Hi!
Introduction ======= Few days ago I thought it would be a great idea to send emails to others in the same LAN (each participant having their own postfix server) and without reaching Internet. Applications of this is: a dynamic during a conference, a workshop, emergency situation (where Internet or centralized server in the LAN is not working), etc. In my first attempt I thought mDNS [1] is very fine for this, to make it work in debian you have to install avahi-daemon [2]. After that, your hostname is appended with .local domain. Mail clients thunderbird [3] and claws mail [4] allow using the /var/mail/user localhost mailbox. The emails are managed very fine but there is a problem trying to send email using postfix as a localhost SMTP server to a mDNS host Showing the config files and testing ====================== When you install avahi-daemon, hosts line in /etc/nsswitch.conf looks like hosts: files mdns4_minimal [NOTFOUND=return] dns myhostname so it first tries to do "nsswitch resolution" with mDNS before contacting dns thing you can check it with command getent (getent - get entries from Name Service Switch libraries), and I think this is equivalent to calling getaddrinfo: $ getent hosts host1.local 192.168.1.124 host1.local from a default debian 10 stable postfix server version 3.4.5, in file /etc/postfix/main.cf I added: (1) ability to run IPs (that works fine, but is not as interesting as mDNS!) and (2) ability to query nsswitch.conf: # allow raw IPs -> src https://serverfault.com/questions/373350/postfix-allow-sending-to-raw-ip-address resolve_numeric_domain = yes # http://www.postfix.org/postconf.5.html#smtp_host_lookup smtp_host_lookup = dns, native In official postfix documentation for "smtp_host_lookup" says "native - Use the native naming service only (nsswitch.conf, or equivalent mechanism)". I thought that it included mDNS but I am pretty sure is not working (and I think is very easy to replicate) <maskeduser@host1.local>: unable to look up host host1.local: Name or service not known for testing purposes, if I added an entry in /etc/hosts like: 192.168.1.24 host1.local and I restart postfix server, then works (interesting: If I quit /etc/hosts entry still works until I restart postfix server again, looks like postfix server only checks /etc/hosts in init time, not dynamically). But /etc/hosts is not so interesting in this scenario because is so static, and for the use case I said in the beginning, very boring. I think the error I'm getting is coming from file src/smtp/smtp_addr.c (sourcecode of postfix 3.4.5) [5]. Postfix is using getaddrinfo, and it should be resolving mDNS lookups, but is not doing it and I don't understand why. I hope we can have this feature and that it does not harm other things Thanks for your time! Pedro [1] https://en.wikipedia.org/wiki/Multicast_DNS [2] https://wiki.debian.org/ZeroConf [3] https://www.thunderbird.net/ [4] https://www.claws-mail.org/ [5] /* * Use the native name service which also looks in /etc/hosts. * * XXX A soft error dominates past and future hard errors. Therefore we * should not clobber a soft error text and status code. */ #define RETRY_AI_ERROR(e) \ ((e) == EAI_AGAIN || (e) == EAI_MEMORY || (e) == EAI_SYSTEM) #ifdef EAI_NODATA #define DSN_NOHOST(e) \ ((e) == EAI_AGAIN || (e) == EAI_NODATA || (e) == EAI_NONAME) #else #define DSN_NOHOST(e) \ ((e) == EAI_AGAIN || (e) == EAI_NONAME) #endif if (smtp_host_lookup_mask & SMTP_HOST_FLAG_NATIVE) { if ((aierr = hostname_to_sockaddr(host, (char *) 0, 0, &res0)) != 0) { dsb_simple(why, (SMTP_HAS_SOFT_DSN(why) || RETRY_AI_ERROR(aierr)) ? (DSN_NOHOST(aierr) ? "4.4.4" : "4.3.0") : (DSN_NOHOST(aierr) ? "5.4.4" : "5.3.0"), "unable to look up host %s: %s", host, MAI_STRERROR(aierr));
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