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traberph
2026-09-27 09:52:12 +02:00
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Dockerfile
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# syntax=docker/dockerfile:1
FROM docker.io/library/alpine:3.24.2
FROM docker.io/library/alpine:3
RUN apk add --no-cache openssh-client tini
COPY --chmod=0755 entrypoint.sh /usr/local/bin/sish-client
# nobody; HOME and state live in /tmp so the root filesystem can be read-only
ENV HOME=/tmp \
STATE_DIR=/tmp/sish-client
# nobody: ssh needs a uid that exists in /etc/passwd, so keep runAsUser at 65534
USER 65534:65534
# -g: forward signals to the whole process group, so ssh stops cleanly too
# tini reaps zombies and forwards SIGTERM to the whole process group (-g), incl. ssh
ENTRYPOINT ["/sbin/tini", "-g", "--", "/usr/local/bin/sish-client"]
HEALTHCHECK --interval=15s --timeout=3s --start-period=30s --retries=2 \
CMD ["/bin/sh", "-c", "test -f \"$STATE_DIR/ready\""]
LABEL org.opencontainers.image.title="sish-client" \
org.opencontainers.image.description="Environment-driven sish connector (SSH reverse tunnels with SNI passthrough)"
README.md
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# sish-client
A small, environment-driven connector for a [sish](https://github.com/antoniomika/sish) edge.
It opens SSH reverse tunnels to the edge, claims hostnames there, and relays their traffic to
local targets. In the default mode sish routes by **SNI without terminating TLS**, and each
connection starts with a **PROXY v2 header** carrying the real client address.
```
client ──TLS──▶ edge :443 (sish, SNI routing) ══ssh══▶ sish-client ──▶ target (terminates TLS)
```
Image: `registry.traberph.de/public/sish-client` (Alpine + OpenSSH client + tini, ~20 MB,
runs as `nobody`, works with a read-only root filesystem).
## Quick start
Opens SSH reverse tunnels to a [sish](https://github.com/antoniomika/sish) edge and reconnects
every 5s when the session drops. Image: `registry.traberph.de/public/sish-client`.
```sh
docker run -d --name sish-client --read-only --tmpfs /tmp \
docker run -d --read-only --tmpfs /tmp \
-e SISH_HOST=edge.example.com \
-e SISH_HOST_KEY="ssh-ed25519 AAAA..." \
-e ROUTES="app.example.com=app:8443" \
-e SISH_ROUTES="app.example.com:443=app:8443" \
-v ./id_ed25519:/secrets/id_ed25519:ro \
registry.traberph.de/public/sish-client:latest
```
The connector's **public** key must be authorized on the edge (sish `--authentication-keys-directory`),
and each hostname must be allowed there (sish `--bind-hosts` or `_sish` TXT records).
## Configuration
| Variable | Default | Description |
| Variable | Default | |
|---|---|---|
| `SISH_HOST` | *required* | Edge hostname or IP |
| `SISH_HOST` | *required* | Edge host |
| `SISH_HOST_KEY` | *required* | Edge host key, `"<type> <base64>"` (first two fields of its `.pub` file) |
| `SISH_ROUTES` | *required* | Comma-separated `host:bind-port=target:port` |
| `SISH_PORT` | `2222` | sish SSH port |
| `SISH_USER` | `connector` | SSH user (sish accepts any; shows up in edge logs) |
| `SISH_KEY` | | Private key contents (e.g. from a Kubernetes `secretKeyRef`). Takes precedence over `SISH_KEY_FILE` |
| `SISH_KEY_FILE` | `/secrets/id_ed25519` | Path to a mounted private key |
| `SISH_HOST_KEY` | | Edge host public key, `"<type> <base64>"` (the first two fields of the `.pub` file) |
| `SISH_KNOWN_HOSTS_FILE` | | Alternative to `SISH_HOST_KEY`: a known_hosts file matching `[SISH_HOST]:SISH_PORT` |
| `SISH_STRICT_HOST_KEY` | `true` | `false` disables host key verification (testing only) |
