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89 lines
2.3 KiB
Go
89 lines
2.3 KiB
Go
package transform
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import (
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"fmt"
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"net"
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"strings"
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)
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func ReverseDomainName(cidr string) (string, error) {
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a, c, err := net.ParseCIDR(cidr)
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if err != nil {
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return "", err
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}
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base, err := reverseaddr(a.String())
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if err != nil {
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return "", err
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}
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base = strings.TrimRight(base, ".")
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bits, total := c.Mask.Size()
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var toTrim int
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if bits == 0 {
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return "", fmt.Errorf("Cannot use /0 in reverse cidr")
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}
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if total == 32 {
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if bits%8 != 0 {
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return "", fmt.Errorf("IPv4 mask must be multiple of 8 bits")
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}
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toTrim = (total - bits) / 8
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} else if total == 128 {
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if bits%4 != 0 {
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return "", fmt.Errorf("IPv6 mask must be multiple of 4 bits")
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}
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toTrim = (total - bits) / 4
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} else {
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return "", fmt.Errorf("Address is not IPv4 or IPv6: %v", cidr)
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}
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parts := strings.SplitN(base, ".", toTrim+1)
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return parts[len(parts)-1], nil
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}
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// copied from go source.
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// https://github.com/golang/go/blob/bfc164c64d33edfaf774b5c29b9bf5648a6447fb/src/net/dnsclient.go#L15
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// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
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// address addr suitable for rDNS (PTR) record lookup or an error if it fails
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// to parse the IP address.
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func reverseaddr(addr string) (arpa string, err error) {
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ip := net.ParseIP(addr)
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if ip == nil {
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return "", &net.DNSError{Err: "unrecognized address", Name: addr}
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}
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if ip.To4() != nil {
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return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
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}
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// Must be IPv6
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buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
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// Add it, in reverse, to the buffer
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for i := len(ip) - 1; i >= 0; i-- {
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v := ip[i]
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buf = append(buf, hexDigit[v&0xF])
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buf = append(buf, '.')
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buf = append(buf, hexDigit[v>>4])
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buf = append(buf, '.')
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}
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// Append "ip6.arpa." and return (buf already has the final .)
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buf = append(buf, "ip6.arpa."...)
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return string(buf), nil
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}
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// Convert unsigned integer to decimal string.
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func uitoa(val uint) string {
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if val == 0 { // avoid string allocation
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return "0"
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}
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var buf [20]byte // big enough for 64bit value base 10
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i := len(buf) - 1
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for val >= 10 {
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q := val / 10
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buf[i] = byte('0' + val - q*10)
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i--
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val = q
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}
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// val < 10
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buf[i] = byte('0' + val)
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return string(buf[i:])
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}
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const hexDigit = "0123456789abcdef"
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