vsftpd.conf may be used to control various aspects of vsftpd's behaviour. By
default, vsftpd looks for this file at the location
/etc/vsftpd.conf.
However, you may override this by specifying a command line argument to
vsftpd. The command line argument is the pathname of the configuration file
for vsftpd. This behaviour is useful because you may wish to use an advanced
inetd such as
xinetd
to launch vsftpd with different configuration files on a per virtual host
basis.
FORMAT
The format of vsftpd.conf is very simple. Each line is either a comment or
a directive. Comment lines start with a # and are ignored. A directive line
has the format:
option=value
It is important to note that it is an error to put any space between the
option, = and value.
Each setting has a compiled in default which may be modified in the
configuration file.
BOOLEAN OPTIONS
Below is a list of boolean options. The value for a boolean option may be set
to
YES
or
NO.
allow_anon_ssl
Only applies if
ssl_enable
is active. If set to YES, anonymous users will be allowed to use secured SSL
connections.
Default: NO
anon_mkdir_write_enable
If set to YES, anonymous users will be permitted to create new directories
under certain conditions. For this to work, the option
write_enable
must be activated, and the anonymous ftp user must have write permission on
the parent directory.
Default: NO
anon_other_write_enable
If set to YES, anonymous users will be permitted to perform write operations
other than upload and create directory, such as deletion and renaming. This
is generally not recommended but included for completeness.
Default: NO
anon_upload_enable
If set to YES, anonymous users will be permitted to upload files under certain
conditions. For this to work, the option
write_enable
must be activated, and the anonymous ftp user must have write permission on
desired upload locations. This setting is also required for virtual users to
upload; by default, virtual users are treated with anonymous (i.e. maximally
restricted) privilege.
Default: NO
anon_world_readable_only
When enabled, anonymous users will only be allowed to download files which
are world readable. This is recognising that the ftp user may own files,
especially in the presence of uploads.
Default: YES
anonymous_enable
Controls whether anonymous logins are permitted or not. If enabled,
both the usernames
ftp
and
anonymous
are recognised as anonymous logins.
Default: YES
ascii_download_enable
When enabled, ASCII mode data transfers will be honoured on downloads.
Default: NO
ascii_upload_enable
When enabled, ASCII mode data transfers will be honoured on uploads.
Default: NO
async_abor_enable
When enabled, a special FTP command known as "async ABOR" will be enabled.
Only ill advised FTP clients will use this feature. Additionally, this feature
is awkward to handle, so it is disabled by default. Unfortunately, some FTP
clients will hang when cancelling a transfer unless this feature is available,
so you may wish to enable it.
Default: NO
background
When enabled, and vsftpd is started in "listen" mode, vsftpd will background
the listener process. i.e. control will immediately be returned to the shell
which launched vsftpd.
Default: NO
check_shell
Note! This option only has an effect for non-PAM builds of vsftpd. If disabled,
vsftpd will not check /etc/shells for a valid user shell for local logins.
Default: YES
chmod_enable
When enables, allows use of the SITE CHMOD command. NOTE! This only applies
to local users. Anonymous users never get to use SITE CHMOD.
Default: YES
chown_uploads
If enabled, all anonymously uploaded files will have the ownership changed
to the user specified in the setting
chown_username.
This is useful from an administrative, and perhaps security, standpoint.
Default: NO
chroot_list_enable
If activated, you may provide a list of local users who are placed in a
chroot() jail in their home directory upon login. The meaning is slightly
different if chroot_local_user is set to YES. In this case, the list becomes
a list of users which are NOT to be placed in a chroot() jail.
By default, the file containing this list is
/etc/vsftpd.chroot_list, but you may override this with the
chroot_list_file
setting.
Default: NO
chroot_local_user
If set to YES, local users will be (by default) placed in a chroot() jail in
their home directory after login.
Warning:
This option has security implications, especially if the users have upload
permission, or shell access. Only enable if you know what you are doing.
Note that these security implications are not vsftpd specific. They apply to
all FTP daemons which offer to put local users in chroot() jails.
Default: NO
connect_from_port_20
This controls whether PORT style data connections use port 20 (ftp-data) on
the server machine. For security reasons, some clients may insist that this
is the case. Conversely, disabling this option enables vsftpd to run with
slightly less privilege.
