Last Change: 2018-09-07, 16:50
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clamav−milter − milter compatible mail scanner
clamav−milter [options] socket_address
Clamav−milter is a filter for sendmail(1) mail server. It uses a mail scanning engine built into clamd(8).
Clamav−milter can use load balancing and fault tolerant techniques to connect to more than one clamd(8) server and seamlessly hot−swap to even the load between different machines and to keep scanning for viruses even when a server goes down. When it is configured to use clamd on the the localhost, when the −−external flag (see below) is not given or LocalSocket in set in clamd.conf(5), clamav−milter verifies that it can communicate with clamd; if it cannot, it terminates.
clamav−milter supports tcpwrappers, the value for daemon_list is "clamav−milter".
The socket_address argument is the socket used to communicate with sendmail(8). It must agree with the entry in sendmail.cf or sendmail.mc. The file associated with the socket must be creatable by clamav−milter, if the User option is set in clamd.conf, then that user must have the rights to create the file.
−a FROM, −−from<=EMAIL>
Source email address of notices. The default is MAILER−DAEMON. If =EMAIL is not given, thus −−from, then the from address is set to the originating email address, however since it is likely that address is forged it must not be relied upon. −h, −−help Output the help information and exit.
−H, −−headers
Include all headers in the content of emails generated by clamav−milter. This is useful for system administrators who may want to look at headers to check if any of their machines are infected.
−V, −−version
Print the version number and exit.
−C DIR, −−chroot=DIR
Run in chroot jail DIR.
You will have to do a lot of fiddling if you want notifications to work, since clamav−milter calls sendmail(8) to handle the notifications and sendmail will run of out the same jail.
−c FILE, −−config−file=FILE
By default clamav−milter uses a default configuration file, this option allows you to specify another one.
−D, −−debug
Enables debugging.
−x n, −−debug−level=n
Set the debug level to n (where n from [0..9]) if clamav−milter was configured and compiled with −−clamav−debug enabled. Will be replaced by −−debug for compatibility with other programs in the suite.
−A, −−advisory
When in advisory mode, clamav−milter flags emails with viruses but still forwards them. The default option is to stop viruses. This mode is incompatible with −−quarantine and −−quarantine−dir.
−b, −−bounce
Send a failure message to the sender, and to the postmaster. [ Warning: most viruses and worms fake their source address, so this option is not recommended, and needs to be enabled at compile−time ]. See also −−noreject.
−B, −−broadcast[=<iface>]
When a virus is intercepted, broadcast a UDP message to the TCPSocket port set in clamd.conf. If the optional iface option is given, broadcasts will be sent on that interface. The default is set by the operating system, usually to the first NIC. A future network management program (yet to be written) will intercept these broadcasts to raise a warning on the operator’s desk.
−d, −−dont−scan−on−error
If a system error occurs pass messages through unscanned, usually when a system error occurs the milter raises a temporary failure which generally causes the message to remain in the queue.
−f, −−force−scan
Always scan, wherever the message came from (see also −−local and −−outgoing). You probably don’t want this.
−e, −−external
Usually clamav−milter scans the emails itself without the use of an external program. The −−external option informs clamav−milter to use an external program such as clamd(8) running either on the local server or other server(s) to perform the scanning.
−k, −−blacklist−time=time
Tells the number of seconds to black list an IP address (IPv4 only). This is especially useful with phishing which often send a number of emails one after the other.
Blacklisting speeds up scanning significantly, however it does have drawbacks since it is possible for a site to be incorrectly blacklisted because of DHCP or an unsafe smart−host. To avoid this, clamav−milter’s blacklist does not last for ever. The recommended value is 60.
Machines on the LAN, the local host, and machines that are our MX peers are never blacklisted.
K, −−dont−blacklist=IP[,IP...]
Instructs clamav−milter to refrain from blacklisting IP the given addresses. This is useful for sites that receive email from upstream servers that are either untrusted or have no virus. Without this option many false positives could occur. This scenario often happens when the upstream server belongs to an ISP that may not have AV software.
