RTFM · Medical healthchecks

lb.saphira.dicom.echo: a wave across the room (C-ECHO)

DICOM's own hello: the check introduces itself with an AE title, asks the scanner or archive for a verification service, and waits for the acknowledgement. In hospital terms; the CT suite's front door is open and the PACS is answering, not just a TCP port.

What it proves

A complete DICOM association: negotiation opens with the right AE titles on both sides, the Verification Service Class (C-ECHO) executes, and the SCP acknowledges. Continuous-safe: read-only, cheap enough to run every few seconds forever.

This is the named capability "the DICOM service is reachable and speaking DICOM", and it never degrades to "the port is open": a TCP proxy in front of a dead PACS fails this check, exactly as it should.

How it works

The check drives echoscu from the DCMTK toolkit; the reference open-source DICOM SCU. The association timeout (-ta) is the shared LB_SAPHIRA_TIMEOUT, and the called/calling AE titles come from the configuration layer or the $6/$7 convenience positionals.

What the wrapper runs

echoscu -aet "$AET" -aec "$AEC" -ta "$LB_TIMEOUT" "$LB_REAL" "${LB_RPORT:-104}"

Configuration

Knobs
Variable / positional	Default	Meaning
$6 or LB_SAPHIRA_DICOM_AEC	ANY-SCP	Called AE title: the AE the SCP expects to be addressed by
$7 or LB_SAPHIRA_DICOM_AET	SAPHIRA-HC	Calling AE title: the check's own name
(target port)	$4, else 104	Many sites use 104, 11112 or 2762; all fine

AE titles are the DICOM equivalent of a door name: a modality may only accept callers addressing a specific title. Per-VIP titles live in the per-service conf file (e.g. dicom.echo/21.conf) so each virtual service can speak as the entity that site expects.

Wiring it up

ldirectord

# ldirectord.cf - DICOM pool over firewall mark 21
virtual = 21
    protocol = fwm
    scheduler = rr
    real = 192.168.20.21:104 gate 1
    real = 192.168.20.22:104 gate 1
    checktype = external
    checkcommand = "/var/lib/lb/saphira/lb.saphira.dicom.echo"

HAProxy

# haproxy.cfg
global
    external-check
backend dicom_pool
    mode tcp
    option external-check
    external-check command "/var/lib/lb/saphira/lb.saphira.dicom.echo"
    server pacs1 192.168.20.21:104 check inter 5s fall 3 rise 2

Run it by hand

Manual run

LB_SAPHIRA_DICOM_AEC=MODALITY-SCP \
  /var/lib/lb/saphira/lb.saphira.dicom.echo ct-pool 104 192.168.20.21 104
echo $?
# 0 = association accepted and C-ECHO acknowledged
# 1 = association refused, timeout, or DICOM rejected

Direct positional form for one-off tests: lb.saphira.dicom.echo 21 104 192.168.20.21 104 MODALITY-SCP SAPHIRA-HC

Failure modes

Association rejected: the SCP answered but refused the AE titles or the service: an acceptor-policy change, and the check caught it
Timeout: the classic PACS wedge: TCP accepts, DICOM never completes
exit 2 never occurs here: this check has full safe defaults

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