Updated: 2022/Sep/29
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POSTTLS-FINGER(1) General Commands Manual POSTTLS-FINGER(1)
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.
-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.
SEE ALSO
smtp-source(1), SMTP/LMTP message source
smtp-sink(1), SMTP/LMTP message dump
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)