Default: NO (but the sample config file enables it)
deny_email_enable
If activated, you may provide a list of anonymous password e-mail responses
which cause login to be denied. By default, the file containing this list is
/etc/vsftpd.banned_emails, but you may override this with the
banned_email_file
setting.
Default: NO
dirlist_enable
If set to NO, all directory list commands will give permission denied.
Default: YES
dirmessage_enable
If enabled, users of the FTP server can be shown messages when they first
enter a new directory. By default, a directory is scanned for the
file .message, but that may be overridden with the configuration setting
message_file.
Default: NO (but the sample config file enables it)
download_enable
If set to NO, all download requests will give permission denied.
Default: YES
dual_log_enable
If enabled, two log files are generated in parallel, going by default to
/var/log/xferlog
and
/var/log/vsftpd.log.
The former is a wu-ftpd style transfer log, parseable by standard tools. The
latter is vsftpd's own style log.
Default: NO
force_dot_files
If activated, files and directories starting with . will be shown in directory
listings even if the "a" flag was not used by the client. This override
excludes the "." and ".." entries.
Default: NO
force_anon_data_ssl
Only applies if
ssl_enable
is activated. If activated, all anonymous logins are forced to use a secure
SSL connection in order to send and receive data on data connections.
Default: NO
force_anon_logins_ssl
Only applies if
ssl_enable
is activated. If activated, all anonymous logins are forced to use a secure
SSL connection in order to send the password.
Default: NO
force_local_data_ssl
Only applies if
ssl_enable
is activated. If activated, all non-anonymous logins are forced to use a secure
SSL connection in order to send and receive data on data connections.
Default: YES
force_local_logins_ssl
Only applies if
ssl_enable
is activated. If activated, all non-anonymous logins are forced to use a secure
SSL connection in order to send the password.
Default: YES
guest_enable
If enabled, all non-anonymous logins are classed as "guest" logins. A guest
login is remapped to the user specified in the
guest_username
setting.
Default: NO
hide_ids
If enabled, all user and group information in directory listings will be
displayed as "ftp".
Default: NO
listen
If enabled, vsftpd will run in standalone mode. This means that vsftpd must
not be run from an inetd of some kind. Instead, the vsftpd executable is
run once directly. vsftpd itself will then take care of listening for and
handling incoming connections.
Default: NO
listen_ipv6
Like the listen parameter, except vsftpd will listen on an IPv6 socket instead
of an IPv4 one. This parameter and the listen parameter are mutually
exclusive.
Default: NO
local_enable
Controls whether local logins are permitted or not. If enabled, normal
user accounts in /etc/passwd (or wherever your PAM config references) may be
used to log in. This must be enable for any non-anonymous login to work,
including virtual users.
Default: NO
lock_upload_files
When enabled, all uploads proceed with a write lock on the upload file. All
downloads proceed with a shared read lock on the download file. WARNING!
Before enabling this, be aware that malicious readers could starve a writer
wanting to e.g. append a file.
Default: NO
log_ftp_protocol
When enabled, all FTP requests and responses are logged, providing the option
xferlog_std_format is not enabled. Useful for debugging.
Default: NO
ls_recurse_enable
When enabled, this setting will allow the use of "ls -R". This is a minor
security risk, because a ls -R at the top level of a large site may consume
a lot of resources.
Default: NO
mdtm_write
When enabled, this setting will allow MDTM to set file modification times
(subject to the usual access checks).
Default: YES
no_anon_password
When enabled, this prevents vsftpd from asking for an anonymous password -
the anonymous user will log straight in.
Default: NO
no_log_lock
When enabled, this prevents vsftpd from taking a file lock when writing to log
files. This option should generally not be enabled. It exists to workaround
operating system bugs such as the Solaris / Veritas filesystem combination
which has been observed to sometimes exhibit hangs trying to lock log files.
Default: NO
one_process_model
If you have a Linux 2.4 kernel, it is possible to use a different security
model which only uses one process per connection. It is a less pure security
model, but gains you performance. You really don't want to enable this unless
you know what you are doing, and your site supports huge numbers of
simultaneously connected users.
Default: NO
passwd_chroot_enable
If enabled, along with
chroot_local_user
, then a chroot() jail location may be specified on a per-user basis. Each
user's jail is derived from their home directory string in /etc/passwd. The
occurrence of /./ in the home directory string denotes that the jail is at that
particular location in the path.
