# Plan: from-header-binding **Status:** agreed 2026-10-12 - the design questions were closed with the owner one at a time and he took it into work **Date:** 2026-10-12 **Version:** `2.4.0` MINOR (one migration; it refuses mail that was accepted before). --- ## Goal An application sends only as itself - in the `From:` line a mail program shows and OpenDKIM chooses the key by, not only in the envelope. Until now Postfix held the envelope sender against the SASL login and nothing held the header. An application of one sending domain could write another registered domain, or the server's own name, into `From:` and have the message signed with that name's key (security review of panel-notifications, 2026-10-12). ## Decisions (owner, 2026-10-12) ### The rule **`From:` is held to what the envelope is held to.** An application set to any address of its domain may write any address of that domain; one with a list of addresses, only those. Anything else is refused: another registered domain, the server's name, an address at a domain this server knows nothing of, no `From:` at all. I proposed the narrower rule - refuse only a `From:` that names a domain this server signs for, and let a stranger's address through unsigned as before - and, for an application with a list, to check the domain alone. The owner chose the wider one both times. What it stops working: a form that puts its visitor's address into `From:`, and an application with a list whose envelope address and header address differ, until the second is added to the list. When the hostname is itself a sending domain its applications write `…@` by right, as the guide already says. ### Where it is checked, and what happens when it cannot be **The panel checks, and the check is strict:** while the panel does not answer, Postfix replies with a temporary error and accepts nothing unchecked. I first proposed the opposite (fail-open, as the journal and the level-2 limit are) and then a small process of its own. The owner's objection to the first: a check that is absent while the panel is down does not close the hole, and the panel only has to be slow - 15 seconds - for Postfix to go on without it. His answer to the second: not yet; measure first. So the check is a role of the panel process on a socket of its own, attached with `default_action=tempfail`. The journal milter keeps its socket and stays fail-open: a slow database still does not defer mail. The check reads no database - it reads the file of sender bindings Postfix itself reads for the envelope, so the two cannot disagree - and moving it into a process of its own later is a change of where it is started. ### Statistics, to decide whether the check needs its own process Per day, kept a year: - **for how long the panel did not answer while Postfix was running.** The panel marks itself alive every few seconds; at start the gap since the last mark is its downtime, counted when Postfix was not restarted within it (a rollout or a reboot stops mail for another reason); - **how many messages were deferred because the check did not answer**, read from `mail.log`. This one also sees a panel that is running and not answering, which the mark does not. Shown on **Server › Health** in a card of its own, *Sender check*: today, the last 30 days, the last 12 months. The card warns when a message was deferred in the last 24 hours; downtime that deferred nothing warns of nothing. ### What a refusal looks like - **To the application:** `550 5.7.1`, permanent, naming the address. A level-2 limit answers 4xx because the message will pass in an hour; a wrong `From:` will not, and a 4xx would have the application send it again for days. - **In the Outbound log:** a row with the reason in the status tag - `wrong From`, beside `rate-limited` and the tag of a refused client address. The built panel wrote `rejected` for all of them; it follows the mockup now. The status filter keeps one entry, `rejected`. - **On the message's page:** a fact *From: line* with the address written in the header, and a History step that says which address was refused, for which application, what that application may write, and that it was told `550 5.7.1`. ### Health, re-laid The owner on the first picture of the new card: too many holes on the screen, the numbers should follow importance, and the boxes should read left to right and top to bottom - with stacked columns the third box was below the fold and the fourth was not. - Order: 01 Processes, 02 Milter sockets, 03 Sender check, 04 TLS certificate, 05 Hostname and reverse DNS, 06 Notification mail, 07 Machine, 08 Configuration. - Layout `cards` (new, on the components page): two stacks, odd numbers on the left and even on the right, each closed up, so a short box leaves no gap beside a tall one and the eye still goes 01, 02, 03. On a phone the boxes come in the order of their numbers. ## To settle while building - How the header is read so that the check and OpenDKIM cannot disagree: one `From:` field, one address, and a cruder second look at the raw value. - What Postfix writes in `mail.log` when a strict milter does not answer, and how a line is told to be this milter's and not OpenDKIM's. - How the panel learns when Postfix started. - Schema: the reason of a refusal and the header address on a send-log row; the table of days. - That a refusal for `From:` is not taken for a refusal under a limit by the notifier. ## Implementation checklist - [x] Mockups: Health card and layout, the log row, help topic; accepted by the owner 2026-10-12 (`design:` commit) - [ ] Migration and store: refusal reason and header address, days of the sender check - [ ] The check: a role of the panel on its own socket, bindings read from Postfix's file; Postfix attaches it strictly; start order - [ ] Statistics: the alive mark and downtime, deferred mail from `mail.log` - [ ] Panel: Health (card, sockets row, layout), Outbound log tags, the message's page, help - [ ] guide.md, architecture.md, security.md (the accepted risk goes), schema-migrations.md - [ ] `go vet`, `go test`, e2e: refused for another domain's `From:`, for the hostname's and for an address off the list; accepted for its own - [ ] Technical review — **Opus** - [ ] Security review — **Fable**