feat(ssh): capture password attempts via pam_exec auth-helper
Real OpenSSH doesn't log attempted passwords — only success/failure
with username — leaving SSH the sole auth-bearing service in the
fleet that contributes nothing to the cred corpus FTP/MySQL/RDP/
VNC/etc. populate. Closes that gap with a tiny pam_exec shim.
A static C helper (~80 LoC, musl, ~38KB stripped) is wired into
/etc/pam.d/sshd as `auth optional pam_exec.so expose_authtok stdout
/usr/sbin/auth-helper`. pam_exec writes the attempted password to
the helper's stdin NUL-terminated; the helper formats an RFC 5424
line in the exact shape templates/syslog_bridge.py produces
(facility local0, PEN 55555, MSGID auth_attempt — same MSGID FTP
uses) and writes it to /proc/1/fd/1 so the existing collector
stdout-reader pipeline picks it up.
Two password fields ride in the SD-block:
- password= RFC 5424 escaped, ASCII-printable only, ? for non-
printables. FTP-compatible — existing dashboard
rendering picks up SSH attempts unchanged.
- password_b64= base64 of the exact PAM_AUTHTOK bytes. Preserves
NUL/0xff/control-byte fingerprinting signal that the
plain field necessarily drops.
Fail-open by design: the PAM line is `optional` so a malfunctioning
helper never blocks sshd auth. Better to miss a cred than break the
honeypot.
Verified end-to-end inside the rebuilt image:
- 38KB static ELF, runs without a dynamic linker
- correct RFC 5424 line for `hunter2` → b64 `aHVudGVyMg==`
- NUL truncation matches pam_exec's contract
- 0xff bytes survive losslessly through password_b64
- empty password produces a well-formed line (e.g. pubkey auth path)
This commit is contained in:
@@ -1,4 +1,17 @@
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ARG BASE_IMAGE=debian:bookworm-slim
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# ── Stage 1: build the static auth-helper credential-capture binary ──────────
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# Compiled against musl so the resulting binary is fully static — runs on
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# any glibc/musl Linux without a libc version match. Stripped at link
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# time via -s so `file /usr/sbin/auth-helper` reports a generic ELF.
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FROM debian:bookworm-slim AS auth-helper-build
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RUN apt-get update && apt-get install -y --no-install-recommends musl-tools \
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&& rm -rf /var/lib/apt/lists/*
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COPY auth-helper.c /tmp/auth-helper.c
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RUN musl-gcc -static -O2 -s -Wall -Wextra \
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-o /auth-helper /tmp/auth-helper.c
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# ── Stage 2: the actual SSH decky image ──────────────────────────────────────
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FROM ${BASE_IMAGE}
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RUN apt-get update && apt-get install -y --no-install-recommends \
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@@ -57,6 +70,17 @@ RUN sed -i \
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-e 's|^\$PrivDropToGroup|#$PrivDropToGroup|' \
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/etc/rsyslog.conf
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# auth-helper: drop the static binary into /usr/sbin and wire pam_exec
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# into the sshd PAM stack so every password attempt (success or fail) is
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# captured before pam_unix runs. `optional` so a malfunctioning helper
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# never blocks auth — fail-open is correct: missed creds are recoverable,
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# a borked honeypot is not. expose_authtok writes the password to the
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# helper's stdin, NUL-terminated.
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COPY --from=auth-helper-build /auth-helper /usr/sbin/auth-helper
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RUN chmod 755 /usr/sbin/auth-helper && \
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sed -i '1i auth optional pam_exec.so expose_authtok stdout /usr/sbin/auth-helper' \
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/etc/pam.d/sshd
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# Sudo: log to syslog (auth facility) AND a local file with full I/O capture
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RUN echo 'Defaults logfile="/var/log/sudo.log"' >> /etc/sudoers && \
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echo 'Defaults syslog=auth' >> /etc/sudoers && \
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173
decnet/templates/ssh/auth-helper.c
Normal file
173
decnet/templates/ssh/auth-helper.c
Normal file
@@ -0,0 +1,173 @@
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/*
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* auth-helper — RFC 5424 cred-capture helper invoked via pam_exec.so.
