This projects aims at implementing
- a SRS library in pure Rust
- Address parser
- Receive rewriting
- Forward rewriting
- Rust Docs
- A daemon exposing above functionality in a Postfix-compatible way
- Postfix-compatible TCP / unix socket lookup table server
- Configuration format & parsing
- Key Rollover
- Logging & Stats
- Rule-based rewriting policy
This project was started by a novice Rust programmer and is still under development.
The Sender Rewriting Scheme is a technique to rewrite the envelope sender address
(SMTP MAIL FROM) in order to not break the Sender Policy Framework on mail relays
that are not whitelisted in the original sender's SPF record.
A part of the return path is encoded in the local part of the rewritten envelope sender address:
SMTP servers implementing SRS support perform the rewriting on forwards and reverse it on reception of a bounce.
A cryptographic hash / HMAC is employed to protect a reversing SMTP server from becoming an open relay for forged requests.
A to B: MAIL FROM user@a
B to C: MAIL FROM SRS0=HHH1=TT=a=user@b
C to D: MAIL FROM SRS1=HHH2=b=HHH1=TT=a=user@c
Subsequent forwardes just change the domain-part of the address.
D to E: MAIL FROM SRS1=HHH2=b=HHH1=TT=a=user@d
E bounces => SRS1=HHH2=b=HHH1=TT=a=user@d is rewritten to SRS0=HHH1=TT=a=user@b
E TO B: MAIL FROM SRS0=HHH1=TT=a=user@b
B rewrites SRS0=HHH1=TT=a=user@b to user@a
B TO A: MAIL FROM user@a
=> the return path is exactly 2 hops long
=> intermediate hosts are short-circuited