Allow IPv6 file names to use = rather than : (#40)

Go doesn't allow colons in file names when module support is enabled.
This restriction is to acommodate systems like MS Windows where colons
in file names may have special meaning. So allow the use of equal-signs
in place of colons in file names that represent IPv6 addresses.

Resolves #39

Author: krader1961

README.md

@@ -67,6 +67,13 @@ to use this more than once per `ipfilter` block.
   of the address. For example, *blacklist/127/0/127.0.0.1* or
   *blacklist/2601/647/2601:647:4601:fa93:1865:4b6c:d055:3f3*.
 
+  **Note:** IPv6 addresses as file names can use
+  colons or equal-signs to separate the components; e.g.,
+  *blacklist/2601/647/2601=647=4601=fa93==3f3*. Using equal-signs in
+  place of colons in the file name may be necessary on platforms like MS
+  Windows which assign special meaning to colons in file names. You have
+  to use one or the other; you cannot mix them in the same file name.
+
   Note that you can also whitelist IP addresses using this mechanism
   by specifying `rule allow`. This may be useful when it follows a more
   general blocking rule (e.g., by country) and you want to selectively

ipfilter.go

@@ -204,12 +204,23 @@ func (ipf IPFilter) PrefixDirBlocked(clientIP net.IP, path IPPath) bool {
 	}
 
 	fname := clientIP.String()
+	fname_variant := ""
+	is_ipv6 := clientIP.To4() == nil
+	if is_ipv6 {
+		fname_variant = strings.ReplaceAll(fname, ":", "=")
+	}
 
 	// Check the "flat" namespace.
 	blacklistPath := filepath.Join(path.PrefixDir, fname)
 	if _, err := os.Stat(blacklistPath); err == nil {
 		return true
 	}
+	if is_ipv6 {
+		blacklistPath := filepath.Join(path.PrefixDir, fname_variant)
+		if _, err := os.Stat(blacklistPath); err == nil {
+			return true
+		}
+	}
 
 	// Check the "sharded" namespace.
 	c := strings.SplitN(fname, ".", 3) // shard IPv4 address
@@ -227,6 +238,12 @@ func (ipf IPFilter) PrefixDirBlocked(clientIP net.IP, path IPPath) bool {
 	if _, err := os.Stat(blacklistPath); err == nil {
 		return true
 	}
+	if is_ipv6 {
+		blacklistPath = filepath.Join(path.PrefixDir, c[0], c[1], fname_variant)
+		if _, err := os.Stat(blacklistPath); err == nil {
+			return true
+		}
+	}
 
 	return false
 }