Default: NO
pasv_addr_resolve
Set to YES if you want to use a hostname (as opposed to IP address) in the
pasv_address
option.
Default: NO
pasv_enable
Set to NO if you want to disallow the PASV method of obtaining a data
connection.
Default: YES
pasv_promiscuous
Set to YES if you want to disable the PASV security check that ensures the
data connection originates from the same IP address as the control connection.
Only enable if you know what you are doing! The only legitimate use for this
is in some form of secure tunnelling scheme, or perhaps to facilitate FXP
support.
Default: NO
port_enable
Set to NO if you want to disallow the PORT method of obtaining a data
connection.
Default: YES
port_promiscuous
Set to YES if you want to disable the PORT security check that ensures that
outgoing data connections can only connect to the client. Only enable if
you know what you are doing!
Default: NO
run_as_launching_user
Set to YES if you want vsftpd to run as the user which launched vsftpd. This is
useful where root access is not available. MASSIVE WARNING! Do NOT enable this
option unless you totally know what you are doing, as naive use of this option
can create massive security problems. Specifically, vsftpd does not / cannot
use chroot technology to restrict file access when this option is set (even if
launched by root). A poor substitute could be to use a
deny_file
setting such as {/*,*..*}, but the reliability of this cannot compare to
chroot, and should not be relied on.
If using this option, many restrictions on other options
apply. For example, options requiring privilege such as non-anonymous logins,
upload ownership changing, connecting from port 20 and listen ports less than
1024 are not expected to work. Other options may be impacted.
Default: NO
secure_email_list_enable
Set to YES if you want only a specified list of e-mail passwords for anonymous
logins to be accepted. This is useful as a low-hassle way of restricting
access to low-security content without needing virtual users. When enabled,
anonymous logins are prevented unless the password provided is listed in the
file specified by the
email_password_file
setting. The file format is one password per line, no extra whitespace. The
default filename is /etc/vsftpd.email_passwords.
Default: NO
session_support
This controls whether vsftpd attempts to maintain sessions for logins. If
vsftpd is maintaining sessions, it will try and update utmp and wtmp. It
will also open a pam_session if using PAM to authenticate, and only close
this upon logout. You may wish to disable this if you do not need session
logging, and you wish to give vsftpd more opportunity to run with less
processes and / or less privilege. NOTE - utmp and wtmp support is only
provided with PAM enabled builds.
Default: NO
setproctitle_enable
If enabled, vsftpd will try and show session status information in the system
process listing. In other words, the reported name of the process will change
to reflect what a vsftpd session is doing (idle, downloading etc). You
probably want to leave this off for security purposes.
Default: NO
ssl_enable
If enabled, and vsftpd was compiled against OpenSSL, vsftpd will support secure
connections via SSL. This applies to the control connection (including login)
and also data connections. You'll need a client with SSL support too. NOTE!!
Beware enabling this option. Only enable it if you need it. vsftpd can make no
guarantees about the security of the OpenSSL libraries. By enabling this
option, you are declaring that you trust the security of your installed
OpenSSL library.
Default: NO
ssl_sslv2
Only applies if
ssl_enable
is activated. If enabled, this option will permit SSL v2 protocol connections.
TLS v1 connections are preferred.
Default: NO
ssl_sslv3
Only applies if
ssl_enable
is activated. If enabled, this option will permit SSL v3 protocol connections.
TLS v1 connections are preferred.
Default: NO
ssl_tlsv1
Only applies if
ssl_enable
is activated. If enabled, this option will permit TLS v1 protocol connections.
TLS v1 connections are preferred.
Default: YES
syslog_enable
If enabled, then any log output which would have gone to /var/log/vsftpd.log
goes to the system log instead. Logging is done under the FTPD facility.
Default: NO
tcp_wrappers
If enabled, and vsftpd was compiled with tcp_wrappers support, incoming
connections will be fed through tcp_wrappers access control. Furthermore,
there is a mechanism for per-IP based configuration. If tcp_wrappers sets
the VSFTPD_LOAD_CONF environment variable, then the vsftpd session will try
and load the vsftpd configuration file specified in this variable.
Default: NO
text_userdb_names
By default, numeric IDs are shown in the user and group fields of directory
listings. You can get textual names by enabling this parameter. It is off
by default for performance reasons.
