There is a difference between ‘valid’ and ‘it works’. It is entirely possible that hostnames aren’t considered valid if they are single characters (my earlier post not withstanding). However, quite a lot of systems do allow them. One major system, Microsoft’s AD/DNS system, has a legacy reason for permitting single character names. Old-school NetBIOS names … Read more
The addresses in an email message header serve different purposes than the envelope sender and recipient (which really aren’t hidden per se, they just aren’t part of the message). The envelope sender and recipient, which you never see in a message, are part of the SMTP protocol, and specify delivery instructions, that is, to which … Read more
Let’s start with a simple example. Let’s say you have an email list, that is going to send out the following RFC2822 content. From: <coolstuff@mymailinglist.example> To: <you@example.com> Subject: Super simple email Reply-To: <coolstuff-threadId=123@mymailinglist.example> This is a very simple body. Now, let’s say you are going to send it from a mailing list, that implements VERP … Read more
It is permissible for routers to connect to each other using RFC1918 or other private addresses, and in fact this is very common for things like point-to-point links, and any routing that takes place inside an AS. Only the border gateways on a network actually need publicly routeable IP addresses for routing to work. If … Read more
Because that is not what you are supposed to be using .invalid for. Like .example it is meant for local testing and documentation. Additionally, using .invalid still causes additional things to happen – additional DNS look ups and queuing on the mail server for retries for one off the top of my head. Using the … Read more
Yes you are correct. The section you quoted says that it must be a quoted string OR a dot atom. Since its clearly not a quoted string (the lack of enclosing ” makes that clear) it must be a dot-atom… That leads us to the definition of dot-atom: Look at this except from RFC 5322 … Read more
There are many reasons why the Header and Envelope From addresses may not match. Most concern automated processes sending mail, where delivery issues need to be reported to an address that is not representative of who sent the mail, or who it was sent on behalf of, or who should be replied to. Mailing lists … Read more
From RFC 5321, section 2.3.11: The standard mailbox naming convention is defined to be “local-part@domain”; contemporary usage permits a much broader set of applications than simple “user names”. Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned … Read more
RFC 822 actually gives an example of this usage. It required (Section 4.4) that the Sender: header be present when it was used. A.2.7. Agent for member of a committee George’s secretary sends out a message which was authored jointly by all the members of a committee. Note that the name of the committee cannot … Read more
The client does not know in advance that the response will be too large, so it will query the server via UDP. The server will respond via UDP and will include as much as possible and set the truncated header bit (“TC” http://www.networksorcery.com/enp/protocol/dns.htm). The client can then resend the request via TCP and get the … Read more