Section: User Commands (1)
Probe the TLS properties of an ESMTP or LMTP server.
posttls-finger [options] [inet:]domain[:port] [match ...]
posttls-finger -S [options] unix:pathname [match ...]
(1) connects to the specified destination
and reports TLS-related information about the server. With SMTP, the
destination is a domainname; with LMTP it is either a domainname
prefixed with inet:
or a pathname prefixed with unix:
Postfix is built without TLS support, the resulting posttls-finger
program has very limited functionality, and only the -a
Note: this is an unsupported test program. No attempt is made
to maintain compatibility between successive versions.
For SMTP servers that don't support ESMTP, only the greeting banner
and the negative EHLO response are reported. Otherwise, the reported
EHLO response details further server capabilities.
If TLS support is enabled when posttls-finger(1) is compiled, and
the server supports STARTTLS, a TLS handshake is attempted.
If DNSSEC support is available, the connection TLS security level
(-l option) defaults to dane; see TLS_README for
details. Otherwise, it defaults to secure. This setting
determines the certificate matching policy.
If TLS negotiation succeeds, the TLS protocol and cipher details are
reported. The server certificate is then verified in accordance with
the policy at the chosen (or default) security level. With public
CA-based trust, when the -L option includes certmatch,
(true by default) name matching is performed even if the certificate
chain is not trusted. This logs the names found in the remote SMTP
server certificate and which if any would match, were the certificate
Note: posttls-finger(1) does not perform any table lookups, so
the TLS policy table and obsolete per-site tables are not consulted.
It does not communicate with the tlsmgr(8) daemon (or any other
Postfix daemons); its TLS session cache is held in private memory, and
disappears when the process exits.
With the -r delay option, if the server assigns a TLS
session id, the TLS session is cached. The connection is then closed
and re-opened after the specified delay, and posttls-finger(1)
then reports whether the cached TLS session was re-used.
When the destination is a load balancer, it may be distributing
load between multiple server caches. Typically, each server returns
its unique name in its EHLO response. If, upon reconnecting with
-r, a new server name is detected, another session is cached
for the new server, and the reconnect is repeated up to a maximum
number of times (default 5) that can be specified via the -m
The choice of SMTP or LMTP (-S option) determines the syntax of
the destination argument. With SMTP, one can specify a service on a
non-default port as host:service, and disable MX (mail
exchanger) DNS lookups with [host] or [host]:port.
The  form is required when you specify an IP address instead of a
hostname. An IPv6 address takes the form [ipv6:address].
The default port for SMTP is taken from the smtp/tcp entry in
/etc/services, defaulting to 25 if the entry is not found.
With LMTP, specify unix:pathname to connect to a local server
listening on a unix-domain socket bound to the specified pathname;
otherwise, specify an optional inet: prefix followed by a
domain and an optional port, with the same syntax as for
SMTP. The default TCP port for LMTP is 24.
- -a family (default: any)
Address family preference: ipv4, ipv6 or any. When
using any, posttls-finger will randomly select one of the two as
the more preferred, and exhaust all MX preferences for the first
address family before trying any addresses for the other.
- -A trust-anchor.pem (default: none)
A list of PEM trust-anchor files that overrides CAfile and CApath
trust chain verification. Specify the option multiple times to
specify multiple files. See the main.cf documentation for
smtp_tls_trust_anchor_file for details.
Disable SMTP chat logging; only TLS-related information is logged.
Print the remote SMTP server certificate trust chain in PEM format.
The issuer DN, subject DN, certificate and public key fingerprints
(see -d mdalg option below) are printed above each PEM
certificate block. If you specify -F CAfile or
-P CApath, the OpenSSL library may augment the chain with
missing issuer certificates. To see the actual chain sent by the
remote SMTP server leave CAfile and CApath unset.
- -d mdalg (default: sha1)
The message digest algorithm to use for reporting remote SMTP server
fingerprints and matching against user provided certificate
fingerprints (with DANE TLSA records the algorithm is specified
in the DNS).
