cfengine-3.19.0/cf-serverd/server_tls.c

/*
  Copyright 2021 Northern.tech AS

  This file is part of CFEngine 3 - written and maintained by Northern.tech AS.

  This program is free software; you can redistribute it and/or modify it
  under the terms of the GNU General Public License as published by the
  Free Software Foundation; version 3.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA

  To the extent this program is licensed as part of the Enterprise
  versions of CFEngine, the applicable Commercial Open Source License
  (COSL) may apply to this file if you as a licensee so wish it. See
  included file COSL.txt.
*/


#include <server_tls.h>
#include <server_common.h>
#include <protocol.h> // ParseProtocolVersionNetwork()

#include <openssl/err.h>                                   /* ERR_get_error */

#include <crypto.h>                                        /* DecryptString */
#include <conversion.h>
#include <signals.h>
#include <item_lib.h>                 /* IsMatchItemIn */
#include <lastseen.h>                 /* LastSaw1 */
#include <net.h>                      /* SendTransaction,ReceiveTransaction */
#include <tls_generic.h>              /* TLSSend */
#include <cf-serverd-enterprise-stubs.h>
#include <connection_info.h>
#include <regex.h>                                       /* StringMatchFull */
#include <known_dirs.h>
#include <file_lib.h>                                           /* IsDirReal */

#include "server_access.h"          /* access_CheckResource, acl_CheckExact */


static SSL_CTX *SSLSERVERCONTEXT = NULL;

#define MAX_ACCEPT_RETRIES 5

/**
 * @param[in]  priv_key private key to use (or %NULL to use the global PRIVKEY)
 * @param[in]  pub_key public key to use (or %NULL to use the global PUBKEY)
 * @param[out] ssl_ctx place to store the SSL context (or %NULL to use the
 *                     global SSL_CTX)
 * @warning Make sure you've called CryptoInitialize() first!
 */
bool ServerTLSInitialize(RSA *priv_key, RSA *pub_key, SSL_CTX **ssl_ctx)
{
    int ret;

    if (priv_key == NULL)
    {
        /* private key not specified, use the global one */
        priv_key = PRIVKEY;
    }
    if (pub_key == NULL)
    {
        /* public key not specified, use the global one */
        pub_key = PUBKEY;
    }

    if (priv_key == NULL || pub_key == NULL)
    {
        Log(LOG_LEVEL_ERR, "Public/private key pair not loaded,"
            " please create one using cf-key");
        return false;
    }

    if (!TLSGenericInitialize())
    {
        return false;
    }

    if (ssl_ctx == NULL)
    {
        ssl_ctx = &SSLSERVERCONTEXT;
    }
    assert(*ssl_ctx == NULL);
    *ssl_ctx = SSL_CTX_new(SSLv23_server_method());
    if (*ssl_ctx == NULL)
    {
        Log(LOG_LEVEL_ERR, "SSL_CTX_new: %s",
            TLSErrorString(ERR_get_error()));
        return false;
    }

    TLSSetDefaultOptions(*ssl_ctx, SERVER_ACCESS.allowtlsversion);

    /*
     * CFEngine is not a web server so it does not need to support many
     * ciphers. It only allows a safe but very common subset by default,
     * extensible via "allowciphers" in body server control. By default
     * the server side allows:
     *
     *     AES256-GCM-SHA384: most high-grade RSA-based cipher from TLSv1.2
     *     AES256-SHA: most backwards compatible but high-grade, from SSLv3
     *     TLS_AES_256_GCM_SHA384: most high-grade RSA-based cipher from TLSv1.3
     *
     * Client side is using the OpenSSL's defaults by default.
     */
    const char *cipher_list = SERVER_ACCESS.allowciphers;
    if (cipher_list == NULL)
    {
#ifdef SSL_OP_NO_TLSv1_3        /* defined if TLS 1.3 is supported */
        cipher_list = "AES256-GCM-SHA384:AES256-SHA:TLS_AES_256_GCM_SHA384";
#else
        cipher_list = "AES256-GCM-SHA384:AES256-SHA";
#endif
    }

    if (!TLSSetCipherList(*ssl_ctx, cipher_list))
    {
        goto err;
    }

    /* Create cert into memory and load it into SSL context. */
    X509 *cert = TLSGenerateCertFromPrivKey(priv_key);
    if (cert == NULL)
    {
        Log(LOG_LEVEL_ERR,
            "Failed to generate in-memory certificate from private key");
        goto err;
    }

    SSL_CTX_use_certificate(*ssl_ctx, cert);
    X509_free(cert);

    ret = SSL_CTX_use_RSAPrivateKey(*ssl_ctx, priv_key);
    if (ret != 1)
    {
        Log(LOG_LEVEL_ERR, "Failed to use RSA private key: %s",
            TLSErrorString(ERR_get_error()));
        goto err;
    }

