21572: Avoid using lsb_release in Debian repository

[arvados.git] / lib / controller / localdb / container_gateway.go

// Copyright (C) The Arvados Authors. All rights reserved.
//
// SPDX-License-Identifier: AGPL-3.0

package localdb

import (
        "bufio"
        "bytes"
        "context"
        "crypto/hmac"
        "crypto/sha256"
        "crypto/subtle"
        "crypto/tls"
        "crypto/x509"
        "errors"
        "fmt"
        "io"
        "io/ioutil"
        "net"
        "net/http"
        "net/http/httputil"
        "net/url"
        "os"
        "strings"

        "git.arvados.org/arvados.git/lib/controller/rpc"
        "git.arvados.org/arvados.git/lib/service"
        "git.arvados.org/arvados.git/lib/webdavfs"
        "git.arvados.org/arvados.git/sdk/go/arvados"
        "git.arvados.org/arvados.git/sdk/go/auth"
        "git.arvados.org/arvados.git/sdk/go/ctxlog"
        "git.arvados.org/arvados.git/sdk/go/httpserver"
        keepweb "git.arvados.org/arvados.git/services/keep-web"
        "github.com/hashicorp/yamux"
        "golang.org/x/net/webdav"
)

var (
        forceProxyForTest       = false
        forceInternalURLForTest *arvados.URL
)