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*
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* Wired into /etc/pam.d/sshd as:
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* auth optional pam_exec.so expose_authtok stdout /usr/sbin/auth-helper
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*
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* Behaviour:
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* - Reads $PAM_USER and $PAM_RHOST from environ (set by pam_exec).
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* - Reads PAM_AUTHTOK from stdin (NUL-terminated, written by pam_exec
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* when invoked with `expose_authtok`).
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* - Emits a single RFC 5424 line on /proc/1/fd/1 in the same shape as
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* templates/syslog_bridge.py:syslog_line() — facility local0, PEN
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* 55555, MSGID `auth_attempt` (matches FTP's existing event type so
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* the parser + dashboard pick it up with zero changes).
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*
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* Two password fields ride in the SD-block:
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* password RFC 5424-escaped ASCII-printable, '?' for non-printables.
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* FTP-compatible; consumed by existing dashboard rendering.
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* password_b64 base64 of the exact PAM_AUTHTOK bytes. Lossless.
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* Preserves NUL/0xff/control bytes that the plain field
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* would silently drop — useful fingerprinting signal.
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*
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* Fail-open: every error path silently exits 0. The PAM line is `optional`
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* so a malfunctioning helper must never break sshd auth.
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*
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* PII discipline: the password value is attacker-supplied bytes. Decky
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* services are not for admin SSH; throwaway creds (root:admin) are the
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* convention. Limitations tracked in development/DEBT.md (DEBT-038).
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*/
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#define _GNU_SOURCE
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#include <fcntl.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <time.h>
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#include <unistd.h>
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#define MAX_USER 256
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#define MAX_HOST 256
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#define MAX_PW 1024
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#define LINE_BUF 8192
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static const char B64[] =
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"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
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/* Standard base64 with '=' padding. NUL-terminates *out*. Returns bytes
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* written (excluding the NUL). On overflow returns 0 and NUL-terminates. */
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static size_t b64_encode(const unsigned char *in, size_t inlen,
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char *out, size_t outcap) {
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size_t i = 0, o = 0;
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while (i + 3 <= inlen) {
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if (o + 4 >= outcap) { out[0] = '\0'; return 0; }
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unsigned x = ((unsigned)in[i] << 16) |
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((unsigned)in[i+1] << 8) |
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(unsigned)in[i+2];
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out[o++] = B64[(x >> 18) & 0x3f];
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out[o++] = B64[(x >> 12) & 0x3f];
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out[o++] = B64[(x >> 6) & 0x3f];
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out[o++] = B64[ x & 0x3f];
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i += 3;
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}
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if (i < inlen) {
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if (o + 4 >= outcap) { out[0] = '\0'; return 0; }
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unsigned x = (unsigned)in[i] << 16;
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if (i + 1 < inlen) x |= (unsigned)in[i+1] << 8;
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out[o++] = B64[(x >> 18) & 0x3f];
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out[o++] = B64[(x >> 12) & 0x3f];
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out[o++] = (i + 1 < inlen) ? B64[(x >> 6) & 0x3f] : '=';
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out[o++] = '=';
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}
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out[o] = '\0';
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return o;
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}
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/* RFC 5424 §6.3.3: in SD-PARAM-VALUE, escape \\ → \\\\, " → \", ] → \].
