Prevent forwarded tcpip requests hanging indefinitely (#50209)

The SSH channel was only being closed in the happy path once
proxying of data completed. This resulted in any connections being
made, and failing for any reason prior to that hanging until the
user terminated the request.

The behavior from the regular.Server was copied to the forward.Server
to ensure that the channel is always closed when handleDirectTCPIPRequest
terminates.

lib/srv/forward/sshserver.go

@@ -24,6 +24,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"os"
 	"strings"
 	"time"
 
@@ -1062,8 +1063,12 @@ func (s *Server) handleDirectTCPIPRequest(ctx context.Context, ch ssh.Channel, r
 	if err != nil {
 		s.log.Errorf("Unable to create connection context: %v.", err)
 		s.stderrWrite(ch, "Unable to create connection context.")
+		if err := ch.Close(); err != nil {
+			s.log.Warnf("Failed to close channel: %v", err)
+		}
 		return
 	}
+	scx.AddCloser(ch)
 	scx.RemoteClient = s.remoteClient
 	scx.ExecType = teleport.ChanDirectTCPIP
 	scx.SrcAddr = sshutils.JoinHostPort(req.Orig, req.OrigPort)
@@ -1095,8 +1100,8 @@ func (s *Server) handleDirectTCPIPRequest(ctx context.Context, ch ssh.Channel, r
 		scx.WithError(err).Warn("Failed to emit port forward event.")
 	}
 
-	if err := utils.ProxyConn(ctx, ch, conn); err != nil {
-		s.log.WithError(err).Warn("Pailed proxying data for port forwarding connection.")
+	if err := utils.ProxyConn(ctx, ch, conn); err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, os.ErrClosed) {
+		s.log.WithError(err).Warn("Failed proxying data for port forwarding connection.")
 	}
 }