    /* Verify cert consistency. */
    ret = SSL_CTX_check_private_key(*ssl_ctx);
    if (ret != 1)
    {
        Log(LOG_LEVEL_ERR, "Inconsistent key and TLS cert: %s",
            TLSErrorString(ERR_get_error()));
        goto err;
    }

    return true;

  err:
    SSL_CTX_free(*ssl_ctx);
    *ssl_ctx = NULL;
    return false;
}

/**
 * @param[in,out] priv_key private key to deinitalize (or %NULL to use the
 *                         global PRIVKEY)
 * @param[in,out] pub_key public key to deinitialize (or %NULL to use the
 *                        global PUBKEY)
 * @param[in,out] ssl_ctx the SSL context to deinitialize (or %NULL to use the
 *                        global SSL_CTX)
 */
void ServerTLSDeInitialize(RSA **priv_key, RSA **pub_key, SSL_CTX **ssl_ctx)
{
    if (priv_key == NULL)
    {
        priv_key = &PRIVKEY;
    }
    if (pub_key == NULL)
    {
        pub_key = &PUBKEY;
    }
    if (ssl_ctx == NULL)
    {
        ssl_ctx = &SSLSERVERCONTEXT;
    }

    if (*pub_key)
    {
        RSA_free(*pub_key);
        *pub_key = NULL;
    }

    if (*priv_key)
    {
        RSA_free(*priv_key);
        *priv_key = NULL;
    }

    if (*ssl_ctx != NULL)
    {
        SSL_CTX_free(*ssl_ctx);
        *ssl_ctx = NULL;
    }
}

/**
 * @brief Set the connection type to CLASSIC or TLS.

 * It is performed by peeking into the TLS connection to read the first bytes,
 * and if it's a CAUTH protocol command use the old protocol loop, else use
 * the TLS protocol loop.
 * This must be the first thing we run on an accepted connection.
 *
 * @return true for success, false otherwise.
 */
bool ServerTLSPeek(ConnectionInfo *conn_info)
{
    assert(conn_info != NULL);

    assert(SSLSERVERCONTEXT != NULL);
    assert(PRIVKEY != NULL);
    assert(PUBKEY  != NULL);

    assert(ConnectionInfoProtocolVersion(conn_info) == CF_PROTOCOL_UNDEFINED);

    const int peek_size = CF_INBAND_OFFSET + sizeof("CAUTH");

    char buf[peek_size];
    ssize_t got = recv(ConnectionInfoSocket(conn_info), buf, sizeof(buf), MSG_PEEK);
    assert(got <= peek_size);
    if (got < 0)
    {
        assert(got == -1);
        Log(LOG_LEVEL_ERR, "TCP receive error: %s", GetErrorStr());
        return false;
    }
    else if (got == 0)
    {
        Log(LOG_LEVEL_INFO,
            "Peer closed TCP connection without sending data!");
        return false;
    }
    else if (got < peek_size)
    {
        Log(LOG_LEVEL_INFO,
            "Peer sent only %zd bytes! Considering the protocol as Classic",
            got);
        ConnectionInfoSetProtocolVersion(conn_info, CF_PROTOCOL_CLASSIC);
    }
    else if (memcmp(&buf[CF_INBAND_OFFSET], "CAUTH", strlen("CAUTH")) == 0)
    {
        Log(LOG_LEVEL_VERBOSE,
            "Peeked CAUTH in TCP stream, considering the protocol as Classic");
        ConnectionInfoSetProtocolVersion(conn_info, CF_PROTOCOL_CLASSIC);
    }
    else                                   /* got==peek_size && not "CAUTH" */
    {
        Log(LOG_LEVEL_VERBOSE,
            "Peeked nothing important in TCP stream, considering the protocol as TLS");
        ConnectionInfoSetProtocolVersion(conn_info, CF_PROTOCOL_TLS);
    }
    LogRaw(LOG_LEVEL_DEBUG, "Peeked data: ", buf, got);

    return true;
}

/**
 * 1. Send "CFE_v%d" server hello.
 * 2. Receive two lines: One "CFE_v%d" with the protocol version the client
 *    wishes to have, and one "IDENTITY USERNAME=blah ..." with identification
 *    information for the client.
 *
 * @note For Identification dialog to end successfully, one "OK WELCOME" line
 *       must be sent right after this function, after identity is verified.
 *
 * @TODO More protocol identity. E.g.
 *       IDENTITY USERNAME=xxx HOSTNAME=xxx CUSTOMNAME=xxx
 *
 * @retval true if protocol version was successfully negotiated and IDENTITY
 *         command was parsed correctly. Identity fields (only #username for
 *         now) have the respective string values, or they are empty if field
 *         was not on IDENTITY line.  #conn_info->protocol has been updated
 *         with the negotiated protocol version.
 * @retval false in case of error.
 */
bool ServerIdentificationDialog(ConnectionInfo *conn_info,
                                char *username, size_t username_size)
{
    int ret;
    char input[1024] = "";