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* Non-printables become '?' so the line stays parser-safe. */
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static size_t sd_escape(const unsigned char *in, size_t inlen,
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char *out, size_t outcap) {
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size_t o = 0;
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for (size_t i = 0; i < inlen; i++) {
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unsigned char c = in[i];
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if (c == '\\' || c == '"' || c == ']') {
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if (o + 3 >= outcap) break;
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out[o++] = '\\';
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out[o++] = c;
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} else if (c >= 0x20 && c < 0x7f) {
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if (o + 2 >= outcap) break;
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out[o++] = c;
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} else {
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if (o + 2 >= outcap) break;
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out[o++] = '?';
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}
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}
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out[o] = '\0';
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return o;
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}
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int main(void) {
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const char *user = getenv("PAM_USER");
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const char *rhost = getenv("PAM_RHOST");
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if (!user) user = "";
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if (!rhost) rhost = "";
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/* Read password until NUL (pam_exec's expose_authtok contract) or EOF. */
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unsigned char pw_raw[MAX_PW];
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size_t pw_len = 0;
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while (pw_len < sizeof(pw_raw)) {
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ssize_t n = read(0, pw_raw + pw_len, sizeof(pw_raw) - pw_len);
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if (n <= 0) break;
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for (ssize_t i = 0; i < n; i++) {
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if (pw_raw[pw_len + i] == 0) {
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pw_len += (size_t)i;
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goto pw_done;
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}
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}
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pw_len += (size_t)n;
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}
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pw_done:;
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/* Timestamp: YYYY-MM-DDThh:mm:ss.uuuuuu+00:00 — matches the shape
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* datetime.now(timezone.utc).isoformat() emits in syslog_bridge.py. */
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struct timespec ts;
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if (clock_gettime(CLOCK_REALTIME, &ts) != 0) return 0;
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struct tm tm;
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if (gmtime_r(&ts.tv_sec, &tm) == NULL) return 0;
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char tsbuf[40];
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snprintf(tsbuf, sizeof(tsbuf),
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"%04d-%02d-%02dT%02d:%02d:%02d.%06ld+00:00",
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tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
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tm.tm_hour, tm.tm_min, tm.tm_sec,
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(long)(ts.tv_nsec / 1000));
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char host[MAX_HOST];
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if (gethostname(host, sizeof(host) - 1) != 0) {
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host[0] = '-'; host[1] = '\0';
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} else {
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host[sizeof(host) - 1] = '\0';
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}
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/* Escape / encode the dynamic fields. Buffers sized 2x source to
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* survive worst-case escape expansion. */
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char user_esc [MAX_USER * 2];
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char rhost_esc[MAX_HOST * 2];
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char pw_esc [MAX_PW * 2];
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char pw_b64 [MAX_PW * 2];
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sd_escape((const unsigned char *)user, strlen(user), user_esc, sizeof(user_esc));
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sd_escape((const unsigned char *)rhost, strlen(rhost), rhost_esc, sizeof(rhost_esc));
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sd_escape(pw_raw, pw_len, pw_esc, sizeof(pw_esc));
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b64_encode(pw_raw, pw_len, pw_b64, sizeof(pw_b64));
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/* Priority: facility=local0(16), severity=INFO(6) → <16*8+6> = <134>.
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* Matches the syslog_bridge.py default exactly. */
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char line[LINE_BUF];
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int n = snprintf(line, sizeof(line),
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"<134>1 %s %s auth-helper - auth_attempt "
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"[relay@55555 username=\"%s\" password=\"%s\" "
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"password_b64=\"%s\" src_ip=\"%s\"]\n",
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tsbuf, host, user_esc, pw_esc, pw_b64, rhost_esc);
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if (n <= 0 || (size_t)n >= sizeof(line)) return 0;
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/* /proc/1/fd/1 is the entrypoint's stdout — the fd Docker captures
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* for `docker logs`. Same channel rsyslog forwards auth.* into via
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* the existing template; we bypass rsyslog entirely so behaviour is
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* deterministic across rsyslog config drift. */
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int fd = open("/proc/1/fd/1", O_WRONLY | O_APPEND);
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if (fd < 0) return 0;
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ssize_t w = write(fd, line, (size_t)n);
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(void)w;
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close(fd);
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return 0;
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}
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