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Check domain name validity in Go
// checkDomain returns an error if the domain name is not valid
// See https://tools.ietf.org/html/rfc1034#section-3.5 and
// https://tools.ietf.org/html/rfc1123#section-2.
func checkDomain(name string) error {
switch {
case len(name) == 0:
return nil // an empty domain name will result in a cookie without a domain restriction
case len(name) > 255:
return fmt.Errorf("cookie domain: name length is %d, can't exceed 255", len(name))
}
var l int
for i := 0; i < len(name); i++ {
b := name[i]
if b == '.' {
// check domain labels validity
switch {
case i == l:
return fmt.Errorf("cookie domain: invalid character '%c' at offset %d: label can't begin with a period", b, i)
case i-l > 63:
return fmt.Errorf("cookie domain: byte length of label '%s' is %d, can't exceed 63", name[l:i], i-l)
case name[l] == '-':
return fmt.Errorf("cookie domain: label '%s' at offset %d begins with a hyphen", name[l:i], l)
case name[i-1] == '-':
return fmt.Errorf("cookie domain: label '%s' at offset %d ends with a hyphen", name[l:i], l)
}
l = i + 1
continue
}
// test label character validity, note: tests are ordered by decreasing validity frequency
if !(b >= 'a' && b <= 'z' || b >= '0' && b <= '9' || b == '-' || b >= 'A' && b <= 'Z') {
// show the printable unicode character starting at byte offset i
c, _ := utf8.DecodeRuneInString(name[i:])
if c == utf8.RuneError {
return fmt.Errorf("cookie domain: invalid rune at offset %d", i)
}
return fmt.Errorf("cookie domain: invalid character '%c' at offset %d", c, i)
}
}
// check top level domain validity
switch {
case l == len(name):
return fmt.Errorf("cookie domain: missing top level domain, domain can't end with a period")
case len(name)-l > 63:
return fmt.Errorf("cookie domain: byte length of top level domain '%s' is %d, can't exceed 63", name[l:], len(name)-l)
case name[l] == '-':
return fmt.Errorf("cookie domain: top level domain '%s' at offset %d begins with a hyphen", name[l:], l)
case name[len(name)-1] == '-':
return fmt.Errorf("cookie domain: top level domain '%s' at offset %d ends with a hyphen", name[l:], l)
case name[l] >= '0' && name[l] <= '9':
return fmt.Errorf("cookie domain: top level domain '%s' at offset %d begins with a digit", name[l:], l)
}
return nil
}
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