| `ROUTES` | | Routes, separated by commas, spaces or newlines (see below) |
| `ROUTES_FILE` | | File with routes, one per line, `#` comments allowed. Combined with `ROUTES` |
| `TARGET` | | Default `host:port` for routes without an explicit target |
| `DEFAULT_BIND_PORT` | `443` | Edge port for routes without an explicit bind port |
| `SNI_PROXY` | `true` | Ask sish to route by SNI instead of terminating TLS |
| `PROXY_PROTOCOL` | `2` | PROXY header version sent to the target: `1`, `2` or `off` |
| `KEEPALIVE_INTERVAL` | `15` | Seconds between SSH keepalives (3 missed = reconnect) |
| `CONNECT_TIMEOUT` | `10` | SSH connect timeout in seconds |
| `BACKOFF_MIN` / `BACKOFF_MAX` | `2` / `60` | Reconnect backoff in seconds (doubles per failure, resets after a successful session) |
| `SSH_EXTRA_OPTS` | | Extra `ssh` arguments, e.g. `-o Ciphers=aes128-gcm@openssh.com` |
| `STATE_DIR` | `/tmp/sish-client` | Writable directory for the key copy, known_hosts and the ready marker |
| `SISH_KEY_FILE` | `/secrets/id_ed25519` | Private key |
| `SISH_SNI_PROXY` | `true` | `true`: sish routes by SNI and passes TLS through. `false`: HTTP routing by `Host` header |
| `SISH_PROXY_PROTOCOL` | `2` | PROXY header sent to the target: `1`, `2` or `off` |
### Routes
HTTP (:80) routes need `SISH_SNI_PROXY=false` and a separate instance from SNI (:443) routes.
If sish rejects any route, the session fails and is retried, so a bad route shows up in the logs.
```
<host>[:<bind-port>][=<target-host>:<target-port>]
```
## Requirements
| Example | Meaning |
|---|---|
| `app.example.com=traefik:8443` | SNI `app.example.com` on edge :443 → `traefik:8443` |
| `app.example.com` | Same, using `TARGET` |
| `*.apps.example.com=traefik:8443` | Wildcard (sish needs `--bind-wildcards`) |
| `app.example.com:80=traefik:8080` | Plain HTTP on edge :80, routed by `Host` header (**needs `SNI_PROXY=false`**, see below) |
- `/tmp` must be writable (e.g. a memory `emptyDir`), the root filesystem can be read-only.
- Run as uid `65534` (the image default). ssh refuses to start for uids missing from `/etc/passwd`.
- The key file must be readable by uid 65534, e.g. a Secret volume with `defaultMode: 0440`
and `fsGroup: 65534`. ssh rejects keys owned by 65534 that others can read.
**HTTP (:80) and SNI (:443) routes cannot share one connector.** With `SNI_PROXY=true` sish
treats every forward as an SNI listener, which collides with its HTTP listener on :80. Run a
second instance for HTTP routes (e.g. for HTTP→HTTPS redirects):
## Security
```sh
SNI_PROXY=false PROXY_PROTOCOL=off ROUTES="app.example.com:80=traefik:8080"
```
If the edge rejects any route, the whole session fails (`ExitOnForwardFailure`) and is retried
with backoff. A misconfigured route is therefore loud (visible in the logs and health status)
instead of silently missing.
Several connectors may claim the same hostname. sish then load-balances between them, which is
how you get redundancy (e.g. one connector per ingress replica).
## Health
The container writes `$STATE_DIR/ready` once sish has confirmed **every** route, and removes it
when the tunnel drops. The image's `HEALTHCHECK` tests this file; in Kubernetes use it as a
readiness probe:
```yaml
readinessProbe:
exec:
command: ["test", "-f", "/tmp/sish-client/ready"]
periodSeconds: 5
```
## Examples
### Docker Compose
```yaml
services:
app:
image: nginx # must terminate TLS and accept PROXY protocol on 8443
connector:
image: registry.traberph.de/public/sish-client:latest
read_only: true
tmpfs: [/tmp]
cap_drop: [ALL]
security_opt: [no-new-privileges:true]
environment:
SISH_HOST: edge.example.com
SISH_HOST_KEY: ssh-ed25519 AAAA...