Lookup the associated DANE TLSA RRset even when a hostname is not an
alias and its address records lie in an unsigned zone. See
smtp_tls_force_insecure_host_tlsa_lookup for details.
- -F CAfile.pem (default: none)
The PEM formatted CAfile for remote SMTP server certificate
verification. By default no CAfile is used and no public CAs
- -g grade (default: medium)
The minimum TLS cipher grade used by posttls-finger. See
smtp_tls_mandatory_ciphers for details.
- -h host_lookup (default: dns)
The hostname lookup methods used for the connection. See the
documentation of smtp_host_lookup for syntax and semantics.
- -H chainfiles (default: none)
List of files with a sequence PEM-encoded TLS client certificate
chains. The list can be built-up incrementally, by specifying
the option multiple times, or all at once via a comma or
whitespace separated list of filenames. Each chain starts with
a private key, which is followed immediately by the
corresponding certificate, and optionally by additional issuer
certificates. Each new key begins a new chain for the
corresponding algorithm. This option is mutually exclusive with
the below -k and -K options.
- -k certfile (default: keyfile)
File with PEM-encoded TLS client certificate chain. This
defaults to keyfile if one is specified.
- -K keyfile (default: certfile)
File with PEM-encoded TLS client private key.
This defaults to certfile if one is specified.
- -l level (default: dane or secure)
The security level for the connection, default dane or
secure depending on whether DNSSEC is available. For syntax
and semantics, see the documentation of smtp_tls_security_level.
When dane or dane-only is supported and selected, if no
TLSA records are found, or all the records found are unusable, the
secure level will be used instead. The fingerprint
security level allows you to test certificate or public-key
fingerprint matches before you deploy them in the policy table.
Note, since posttls-finger does not actually deliver any email,
the none, may and encrypt security levels are not
very useful. Since may and encrypt don't require peer
certificates, they will often negotiate anonymous TLS ciphersuites,
so you won't learn much about the remote SMTP server's certificates
at these levels if it also supports anonymous TLS (though you may
learn that the server supports anonymous TLS).
- -L logopts (default: routine,certmatch)
Fine-grained TLS logging options. To tune the TLS features logged
during the TLS handshake, specify one or more of:
- 0, none
These yield no TLS logging; you'll generally want more, but this
is handy if you just want the trust chain:
$ posttls-finger -cC -L none destination
- 1, routine, summary
These synonymous values yield a normal one-line summary of the TLS
- 2, debug
These synonymous values combine routine, ssl-debug, cache and verbose.
- 3, ssl-expert
These synonymous values combine debug with ssl-handshake-packet-dump.
For experts only.
- 4, ssl-developer
These synonymous values combine ssl-expert with ssl-session-packet-dump.
For experts only, and in most cases, use wireshark instead.
Turn on OpenSSL logging of the progress of the SSL handshake.
Log hexadecimal packet dumps of the SSL handshake; for experts only.
Log hexadecimal packet dumps of the entire SSL session; only useful
to those who can debug SSL protocol problems from hex dumps.
Logs trust chain verification problems. This is turned on
automatically at security levels that use peer names signed
by Certification Authorities to validate certificates. So while
this setting is recognized, you should never need to set it
This logs a one line summary of the remote SMTP server certificate
subject, issuer, and fingerprints.
This logs remote SMTP server certificate matching, showing the CN
and each subjectAltName and which name matched. With DANE, logs
matching of TLSA record trust-anchor and end-entity certificates.
This logs session cache operations, showing whether session caching
is effective with the remote SMTP server. Automatically used when
reconnecting with the -r option; rarely needs to be set
Enables verbose logging in the Postfix TLS driver; includes all of
peercert..cache and more.
The default is routine,certmatch. After a reconnect,
peercert, certmatch and verbose are automatically
disabled while cache and summary are enabled.
