Run the server process with seikan-server run (systemd uses ExecStart=seikan-server run); the bare command prints help. Flags go after run (seikan-server run --admin-host …), and every flag has an SEIKAN_* env equivalent.

Flag Env var Default Description
--state-dir SEIKAN_STATE_DIR /var/lib/seikan bbolt DB + autocert cache.
--admin-host SEIKAN_ADMIN_HOST Reserved hostname for the admin API on :443.
--acme-email SEIKAN_ACME_EMAIL Let's Encrypt contact email.
--acme-staging SEIKAN_ACME_STAGING false Use the LE staging directory.
--self-signed SEIKAN_SELF_SIGNED false Dev: serve self-signed certs instead of ACME.
--verbose SEIKAN_VERBOSE false Start with per-request access logging on (toggle live with verbose on/off).
--http-addr SEIKAN_HTTP_ADDR :80 ACME HTTP-01 + redirect listener.
--https-addr SEIKAN_HTTPS_ADDR :443 Public HTTPS (frontends + admin API).
--tunnel-addr SEIKAN_TUNNEL_ADDR :7000 Tunnel control listener (clients dial in).
--tcp-range SEIKAN_TCP_RANGE 10000-20000 Public ports auto-assigned to TCP frontends.
--metrics-addr SEIKAN_METRICS_ADDR 127.0.0.1:9090 Metrics + pprof endpoint ("" disables).
--perf-profile SEIKAN_PERF_PROFILE balanced Performance preset (see Performance).
SEIKAN_MAX_CONNS 8192 Max concurrent public HTTPS connections (0 = unlimited).
SEIKAN_MAX_TUNNELS 1024 Max concurrent client tunnels (0 = unlimited).
SEIKAN_MAX_FRONTENDS 0 Max total frontends (0 = unlimited).
SEIKAN_APIKEY_PREFIX sk_ Prefix on generated admin API keys.
SEIKAN_TOKEN_PREFIX sk_t_ Prefix on generated per-frontend connect tokens.
SEIKAN_AFFINITY_COOKIE sk_aff Sticky-session affinity cookie name.
SEIKAN_KEEPALIVE profile Reuse tunnel streams across HTTP requests.
SEIKAN_FLUSH_INTERVAL profile ReverseProxy flush interval (-1/immediate or a duration).
SEIKAN_STREAM_WINDOW profile yamux per-stream window in bytes (min 262144).
SEIKAN_COPY_BUFFER profile Pipe copy buffer in bytes.
LOG_LEVEL info debug | info | warn | error.

seikan-server install writes every setting above to /etc/seikan-server.env at its effective value (the perf knobs commented, since they default from the profile). Edit it with seikan-server configure; upgrades backfill any newly-added settings.