`nx_email_mta.nx` source

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1// nx_email_mta.nx -- EMAIL RUNG R7: receiving MTA (SMTP server logic).
2//
3// module: nishi-core.email.mta
4// depends: (none -- pure RFC 5321 server-side command parse + policy + FSM)
5// capability: CORE_EMAIL
6//
7// The server end of SMTP: classify inbound commands, enforce the RCPT
8// relay policy (accept mail for OUR domain, refuse to relay for others --
9// the open-relay defense), and drive the receive state machine emitting
10// the correct reply codes. Pure logic; the integration capstone (accept
11// loop over nx_socket, dot-unstuff the DATA via R4's nx_pop3_dedot, and
12// deliver into R3's nx_email_mailbox) is exercised end-to-end in the gate.
13//
14// license_tier: INDEPENDENT_REDERIVE
15// genealogy_id: international-research-sources/ietf/rfc_5321
16// lineage_id:   nishi_email_mta_r7
17//
18// nx_safety_envelope:
19//   intended_use:       "Inbound SMTP command parse + RCPT relay policy +
20//                        server reply-code FSM. Receiving MTA substrate."
21//   sil_target:         SIL2     (open relay / misdelivery)
22//   evidence:           [cmd_classify, addr_extract, relay_refused,
23//                        out_of_sequence_503, reply_code_fsm]
24//   hazard_register:    [bug-tape-open-relay, bug-tape-smtp-state-confusion]
25//   residual_risk:      "Greylisting/rate-limit/SPF-at-RCPT (compose R5)
26//                        + live accept loop are upstream/_io concerns."
27//   verdict:            NOT_YET_EVALUATED
28
29// ---- inbound command enum ----
30const MTA_C_UNKNOWN: i64 = 0
31const MTA_C_EHLO:    i64 = 1
32const MTA_C_HELO:    i64 = 2
33const MTA_C_MAIL:    i64 = 3
34const MTA_C_RCPT:    i64 = 4
35const MTA_C_DATA:    i64 = 5
36const MTA_C_QUIT:    i64 = 6
37const MTA_C_RSET:    i64 = 7
38const MTA_C_NOOP:    i64 = 8
39
40// ---- server states ----
41const MTA_S_GREET: i64 = 0   // 220 sent, await EHLO
42const MTA_S_READY: i64 = 1   // after EHLO, await MAIL
43const MTA_S_MAIL:  i64 = 2   // after MAIL, await RCPT
44const MTA_S_RCPT:  i64 = 3   // >=1 RCPT accepted, await RCPT/DATA
45const MTA_S_DATA:  i64 = 4   // in DATA collection
46const MTA_S_QUIT:  i64 = 5
47
48func mta_upper(c: i64) -> i64 { if c >= 97 && c <= 122 { return c - 32 } return c }
49
50func mta_verb_eq(line: *u8, n: i64, v: *u8) -> i64 {
51    if n < 4 { return 0 }
52    var i: i64 = 0
53    while i < 4 { if mta_upper(line[i] & 0xff) != (v[i] & 0xff) { return 0 } i = i + 1 }
54    return 1
55}
56
57// Classify an inbound command line.
58func nx_mta_parse_cmd(line: *u8, n: i64) -> i64 {
59    if mta_verb_eq(line, n, "EHLO" as *u8) == 1 { return MTA_C_EHLO }
60    if mta_verb_eq(line, n, "HELO" as *u8) == 1 { return MTA_C_HELO }
61    if mta_verb_eq(line, n, "MAIL" as *u8) == 1 { return MTA_C_MAIL }
62    if mta_verb_eq(line, n, "RCPT" as *u8) == 1 { return MTA_C_RCPT }
63    if mta_verb_eq(line, n, "DATA" as *u8) == 1 { return MTA_C_DATA }
64    if mta_verb_eq(line, n, "QUIT" as *u8) == 1 { return MTA_C_QUIT }
65    if mta_verb_eq(line, n, "RSET" as *u8) == 1 { return MTA_C_RSET }
66    if mta_verb_eq(line, n, "NOOP" as *u8) == 1 { return MTA_C_NOOP }
67    return MTA_C_UNKNOWN
68}
69
70// Extract the address between '<' and '>' into out (null-terminated).
71// Returns address length or -1 if absent/malformed.
72func nx_mta_extract_addr(line: *u8, n: i64, out: *u8, cap: i64) -> i64 {
73    var lt: i64 = 0 - 1
74    var i: i64 = 0
75    while i < n { if (line[i] & 0xff) == 60 { lt = i; i = n } else { i = i + 1 } }
76    if lt < 0 { return 0 - 1 }
77    var oi: i64 = 0
78    var j: i64 = lt + 1
79    while j < n && (line[j] & 0xff) != 62 && oi < cap - 1 { out[oi] = line[j]; oi = oi + 1; j = j + 1 }
80    if j >= n { return 0 - 1 }
81    if (line[j] & 0xff) != 62 { return 0 - 1 }
82    out[oi] = 0 as u8
83    return oi
84}
85
86// RCPT relay policy: accept iff the recipient's domain (after the last
87// '@') matches OUR domain (case-insensitive). 1 accept / 0 refuse-relay.
88func nx_mta_rcpt_accept(addr: *u8, alen: i64, domain: *u8, dlen: i64) -> i64 {
89    var at: i64 = 0 - 1
90    var i: i64 = 0
91    while i < alen { if (addr[i] & 0xff) == 64 { at = i } i = i + 1 }
92    if at < 0 { return 0 }
93    let ds: i64 = at + 1
94    let dl: i64 = alen - ds
95    if dl != dlen { return 0 }
96    var k: i64 = 0
97    while k < dl {
98        if mta_upper(addr[ds + k] & 0xff) != mta_upper(domain[k] & 0xff) { return 0 }
99        k = k + 1
100    }
101    return 1
102}
103
104// Server FSM: given current state + parsed command (+ whether the RCPT
105// passed policy), set *out_next and return the SMTP reply code.
106func nx_mta_reply_for(state: i64, cmd: i64, rcpt_ok: i64, out_next: *i64) -> i64 {
107    if cmd == MTA_C_QUIT { *out_next = MTA_S_QUIT; return 221 }
108    if cmd == MTA_C_RSET { *out_next = MTA_S_READY; return 250 }
109    if cmd == MTA_C_NOOP { *out_next = state; return 250 }
110    if cmd == MTA_C_EHLO || cmd == MTA_C_HELO { *out_next = MTA_S_READY; return 250 }
111    if cmd == MTA_C_MAIL {
112        if state == MTA_S_READY { *out_next = MTA_S_MAIL; return 250 }
113        *out_next = state; return 503
114    }
115    if cmd == MTA_C_RCPT {
116        if state == MTA_S_MAIL || state == MTA_S_RCPT {
117            if rcpt_ok == 1 { *out_next = MTA_S_RCPT; return 250 }
118            *out_next = state; return 550          // relay denied
119        }
120        *out_next = state; return 503
121    }
122    if cmd == MTA_C_DATA {
123        if state == MTA_S_RCPT { *out_next = MTA_S_DATA; return 354 }
124        *out_next = state; return 503
125    }
126    *out_next = state; return 500
127}
128
129// Reply code after a DATA body has been received + delivered.
130func nx_mta_data_delivered(stored_ok: i64, out_next: *i64) -> i64 {
131    *out_next = MTA_S_READY
132    if stored_ok == 1 { return 250 }
133    return 451
134}
nx_email_mta.nx source

`nx_email_mta.nx` source
