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Add support for AUTH PLAIN #148

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Add support for AUTH PLAIN #148

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3e45943
Add support for AUTH PLAIN
enndubyu Jan 13, 2026
dc6bbc5
Add clearer syslog output when server doesn't support any suitable au…
enndubyu Jan 14, 2026
34ec96f
fixup! Add support for AUTH PLAIN
enndubyu Feb 13, 2026
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1 change: 1 addition & 0 deletions dma.h
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Original file line number Diff line number Diff line change
Expand Up @@ -166,6 +166,7 @@ struct mx_hostentry {
struct smtp_auth_mechanisms {
int cram_md5;
int login;
int plain;
};

struct smtp_features {
Expand Down
77 changes: 53 additions & 24 deletions net.c
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Original file line number Diff line number Diff line change
Expand Up @@ -281,44 +281,67 @@ smtp_login(int fd, char *login, char* password, const struct smtp_features* feat
}
}

// LOGIN
if (features->auth.login) {
// LOGIN or PLAIN
if (features->auth.login || features->auth.plain) {
if ((config.features & INSECURE) != 0 ||
(config.features & SECURETRANSFER) != 0) {
/* Send AUTH command according to RFC 2554 */
send_remote_command(fd, "AUTH LOGIN");
const char *auth_mechanism = features->auth.login ? "LOGIN" : "PLAIN";
send_remote_command(fd, "AUTH %s", auth_mechanism);
if (read_remote(fd, 0, NULL) != 3) {
syslog(LOG_NOTICE, "remote delivery deferred:"
" AUTH login not available: %s",
neterr);
" AUTH %s not available: %s",
auth_mechanism, neterr);
return (1);
}

len = base64_encode(login, strlen(login), &temp);
if (len < 0) {
if (features->auth.login) {
// LOGIN mechanism
len = base64_encode(login, strlen(login), &temp);
if (len < 0) {
encerr:
syslog(LOG_ERR, "can not encode auth reply: %m");
return (1);
}
syslog(LOG_ERR, "can not encode auth reply: %m");
return (1);
}

send_remote_command(fd, "%s", temp);
free(temp);
res = read_remote(fd, 0, NULL);
if (res != 3) {
syslog(LOG_NOTICE, "remote delivery %s: AUTH login failed: %s",
res == 5 ? "failed" : "deferred", neterr);
return (res == 5 ? -1 : 1);
}
send_remote_command(fd, "%s", temp);
free(temp);
res = read_remote(fd, 0, NULL);
if (res != 3) {
syslog(LOG_NOTICE, "remote delivery %s: AUTH LOGIN failed: %s",
res == 5 ? "failed" : "deferred", neterr);
return (res == 5 ? -1 : 1);
}

len = base64_encode(password, strlen(password), &temp);
if (len < 0)
goto encerr;

send_remote_command(fd, "%s", temp);
free(temp);
} else if (features->auth.plain) {
// PLAIN mechanism
size_t buflen = strlen(login) + strlen(password) + 3;
char *plainbuf = malloc(buflen);
if (plainbuf == NULL) {
syslog(LOG_ERR, "remote delivery deferred: unable to allocate memory");
return (1);
}

snprintf(plainbuf, buflen, "%c%s%c%s", '\0', login, '\0', password);

len = base64_encode(password, strlen(password), &temp);
if (len < 0)
goto encerr;
len = base64_encode(plainbuf, buflen, &temp);
free(plainbuf);
if (len < 0)
goto encerr;

send_remote_command(fd, "%s", temp);
free(temp);
}

send_remote_command(fd, "%s", temp);
free(temp);
res = read_remote(fd, 0, NULL);
if (res != 2) {
syslog(LOG_NOTICE, "remote delivery %s: Authentication failed: %s",
syslog(LOG_NOTICE, "remote delivery %s: AUTH PLAIN failed: %s",
res == 5 ? "failed" : "deferred", neterr);
return (res == 5 ? -1 : 1);
}
Expand Down Expand Up @@ -381,6 +404,9 @@ static void parse_auth_line(char* line, struct smtp_auth_mechanisms* auth) {
else if (strcmp(method, "LOGIN") == 0)
auth->login = 1;

else if (strcmp(method, "PLAIN") == 0)
auth->plain = 1;

method = strtok(NULL, " ");
}
}
Expand Down Expand Up @@ -469,6 +495,9 @@ int perform_server_greeting(int fd, struct smtp_features* features) {
if (features->auth.login) {
syslog(LOG_DEBUG, " Server supports LOGIN authentication");
}
if (features->auth.plain) {
syslog(LOG_DEBUG, " Server supports PLAIN authentication");
}

return 0;
}
Expand Down