// ContainerRequestLog returns a WebDAV handler that reads logs from
// the indicated container request. It works by proxying the incoming
// HTTP request to
//
//   - the container gateway, if there is an associated container that
//     is running
//
//   - a different controller process, if there is a running container
//     whose gateway is accessible through a tunnel to a different
//     controller process
//
//   - keep-web, if saved logs exist and there is no gateway (or the
//     associated container is finished)
//
//   - an empty-collection stub, if there is no gateway and no saved
//     log
//
// For an incoming request
//
//      GET /arvados/v1/container_requests/{cr_uuid}/log/{c_uuid}{/c_log_path}
//
// The upstream request may be to {c_uuid}'s container gateway
//
//      GET /arvados/v1/container_requests/{cr_uuid}/log/{c_uuid}{/c_log_path}
//      X-Webdav-Prefix: /arvados/v1/container_requests/{cr_uuid}/log/{c_uuid}
//      X-Webdav-Source: /log
//
// ...or the upstream request may be to keep-web (where {cr_log_uuid}
// is the container request log collection UUID)
//
//      GET /arvados/v1/container_requests/{cr_uuid}/log/{c_uuid}{/c_log_path}
//      Host: {cr_log_uuid}.internal
//      X-Webdav-Prefix: /arvados/v1/container_requests/{cr_uuid}/log
//      X-Arvados-Container-Uuid: {c_uuid}
//
// ...or the request may be handled locally using an empty-collection
// stub.
func (conn *Conn) ContainerRequestLog(ctx context.Context, opts arvados.ContainerLogOptions) (http.Handler, error) {
        if opts.Method == "OPTIONS" && opts.Header.Get("Access-Control-Request-Method") != "" {
                return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                        if !keepweb.ServeCORSPreflight(w, opts.Header) {
                                // Inconceivable.  We already checked
                                // for the only condition where
                                // ServeCORSPreflight returns false.
                                httpserver.Error(w, "unhandled CORS preflight request", http.StatusInternalServerError)
                        }
                }), nil
        }
        cr, err := conn.railsProxy.ContainerRequestGet(ctx, arvados.GetOptions{UUID: opts.UUID, Select: []string{"uuid", "container_uuid", "log_uuid"}})
        if err != nil {
                if se := httpserver.HTTPStatusError(nil); errors.As(err, &se) && se.HTTPStatus() == http.StatusUnauthorized {
                        // Hint to WebDAV client that we accept HTTP basic auth.
                        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                                w.Header().Set("Www-Authenticate", "Basic realm=\"collections\"")
                                w.WriteHeader(http.StatusUnauthorized)
                        }), nil
                }
                return nil, err
        }
        ctr, err := conn.railsProxy.ContainerGet(ctx, arvados.GetOptions{UUID: cr.ContainerUUID, Select: []string{"uuid", "state", "gateway_address"}})
        if err != nil {
                return nil, err
        }
        // .../log/{ctr.UUID} is a directory where the currently
        // assigned container's log data [will] appear (as opposed to
        // previous attempts in .../log/{previous_ctr_uuid}). Requests
        // that are outside that directory, and requests on a
        // non-running container, are proxied to keep-web instead of
        // going through the container gateway system.
        //
        // Side note: a depth>1 directory tree listing starting at
        // .../{cr_uuid}/log will only include subdirectories for
        // finished containers, i.e., will not include a subdirectory
        // with log data for a current (unfinished) container UUID.
        // In order to access live logs, a client must look up the
        // container_uuid field of the container request record, and
        // explicitly request a path under .../{cr_uuid}/log/{c_uuid}.
        if ctr.GatewayAddress == "" ||
                (ctr.State != arvados.ContainerStateLocked && ctr.State != arvados.ContainerStateRunning) ||
                !(opts.Path == "/"+ctr.UUID || strings.HasPrefix(opts.Path, "/"+ctr.UUID+"/")) {
                return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                        conn.serveContainerRequestLogViaKeepWeb(opts, cr, w, r)
                }), nil
        }
        dial, arpc, err := conn.findGateway(ctx, ctr, opts.NoForward)
        if err != nil {
                return nil, err
        }
        if arpc != nil {
                opts.NoForward = true
                return arpc.ContainerRequestLog(ctx, opts)
        }
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                r = r.WithContext(ctx)
                var proxyReq *http.Request
                var proxyErr error
                var expectRespondAuth string
                proxy := &httputil.ReverseProxy{
                        // Our custom Transport:
                        //
                        // - Uses a custom dialer to connect to the
                        // gateway (either directly or through a
                        // tunnel set up though ContainerTunnel)
                        //
                        // - Verifies the gateway's TLS certificate
                        // using X-Arvados-Authorization headers.
                        //
                        // This involves modifying the outgoing
                        // request header in DialTLSContext.
                        // (ReverseProxy certainly doesn't expect us
                        // to do this, but it works.)
                        Transport: &http.Transport{
                                DialTLSContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
                                        tlsconn, requestAuth, respondAuth, err := dial()
                                        if err != nil {
                                                return nil, err
                                        }
                                        proxyReq.Header.Set("X-Arvados-Authorization", requestAuth)
                                        expectRespondAuth = respondAuth
                                        return tlsconn, nil
                                },
                        },
                        Director: func(r *http.Request) {
                                // Scheme/host of incoming r.URL are
                                // irrelevant now, and may even be
                                // missing. Host is ignored by our
                                // DialTLSContext, but we need a
                                // generic syntactically correct URL
                                // for net/http to work with.
                                r.URL.Scheme = "https"
                                r.URL.Host = "0.0.0.0:0"
                                r.Header.Set("X-Arvados-Container-Gateway-Uuid", ctr.UUID)
                                r.Header.Set("X-Webdav-Prefix", "/arvados/v1/container_requests/"+cr.UUID+"/log/"+ctr.UUID)
                                r.Header.Set("X-Webdav-Source", "/log")
                                proxyReq = r
                        },
                        ModifyResponse: func(resp *http.Response) error {
                                if resp.Header.Get("X-Arvados-Authorization-Response") != expectRespondAuth {
                                        // Note this is how we detect
                                        // an attacker-in-the-middle.
                                        return httpserver.ErrorWithStatus(errors.New("bad X-Arvados-Authorization-Response header"), http.StatusBadGateway)
                                }
                                resp.Header.Del("X-Arvados-Authorization-Response")
                                preemptivelyDeduplicateHeaders(w.Header(), resp.Header)
                                return nil
                        },
                        ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
                                proxyErr = err
                        },
                }
                proxy.ServeHTTP(w, r)
                if proxyErr == nil {
                        // proxy succeeded
                        return
                }
                // If proxying to the container gateway fails, it
                // might be caused by a race where crunch-run exited
                // after we decided (above) the log was not final.
                // In that case we should proxy to keep-web.
                ctr, err := conn.railsProxy.ContainerGet(ctx, arvados.GetOptions{
                        UUID:   ctr.UUID,
                        Select: []string{"uuid", "state", "gateway_address", "log"},
                })
                if err != nil {
                        // Lost access to the container record?
                        httpserver.Error(w, "error re-fetching container record: "+err.Error(), http.StatusServiceUnavailable)
                } else if ctr.State == arvados.ContainerStateLocked || ctr.State == arvados.ContainerStateRunning {
                        // No race, proxyErr was the best we can do
                        httpserver.Error(w, "proxy error: "+proxyErr.Error(), http.StatusServiceUnavailable)
                } else {
                        conn.serveContainerRequestLogViaKeepWeb(opts, cr, w, r)
                }
        }), nil
}

