[PATCH v5 1/3] send-mail: improve checks for valid_fqdn

The current implementation of a valid Fully Qualified Domain Name is not that strict. It just checks whether it has a dot (.) and if using macOS, it should not end with .local. As per RFC1035[1], from what I understood, the following checks need to be done:

- The domain must contain atleast one dot
- Each label (separated by dots) must be 1-63 characters long
- Labels must start and end with an alphanumeric character
- Labels can contain alphanumeric characters and hyphens

Here are some examples of valid and invalid labels:

'example.com', # Valid 'sub.example.com', # Valid 'my-domain.org', # Valid 'localhost', # Invalid (no dot) 'MacBook..', # Invalid (double dots) '-example.com', # Invalid (starts with a hyphen) 'example-.com', # Invalid (ends with a hyphen) 'example..com', # Invalid (double dots) 'example', # Invalid (no TLD) 'example.local', # Invalid on macOS 'valid-domain.co.uk', # Valid '123.example.com', # Valid 'example.com.', # Invalid (trailing dot) 'toolonglabeltoolonglabeltoolonglabeltoolonglabeltoolonglabeltoolonglabel.com', # Invalid (label > 63 chars)

Due to current implementation, I was not able to send emails from Ubuntu. Upon debugging, I found that the SMTP domain being passed to Outlook's servers was not valid.

Net::SMTP=GLOB(0x5db4351225f8)>>> EHLO MacBook.. Net::SMTP=GLOB(0x5db4351225f8)<<< 501 5.5.4 Invalid domain name Net::SMTP=GLOB(0x5db4351225f8)>>> HELO MacBook..

Notice that an invalid domain name "MacBook.." is sent by git-send-email. We have a fallback code that checks output from Net::Domain::domainname() or asking domain method of an Net::SMTP instance to detect a misconfigured hostname and replace it with fallback "localhost.localdomain", but the valid_fqdn apparently is failing to say "MacBook.." is not a valid fqdn.

With this patch, the rule used in valid_fqdn is tightened, the beginning part of the SMTP exchange looked like this:

Net::SMTP=GLOB(0x58c8af71e930)>>> EHLO localhost.localdomain Net::SMTP=GLOB(0x58c8af71e930)<<< 250-PN4P287CA0064.outlook.office365.com Hello

[1]: https://datatracker.ietf.org/doc/html/rfc1035

Signed-off-by: Aditya Garg <gargaditya08@live.com>
---
 git-send-email.perl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)