−l, −−local
Also scan messages sent from LAN. You probably want this especially if your LAN is populated by machines running Windows or DOS.
Machines with IP addresses within the ranges 192.168.0.0/16, 10.0.0.0/8, 172.16.0.0/12 and 169.254.0.0/16 are defined as ’local’. Messages from other machines are always scanned. Up to 8 extra ranges may be added with the −−ignore option.
−M, −−freshclam−monitor
When not running in external mode, this option tells clamav−milter how often to check that the virus database has been updated, probably by freshclam(1). The option takes one parameter, which is a number in seconds. The default is 300 seconds. The checking cannot be disabled, a value less than or equal to zero will be rejected.
−n, −−noxheader
Usually clamav−milter adds headings to messages that are scanned. The headers are of the form "X−Virus−Scanned: version", and "X−Virus−Status: clean/infected/not−scanned". This option instructs clamav−milter to refrain from adding this heading.
−N, −−noreject
When clamav−milter processes an e−mail which contains a virus it rejects the e−mail by using the SMTP code 550 or 554 depending on the state machine. This option causes clamav−milter to silently discard such messages. It is recommended that system administrators use this option when NOT using the −−bounce option.
−o, −−outgoing
Scan messages generated from this machine. You probably don’t need this.
−i, −−pidfile=FILE
Notifies clamav−milter to store its process ID in FILE. The file must be creatable by clamav−milter, if the User option is set in clamd.conf(5), then that user must have the rights to create the file.
−p, −−postmaster=EMAILADDRESS
Sets the e−mail address that receives notifications of viruses caught, when the −−quiet option is not given.
−P, −−postmaster−only
When the −−quiet option is not given, send a notification to the postmaster. Setting this flag will include the ID of the message in the email’s body which can ease searching through system logs if the administrator believes it is a locally sourced virus. Without this option, the intended recipient of the email will also receive a copy of the notification of the interception.
−q, −−quiet
Don’t send any notification messages when a virus or worm is detected. This option overrides the −−bounce and −−postmaster−only options, and is the way to turn off notification to the postmaster.
−Q, −−quarantine=EMAILADDRESS
If this e−mail address is given, messages containing a virus or worm are redirected to it.
−r, −−report−phish=EMAILADDRESS
Report caught phishing to an anti−phish organisation’s email address such as pirt_clamav AT castlecops DOT com and reportphishing AT antiphishing DOT org.
−R, −−report−phish−false−positives=EMAILADDRESS
Report phish false positves to an email address, such as bugs AT clamav DOT net.
−U, −−quarantine−dir=DIR
If this option is given, infected files are left in this directory. The directory must not be publicly readable or writable, if it is, clamav−milter will issue an error and fail to start. Note − this option only works when using LocalSocket.
−−server=HOSTNAME/ADDRESS, −s HOSTNAME/ADDRESS
IP address or hostname of server(s) running clamd (when using TCPsocket and −−external). More than one server may be specified, separating the server’s names by colons. If more than one server is specified, clamav−milter will load balance between the available servers. All the servers must be up when clamav−milter starts, however afterwards it is fault tolerant to a server becoming unavailable, and will only raise an error if all of the servers cannot be reached. The default value for ADDRESS is 127.0.0.1 (localhost).
−−sign, −S
Add a hard−coded signature to each scanned file. It is likely that this signature will only display on the end user’s terminal if the message is plain/text or not encoded.
−−signature−file, −F
Location of file to be appended to each scanned message. Overrides −S.
−−max−children=n, −m n
Set a hint of the maximum number of children. If the number is hit the maximum time a pending thread will be held up is set by −−timeout, so the number of threads can exceed this number for short periods of time. There is no default, if this argument is not clamav−milter will spawn as many children as is necessary up to the MaxThreads limit set in clamd.conf. When clamav−milter has been built with SESSION mode this argument is mandatory since it tells clamav−milter the number of sessions to keep open to clamd servers. When not built with in SESSION mode it is unlikely that you will need this unless your system is under great load. Note, however, that the default build is for SESSION to be disabled.