    /* Send "CFE_v%d cf-serverd version". */
    char version_string[CF_MAXVARSIZE];
    int len = snprintf(version_string, sizeof(version_string),
                       "CFE_v%d cf-serverd %s\n",
                       CF_PROTOCOL_LATEST, VERSION);

    ret = TLSSend(conn_info->ssl, version_string, len);
    if (ret != len)
    {
        Log(LOG_LEVEL_NOTICE, "Connection was hung up!");
        return false;
    }

    /* Receive CFE_v%d ... \n IDENTITY USERNAME=... */
    int input_len = TLSRecvLines(conn_info->ssl, input, sizeof(input));
    if (input_len <= 0)
    {
        Log(LOG_LEVEL_NOTICE,
            "Client closed connection early! He probably does not trust our key...");
        return false;
    }

    const ProtocolVersion protocol = ParseProtocolVersionNetwork(input);
    if (ProtocolIsUndefined(protocol))
    {
        Log(LOG_LEVEL_NOTICE,
            "Protocol version negotiation failed! Received: %s",
            input);
        return false;
    }

    /* This is already inside TLS code, so TLS is required at this point*/
    if (ProtocolIsClassic(protocol))
    {
        Log(LOG_LEVEL_NOTICE,
            "Client advertises disallowed protocol version: %d",
            protocol);
        return false;
    }

    if (ProtocolIsTooNew(protocol))
    {
        Log(LOG_LEVEL_NOTICE,
            "Client attempted a protocol version which is too new for us: %d",
            protocol);
        return false;
    }

    assert(ProtocolIsKnown(protocol));

    /* Did we receive 2nd line or do we need to receive again? */
    const char id_line[] = "\nIDENTITY ";
    char *line2 = memmem(input, input_len, id_line, strlen(id_line));
    if (line2 == NULL)
    {
        /* Wait for 2nd line to arrive. */
        input_len = TLSRecvLines(conn_info->ssl, input, sizeof(input));
        if (input_len <= 0)
        {
            Log(LOG_LEVEL_NOTICE,
                "Client closed connection during identification dialog!");
            return false;
        }
        line2 = input;
    }
    else
    {
        line2++;                                               /* skip '\n' */
    }

    /***** Parse all IDENTITY fields from line2 *****/

    char word1[1024], word2[1024];
    int line2_pos = 0, chars_read = 0;

    /* Reset all identity variables, we'll set them according to fields
     * on IDENTITY line. For now only "username" setting exists... */
    username[0] = '\0';

    /* Assert sscanf() is safe to use. */
    nt_static_assert(sizeof(word1) >= sizeof(input));
    nt_static_assert(sizeof(word2) >= sizeof(input));

    ret = sscanf(line2, "IDENTITY %[^=]=%s%n", word1, word2, &chars_read);
    while (ret >= 2)
    {
        /* Found USERNAME identity setting */
        if (strcmp(word1, "USERNAME") == 0)
        {
            if ((strlen(word2) < username_size) && (IsUserNameValid(word2) == true))
            {
                strcpy(username, word2);
            }
            else
            {
                Log(LOG_LEVEL_NOTICE, "Received invalid IDENTITY: %s=%s",
                    word1, word2);
                return false;
            }
            Log(LOG_LEVEL_VERBOSE, "Setting IDENTITY: %s=%s",
                word1, word2);
        }
        /* ... else if (strcmp()) for other acceptable IDENTITY parameters. */
        else
        {
            Log(LOG_LEVEL_VERBOSE, "Received unknown IDENTITY parameter: %s=%s",
                word1, word2);
        }

        line2_pos += chars_read;
        ret = sscanf(&line2[line2_pos], " %[^=]=%s%n", word1, word2, &chars_read);
    }

    /* Version client and server agreed on. */
    assert(ProtocolIsKnown(protocol));
    conn_info->protocol = protocol;

    return true;
}

bool ServerSendWelcome(const ServerConnectionState *conn)
{
    char s[1024] = "OK WELCOME";
    size_t len = strlen(s);
    int ret;