Default: NO
tilde_user_enable
If enabled, vsftpd will try and resolve pathnames such as ~chris/pics, i.e. a
tilde followed by a username. Note that vsftpd will always resolve the
pathnames ~ and ~/something (in this case the ~ resolves to the initial
login directory). Note that ~user paths will only resolve if the file
/etc/passwd
may be found within the _current_ chroot() jail.
Default: NO
use_localtime
If enabled, vsftpd will display directory listings with the time in your
local time zone. The default is to display GMT. The times returned by the
MDTM FTP command are also affected by this option.
Default: NO
use_sendfile
An internal setting used for testing the relative benefit of using the
sendfile() system call on your platform.
Default: YES
userlist_deny
This option is examined if
userlist_enable
is activated. If you set this setting to NO, then users will be denied login
unless they are explicitly listed in the file specified by
userlist_file.
When login is denied, the denial is issued before the user is asked for a
password.
Default: YES
userlist_enable
If enabled, vsftpd will load a list of usernames, from the filename given by
userlist_file.
If a user tries to log in using a name in this file, they will be denied
before they are asked for a password. This may be useful in preventing
cleartext passwords being transmitted. See also
userlist_deny.
Default: NO
virtual_use_local_privs
If enabled, virtual users will use the same privileges as local users. By
default, virtual users will use the same privileges as anonymous users, which
tends to be more restrictive (especially in terms of write access).
Default: NO
write_enable
This controls whether any FTP commands which change the filesystem are allowed
or not. These commands are: STOR, DELE, RNFR, RNTO, MKD, RMD, APPE and SITE.
Default: NO
xferlog_enable
If enabled, a log file will be maintained detailling uploads and downloads.
By default, this file will be placed at /var/log/vsftpd.log, but this location
may be overridden using the configuration setting
vsftpd_log_file.
Default: NO (but the sample config file enables it)
xferlog_std_format
If enabled, the transfer log file will be written in standard xferlog format,
as used by wu-ftpd. This is useful because you can reuse existing transfer
statistics generators. The default format is more readable, however. The
default location for this style of log file is /var/log/xferlog, but you may
change it with the setting
xferlog_file.
Default: NO
NUMERIC OPTIONS
Below is a list of numeric options. A numeric option must be set to a non
negative integer. Octal numbers are supported, for convenience of the umask
options. To specify an octal number, use 0 as the first digit of the number.
accept_timeout
The timeout, in seconds, for a remote client to establish connection with
a PASV style data connection.
Default: 60
anon_max_rate
The maximum data transfer rate permitted, in bytes per second, for anonymous
clients.
Default: 0 (unlimited)
anon_umask
The value that the umask for file creation is set to for anonymous users. NOTE! If you want to specify octal values, remember the "0" prefix otherwise the
value will be treated as a base 10 integer!
Default: 077
connect_timeout
The timeout, in seconds, for a remote client to respond to our PORT style
data connection.
Default: 60
data_connection_timeout
The timeout, in seconds, which is roughly the maximum time we permit data
transfers to stall for with no progress. If the timeout triggers, the remote
client is kicked off.
Default: 300
delay_failed_login
The number of seconds to pause prior to reporting a failed login.
Default: 1
delay_successful_login
The number of seconds to pause prior to allowing a successful login.
Default: 0
file_open_mode
The permissions with which uploaded files are created. Umasks are applied
on top of this value. You may wish to change to 0777 if you want uploaded
files to be executable.
Default: 0666
ftp_data_port
The port from which PORT style connections originate (as long as the poorly
named
connect_from_port_20
is enabled).
Default: 20
idle_session_timeout
The timeout, in seconds, which is the maximum time a remote client may spend
between FTP commands. If the timeout triggers, the remote client is kicked
off.
Default: 300
listen_port
If vsftpd is in standalone mode, this is the port it will listen on for
incoming FTP connections.
Default: 21
local_max_rate
The maximum data transfer rate permitted, in bytes per second, for local
authenticated users.
Default: 0 (unlimited)
local_umask
The value that the umask for file creation is set to for local users. NOTE! If
you want to specify octal values, remember the "0" prefix otherwise the value
will be treated as a base 10 integer!
Default: 077
max_clients
If vsftpd is in standalone mode, this is the maximum number of clients which
may be connected. Any additional clients connecting will get an error message.