// serveContainerLogViaKeepWeb handles a request for saved container
// log content by proxying to one of the configured keep-web servers.
//
// It tries to choose a keep-web server that is running on this host.
func (conn *Conn) serveContainerRequestLogViaKeepWeb(opts arvados.ContainerLogOptions, cr arvados.ContainerRequest, w http.ResponseWriter, r *http.Request) {
        if cr.LogUUID == "" {
                // Special case: if no log data exists yet, we serve
                // an empty collection by ourselves instead of
                // proxying to keep-web.
                conn.serveEmptyDir("/arvados/v1/container_requests/"+cr.UUID+"/log", w, r)
                return
        }
        myURL, _ := service.URLFromContext(r.Context())
        u := url.URL(myURL)
        myHostname := u.Hostname()
        var webdavBase arvados.URL
        var ok bool
        for webdavBase = range conn.cluster.Services.WebDAV.InternalURLs {
                ok = true
                u := url.URL(webdavBase)
                if h := u.Hostname(); h == "127.0.0.1" || h == "0.0.0.0" || h == "::1" || h == myHostname {
                        // Prefer a keep-web service running on the
                        // same host as us. (If we don't find one, we
                        // pick one arbitrarily.)
                        break
                }
        }
        if !ok {
                httpserver.Error(w, "no internalURLs configured for WebDAV service", http.StatusInternalServerError)
                return
        }
        proxy := &httputil.ReverseProxy{
                Director: func(r *http.Request) {
                        r.URL.Scheme = webdavBase.Scheme
                        r.URL.Host = webdavBase.Host
                        // Outgoing Host header specifies the
                        // collection ID.
                        r.Host = cr.LogUUID + ".internal"
                        // We already checked permission on the
                        // container, so we can use a root token here
                        // instead of counting on the "access to log
                        // via container request and container"
                        // permission check, which can be racy when a
                        // request gets retried with a new container.
                        r.Header.Set("Authorization", "Bearer "+conn.cluster.SystemRootToken)
                        // We can't change r.URL.Path without
                        // confusing WebDAV (request body and response
                        // headers refer to the same paths) so we tell
                        // keep-web to map the log collection onto the
                        // containers/X/log/ namespace.
                        r.Header.Set("X-Webdav-Prefix", "/arvados/v1/container_requests/"+cr.UUID+"/log")
                        if len(opts.Path) >= 28 && opts.Path[6:13] == "-dz642-" {
                                // "/arvados/v1/container_requests/{crUUID}/log/{cUUID}..."
                                // proxies to
                                // "/log for container {cUUID}..."
                                r.Header.Set("X-Webdav-Prefix", "/arvados/v1/container_requests/"+cr.UUID+"/log/"+opts.Path[1:28])
                                r.Header.Set("X-Webdav-Source", "/log for container "+opts.Path[1:28]+"/")
                        }
                },
                ModifyResponse: func(resp *http.Response) error {
                        preemptivelyDeduplicateHeaders(w.Header(), resp.Header)
                        return nil
                },
        }
        if conn.cluster.TLS.Insecure {
                proxy.Transport = &http.Transport{
                        TLSClientConfig: &tls.Config{
                                InsecureSkipVerify: conn.cluster.TLS.Insecure,
                        },
                }
        }
        proxy.ServeHTTP(w, r)
}