ROUTES: app.example.com=app:8443
volumes:
- ./id_ed25519:/secrets/id_ed25519:ro
restart: unless-stopped
```
The key file must be readable by uid `65534` inside the container. Compose ignores `uid`/`mode`
on file secrets, so either:
- make the key readable by a group and run with `user: "65534:<gid>"`. With **rootless** Docker
your host files appear as owned by root inside the container, so the gid is `0`.
- or pass the key via `SISH_KEY` instead of a file.
### Kubernetes sidecar (next to an ingress controller)
```yaml
- name: sish-client
image: registry.traberph.de/public/sish-client:20260926154233-1a2b3c4 # {"$imagepolicy": "flux-system:sish-client"}
env:
- { name: SISH_HOST, value: edge.example.com }
- { name: SISH_HOST_KEY, value: "ssh-ed25519 AAAA..." }
- { name: ROUTES_FILE, value: /etc/sish-client/routes.txt }
- { name: TARGET, value: "127.0.0.1:9443" } # ingress entrypoint with PROXY protocol
- name: SISH_KEY
valueFrom: { secretKeyRef: { name: sish-client, key: id_ed25519 } }
securityContext:
runAsNonRoot: true
readOnlyRootFilesystem: true
allowPrivilegeEscalation: false
capabilities: { drop: [ALL] }
volumeMounts:
- { name: tmp, mountPath: /tmp }
- { name: routes, mountPath: /etc/sish-client, readOnly: true }
readinessProbe:
exec: { command: ["test", "-f", "/tmp/sish-client/ready"] }
```
Routes are only read at connect time. Roll the pods after changing them (e.g. with a config
hash annotation).
## Image tags
The Gitea workflow (`.gitea/workflows/build.yaml`) builds `linux/amd64` and `linux/arm64` and pushes:
| Trigger | Tags |
|---|---|
| Push to `main` | `<yyyymmddHHMMSS>-<sha7>` (UTC) and `latest` |
| Git tag `v*` | `<yyyymmddHHMMSS>-<sha7>` and the git tag, e.g. `v1.0.0` |
Repository secrets required: `REGISTRY_USERNAME`, `REGISTRY_TOKEN` (a token with push access
to `registry.traberph.de/public`). The runner needs Docker (Buildx + QEMU for arm64).
### Flux image automation
```yaml
apiVersion: image.toolkit.fluxcd.io/v1
kind: ImageRepository
metadata: { name: sish-client, namespace: flux-system }
spec:
image: registry.traberph.de/public/sish-client
interval: 10m
---
apiVersion: image.toolkit.fluxcd.io/v1
kind: ImagePolicy
metadata: { name: sish-client, namespace: flux-system }
spec:
imageRepositoryRef: { name: sish-client }
filterTags:
pattern: '^(?P<ts>[0-9]{14})-[0-9a-f]{7}$'
extract: '$ts'
policy:
numerical: { order: asc }
```
## Build locally
```sh
docker build -t registry.traberph.de/public/sish-client:dev .
```
- The edge is authenticated only by `SISH_HOST_KEY`. There is no trust-on-first-use and no
fallback: a wrong or missing key fails the connection.
- All ssh_config files are ignored (`-F /dev/null`). Agent and X11 forwarding are off, so the
edge can only open connections to the configured route targets.
- **PROXY protocol:** the target trusts the PROXY header for the client IP. It must only accept
PROXY connections from this client (e.g. listen on `127.0.0.1` in the sidecar and set trusted
IPs), otherwise anything that can reach that port can spoof client IPs. If the target does not
expect a PROXY header, set `SISH_PROXY_PROTOCOL=off`.
- Whoever controls the edge sees HTTP traffic, and with `SISH_SNI_PROXY=true` still sees the
SNI hostnames and client IPs, but not TLS contents.