    /* "OK WELCOME" is the important part. The rest is just extra verbosity. */
    if (conn->username[0] != '\0')
    {
        ret = snprintf(&s[len], sizeof(s) - len, " %s=%s",
                           "USERNAME", conn->username);
        if (ret < 0)
        {
            Log(LOG_LEVEL_ERR,
                "Unexpected failure from snprintf (%d - %s) while "
                "constructing OK WELCOME message (ServerSendWelcome)",
                errno, GetErrorStr());
            return false;
        }
        else if ((size_t) ret >= sizeof(s) - len)
        {
            Log(LOG_LEVEL_NOTICE, "Sending OK WELCOME message truncated: %s", s);
            return false;
        }
        len += ret;
    }

    /* Overwrite the terminating '\0', we don't need it anyway. */
    s[len] = '\n';
    len++;

    ret = TLSSend(conn->conn_info->ssl, s, len);
    if (ret == -1)
    {
        return false;
    }

    return true;
}

/**
 * @brief Accept a TLS connection and authenticate and identify.
 *
 * Doesn't include code for verifying key and lastseen
 *
 * @param conn    connection state
 * @param ssl_ctx SSL context to use for the session (or %NULL to use the
 *                default SSLSERVERCONTEXT)
 *
 * @see ServerTLSSessionEstablish
 * @return true for success false otherwise
 */
bool BasicServerTLSSessionEstablish(ServerConnectionState *conn, SSL_CTX *ssl_ctx)
{
    if (conn->conn_info->status == CONNECTIONINFO_STATUS_ESTABLISHED)
    {
        return true;
    }
    if (ssl_ctx == NULL)
    {
        ssl_ctx = SSLSERVERCONTEXT;
    }
    assert(ConnectionInfoSSL(conn->conn_info) == NULL);
    SSL *ssl = SSL_new(ssl_ctx);
    if (ssl == NULL)
    {
        Log(LOG_LEVEL_ERR, "SSL_new: %s",
            TLSErrorString(ERR_get_error()));
        return false;
    }
    ConnectionInfoSetSSL(conn->conn_info, ssl);

    /* Pass conn_info inside the ssl struct for TLSVerifyCallback(). */
    SSL_set_ex_data(ssl, CONNECTIONINFO_SSL_IDX, conn->conn_info);

    /* Now we are letting OpenSSL take over the open socket. */
    SSL_set_fd(ssl, ConnectionInfoSocket(conn->conn_info));

    int remaining_tries = MAX_ACCEPT_RETRIES;
    int ret = -1;
    bool should_retry = true;
    while ((ret < 0) && should_retry)
    {
        ret = SSL_accept(ssl);
        if (ret < 0)
        {
            int code = TLSLogError(ssl, LOG_LEVEL_VERBOSE, "SSL accept failed", ret);
            should_retry = ((remaining_tries > 0) &&
                            ((code == SSL_ERROR_WANT_READ) || (code == SSL_ERROR_WANT_WRITE)));

        }
        if ((ret < 0) && should_retry)
        {
            sleep(1);
            remaining_tries--;
        }
    }
    if (ret <= 0)
    {
        TLSLogError(ssl, LOG_LEVEL_ERR,
                    "Failed to accept TLS connection", ret);
        return false;
    }

    Log(LOG_LEVEL_VERBOSE, "TLS version negotiated: %8s; Cipher: %s,%s",
        SSL_get_version(ssl),
        SSL_get_cipher_name(ssl),
        SSL_get_cipher_version(ssl));

    return true;
}

/**
 * @brief Accept a TLS connection and authenticate and identify.
 *
 * This function uses trustkeys to trust new keys and updates lastseen
 *
 * @param conn    connection state
 * @param ssl_ctx SSL context to use for the session (or %NULL to use the
 *                default SSLSERVERCONTEXT)
 *
 * @see BasicServerTLSSessionEstablish
 * @note Various fields in #conn are set, like username and keyhash.
 * @return true for success false otherwise
 */
bool ServerTLSSessionEstablish(ServerConnectionState *conn, SSL_CTX *ssl_ctx)
{
    if (conn->conn_info->status == CONNECTIONINFO_STATUS_ESTABLISHED)
    {
        return true;
    }

    bool established = BasicServerTLSSessionEstablish(conn, ssl_ctx);
    if (!established)
    {
        return false;
    }

    Log(LOG_LEVEL_VERBOSE, "TLS session established, checking trust...");

    /* Send/Receive "CFE_v%d" version string, agree on version, receive
       identity (username) of peer. */
    char username[sizeof(conn->username)] = "";
    bool id_success = ServerIdentificationDialog(conn->conn_info,
                                                 username, sizeof(username));
    if (!id_success)
    {
        return false;
    }

