docker/cli · error

source is required

Error message

source is required

What it means

Thrown by ConfigOpt.Set when parsing a --config request in key=value form but no source/src key was provided. A Swarm config reference requires a source config name to mount; the parser only validates this after consuming all fields, so a missing source is caught at the end. The simple short form (--config foo) auto-sets source and never triggers this.

Solutions

1. Add source=<configname> (or src=) to the --config spec.
2. Use the short form --config <configname> if no other overrides are needed.
3. Verify the config exists with docker config ls before referencing it.

Example fix

// before
--config target=/app.conf,uid=33
// after
--config source=appcfg,target=/app.conf,uid=33

hasSource := false
for _, f := range strings.Split(spec, ",") {
    k, _, _ := strings.Cut(strings.ToLower(f), "=")
    if k == "source" || k == "src" {
        hasSource = true
    }
}
if !hasSource && !(len(strings.Split(spec, ",")) == 1 && !strings.Contains(spec, "=")) {
    return errors.New("config spec requires source=")
}

if err := configOpt.Set(spec); err != nil {
    return err
}

When it happens

Trigger: Passing --config target=/app.conf without a source=, or any key=value config spec omitting source/src.

Common situations: Mistakenly believing target alone identifies the config, or copy-pasting a partial spec that lost the source field.

Related errors

• error reading from STDIN: data is empty
• config file is required
• cannot supply extra formatting options to the pretty…
• hostip is not supported
• invalid field in secret request

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/b50c47268cb1585d. Report an issue: GitHub.

Appendix: source

Thrown at opts/swarmopts/config.go:72

			options.File.Name = val
		case "uid":
			options.File.UID = val
		case "gid":
			options.File.GID = val
		case "mode":
			m, err := strconv.ParseUint(val, 0, 32)
			if err != nil {
				return fmt.Errorf("invalid mode specified: %v", err)
			}

			options.File.Mode = os.FileMode(m)
		default:
			return fmt.Errorf("invalid field in config request: %s", key)
		}
	}

	if options.ConfigName == "" {
		return errors.New("source is required")
	}
	if options.File.Name == "" {
		options.File.Name = options.ConfigName
	}

	o.values = append(o.values, options)
	return nil
}

// Type returns the type of this option
func (*ConfigOpt) Type() string {
	return "config"
}

// String returns a string repr of this option
func (o *ConfigOpt) String() string {
	configs := make([]string, 0, len(o.values))
	for _, config := range o.values {

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