Ubuntu Manpage: posttls-finger - Probe the TLS properties of an ESMTP or LMTP server.

Provided by: postfix_3.8.6-1build2_amd64

NAME

       posttls-finger - Probe the TLS properties of an ESMTP or LMTP server.

SYNOPSIS

       posttls-finger [options] [inet:]domain[:port] [match ...]
       posttls-finger -S [options] unix:pathname [match ...]

DESCRIPTION

posttls-finger(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:. If Postfix is built without TLS support, the resulting
posttls-finger(1) program has very limited functionality, and only the -a, -c, -h, -o, -S, -t, -T and -v
options are available.

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 chain trusted.

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 option.

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.

Arguments:

-a family (default: any)
Address family preference: ipv4, ipv6 or any. When using any, posttls-finger(1) 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.

-c Disable SMTP chat logging; only TLS-related information is logged.

-C 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: $smtp_tls_fingerprint_digest)
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). In Postfix versions prior to 3.6, the default value was "md5".

-f 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 are trusted.

-g grade (default: medium)
The minimum TLS cipher grade used by posttls-finger(1). 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(1) 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 connection.

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.

ssl-debug
Turn on OpenSSL logging of the progress of the SSL handshake.

ssl-handshake-packet-dump
Log hexadecimal packet dumps of the SSL handshake; for experts only.

ssl-session-packet-dump
Log hexadecimal packet dumps of the entire SSL session; only useful to those who can debug
SSL protocol problems from hex dumps.

untrusted
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 explicitly.

peercert
This logs a one line summary of the remote SMTP server certificate subject, issuer, and
fingerprints.

certmatch
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.

cache 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 explicitly.

verbose
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_dane_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: >=TLSv1)
TLS protocols that posttls-finger(1) 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.

-R Use SRV lookup instead of MX.

-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.

-S 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
argument.

-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.

-v Enable verbose Postfix logging. Specify more than once to increase the level of verbose logging.

-w Enable outgoing TLS wrapper mode, or SUBMISSIONS/SMTPS support. This is typically provided on
port 465 by servers that are compatible with the SMTP-in-SSL protocol, rather than the STARTTLS
protocol. The destination domain:port must of course provide such a service.

-X Enable tlsproxy(8) mode. This is an unsupported mode, for program development only.

[inet:]domain[:port]
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).

unix:pathname
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.

ENVIRONMENT

       MAIL_CONFIG
              Read configuration parameters from a non-default location.

       MAIL_VERBOSE
              Same as -v option.

README FILES

       Use "postconf readme_directory" or "postconf html_directory" to locate this information.
       TLS_README, Postfix STARTTLS howto

LICENSE

       The Secure Mailer license must be distributed with this software.

AUTHOR(S)

       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA

       Wietse Venema
       Google, Inc.
       111 8th Avenue
       New York, NY 10011, USA

       Viktor Dukhovni

                                                                                               POSTTLS-FINGER(1)