- The base image follows `alpine:3`. Rebuild regularly to pick up OpenSSH fixes.
entrypoint.sh
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#!/bin/sh
# sish-client: environment-driven connector for a sish edge.
# Claims hostnames on the edge via SSH reverse forwards and relays them to local
# targets. Reconnects forever with exponential backoff. See README.md.
# Opens SSH reverse tunnels to a sish edge and reconnects forever. See README.md.
set -eu
set -f # routes may contain wildcards (*.example.com); never glob-expand them
set -f # no globbing: routes may contain wildcards (*.example.com)
ESC=$(printf '\033')
die() { echo "sish-client: $*" >&2; exit 64; }
log() { printf '%s sish-client: %s\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)" "$*"; }
die() { log "error: $*"; exit 64; }
is_true() {
case "$(printf '%s' "$1" | tr 'A-Z' 'a-z')" in
1 | true | yes | on) return 0 ;;
*) return 1 ;;
esac
}
is_uint() { case "$1" in '' | *[!0-9]*) return 1 ;; *) return 0 ;; esac; }
: "${SISH_HOST:?is required}" "${SISH_HOST_KEY:?is required}" "${SISH_ROUTES:?is required}"
SNI_PROXY=${SISH_SNI_PROXY:-true}
PROXY_PROTOCOL=${SISH_PROXY_PROTOCOL:-2}
KEY=${SISH_KEY_FILE:-/secrets/id_ed25519}
# --- configuration -----------------------------------------------------------
case $SNI_PROXY in true | false) ;; *) die "SISH_SNI_PROXY must be true or false" ;; esac
case $PROXY_PROTOCOL in 1 | 2 | off) ;; *) die "SISH_PROXY_PROTOCOL must be 1, 2 or off" ;; esac
[ -r "$KEY" ] || die "cannot read private key $KEY"
SISH_HOST=${SISH_HOST:-}
SISH_PORT=${SISH_PORT:-2222}
SISH_USER=${SISH_USER:-connector}
SISH_KEY=${SISH_KEY:-}
SISH_KEY_FILE=${SISH_KEY_FILE:-/secrets/id_ed25519}
SISH_HOST_KEY=${SISH_HOST_KEY:-}
SISH_KNOWN_HOSTS_FILE=${SISH_KNOWN_HOSTS_FILE:-}
SISH_STRICT_HOST_KEY=${SISH_STRICT_HOST_KEY:-true}
# Pin the edge host key: the only entry ssh will trust. A newline would allow
# extra known_hosts lines (e.g. a wildcard entry), so the key must be one line.
case $SISH_HOST_KEY in *'
'*) die "SISH_HOST_KEY must be a single line" ;; esac
KNOWN_HOSTS=$(mktemp) # private 0600 file with a random name, safe in a shared /tmp
printf 'sish-edge %s\n' "$SISH_HOST_KEY" > "$KNOWN_HOSTS"
ROUTES=${ROUTES:-}
ROUTES_FILE=${ROUTES_FILE:-}
TARGET=${TARGET:-}
DEFAULT_BIND_PORT=${DEFAULT_BIND_PORT:-443}
SNI_PROXY=${SNI_PROXY:-true}
PROXY_PROTOCOL=${PROXY_PROTOCOL:-2}
KEEPALIVE_INTERVAL=${KEEPALIVE_INTERVAL:-15}
CONNECT_TIMEOUT=${CONNECT_TIMEOUT:-10}
BACKOFF_MIN=${BACKOFF_MIN:-2}
BACKOFF_MAX=${BACKOFF_MAX:-60}
SSH_EXTRA_OPTS=${SSH_EXTRA_OPTS:-}
STATE_DIR=${STATE_DIR:-/tmp/sish-client}
[ -n "$SISH_HOST" ] || die "SISH_HOST is required"
for v in SISH_PORT DEFAULT_BIND_PORT KEEPALIVE_INTERVAL CONNECT_TIMEOUT BACKOFF_MIN BACKOFF_MAX; do
eval "val=\$$v"
is_uint "$val" || die "$v must be a non-negative integer (got '$val')"