    /* We *now* (maybe a bit late) verify the key that the client sent us in
     * the TLS handshake, since we need the username to do so. TODO in the
     * future store keys irrelevant of username, so that we can match them
     * before IDENTIFY. */
    int ret = TLSVerifyPeer(conn->conn_info, conn->ipaddr, username);
    if (ret == -1)                                      /* error */
    {
        return false;
    }

    if (ret == 1)                                    /* trusted key */
    {
        Log(LOG_LEVEL_VERBOSE,
            "%s: Client is TRUSTED, public key MATCHES stored one.",
            KeyPrintableHash(ConnectionInfoKey(conn->conn_info)));
    }

    if (ret == 0)                                  /* untrusted key */
    {
        if ((SERVER_ACCESS.trustkeylist != NULL) &&
            (IsMatchItemIn(SERVER_ACCESS.trustkeylist, conn->ipaddr)))
        {
            Log(LOG_LEVEL_VERBOSE,
                "Peer was found in \"trustkeysfrom\" list");
            Log(LOG_LEVEL_NOTICE, "Trusting new key: %s",
                KeyPrintableHash(ConnectionInfoKey(conn->conn_info)));

            SavePublicKey(username, KeyPrintableHash(conn->conn_info->remote_key),
                          KeyRSA(ConnectionInfoKey(conn->conn_info)));
        }
        else
        {
            Log(LOG_LEVEL_NOTICE,
                "TRUST FAILED, peer presented an untrusted key, dropping connection!");
            Log(LOG_LEVEL_VERBOSE,
                "Add peer to \"trustkeysfrom\" if you really want to start trusting this new key.");
            return false;
        }
    }

    /* All checks succeeded, set conn->uid (conn->sid for Windows)
     * according to the received USERNAME identity. */
    SetConnIdentity(conn, username);

    /* No CAUTH, SAUTH in non-classic protocol. */
    conn->user_data_set = true;
    conn->rsa_auth = true;

    LastSaw1(conn->ipaddr, KeyPrintableHash(ConnectionInfoKey(conn->conn_info)),
             LAST_SEEN_ROLE_ACCEPT);

    ServerSendWelcome(conn);
    return true;
}

//*******************************************************************
// COMMANDS
//*******************************************************************

ProtocolCommandNew GetCommandNew(char *str)
{
    int i;
    for (i = 0; PROTOCOL_NEW[i] != NULL; i++)
    {
        int cmdlen = strlen(PROTOCOL_NEW[i]);
        if ((strncmp(str, PROTOCOL_NEW[i], cmdlen) == 0) &&
            (str[cmdlen] == ' ' || str[cmdlen] == '\0'))
        {
            return i;
        }
    }
    assert (i == PROTOCOL_COMMAND_BAD);
    return i;
}


/**
 * Currently this function returns false when we want the connection
 * closed, and true, when we want to proceed further with requests.
 *
 * @TODO So we need this function to return more than true/false, because now
 * we return true even when access is denied! E.g. return -1 for error, 0 on
 * success, 1 on access denied. It can be an option if connection will close
 * on denial.
 */
bool BusyWithNewProtocol(EvalContext *ctx, ServerConnectionState *conn)
{
    assert(conn != NULL);

    /* The CF_BUFEXT extra space is there to ensure we're not *reading* out of
     * bounds in commands that carry extra binary arguments, like MD5. */
    char recvbuffer[CF_BUFSIZE + CF_BUFEXT] = { 0 };
    /* This size is the max we can SendTransaction(). */
    char sendbuffer[CF_BUFSIZE - CF_INBAND_OFFSET] = { 0 };
    char filename[CF_BUFSIZE + 1];      /* +1 for appending slash sometimes */
    ServerFileGetState get_args = { 0 };

    /* We already encrypt because of the TLS layer, no need to encrypt more. */
    const int encrypted = 0;

    /* Legacy stuff only for old protocol. */
    assert(conn->rsa_auth == true);
    assert(conn->user_data_set == true);

    /* Receive up to CF_BUFSIZE - 1 bytes. */
    const int received = ReceiveTransaction(conn->conn_info,
                                            recvbuffer, NULL);

    if (received == -1)
    {
        /* Already Log()ged in case of error. */
        return false;
    }
    if (received > CF_BUFSIZE - 1)
    {
        UnexpectedError("Received transaction of size %d", received);
        return false;
    }

    if (strlen(recvbuffer) == 0)
    {
        Log(LOG_LEVEL_WARNING,
            "Got NULL transmission (of size %d)", received);
        return true;
    }
    /* Don't process request if we're signalled to exit. */
    if (IsPendingTermination())
    {
        Log(LOG_LEVEL_VERBOSE, "Server must exit, closing connection");
        return false;
    }