Default: 0 (unlimited)
max_login_fails
After this many login failures, the session is killed.
Default: 3
max_per_ip
If vsftpd is in standalone mode, this is the maximum number of clients which
may be connected from the same source internet address. A client will get an
error message if they go over this limit.
Default: 0 (unlimited)
pasv_max_port
The maximum port to allocate for PASV style data connections. Can be used to
specify a narrow port range to assist firewalling.
Default: 0 (use any port)
pasv_min_port
The minimum port to allocate for PASV style data connections. Can be used to
specify a narrow port range to assist firewalling.
Default: 0 (use any port)
trans_chunk_size
You probably don't want to change this, but try setting it to something like
8192 for a much smoother bandwidth limiter.
Default: 0 (let vsftpd pick a sensible setting)
STRING OPTIONS
Below is a list of string options.
anon_root
This option represents a directory which vsftpd will try to change into
after an anonymous login. Failure is silently ignored.
Default: (none)
banned_email_file
This option is the name of a file containing a list of anonymous e-mail
passwords which are not permitted. This file is consulted if the option
deny_email_enable
is enabled.
Default: /etc/vsftpd.banned_emails
banner_file
This option is the name of a file containing text to display when someone
connects to the server. If set, it overrides the banner string provided by
the
ftpd_banner
option.
Default: (none)
chown_username
This is the name of the user who is given ownership of anonymously uploaded
files. This option is only relevant if another option,
chown_uploads,
is set.
Default: root
chroot_list_file
The option is the name of a file containing a list of local users which
will be placed in a chroot() jail in their home directory. This option is
only relevant if the option
chroot_list_enable
is enabled. If the option
chroot_local_user
is enabled, then the list file becomes a list of users to NOT place in a
chroot() jail.
Default: /etc/vsftpd.chroot_list
cmds_allowed
This options specifies a comma separated list of allowed FTP commands (post
login. USER, PASS and QUIT are always allowed pre-login). Other
commands are rejected. This is a powerful method of really locking down an
FTP server. Example: cmds_allowed=PASV,RETR,QUIT
Default: (none)
deny_file
This option can be used to set a pattern for filenames (and directory names
etc.) which should not be accessible in any way. The affected items are not
hidden, but any attempt to do anything to them (download, change into
directory, affect something within directory etc.) will be denied. This option
is very simple, and should not be used for serious access control - the
filesystem's permissions should be used in preference. However, this option
may be useful in certain virtual user setups. In particular aware that if
a filename is accessible by a variety of names (perhaps due to symbolic
links or hard links), then care must be taken to deny access to all the names.
Access will be denied to items if their name contains the string given by
hide_file, or if they match the regular expression specified by hide_file.
Note that vsftpd's regular expression matching code is a simple implementation
which is a subset of full regular expression functionality. Because of this,
you will need to carefully and exhaustively test any application of this
option. And you are recommended to use filesystem permissions for any
important security policies due to their greater reliability. Supported
regex syntax is any number of *, ? and unnested {,} operators. Regex
matching is only supported on the last component of a path, e.g. a/b/? is
supported but a/?/c is not.
Example: deny_file={*.mp3,*.mov,.private}
Default: (none)
dsa_cert_file
This option specifies the location of the DSA certificate to use for SSL
encrypted connections.
Default: (none - an RSA certificate suffices)
dsa_private_key_file
This option specifies the location of the DSA private key to use for SSL
encrypted connections. If this option is not set, the private key is expected
to be in the same file as the certificate.
Default: (none)
email_password_file
This option can be used to provide an alternate file for usage by the
secure_email_list_enable
setting.
Default: /etc/vsftpd.email_passwords
ftp_username
This is the name of the user we use for handling anonymous FTP. The home
directory of this user is the root of the anonymous FTP area.
Default: ftp
ftpd_banner
This string option allows you to override the greeting banner displayed
by vsftpd when a connection first comes in.
Default: (none - default vsftpd banner is displayed)
guest_username
See the boolean setting
guest_enable
for a description of what constitutes a guest login. This setting is the
real username which guest users are mapped to.
Default: ftp
hide_file
This option can be used to set a pattern for filenames (and directory names
etc.) which should be hidden from directory listings. Despite being hidden,
the files / directories etc. are fully accessible to clients who know what
names to actually use. Items will be hidden if their names contain the string
given by hide_file, or if they match the regular expression specified by
hide_file. Note that vsftpd's regular expression matching code is a simple
implementation which is a subset of full regular expression functionality.
