The domain name used for EHLO would like to be fully-qualified

[un-clouded]

If HAVE_UNAME, it seems that session->localhost will be the local host name without the domain name on the end. This causes grumpiness with any postfix installation where reject_non_fqdn_helo_hostname is configured, and maybe in other situations also. It seems from RFC5321 that the EHLO should be fully-qualified (or an IP address). Is there any chance that do_session could read /etc/mailname for the EHLO domain name? That'd let users choose what to send without having to re-compile libESMTP. Cheers!

[libesmtp]

Having thought about this, I feel that the application should always set the localhost using smtp_set_hostname(). I'd prefer to avoid assuming the names of files in library code (OpenSSL somewhat forced my hand on this issue).

When smtp_set_hostname() is not used libESMTP attempts to determine the local hostname. As you point out, RFC5321 requires a FQDN which neither uname() nor gethostname() provide, however it might be possible to use getaddrinfo() to find the domain name. My preferred course of action would be to investigate this approach, or force the application to set the localhost's FQDN.

[un-clouded]

Having thought about this, I feel that the application should always set the localhost using smtp_set_hostname().

That sounds reasonable, though it does somewhat pass the buck when the application (collectd in my case) sets an unqualified hostname, even though a FQDN is required for EHLO. In this case, a user that can't recompile is left without an option.

I'd prefer to avoid assuming the names of files in library code (OpenSSL somewhat forced my hand on this issue).

That's a good point, though doesn't libc itself refer to /etc/hosts? Though I've just noticed that gethostbyname allows the path to the file to be overridden with the HOSTALIASES environment variable. Would that approach be acceptable?

When smtp_set_hostname() is not used libESMTP attempts to determine the local hostname. As you point out, RFC5321 requires a FQDN which neither uname() nor gethostname() provide, however it might be possible to use getaddrinfo() to find the domain name. My preferred course of action would be to investigate this approach, or force the application to set the localhost's FQDN.

I had a go with getaddrinfo(). Perhaps I'm using it wrong, but I couldn't get anything other than ai_canonname=(null) with it. I guess it could be used to obtain an IP address, which could be used for EHLO? I was doing it wrong. Using AI_CANONNAME in hints.ai_flags does provide the canonical hostname.

[un-clouded]

The PR has been updated with a proposal to use getaddrinfo(), but also to permit the user to override session->localhost when necessary. It does not make changes to session->localhost when the application is well-behaved by specifying a FQDN with smtp_set_hostname(). The /etc/mailname patch is not yet removed, but can be if the other changes are acceptable.

[libesmtp]

I can review this in the next few days as I'm busy till then. I realised in retrospect that getnameinfo() is closer to what I had in mind for this. But I have some reading to do first as I want to be careful about semantics.