    /* TODO break recvbuffer here: command, param1, param2 etc. */

    switch (GetCommandNew(recvbuffer))
    {
    case PROTOCOL_COMMAND_EXEC:
    {
        const size_t EXEC_len = strlen(PROTOCOL_NEW[PROTOCOL_COMMAND_EXEC]);
        /* Assert recvbuffer starts with EXEC. */
        assert(strncmp(PROTOCOL_NEW[PROTOCOL_COMMAND_EXEC],
                       recvbuffer, EXEC_len) == 0);

        char *args = &recvbuffer[EXEC_len];
        args += strspn(args, " \t");                       /* bypass spaces */

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Received:", "EXEC", args);

        bool b = DoExec2(ctx, conn, args,
                         sendbuffer, sizeof(sendbuffer));

        /* In the end we might keep the connection open (return true) to be
         * ready for next requests, but we must always send the TERMINATOR
         * string so that the client can close the connection at will. */
        Terminate(conn->conn_info);

        return b;
    }
    case PROTOCOL_COMMAND_VERSION:

        snprintf(sendbuffer, sizeof(sendbuffer), "OK: %s", Version());
        SendTransaction(conn->conn_info, sendbuffer, 0, CF_DONE);
        return true;

    case PROTOCOL_COMMAND_GET:
    {
        int ret = sscanf(recvbuffer, "GET %d %[^\n]",
                         &(get_args.buf_size), filename);

        if (ret != 2 ||
            get_args.buf_size <= 0 || get_args.buf_size > CF_BUFSIZE)
        {
            goto protocol_error;
        }

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Received:", "GET", filename);

        /* TODO batch all the following in one function since it's very
         * similar in all of GET, OPENDIR and STAT. */

        size_t zret = ShortcutsExpand(filename, sizeof(filename),
                                     SERVER_ACCESS.path_shortcuts,
                                     conn->ipaddr, conn->revdns,
                                     KeyPrintableHash(ConnectionInfoKey(conn->conn_info)));
        if (zret == (size_t) -1)
        {
            goto protocol_error;
        }

        zret = PreprocessRequestPath(filename, sizeof(filename));
        if (zret == (size_t) -1)
        {
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        PathRemoveTrailingSlash(filename, strlen(filename));

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Translated to:", "GET", filename);

        if (acl_CheckPath(paths_acl, filename,
                          conn->ipaddr, conn->revdns,
                          KeyPrintableHash(ConnectionInfoKey(conn->conn_info)))
            == false)
        {
            Log(LOG_LEVEL_INFO, "access denied to GET: %s", filename);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        memset(sendbuffer, 0, sizeof(sendbuffer));

        if (get_args.buf_size >= CF_BUFSIZE)
        {
            get_args.buf_size = 2048;
        }

        /* TODO eliminate! */
        get_args.conn = conn;
        get_args.encrypt = false;
        get_args.replybuff = sendbuffer;
        get_args.replyfile = filename;

        CfGetFile(&get_args);

        return true;
    }
    case PROTOCOL_COMMAND_OPENDIR:
    {
        memset(filename, 0, sizeof(filename));
        int ret = sscanf(recvbuffer, "OPENDIR %[^\n]", filename);
        if (ret != 1)
        {
            goto protocol_error;
        }

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Received:", "OPENDIR", filename);

        /* sizeof()-1 because we need one extra byte for
           appending '/' afterwards. */
        size_t zret = ShortcutsExpand(filename, sizeof(filename) - 1,
                                      SERVER_ACCESS.path_shortcuts,
                                      conn->ipaddr, conn->revdns,
                                      KeyPrintableHash(ConnectionInfoKey(conn->conn_info)));
        if (zret == (size_t) -1)
        {
            goto protocol_error;
        }

        zret = PreprocessRequestPath(filename, sizeof(filename) - 1);
        if (zret == (size_t) -1)
        {
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        /* OPENDIR *must* be directory. */
        PathAppendTrailingSlash(filename, strlen(filename));

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Translated to:", "OPENDIR", filename);

        if (acl_CheckPath(paths_acl, filename,
                          conn->ipaddr, conn->revdns,
                          KeyPrintableHash(ConnectionInfoKey(conn->conn_info)))
            == false)
        {
            Log(LOG_LEVEL_INFO, "access denied to OPENDIR: %s", filename);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        CfOpenDirectory(conn, sendbuffer, filename);
        return true;
    }
    case PROTOCOL_COMMAND_SYNCH:
    {
        long time_no_see = 0;
        memset(filename, 0, sizeof(filename));
        int ret = sscanf(recvbuffer, "SYNCH %ld STAT %[^\n]",
                         &time_no_see, filename);

        if (ret != 2 || filename[0] == '\0')
        {
            goto protocol_error;
        }

        time_t tloc = time(NULL);
        if (tloc == -1)
        {
            /* Should never happen. */
            Log(LOG_LEVEL_ERR, "Couldn't read system clock. (time: %s)", GetErrorStr());
            SendTransaction(conn->conn_info, "BAD: clocks out of synch", 0, CF_DONE);
            return true;
        }

        time_t trem = (time_t) time_no_see;
        int drift = (int) (tloc - trem);