- -m count (default: 5)
When the -r delay option is specified, the -m option
determines the maximum number of reconnect attempts to use with
a server behind a load balancer, to see whether connection caching
is likely to be effective for this destination. Some MTAs
don't expose the underlying server identity in their EHLO
response; with these servers there will never be more than
1 reconnection attempt.
- -M insecure_mx_policy (default: dane)
The TLS policy for MX hosts with "secure" TLSA records when the
nexthop destination security level is dane, but the MX
record was found via an "insecure" MX lookup. See the main.cf
documentation for smtp_tls_insecure_mx_policy for details.
- -o name=value
Specify zero or more times to override the value of the main.cf
parameter name with value. Possible use-cases include
overriding the values of TLS library parameters, or "myhostname" to
configure the SMTP EHLO name sent to the remote server.
- -p protocols (default: !SSLv2)
List of TLS protocols that posttls-finger will exclude or include. See
smtp_tls_mandatory_protocols for details.
- -P CApath/ (default: none)
The OpenSSL CApath/ directory (indexed via c_rehash(1)) for remote
SMTP server certificate verification. By default no CApath is used
and no public CAs are trusted.
- -r delay
With a cacheable TLS session, disconnect and reconnect after delay
seconds. Report whether the session is re-used. Retry if a new server
is encountered, up to 5 times or as specified with the -m option.
By default reconnection is disabled, specify a positive delay to
enable this behavior.
- -s servername
The server name to send with the TLS Server Name Indication (SNI)
extension. When the server has DANE TLSA records, this parameter
is ignored and the TLSA base domain is used instead. Otherwise, SNI is
not used by default, but can be enabled by specifying the desired value
with this option.
Disable SMTP; that is, connect to an LMTP server. The default port for
LMTP over TCP is 24. Alternative ports can specified by appending
":servicename" or ":portnumber" to the destination
- -t timeout (default: 30)
The TCP connection timeout to use. This is also the timeout for
reading the remote server's 220 banner.
- -T timeout (default: 30)
The SMTP/LMTP command timeout for EHLO/LHLO, STARTTLS and QUIT.
Enable verbose Postfix logging. Specify more than once to increase
the level of verbose logging.
Enable outgoing TLS wrapper mode, or SMTPS support. This is typically
provided on port 465 by servers that are compatible with the ad-hoc
SMTP in SSL protocol, rather than the standard STARTTLS protocol.
The destination domain:port should of course provide such
Enable tlsproxy(8) mode. This is an unsupported mode,
for program development only.
Connect via TCP to domain domain, port port. The default
port is smtp (or 24 with LMTP). With SMTP an MX lookup is
performed to resolve the domain to a host, unless the domain is
enclosed in . If you want to connect to a specific MX host,
for instance mx1.example.com, specify [mx1.example.com]
as the destination and example.com as a match argument.
When using DNS, the destination domain is assumed fully qualified
and no default domain or search suffixes are applied; you must use
fully-qualified names or also enable native host lookups
(these don't support dane or dane-only as no DNSSEC
validation information is available via native lookups).
Connect to the UNIX-domain socket at pathname. LMTP only.
- match ...
With no match arguments specified, certificate peername matching uses
the compiled-in default strategies for each security level. If you
specify one or more arguments, these will be used as the list of
certificate or public-key digests to match for the fingerprint
level, or as the list of DNS names to match in the certificate at the
verify and secure levels. If the security level is
dane, or dane-only the match names are ignored, and
hostname, nexthop strategies are used.
Read configuration parameters from a non-default location.
Same as -v option.
smtp-source(1), SMTP/LMTP message source
smtp-sink(1), SMTP/LMTP message dump
Use "postconf readme_directory
" or "postconf
" to locate this information.
TLS_README, Postfix STARTTLS howto
The Secure Mailer license must be distributed with this software.
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
111 8th Avenue
New York, NY 10011, USA