# host:bind-port=target:port -> -R host:bind-port:target:port
# Each route stays a single argument to -R, so it cannot inject ssh options.
forwards=""
for r in $(printf '%s' "$SISH_ROUTES" | tr , ' '); do
case $r in *=*) ;; *) die "bad route '$r', expected host:port=target:port" ;; esac
forwards="$forwards -R ${r%%=*}:${r#*=}"
done
case "$PROXY_PROTOCOL" in 1 | 2 | off | none | false | 0) ;; *) die "PROXY_PROTOCOL must be 1, 2 or off" ;; esac
[ -n "$forwards" ] || die "SISH_ROUTES contains no routes"
umask 077
mkdir -p "$STATE_DIR" || die "STATE_DIR $STATE_DIR is not writable (mount a tmpfs on /tmp with a read-only root fs)"
READY_FILE=$STATE_DIR/ready
FIFO=$STATE_DIR/ssh.out
KEY=$STATE_DIR/id_key
KNOWN_HOSTS=$STATE_DIR/known_hosts
rm -f "$READY_FILE"
# sish session options, sent as the remote command
opts=""
if [ "$SNI_PROXY" = true ]; then opts="sni-proxy=true"; fi
if [ "$PROXY_PROTOCOL" != off ]; then opts="$opts proxy-protocol=$PROXY_PROTOCOL"; fi
# --- private key: always copied so ssh sees a private 0600 file we own -------
if [ -n "$SISH_KEY" ]; then
printf '%s\n' "$SISH_KEY" > "$KEY"
elif [ -r "$SISH_KEY_FILE" ]; then
cat "$SISH_KEY_FILE" > "$KEY"
else
die "no private key: set SISH_KEY or mount a readable key at SISH_KEY_FILE ($SISH_KEY_FILE)"
fi
chmod 600 "$KEY"
# --- host key verification ---------------------------------------------------
HOST_KEY_OPTS=""
if [ -n "$SISH_HOST_KEY" ]; then
printf 'sish-edge %s\n' "$SISH_HOST_KEY" > "$KNOWN_HOSTS"
HOST_KEY_OPTS="-o HostKeyAlias=sish-edge -o UserKnownHostsFile=$KNOWN_HOSTS -o StrictHostKeyChecking=yes"
elif [ -n "$SISH_KNOWN_HOSTS_FILE" ]; then
[ -r "$SISH_KNOWN_HOSTS_FILE" ] || die "SISH_KNOWN_HOSTS_FILE $SISH_KNOWN_HOSTS_FILE is not readable"
HOST_KEY_OPTS="-o UserKnownHostsFile=$SISH_KNOWN_HOSTS_FILE -o StrictHostKeyChecking=yes"
elif is_true "$SISH_STRICT_HOST_KEY"; then
die "no host key: set SISH_HOST_KEY (e.g. 'ssh-ed25519 AAAA...') or SISH_KNOWN_HOSTS_FILE"
else
log "WARNING: host key verification disabled (SISH_STRICT_HOST_KEY=false)"
HOST_KEY_OPTS="-o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no"
