diff --git a/server/clone.go b/server/clone.go
index 5b380c6..8e813ec 100644
--- a/server/clone.go
+++ b/server/clone.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net"
 	"time"
 
 	mocoagent "github.com/cybozu-go/moco-agent"
@@ -56,7 +57,9 @@ func (a *Agent) Clone(ctx context.Context, req *proto.CloneRequest) error {
 		metrics.CloneDurationSeconds.Observe(time.Since(startTime).Seconds())
 	}()
 
-	donorAddr := fmt.Sprintf("%s:%d", req.Host, req.Port)
+	// Unfortunately, MySQL 8.0 does not support IPv6 address format.
+	// https://dev.mysql.com/doc/refman/8.0/en/clone-plugin-options-variables.html#sysvar_clone_valid_donor_list
+	donorAddr := net.JoinHostPort(req.Host, fmt.Sprint(req.Port))
 	if _, err := a.db.ExecContext(ctx, `SET GLOBAL clone_valid_donor_list = ?`, donorAddr); err != nil {
 		return status.Errorf(codes.Internal, "failed to set clone_valid_donor_list: %+v", err)
 	}
diff --git a/server/connect.go b/server/connect.go
index c35e705..413413d 100644
--- a/server/connect.go
+++ b/server/connect.go
@@ -3,6 +3,7 @@ package server
 import (
 	"errors"
 	"fmt"
+	"net"
 	"time"
 
 	mocoagent "github.com/cybozu-go/moco-agent"
@@ -15,7 +16,7 @@ func getMySQLConn(config MySQLAccessorConfig) (*sqlx.DB, error) {
 	conf.User = mocoagent.AgentUser
 	conf.Passwd = config.Password
 	conf.Net = "tcp"
-	conf.Addr = fmt.Sprintf("%s:%d", config.Host, config.Port)
+	conf.Addr = net.JoinHostPort(config.Host, fmt.Sprint(config.Port))
 	conf.Timeout = config.ConnectionTimeout
 	conf.ReadTimeout = config.ReadTimeout
 	conf.InterpolateParams = true