.TH s/qmail: qmail-dkim 8
.SH "NAME"
qmail-dkim \- libdkim implementation for s/qmail
.SH "SYNOPSIS"
.B qmail-dkim
[
.I -h
.I -v
.I -V
.I -s[ecckey]
.I -b[1|2|3]
.I -c[s|t|u]
.I -d domain
.I -i identity
.I -l 
.I -q
.I -t
.I -x expire_time
.I -y selector
.I -Y selector2
.I -z[1|2|3|4|5]
]
.I in_message
.I RSA_private_key
.I out_message
.I Ed25519_private_key
.SH "DESCRIPTION"
.B qmail-dkim
is the implementation of
.B libdkim
for s/qmail providing API compatibility 
and supporting RSA and Ed25519 DKIM signatures
in single or hybrid mode.
In hybrid mode, two 
.I private keys
and two
.I selectors 
need to be provided.
.B qmail-dkim
supports distinct operations:
.TP 5
.B qmail-dkim \fI-s in_message RSA_private_key out_message\fR
DKIM signes 
.I in_message
with the given
.I private_key 
and returns
.IR out_message .
.TP 5 
.B qmail-dkim \fI-s in_message RSA_private_key out_message Ed255_private_key\fR
signs 
.I in_message 
with both a RSA 
.I RSA_private_key 
and a
.IR Ed25519_private_key.
Here, the RSA default selector is \fIdefault\fR and the 
Ed25519 default selector is \fIeddy\fR; both subject of change.
.TP 5
.B qmail-dkim  \fI-v in_message\fR
verifies the 
.IR in_message .
.SH "DKIM FORMATS"
DKIM needs a common understanding of the attributes
subject for signing and verification. 
The following attributes can be set:
.TP 5
-c
is the 'canonicalization', thus how a validiation client
should deal with signature verification of the 
message headers and/or body. Here, the choices are given 
via an appended character:
.I r
relax on header,
.I s
simple (strict) on message body,
.I t
relax/simple, or eventually
.I u
simple relaxed.
Finally, the hash function to be used in the signature
can be given as
.TP 5
-z
following either with
.I 1
using sha1, or 
.I 2
using sha256, or finally as default
.I 3
providing both signature values in the mail header.
.I 4
telling 
.B qmail-dkim
to use the Ed25519 signature scheme.
.I 5
allows
.B qmail-dkim 
to attach both a 
.I RSA-SHA256
as well as a
.I Ed25519
signature to the message, which considered to be a
.I hybrid
mode.

.SH "DKIM SIGNING"
.B qmail-dkim
will include (several) message headers detailing the
.B DKIM signature
with at least the following fields:
.TP 3
a
=<signature type>
.TP 3
c
=<used canoncicalization>
.TP 3
s
=<selector>
.TP 3
d
=<identity>
.TP 3
i
=<identifier>
.TP 3
h
=<included header1:header2:...>
.TP 3
bh
=<hash of the canonicalized body until its upper limit length; if given>
.TP 3
b
=<base64 encoded signature>
.P
Additional settings can be achieved using the following options:
.TP 5
.I -d domain
is the signer's domain name and together with the prepended
.TP 5
.I -y selector
it is used for the DNS TXT lookup of the public key; supporting
mainly key roll-over. The first selector is used for RSA signatures.
.TP 5
.I -Y selector2
Same as \fI-y\fR but now for Ed25519 signatures.
.TP 5
.I -I identifier
giving an additional hint about the agent or identifier
responsible for the signing like 'postmaster@domain'; defaults to 
.IR domain .
.TP 5
.I -t expire_time
given in seconds, tells how log the signature is valid. 
It defaults to 
.I 604800
secconds (seven days).
.P
Further, some more option fields can be displayed in the header:
.TP 5
.I -l
include a body length tag.
.TP 5
.I -q
include the query method tag.

.SH "DKIM VERIFICATION"
.B qmail-dkim
as invoked by
.B qmail-dkverify
extracting the received DKIM header fields,
and following the signature verification procedure
as given here, while fetching the signer's
.I public key
using a DNS TXT lookup.
Now, the respective header lines, and/or
the message body will be hashed and compared
against the values taken from the signatures.

The results will be indicated by either return code
.I 0 
in case of success, 
.I 1
in case of mismatch, or
.I -1
if other failures were encountered.

Given the call argument
.TP 3
-v 
.B qmail-dkim
will provide the DKIM results
.I pass
or
.I fail
including verbose reasons on the commmand line. 
This is the legacy mode.

.RE
Rather, invoking
.B qmail-dkim
with argument
.TP 3
-V
it communicates the results over a file interface
to be picked up by 
.IR qmail-dkverify .

.SH "SEE ALSO"
qmail-queue(8),
qmail-remote(8),
qmail-dksign(8),
qmail-dkverify(8),
qmail-send(8),
qmail-log(8).