See
deny_file
for details of exactly what regex syntax is supported.
Example: hide_file={*.mp3,.hidden,hide*,h?}
Default: (none)
listen_address
If vsftpd is in standalone mode, the default listen address (of all local
interfaces) may be overridden by this setting. Provide a numeric IP address.
Default: (none)
listen_address6
Like listen_address, but specifies a default listen address for the IPv6
listener (which is used if listen_ipv6 is set). Format is standard IPv6
address format.
Default: (none)
local_root
This option represents a directory which vsftpd will try to change into
after a local (i.e. non-anonymous) login. Failure is silently ignored.
Default: (none)
message_file
This option is the name of the file we look for when a new directory is
entered. The contents are displayed to the remote user. This option is
only relevant if the option
dirmessage_enable
is enabled.
Default: .message
nopriv_user
This is the name of the user that is used by vsftpd when it wants to be
totally unprivileged. Note that this should be a dedicated user, rather
than nobody. The user nobody tends to be used for rather a lot of important
things on most machines.
Default: nobody
pam_service_name
This string is the name of the PAM service vsftpd will use.
Default: vsftpd
pasv_address
Use this option to override the IP address that vsftpd will advertise in
response to the PASV command. Provide a numeric IP address, unless
pasv_addr_resolve
is enabled, in which case you can provide a hostname which will be DNS
resolved for you at startup.
Default: (none - the address is taken from the incoming connected socket)
rsa_cert_file
This option specifies the location of the RSA certificate to use for SSL
encrypted connections.
Default: /usr/share/ssl/certs/vsftpd.pem
rsa_private_key_file
This option specifies the location of the RSA private key to use for SSL
encrypted connections. If this option is not set, the private key is expected
to be in the same file as the certificate.
Default: (none)
secure_chroot_dir
This option should be the name of a directory which is empty. Also, the
directory should not be writable by the ftp user. This directory is used
as a secure chroot() jail at times vsftpd does not require filesystem access.
Default: /var/run/vsftpd
ssl_ciphers
This option can be used to select which SSL ciphers vsftpd will allow for
encrpyted SSL connections. See the
ciphers
man page for further details. Note that restricting ciphers can be a useful
security precaution as it prevents malicious remote parties forcing a cipher
which they have found problems with.
Default: DES-CBC3-SHA
user_config_dir
This powerful option allows the override of any config option specified in
the manual page, on a per-user basis. Usage is simple, and is best illustrated
with an example. If you set
user_config_dir
to be
/etc/vsftpd_user_conf
and then log on as the user "chris", then vsftpd will apply the settings in
the file
/etc/vsftpd_user_conf/chris
for the duration of the session. The format of this file is as detailed in
this manual page! PLEASE NOTE that not all settings are effective on a
per-user basis. For example, many settings only prior to the user's session
being started. Examples of settings which will not affect any behviour on
a per-user basis include listen_address, banner_file, max_per_ip, max_clients,
xferlog_file, etc.
Default: (none)
user_sub_token
This option is useful is conjunction with virtual users. It is used to
automatically generate a home directory for each virtual user, based on a
template. For example, if the home directory of the real user specified via
guest_username
is
/home/virtual/$USER,
and
user_sub_token
is set to
$USER,
then when virtual user fred logs in, he will end up (usually chroot()'ed) in
the directory
/home/virtual/fred.
This option also takes affect if
local_root
contains
user_sub_token.
Default: (none)
userlist_file
This option is the name of the file loaded when the
userlist_enable
option is active.
Default: /etc/vsftpd.user_list
vsftpd_log_file
This option is the name of the file to which we write the vsftpd style
log file. This log is only written if the option
xferlog_enable
is set, and
xferlog_std_format
is NOT set. Alternatively, it is written if you have set the option
dual_log_enable.
One further complication - if you have set
syslog_enable,
then this file is not written and output is sent to the system log instead.
Default: /var/log/vsftpd.log
xferlog_file
This option is the name of the file to which we write the wu-ftpd style
transfer log. The transfer log is only written if the option
xferlog_enable
is set, along with
xferlog_std_format.
Alternatively, it is written if you have set the option
dual_log_enable.