path: root/man/qmail-dksign.9

.TH s/qmail: qmail-dksign 8
.SH "NAME"
qmail-dksign \- DKIM sign outgoing messages
.SH "SYNOPSIS"
.B qmail-dksign 
.I host
.I sender
.I recip
[
.I recip ...
]
.SH "DESCRIPTION"
.B qmail-dksign
is a stub routine to be invoked by 
.B qmail-spawn
in place of
.B qmail-remote
and is required to customize the signing policy
for outgoing emails according to RFC 6893/8463 by means of 
.B qmail-dkim
and finally to invoke
.B qmail-remote
for subsequent message delivery.

.B qmail-dksign
is also an extension to
.B qmail-queue
(with comparable permissions) using
.I queue/dkim/<n>/<m>
to provide a temporary but persistent staging
area for outgoing messages to be DKIM signed.
.SH "CONTROL FILE"
.B qmail-dksign 
will be only called by 
.B qmail-rspawn 
if 
.I SQMAIL/control/dkimdomains
is present.

.IR dkimdomains :
\'domain:selector[,selector2]|sdid|[auid|~]|expire|c:z:l\'
allows multitenant and hybrid DKIM signing settings per sending
.IR domain .

.I domain 
is the sender's envelope domain in order to fetch the 
individually tailored DKIM signing paramaters for these.

The following DKIM parameters can be specified:
.TP 5
.I selector
is used as prepending name label for 
.IR domain :
.IR selector._domainkey.domain .
If not explicitely given, it defaults to
.I default
and is mostly used to support the key roll-over.
.TP 5
.I selector,selector2
defines a hybrid selector and allows to provide
two different selectors together
with their private keys for concurrently signing of messages 
according to both the RSA-SHA256 and the Ed25519 algorithm.
.TP 5
.I sdid 
Here, you can overwrite the 'Signing Domain Identifier' (SDID),
thus decouple the information given in the DKIM header from
the envelope domain sender. This allows to setup common DNS
public keys for several domains irrespectively of the sending
.IR domain .
.TP 5
.I auid
is the 'Agent/User Identifier' of the signer, 
in case it is not the sending
.IR domain .
In most cases it can be neglected and is obsolete.
Rather, you can specifiy that the 
.I auid 
is always included as
.I originator 
of the mail while providing the tilde symbol
.I ~ 
here as generic substitude.
.TP 5
.I expire
determins the validity period of the signature in DKIM signed
message. Due to the assumed key-rollover, it is limited 
and defaults to 
.I 604800
secs since the email was signed.
.TP 5
.I c
is the 'canonicalization'; thus how a validation client
should deal with signature verification of the received
message header and/or body. Here, the choices are
.I r
relax (allow mangling of whitespaces and cases; default)
.I s
simple (=strict)
.I t
relax on header, simple on body,
.I u
simple on header, relax on body.
.TP 5
.I z
The signature algorithm can be specified as
.I 1
RSA with sha1, 
.I 2
RSA with sha256 (as default), or
.I 3
providing both signature values in the mail header;
.I 4
Ed25519 ECC signatures.
.I 5
tells
.B qmail-dksign
to include both 
.I RSA-SHA256
and 
.I Ed25519
signatures in the mail header.
Here, you need two different 
.I selectors
and 
.IR private\ keys.
Finally, setting 
.TP 5
.I l
(literal) advices 
.I qmail-dkim
to include the body hash length (after canonicalization)
to the DKIM header. This might be useful to cope with programs
like mailing list servers adding a 'footer' to the mail 
after the signing operation has been completed.

.RE
RSA and Ed25519 signatures can now be used simultaneously
while providing different keys available as distinct selectors.
Those settings are handed-over to
.B qmail-dkim 
to provide the signing of emails.
.B qmail-dksign
calls 
.B qmail-dkim 
to automatically include the query method
.I q=dns/txt 
in the DKIM header.
.SH "SELECTING DOMAINS FOR SIGNING"
.B qmail-dksign
can be instructed to sign all outgoing mails with the
MTA's private key. This is achieved by simply using
.I *:
in 
.IR control/dkimdomains .
Rather, the signing operation can be restricted for domains
.B s/qmail 
has responsibility for, as given in 
.IR rcpthosts .
This is commanded via
.IR =: .
Alternatively, in multitenant mode
.B qmail-dksign
may use domain specific DKIM settings and private keys 
for the sending domains and permitting parenting.
Particular domains for which outgoing emails shall
not be DKIM signed can be given as:
.IR !nodkim.org .