// httputil.ReverseProxy uses (http.Header)Add() to copy headers from
// the upstream Response to the downstream ResponseWriter. If headers
// have already been set on the downstream ResponseWriter, Add() will
// result in duplicate headers. For example, if we set CORS headers
// and then use ReverseProxy with an upstream that also sets CORS
// headers, our client will receive
//
//      Access-Control-Allow-Origin: *
//      Access-Control-Allow-Origin: *
//
// ...which is incorrect.
//
// preemptivelyDeduplicateHeaders, when called from a ModifyResponse
// hook, solves this by removing any conflicting headers from
// ResponseWriter. This way, when ReverseProxy calls Add(), it will
// assign the new values without causing duplicates.
//
// dst is the downstream ResponseWriter's Header(). src is the
// upstream resp.Header.
func preemptivelyDeduplicateHeaders(dst, src http.Header) {
        for hdr := range src {
                dst.Del(hdr)
        }
}

// serveEmptyDir handles read-only webdav requests as if there was an
// empty collection rooted at the given path. It's equivalent to
// proxying to an empty collection in keep-web, but avoids the extra
// hop.
func (conn *Conn) serveEmptyDir(path string, w http.ResponseWriter, r *http.Request) {
        wh := webdav.Handler{
                Prefix:     path,
                FileSystem: webdav.NewMemFS(),
                LockSystem: webdavfs.NoLockSystem,
                Logger: func(r *http.Request, err error) {
                        if err != nil && !os.IsNotExist(err) {
                                ctxlog.FromContext(r.Context()).WithError(err).Info("webdav error on empty collection fs")
                        }
                },
        }
        wh.ServeHTTP(w, r)
}

// ContainerSSH returns a connection to the SSH server in the
// appropriate crunch-run process on the worker node where the
// specified container is running.
//
// If the returned error is nil, the caller is responsible for closing
// sshconn.Conn.
func (conn *Conn) ContainerSSH(ctx context.Context, opts arvados.ContainerSSHOptions) (sshconn arvados.ConnectionResponse, err error) {
        user, err := conn.railsProxy.UserGetCurrent(ctx, arvados.GetOptions{})
        if err != nil {
                return sshconn, err
        }
        ctr, err := conn.railsProxy.ContainerGet(ctx, arvados.GetOptions{UUID: opts.UUID, Select: []string{"uuid", "state", "gateway_address", "interactive_session_started"}})
        if err != nil {
                return sshconn, err
        }
        ctxRoot := auth.NewContext(ctx, &auth.Credentials{Tokens: []string{conn.cluster.SystemRootToken}})
        if !user.IsAdmin || !conn.cluster.Containers.ShellAccess.Admin {
                if !conn.cluster.Containers.ShellAccess.User {
                        return sshconn, httpserver.ErrorWithStatus(errors.New("shell access is disabled in config"), http.StatusServiceUnavailable)
                }
                crs, err := conn.railsProxy.ContainerRequestList(ctxRoot, arvados.ListOptions{Limit: -1, Filters: []arvados.Filter{{"container_uuid", "=", opts.UUID}}})
                if err != nil {
                        return sshconn, err
                }
                for _, cr := range crs.Items {
                        if cr.ModifiedByUserUUID != user.UUID {
                                return sshconn, httpserver.ErrorWithStatus(errors.New("permission denied: container is associated with requests submitted by other users"), http.StatusForbidden)
                        }
                }
                if crs.ItemsAvailable != len(crs.Items) {
                        return sshconn, httpserver.ErrorWithStatus(errors.New("incomplete response while checking permission"), http.StatusInternalServerError)
                }
        }

        if ctr.State == arvados.ContainerStateQueued || ctr.State == arvados.ContainerStateLocked {
                return sshconn, httpserver.ErrorWithStatus(fmt.Errorf("container is not running yet (state is %q)", ctr.State), http.StatusServiceUnavailable)
        } else if ctr.State != arvados.ContainerStateRunning {
                return sshconn, httpserver.ErrorWithStatus(fmt.Errorf("container has ended (state is %q)", ctr.State), http.StatusGone)
        }

        dial, arpc, err := conn.findGateway(ctx, ctr, opts.NoForward)
        if err != nil {
                return sshconn, err
        }
        if arpc != nil {
                opts.NoForward = true
                return arpc.ContainerSSH(ctx, opts)
        }

        tlsconn, requestAuth, respondAuth, err := dial()
        if err != nil {
                return sshconn, err
        }
        bufr := bufio.NewReader(tlsconn)
        bufw := bufio.NewWriter(tlsconn)

