor_exits",
+			Description: "A Tor exit node, where traffic from the Tor network leaves",
+			URL:         "https://check.torproject.org/torbulkexitlist",
+			Interval:    torExitsInterval,
+			Format:      FormatLines,
+		},
+	}
+}