.EE
 *:
 =:default,eddy||~||:5
 .heaven.com:||me@devil.com|500000|r:3
 cloud1.com:january|postmaster@cloud.com|||t::l
 cloud7.com:february|postmaster@cloud.com|||u:1
 mybuddy.org:eddy||||:4
 !nodkim.org:
.EX

Note: The owner of the crypto material (public and private keys) is 
.IR qmailq .
.SH "CRYPTO MATERIAL"
.B qmail-dksign
follows the conventions from 
.B qmail-remote
to use the directory
.I SQMAIL/ssl/domainkeys
to store public and private keys.

Each 
.I domain
may have its own key material resulting in a structure
.IR SQMAIL/ssl/domainkeys/<domain>/ ,
where the following keyfiles are expected:
.TP 5
.IR <selector>\ (default:\ 'default') 
is a mandatory symbolic link to
.I [rsa|ed25519].private_<selector>
used for signing.
.TP 5
.I rsa.public_<selector>
is the DER-header enriched and base64 encoded RSA public key.
.TP 5
.I ed25519.public_<selector>
is the 'naked' base64 encoded Ed25519 public key.

.RE
Here,
.I <selector>
is the name of the current 
.IR selector .
After having generated keys and providing a new 
.IR selector , 
this name has to be included as 
.I selector
for the given domain in
.I SQMAIL/control/dkimdomains
in order to become active for signing.

In case of 
.I hybrid\ signatures
different selectors need to be given for the
RSA and the Ed25519 keys each. 
They have to be provided concatinated by a colon in 
.IR dkimdomains .
White spaces are not allowed. If the RSA selector is
.IR default ,
it can be omitted while followed by the colon and the 
Ed25519 selector name.

.SH "SHARING KEYS FOR DIFFERENT DOMAINS"
Different 
.I domains
may however share common keys for signing and verification.
In order to allow a common private key for signing, simply
create symlinks for the others domains under
.I SQMAIL/ssl/domainkeys/ 
to the master one.
.B qmail-dksign
will now pick up those and use the provided key for signing.

However, in general this reqires to deploy DKIM records
for those domains sharing the same public key but require
different domain names as distinguished DNS TXT records.

Rather, you may want to publish just one
DKIM DNS TXT record which is commonly shared for all 
concerning domains. Since the 
.I sending\ domain
is used as default for the
.IR SDID ,
you need now to provide the same
.I SDID
explicitely for each domain of concern in
.IR control/dkimdomains .

The '<selector>' - and not the SDID - 
together with the literal
.I ._domainkey.
and the domain name defines the binding of the
private key with the DKIM TXT record:
.IR <selector>._domainkey.<domain> .

.SH "GNERATING CRYPTO MATERIAL"
Public/private keys can be generated by
.I OpenSSL
or 
.I LibreSSL
or compatible TLS implementations and
shall be provided in canonical format.
The directory
.I SQMAIL/ssl/domainkeys/
and the resulting key needs to be readable by
.IR qmailq ,
the user 
.B qmail-dksign 
and 
.B qmail-dkim 
runs under. The private key shall
.B NEVER
exposed to the public. 

The script
.B mkdkimkey 
is enabled to generate 
.I RSA 
or
.I Ed25519
private and public keys in the required format
together with a 
.I BIND
compliant DKIM DNS TXT record.
.SH "RESPONSES"
.B qmail-dksign
may provide the following responses indicating an error:
.TP 5
Z
Unable to switch to target directory.
.TP 5
Z
Unable to create DKIM stage file: <file>
.TP 5
Z
Unable to unlink DKIM stage file.
.TP 5
Z
Unable to read control files.
.TP 5
Z
Unable to read message.
.TP 5
D
SMTP cannot transfer messages with partial final lines.
.TP 5
K
can't read private file: <file> continue without signing.
.TP 5
Z
unable to run qmail-remote. (=> configuration/permission error)
.SH "SYSTEM IMPACT"
.B qmail-dksign
makes heavy use of system file descriptors. 
Given a high
.I concurrencyremote
you may run out of file descriptors which thus need to be enhanced
either system-wide or for the specific users
.I qmailr
and 
.IR qmails .
.SH "SEE ALSO"
qmail-queue(8),
qmail-remote(8),
qmail-dkim(8),
qmail-dkverify(8),
qmail-log(8).