        u := url.URL{
                Scheme: "http",
                Host:   tlsconn.RemoteAddr().String(),
                Path:   "/ssh",
        }
        postform := url.Values{
                // uuid is only needed for older crunch-run versions
                // (current version uses X-Arvados-* header below)
                "uuid":           {opts.UUID},
                "detach_keys":    {opts.DetachKeys},
                "login_username": {opts.LoginUsername},
                "no_forward":     {fmt.Sprintf("%v", opts.NoForward)},
        }
        postdata := postform.Encode()
        bufw.WriteString("POST " + u.String() + " HTTP/1.1\r\n")
        bufw.WriteString("Host: " + u.Host + "\r\n")
        bufw.WriteString("Upgrade: ssh\r\n")
        bufw.WriteString("X-Arvados-Container-Gateway-Uuid: " + opts.UUID + "\r\n")
        bufw.WriteString("X-Arvados-Authorization: " + requestAuth + "\r\n")
        bufw.WriteString("Content-Type: application/x-www-form-urlencoded\r\n")
        fmt.Fprintf(bufw, "Content-Length: %d\r\n", len(postdata))
        bufw.WriteString("\r\n")
        bufw.WriteString(postdata)
        bufw.Flush()
        resp, err := http.ReadResponse(bufr, &http.Request{Method: "POST"})
        if err != nil {
                tlsconn.Close()
                return sshconn, httpserver.ErrorWithStatus(fmt.Errorf("error reading http response from gateway: %w", err), http.StatusBadGateway)
        }
        defer resp.Body.Close()
        if resp.StatusCode != http.StatusSwitchingProtocols {
                body, _ := ioutil.ReadAll(io.LimitReader(resp.Body, 1000))
                tlsconn.Close()
                return sshconn, httpserver.ErrorWithStatus(fmt.Errorf("unexpected status %s %q", resp.Status, body), http.StatusBadGateway)
        }
        if strings.ToLower(resp.Header.Get("Upgrade")) != "ssh" ||
                strings.ToLower(resp.Header.Get("Connection")) != "upgrade" {
                tlsconn.Close()
                return sshconn, httpserver.ErrorWithStatus(errors.New("bad upgrade"), http.StatusBadGateway)
        }
        if resp.Header.Get("X-Arvados-Authorization-Response") != respondAuth {
                tlsconn.Close()
                return sshconn, httpserver.ErrorWithStatus(errors.New("bad X-Arvados-Authorization-Response header"), http.StatusBadGateway)
        }

        if !ctr.InteractiveSessionStarted {
                _, err = conn.railsProxy.ContainerUpdate(ctxRoot, arvados.UpdateOptions{
                        UUID: opts.UUID,
                        Attrs: map[string]interface{}{
                                "interactive_session_started": true,
                        },
                })
                if err != nil {
                        tlsconn.Close()
                        return sshconn, httpserver.ErrorWithStatus(err, http.StatusInternalServerError)
                }
        }

        sshconn.Conn = tlsconn
        sshconn.Bufrw = &bufio.ReadWriter{Reader: bufr, Writer: bufw}
        sshconn.Logger = ctxlog.FromContext(ctx)
        sshconn.Header = http.Header{"Upgrade": {"ssh"}}
        return sshconn, nil
}

// ContainerGatewayTunnel sets up a tunnel enabling us (controller) to
// connect to the caller's (crunch-run's) gateway server.
func (conn *Conn) ContainerGatewayTunnel(ctx context.Context, opts arvados.ContainerGatewayTunnelOptions) (resp arvados.ConnectionResponse, err error) {
        h := hmac.New(sha256.New, []byte(conn.cluster.SystemRootToken))
        fmt.Fprint(h, opts.UUID)
        authSecret := fmt.Sprintf("%x", h.Sum(nil))
        if subtle.ConstantTimeCompare([]byte(authSecret), []byte(opts.AuthSecret)) != 1 {
                ctxlog.FromContext(ctx).Info("received incorrect auth_secret")
                return resp, httpserver.ErrorWithStatus(errors.New("authentication error"), http.StatusUnauthorized)
        }