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Received:", "STAT", filename);

        /* sizeof()-1 because we need one extra byte for
           appending '/' afterwards. */
        size_t zret = ShortcutsExpand(filename, sizeof(filename) - 1,
                                      SERVER_ACCESS.path_shortcuts,
                                      conn->ipaddr, conn->revdns,
                                      KeyPrintableHash(ConnectionInfoKey(conn->conn_info)));
        if (zret == (size_t) -1)
        {
            goto protocol_error;
        }

        zret = PreprocessRequestPath(filename, sizeof(filename) - 1);
        if (zret == (size_t) -1)
        {
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        if (IsDirReal(filename))
        {
            PathAppendTrailingSlash(filename, strlen(filename));
        }
        else
        {
            PathRemoveTrailingSlash(filename, strlen(filename));
        }

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Translated to:", "STAT", filename);

        if (acl_CheckPath(paths_acl, filename,
                          conn->ipaddr, conn->revdns,
                          KeyPrintableHash(ConnectionInfoKey(conn->conn_info)))
            == false)
        {
            Log(LOG_LEVEL_INFO, "access denied to STAT: %s", filename);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        Log(LOG_LEVEL_DEBUG, "Clocks were off by %ld",
            (long) tloc - (long) trem);

        if (DENYBADCLOCKS && (drift * drift > CLOCK_DRIFT * CLOCK_DRIFT))
        {
            snprintf(sendbuffer, sizeof(sendbuffer),
                     "BAD: Clocks are too far unsynchronized %ld/%ld",
                     (long) tloc, (long) trem);
            Log(LOG_LEVEL_INFO, "denybadclocks %s", sendbuffer);
            SendTransaction(conn->conn_info, sendbuffer, 0, CF_DONE);
            return true;
        }

        StatFile(conn, sendbuffer, filename);

        return true;
    }
    case PROTOCOL_COMMAND_MD5:
    {
        int ret = sscanf(recvbuffer, "MD5 %[^\n]", filename);
        if (ret != 1)
        {
            goto protocol_error;
        }

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Received:", "MD5", filename);

        /* TODO batch all the following in one function since it's very
         * similar in all of GET, OPENDIR and STAT. */

        size_t zret = ShortcutsExpand(filename, sizeof(filename),
                                     SERVER_ACCESS.path_shortcuts,
                                     conn->ipaddr, conn->revdns,
                                     KeyPrintableHash(ConnectionInfoKey(conn->conn_info)));
        if (zret == (size_t) -1)
        {
            goto protocol_error;
        }

        zret = PreprocessRequestPath(filename, sizeof(filename));
        if (zret == (size_t) -1)
        {
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        PathRemoveTrailingSlash(filename, strlen(filename));

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Translated to:", "MD5", filename);

        if (acl_CheckPath(paths_acl, filename,
                          conn->ipaddr, conn->revdns,
                          KeyPrintableHash(ConnectionInfoKey(conn->conn_info)))
            == false)
        {
            Log(LOG_LEVEL_INFO, "access denied to file: %s", filename);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        assert(CF_DEFAULT_DIGEST_LEN <= EVP_MAX_MD_SIZE);
        unsigned char digest[EVP_MAX_MD_SIZE + 1];

        assert(CF_BUFSIZE + CF_SMALL_OFFSET + (size_t) CF_DEFAULT_DIGEST_LEN
               <= sizeof(recvbuffer));
        memcpy(digest, recvbuffer + strlen(recvbuffer) + CF_SMALL_OFFSET,
               CF_DEFAULT_DIGEST_LEN);

        CompareLocalHash(filename, digest, sendbuffer);
        SendTransaction(conn->conn_info, sendbuffer, 0, CF_DONE);

        return true;
    }
    case PROTOCOL_COMMAND_VAR:
    {
        char var[256];
        int ret = sscanf(recvbuffer, "VAR %255[^\n]", var);
        if (ret != 1)
        {
            goto protocol_error;
        }

        /* TODO if this is literals_acl, then when should I check vars_acl? */
        if (acl_CheckExact(literals_acl, var,
                           conn->ipaddr, conn->revdns,
                           KeyPrintableHash(ConnectionInfoKey(conn->conn_info)))
            == false)
        {
            Log(LOG_LEVEL_INFO, "access denied to variable: %s", var);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        GetServerLiteral(ctx, conn, sendbuffer, recvbuffer, encrypted);
        return true;
    }
    case PROTOCOL_COMMAND_CONTEXT:
    {
        char client_regex[256];
        int ret = sscanf(recvbuffer, "CONTEXT %255[^\n]", client_regex);
        if (ret != 1)
        {
            goto protocol_error;
        }