−−dont−wait
Tells clamav−milter what do to if the max−children number is exceeded. Usually clamav−milter waits until a child dies or the timeout value has been exceeded, which ever comes first, however with dont−wait enabled, clamav−milter will inform the remote SMTP client to retry later.
−−dont−sanitise
Allow semicolon and pipe characters in email addresses.
−−ignore net, −I net
net is taken to be an extra IPv4 or IPv6 network in prefix/length notation (for example 192.0.2.0/24 or 2001:db8::/32) which is treated as being on the LAN for the purposes of the −−local argument. Up to eight nets can be specified.
−−template−file=file −t file
File points to a file whose contents is sent as the warning message whenever a virus is intercepted. Occurrences of %v within the file is replaced with the message returned from clamd, which includes the name of the virus. Occurrences of %h are replaced with the message’s headers. The %v string can be escaped thus, \%v, to send the string %v. The % character can be escaped thus, %%, to send the % character. Any occurrence of strings in dollar signs are replaced with the appropriate sendmail−variable, e.g. ${if_addr}$. If the −t option is not given, clamav−milter defaults to a hard−coded message. Note that to send warning messages, clamav−milter must be able to execute sendmail.
−−template−headers=file
File points to a file whose contents are added to the headers of the warning message given to the −−template−file option. For example, to state the character set of the message, put "Content−Type: text/plain; charset=koi8−r" into the file.
−−timeout=n −T n
Used in conjunction with max−children. If clamav−milter waits for more than n seconds (default 300) it proceeds with scanning. Setting n to zero will turn off the timeout and clamav−milter will wait indefinitely for the scanning to quit. In practice the timeout set by sendmail will then take over.
−−detect−forged−local−address −L
When neither −−force, −−local nor −−outgoing is given, this option intercepts incoming mails that incorrectly claim to be from the local domain.
−−whitelist−file=FILE, −W file
This option specifies a file which contains a list of e−mail addresses. E−mails sent to or from these addresses will NOT be checked. While this is not an Anti−Virus function, it is quite useful for some systems. The address given to the −−quarantine directive is always whitelisted.
The file consists of a list of addresses, each address on a line enclosed in angle brackets (e.g. <foo AT bar DOT com>). Optionally each line can start with the string To: or From: indicating if it is the sender or recipient that is to be whitelisted. If the field is missing, the default is To. Lines starting with #, : or ! are ignored.
−−sendmail−cf=FILE
When starting, clamav−milter runs some sanity checks against the sendmail.cf file, usually in /etc/sendmail.cf or /etc/mail/sendmail.cf. This directive tells clamav−milter where to find the sendmail.cf file.
−−black−hole−mode
Since sendmail calls its milters before it looks in its alias and virtuser tables, clamav−milter can spend time looking for malware that’s going to be thrown away even if the message is clean.
Enabling this stops these messages from being scanned (in practice clamav−milter will discard these messages so the message doesn’t go further down the milter call chain). Only enable this if your site has many addresses aliased to /dev/null.
To enable this mode clamav−milter must have certain sendmail rights: it needs to run as a TrustedUser as defined by sendmail (see http://www.sendmail.org/m4/tweaking_config.html) by the use of the User directive in clamd.conf, the clamav user must be able read the mail queue (often /var/spool/mqueue), and AllowSupplementaryGroups must be enabled in clamd.conf. Some operating systems set /var/spool/mqueue to be mode 700 forcing you to run clamav−milter as root for black−hole−mode. This is always unadvisable, it is better to have /var/spool/mqueue as mode 750.
There is no support for IPv6.
clamav−milter −o local:/var/run/clamav/clmilter.sock
Nigel Horne <njh AT bandsman DOT co DOT uk>
clamd(8), clamscan(1), freshclam(1), sigtool(1), clamd.conf(5), hosts_access(5), sendmail(8)