fi
# --- routes ------------------------------------------------------------------
# Entry: <host>[:<bind-port>][=<target-host>:<target-port>]
# Without "=target", TARGET is used. Bind port defaults to DEFAULT_BIND_PORT.
route_entries() {
{
printf '%s\n' "$ROUTES" | tr ', ' '\n\n'
if [ -n "$ROUTES_FILE" ]; then
[ -r "$ROUTES_FILE" ] || die "ROUTES_FILE $ROUTES_FILE is not readable"
sed 's/#.*//' "$ROUTES_FILE" | tr ', \t' '\n\n\n'
fi
} | sed '/^[[:space:]]*$/d'
}
FORWARDS=""
ROUTE_COUNT=0
for entry in $(route_entries); do
case "$entry" in
*=*) bind=${entry%%=*} target=${entry#*=} ;;
*) bind=$entry target=$TARGET ;;
esac
[ -n "$target" ] || die "route '$entry' has no target and TARGET is not set"
case "$target" in *:*) ;; *) die "target '$target' of route '$entry' must be host:port" ;; esac
case "$bind" in *:*) ;; *) bind="$bind:$DEFAULT_BIND_PORT" ;; esac
FORWARDS="$FORWARDS -R $bind:$target"
ROUTE_COUNT=$((ROUTE_COUNT + 1))
log "route: $bind -> $target"
done
[ "$ROUTE_COUNT" -gt 0 ] || die "no routes: set ROUTES and/or ROUTES_FILE"
# --- sish session options (sent as the remote "command") ---------------------
REMOTE_CMD=""
is_true "$SNI_PROXY" && REMOTE_CMD="$REMOTE_CMD sni-proxy=true"
case "$PROXY_PROTOCOL" in 1 | 2) REMOTE_CMD="$REMOTE_CMD proxy-protocol=$PROXY_PROTOCOL" ;; esac
SSH_OPTS="-T -p $SISH_PORT -i $KEY
-o IdentitiesOnly=yes -o BatchMode=yes -o LogLevel=ERROR
-o ExitOnForwardFailure=yes
-o ConnectTimeout=$CONNECT_TIMEOUT
-o ServerAliveInterval=$KEEPALIVE_INTERVAL -o ServerAliveCountMax=3
$HOST_KEY_OPTS $SSH_EXTRA_OPTS"
# --- run loop ----------------------------------------------------------------
SSH_PID=""
shutdown() {
log "received signal, shutting down"
rm -f "$READY_FILE"
[ -n "$SSH_PID" ] && kill "$SSH_PID" 2>/dev/null
exit 0
}
trap shutdown TERM INT
# Runs one ssh session. Marks ready once sish confirmed every forward.
WAS_READY=0
run_once() {
WAS_READY=0
rm -f "$READY_FILE" "$FIFO"
mkfifo "$FIFO"
# shellcheck disable=SC2086 # word splitting of option strings is intended
ssh $SSH_OPTS $FORWARDS "$SISH_USER@$SISH_HOST" $REMOTE_CMD > "$FIFO" 2>&1 &
SSH_PID=$!
confirmed=0
while IFS= read -r line; do
line=$(printf '%s' "$line" | sed "s/${ESC}\[[0-9;]*m//g; s/\r\$//")
[ -n "$line" ] || continue
log "edge: $line"
case "$line" in
*"Starting SSH Forwarding service"*)
confirmed=$((confirmed + 1))
if [ "$confirmed" -ge "$ROUTE_COUNT" ] && [ "$WAS_READY" -eq 0 ]; then
WAS_READY=1
: > "$READY_FILE"
log "ready: $confirmed/$ROUTE_COUNT forward(s) active"
fi
;;
esac
done < "$FIFO"
rc=0
wait "$SSH_PID" || rc=$?
SSH_PID=""
rm -f "$READY_FILE" "$FIFO"
return "$rc"
}
delay=$BACKOFF_MIN
# -F /dev/null: ignore all ssh_config files, every option is set here.
# The server can only open channels for the forwards requested above; agent and
# X11 forwarding are off by default.
while :; do
log "connecting to $SISH_USER@$SISH_HOST:$SISH_PORT ($ROUTE_COUNT route(s))"
rc=0
run_once || rc=$?
[ "$WAS_READY" -eq 1 ] && delay=$BACKOFF_MIN
log "tunnel closed (exit $rc), reconnecting in ${delay}s"
sleep "$delay" &
wait $! || true
delay=$((delay * 2))
[ "$delay" -le "$BACKOFF_MAX" ] || delay=$BACKOFF_MAX
# shellcheck disable=SC2086 # $forwards and $opts are split on purpose
ssh -F /dev/null -T -p "${SISH_PORT:-2222}" -i "$KEY" \
-o BatchMode=yes -o IdentitiesOnly=yes -o ExitOnForwardFailure=yes \
-o ServerAliveInterval=15 -o ServerAliveCountMax=3 \
-o HostKeyAlias=sish-edge -o StrictHostKeyChecking=yes \
-o UserKnownHostsFile="$KNOWN_HOSTS" -o GlobalKnownHostsFile=/dev/null \
$forwards "connector@$SISH_HOST" $opts \
|| echo "sish-client: ssh exited ($?), reconnecting in 5s" >&2
sleep 5
done