Fix the domain name length limit

I previously copied the domain name length limit from the RFCs, but I misunderstood that "octets" in the RFCs didn't mean the number of characters in the ASCII domain name but the number of bytes as transmitted. When transmitted, the domain name has one byte for each label (part between periods) giving the label length. Those bytes correspond to the dots, except for the last label which doesn't have a dot, and the empty label which isn't printed. So the longest domain name length in characters is two less than what I thought.

See https://stackoverflow.com/questions/32290167/what-is-the-maximum-length-of-a-dns-name for explanation.

I noticed this when I saw that the idna package was rejecting domain names with 254 characters which this library accepted.

Author: JoshData

CHANGELOG.md

@@ -1,6 +1,7 @@
 In Development
 --------------
 
+* The domain name length limit is corrected from 255 to 253 IDNA ASCII characters. I misread the RFCs.
 * When a domain name has no MX record but does have an A or AAAA record, if none of the IP addresses in the response are globally reachable (i.e. not Private-Use, Loopback, etc.), the response is treated as if there was no A/AAAA response and the email address will fail the deliverability check.
 * When a domain name has no MX record but does have an A or AAAA record, the mx field in the object returned by validate_email incorrectly held the IP addresses rather than the domain itself.
 * Fixes in tests.

email_validator/rfc_constants.py

@@ -42,7 +42,7 @@
 EMAIL_MAX_LENGTH = 254
 LOCAL_PART_MAX_LENGTH = 64
 DNS_LABEL_LENGTH_LIMIT = 63  # in "octets", RFC 1035 2.3.1
-DOMAIN_MAX_LENGTH = 255  # in "octets", RFC 1035 2.3.4 and RFC 5321 4.5.3.1.2
+DOMAIN_MAX_LENGTH = 253  # in "octets" as transmitted, RFC 1035 2.3.4 and RFC 5321 4.5.3.1.2, and see https://stackoverflow.com/questions/32290167/what-is-the-maximum-length-of-a-dns-name
 
 # RFC 2142
 CASE_INSENSITIVE_MAILBOX_NAMES = [

tests/test_syntax.py

@@ -343,9 +343,9 @@ def test_domain_literal():
         ('obsolete."quoted".atom@example.com', 'The email address contains invalid characters before the @-sign: \'"\'.'),
         ('11111111112222222222333333333344444444445555555555666666666677777@example.com', 'The email address is too long before the @-sign (1 character too many).'),
         ('111111111122222222223333333333444444444455555555556666666666777777@example.com', 'The email address is too long before the @-sign (2 characters too many).'),
-        ('me@1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.111111111122222222223333333333444444444455555555556.com', 'The email address is too long (4 characters too many).'),
-        ('me@1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555566.com', 'The email address is too long after the @-sign (1 character too many).'),
-        ('me@中1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555566.com', 'The email address is too long after the @-sign.'),
+        ('meme@1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.com', 'The email address is too long (4 characters too many).'),
+        ('me@1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.11111111112222222222333333333344444444445555555555.com', 'The email address is too long after the @-sign (1 character too many).'),
+        ('me@中1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444.com', 'The email address is too long after the @-sign.'),
         ('my.long.address@1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.11111111112222222222333333333344444.info', 'The email address is too long (2 characters too many).'),
         ('my.long.address@λ111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.11111111112222222222333333.info', 'The email address is too long (when converted to IDNA ASCII).'),
         ('my.long.address@λ111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444444444555555555.6666666666777777777788888888889999999999000000000.1111111111222222222233333333334444.info', 'The email address is too long (at least 1 character too many).'),