        muxconn, clientconn := net.Pipe()
        tunnel, err := yamux.Server(muxconn, nil)
        if err != nil {
                clientconn.Close()
                return resp, httpserver.ErrorWithStatus(err, http.StatusInternalServerError)
        }

        conn.gwTunnelsLock.Lock()
        if conn.gwTunnels == nil {
                conn.gwTunnels = map[string]*yamux.Session{opts.UUID: tunnel}
        } else {
                conn.gwTunnels[opts.UUID] = tunnel
        }
        conn.gwTunnelsLock.Unlock()

        go func() {
                <-tunnel.CloseChan()
                conn.gwTunnelsLock.Lock()
                if conn.gwTunnels[opts.UUID] == tunnel {
                        delete(conn.gwTunnels, opts.UUID)
                }
                conn.gwTunnelsLock.Unlock()
        }()

        // Assuming we're acting as the backend of an http server,
        // lib/controller/router will call resp's ServeHTTP handler,
        // which upgrades the incoming http connection to a raw socket
        // and connects it to our yamux.Server through our net.Pipe().
        resp.Conn = clientconn
        resp.Bufrw = &bufio.ReadWriter{Reader: bufio.NewReader(&bytes.Buffer{}), Writer: bufio.NewWriter(&bytes.Buffer{})}
        resp.Logger = ctxlog.FromContext(ctx)
        resp.Header = http.Header{"Upgrade": {"tunnel"}}
        if u, ok := service.URLFromContext(ctx); ok {
                resp.Header.Set("X-Arvados-Internal-Url", u.String())
        } else if forceInternalURLForTest != nil {
                resp.Header.Set("X-Arvados-Internal-Url", forceInternalURLForTest.String())
        }
        return
}

type gatewayDialer func() (conn net.Conn, requestAuth, respondAuth string, err error)

// findGateway figures out how to connect to ctr's gateway.
//
// If the gateway can be contacted directly or through a tunnel on
// this instance, the first return value is a non-nil dialer.
//
// If the gateway is only accessible through a tunnel through a
// different controller process, the second return value is a non-nil
// *rpc.Conn for that controller.
func (conn *Conn) findGateway(ctx context.Context, ctr arvados.Container, noForward bool) (gatewayDialer, *rpc.Conn, error) {
        conn.gwTunnelsLock.Lock()
        tunnel := conn.gwTunnels[ctr.UUID]
        conn.gwTunnelsLock.Unlock()

        myURL, _ := service.URLFromContext(ctx)

        if host, _, splitErr := net.SplitHostPort(ctr.GatewayAddress); splitErr == nil && host != "" && host != "127.0.0.1" {
                // If crunch-run provided a GatewayAddress like
                // "ipaddr:port", that means "ipaddr" is one of the
                // external interfaces where the gateway is
                // listening. In that case, it's the most
                // reliable/direct option, so we use it even if a
                // tunnel might also be available.
                return func() (net.Conn, string, string, error) {
                        rawconn, err := (&net.Dialer{}).DialContext(ctx, "tcp", ctr.GatewayAddress)
                        if err != nil {
                                return nil, "", "", httpserver.ErrorWithStatus(err, http.StatusServiceUnavailable)
                        }
                        return conn.dialGatewayTLS(ctx, ctr, rawconn)
                }, nil, nil
        }
        if tunnel != nil && !(forceProxyForTest && !noForward) {
                // If we can't connect directly, and the gateway has
                // established a yamux tunnel with us, connect through
                // the tunnel.
                //
                // ...except: forceProxyForTest means we are emulating
                // a situation where the gateway has established a
                // yamux tunnel with controller B, and the
                // ContainerSSH request arrives at controller A. If
                // noForward==false then we are acting as A, so
                // we pretend not to have a tunnel, and fall through
                // to the "tunurl" case below. If noForward==true
                // then the client is A and we are acting as B, so we
                // connect to our tunnel.
                return func() (net.Conn, string, string, error) {
                        rawconn, err := tunnel.Open()
                        if err != nil {
                                return nil, "", "", httpserver.ErrorWithStatus(err, http.StatusServiceUnavailable)
                        }
                        return conn.dialGatewayTLS(ctx, ctr, rawconn)
                }, nil, nil
        }
        if tunurl := strings.TrimPrefix(ctr.GatewayAddress, "tunnel "); tunurl != ctr.GatewayAddress &&
                tunurl != "" &&
                tunurl != myURL.String() &&
                !noForward {
                // If crunch-run provided a GatewayAddress like
                // "tunnel https://10.0.0.10:1010/", that means the
                // gateway has established a yamux tunnel with the
                // controller process at the indicated InternalURL
                // (which isn't us, otherwise we would have had
                // "tunnel != nil" above). We need to proxy through to
                // the other controller process in order to use the
                // tunnel.
                for u := range conn.cluster.Services.Controller.InternalURLs {
                        if u.String() == tunurl {
                                ctxlog.FromContext(ctx).Debugf("connecting to container gateway through other controller at %s", u)
                                u := url.URL(u)
                                return nil, rpc.NewConn(conn.cluster.ClusterID, &u, conn.cluster.TLS.Insecure, rpc.PassthroughTokenProvider), nil
                        }
                }
                ctxlog.FromContext(ctx).Warnf("container gateway provided a tunnel endpoint %s that is not one of Services.Controller.InternalURLs", tunurl)
                return nil, nil, httpserver.ErrorWithStatus(errors.New("container gateway is running but tunnel endpoint is invalid"), http.StatusServiceUnavailable)
        }
        if ctr.GatewayAddress == "" {
                return nil, nil, httpserver.ErrorWithStatus(errors.New("container is running but gateway is not available"), http.StatusServiceUnavailable)
        } else {
                return nil, nil, httpserver.ErrorWithStatus(errors.New("container is running but tunnel is down"), http.StatusServiceUnavailable)
        }
}

