GoSendmail

A subset of sendmail functionality for modern self hosting.

Motivation

Traditional models of email security are designed around a trusted mail server, and semi-trusted user agents. The mail server should continue to perform reasonably even if an end-user machine is compromised. For single-user domains, realistic threat models are more likely to involve compromise of the publicly connected server, motivating a design minimizing trust in the front server.

GoSendmail is an originating mail transfer agent for this threat model.

It is designed as a counterpoint to maildiranasaurus, which receives email on a semi-trusted server.

Features

• Sends from an authoritative / stable IP, supporting StartTLS, and with client certificates proving the authoritative sender.
• Mail DKIM signed with a key that isn't on the authoritative server.

Design

gosendmail provides two binaries which together provide:

• Santitizing / writing a message ID to identify a new message.
• DKIM signing / authorization of email.
• Finding the destination mail server(s) for the message.
• Speaking the SMTP protocol over secured TLS for delivery.

signmail parses stdin for mail sent via a mutt-like program. It runs santization and dkim signing, and then passes the signed message to another process, based on the sending domain. The sub-process interaction is designed to be interoperable with the semantics of sendmail. (e.g. the final email body is passed to stdin.)

sendmail runs on a semi-trusted server, takes an already signed message, and performs the actual sending to remote MTSs.

Usage

• go build ./cmd/sendmail ./cmd/signmail
• copy sendmail to your cloud server (or build it there).
• Modify config.json for relevant keys and domain(s).
• Configure Mutt or your MTA to send using the signmail binary.
• Use the environmental variables GOSENDMAIL_TLS and GOSENDMAIL_SELFSIGNED when insecure mail delivery is desirable. These variable will be read by the sendmail binary, a can be propagated through SSH.

Configuration Options

Environmental Variables

• GOSENDMAIL_TLS - set to a false-y ("false", "0") value to skip StartTLS
• GOSENDMAIL_SELFSIGNED - set to a true value ("true", "1") to allow TLS handshakes with servers that present invalid certificates.
• GOSENDMAIL_RECIPIENTS - overrides the addresses the message will be sent to. This helps support partial resumption of remaining recipients and BCC. If not specified, recipients will be loaded from the To, CC, and BCC fields.

Configuration Options (signmail)

• DkimKeyCmd The subprocess to execute to retrieve the bytes of the dkim signing key.
• DkimSelector The DKIM selector, a part of the DKIM dns record. (default: 'default')
• SendCommand The subprocess to use to send signed messages via the semi-trusted server.

Configuration Options (sendmail)

• DialerProxy A URL (e.g. socks5://...) that connections to remote MTAs will be dialed through.
• TLSCert The certificate file for the sender (client) to use for self authentication.
• TLSKey The corresponding private key file for the sending client to use.

DKIM setup

openssl genpkey -algorithm ed25519 -out domain.dkim.pem
openssl pkey -in domain.dkim.pem -pubout -out domain.dkim.pub
P=openssl asn1parse -in domain.dkim.pub -offset 12 -noout -out /dev/stdout | openssl base64

then the record is v=DKIM1; k=ed25519; p=$P