go-sql-driver/mysql · error

invalid DSN: missing the slash separating the database name

Error message

invalid DSN: missing the slash separating the database name

What it means

ParseDSN scans the DSN from the right looking for a '/' that separates the connection spec from the database name. If the string is non-empty yet contains no '/' at all, it returns errInvalidDSNNoSlash at dsn.go:473. The slash is mandatory even when the database name is empty.

Source

Thrown at dsn.go:31

	"context"
	"crypto/rsa"
	"crypto/tls"
	"errors"
	"fmt"
	"maps"
	"math/big"
	"net"
	"net/url"
	"sort"
	"strconv"
	"strings"
	"time"
)

var (
	errInvalidDSNUnescaped       = errors.New("invalid DSN: did you forget to escape a param value?")
	errInvalidDSNAddr            = errors.New("invalid DSN: network address not terminated (missing closing brace)")
	errInvalidDSNNoSlash         = errors.New("invalid DSN: missing the slash separating the database name")
	errInvalidDSNUnsafeCollation = errors.New("invalid DSN: interpolateParams can not be used with unsafe collations")
)

// Config is a configuration parsed from a DSN string.
// If a new Config is created instead of being parsed from a DSN string,
// the NewConfig function should be used, which sets default values.
type Config struct {
	// non boolean fields

	User                 string            // Username
	Passwd               string            // Password (requires User)
	Net                  string            // Network (e.g. "tcp", "tcp6", "unix". default: "tcp")
	Addr                 string            // Address (default: "127.0.0.1:3306" for "tcp" and "/tmp/mysql.sock" for "unix")
	DBName               string            // Database name
	Params               map[string]string // Connection parameters
	ConnectionAttributes string            // Connection Attributes, comma-delimited string of user-defined "key:value" pairs
	Collation            string            // Connection collation. When set, this will be set in SET NAMES <charset> COLLATE <collation> query
	Loc                  *time.Location    // Location for time.Time values

View on GitHub (pinned to c426bd9379)

Solutions

1. Append '/' (optionally followed by the database name) to the DSN: 'user:pass@tcp(host:3306)/'.
2. Build the DSN from a *mysql.Config via cfg.FormatDSN() so the slash is always emitted correctly.
3. If the DSN comes from an env var, validate it contains a '/' before passing it to sql.Open.

Example fix

// before
dsn := "user:pass@tcp(localhost:3306)" // missing slash
// after
dsn := "user:pass@tcp(localhost:3306)/" // or .../mydb

// A valid mysql DSN must contain at least one '/'.
func hasDSNSlash(dsn string) bool {
    return strings.Contains(dsn, "/")
}

// errInvalidDSNNoSlash is unexported; match by message.
if _, err := mysql.ParseDSN(dsn); err != nil && strings.Contains(err.Error(), "missing the slash") {
    dsn = strings.TrimRight(dsn, "") + "/"
}

When it happens

Trigger: ParseDSN or sql.Open('mysql', 'user:pass@tcp(host:3306)') with no trailing slash; or any non-empty DSN string with zero '/' characters.

Common situations: Concatenating host/user segments but forgetting the '/dbname' suffix; an env var that got its trailing '/' trimmed; a config template that conditionally omits the database segment.

Related errors

• invalid DSN: did you forget to escape a param value?
• default addr for network '{cfg.Net}' unknown
• invalid value / unknown config name: {cfg.TLSConfig}
• invalid value / unknown server pub key name: {cfg.ServerPubK
• invalid bool value: {value}

AI-assisted analysis of go-sql-driver/mysql@c426bd9379 (2026-08-04). Data as JSON: /data/errors/05d8b1fadb24b4d7.json. Report an issue: GitHub.