// dialGatewayTLS negotiates a TLS connection to a container gateway
// over the given raw connection.
func (conn *Conn) dialGatewayTLS(ctx context.Context, ctr arvados.Container, rawconn net.Conn) (*tls.Conn, string, string, error) {
        // crunch-run uses a self-signed / unverifiable TLS
        // certificate, so we use the following scheme to ensure we're
        // not talking to an attacker-in-the-middle.
        //
        // 1. Compute ctrKey = HMAC-SHA256(sysRootToken,ctrUUID) --
        // this will be the same ctrKey that a-d-c supplied to
        // crunch-run in the GatewayAuthSecret env var.
        //
        // 2. Compute requestAuth = HMAC-SHA256(ctrKey,serverCert) and
        // send it to crunch-run as the X-Arvados-Authorization
        // header, proving that we know ctrKey. (Note a MITM cannot
        // replay the proof to a real crunch-run server, because the
        // real crunch-run server would have a different cert.)
        //
        // 3. Compute respondAuth = HMAC-SHA256(ctrKey,requestAuth)
        // and ensure the server returns it in the
        // X-Arvados-Authorization-Response header, proving that the
        // server knows ctrKey.
        var requestAuth, respondAuth string
        tlsconn := tls.Client(rawconn, &tls.Config{
                InsecureSkipVerify: true,
                VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
                        if len(rawCerts) == 0 {
                                return errors.New("no certificate received, cannot compute authorization header")
                        }
                        h := hmac.New(sha256.New, []byte(conn.cluster.SystemRootToken))
                        fmt.Fprint(h, ctr.UUID)
                        authKey := fmt.Sprintf("%x", h.Sum(nil))
                        h = hmac.New(sha256.New, []byte(authKey))
                        h.Write(rawCerts[0])
                        requestAuth = fmt.Sprintf("%x", h.Sum(nil))
                        h.Reset()
                        h.Write([]byte(requestAuth))
                        respondAuth = fmt.Sprintf("%x", h.Sum(nil))
                        return nil
                },
        })
        err := tlsconn.HandshakeContext(ctx)
        if err != nil {
                return nil, "", "", httpserver.ErrorWithStatus(fmt.Errorf("TLS handshake failed: %w", err), http.StatusBadGateway)
        }
        if respondAuth == "" {
                tlsconn.Close()
                return nil, "", "", httpserver.ErrorWithStatus(errors.New("BUG: no respondAuth"), http.StatusInternalServerError)
        }
        return tlsconn, requestAuth, respondAuth, nil
}