        Log(LOG_LEVEL_VERBOSE, "%14s %7s %s",
            "Received:", "CONTEXT", client_regex);

        /* WARNING: this comes from legacy code and must be killed if we care
         * about performance. We should not accept regular expressions from
         * the client, but this will break backwards compatibility.
         *
         * I replicated the code in raw form here to emphasize complexity,
         * it's the only *slow* command currently in the protocol.  */

        Item *persistent_classes = ListPersistentClasses();
        Item *matched_classes = NULL;

        /* For all persistent classes */
        for (Item *ip = persistent_classes; ip != NULL; ip = ip->next)
        {
            const char *class_name = ip->name;

            /* Does this class match the regex the client sent? */
            if (StringMatchFull(client_regex, class_name))
            {
                /* Is this class allowed to be given to the specific
                 * host, according to the regexes in the ACLs? */
                if (acl_CheckRegex(classes_acl, class_name,
                                   conn->ipaddr, conn->revdns,
                                   KeyPrintableHash(ConnectionInfoKey(conn->conn_info)),
                                   NULL)
                    == true)
                {
                    Log(LOG_LEVEL_DEBUG, "Access granted to class: %s",
                        class_name);
                    PrependItem(&matched_classes, class_name, NULL);
                }
            }
        }

        if (matched_classes == NULL)
        {
            Log(LOG_LEVEL_INFO,
                "No allowed classes for remoteclassesmatching: %s",
                client_regex);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        ReplyServerContext(conn, encrypted, matched_classes);
        return true;
    }
    case PROTOCOL_COMMAND_QUERY:
    {
        char query[256], name[128];
        int ret1 = sscanf(recvbuffer, "QUERY %255[^\n]", query);
        int ret2 = sscanf(recvbuffer, "QUERY %127s", name);
        if (ret1 != 1 || ret2 != 1)
        {
            goto protocol_error;
        }

        const char *hostkey = KeyPrintableHash(
            ConnectionInfoKey(conn->conn_info));
        const bool access_to_query = acl_CheckExact(
            query_acl, name, conn->ipaddr, conn->revdns, hostkey);

        if (!access_to_query)
        {
            Log(LOG_LEVEL_INFO, "access denied to query: %s", query);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        if (GetServerQuery(conn, recvbuffer, encrypted))
        {
            return true;
        }

        break;
    }
    case PROTOCOL_COMMAND_COOKIE:
    {
        if (ConnectionInfoProtocolVersion(conn->conn_info) < CF_PROTOCOL_COOKIE)
        {
            goto protocol_error;
        }

        const char *hostkey = KeyPrintableHash(
            ConnectionInfoKey(conn->conn_info));
        const bool access_to_query_delta = acl_CheckExact(
            query_acl, "delta", conn->ipaddr, conn->revdns, hostkey);

        if (!access_to_query_delta)
        {
            Log(LOG_LEVEL_INFO, "access denied to cookie query: %s", recvbuffer);
            RefuseAccess(conn, recvbuffer);
            return true;
        }

        if (ReturnCookies(conn))
        {
            return true;
        }

        break;
    }
    case PROTOCOL_COMMAND_CALL_ME_BACK:
        /* Server side, handing the collect call off to cf-hub. */

        if (acl_CheckExact(query_acl, "collect_calls",
                           conn->ipaddr, conn->revdns,
                           KeyPrintableHash(ConnectionInfoKey(conn->conn_info)))
            == false)
        {
            Log(LOG_LEVEL_INFO,
                "access denied to Call-Collect, check the ACL for class: collect_calls");
            return false;
        }

        ReceiveCollectCall(conn);
        /* On success that returned true; otherwise, it did all
         * relevant Log()ging.  Either way, we're no longer busy with
         * it and our caller can close the connection: */
        return false;

    case PROTOCOL_COMMAND_BAD:

        Log(LOG_LEVEL_WARNING, "Unexpected protocol command: %s", recvbuffer);
    }

    /* We should only reach this point if something went really bad, and
     * close connection. In all other cases (like access denied) connection
     * shouldn't be closed.
     */

protocol_error:
    strcpy(sendbuffer, "BAD: Request denied");
    SendTransaction(conn->conn_info, sendbuffer, 0, CF_DONE);
    Log(LOG_LEVEL_INFO,
        "Closing connection due to illegal request: %s", recvbuffer);
    return false;
}