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More for MTA-STS for incoming mail

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* Create the mta_sts A/AAAA records even if there is no valid TLS certificate because we can't get a TLS certificate if we don't set up the domains.
* Make the policy id in the TXT record stable by using a hash of the policy file so that the DNS record doesn't change every day, which means no nightly notification and also it allows for longer caching by sending MTAs.
This commit is contained in:
Joshua Tauberer 2020-05-30 08:04:07 -04:00
parent 37dad9d4bb
commit 3a4b8da8fd
1 changed files with 23 additions and 13 deletions
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28
management/dns_update.py
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@ -308,17 +308,23 @@ def build_zone(domain, all_domains, additional_records, www_redirect_domains, en
# this domain name, then the domain name is a MTA-STS (https://tools.ietf.org/html/rfc8461)
# Policy Domain.
#
# A "_mta-sts" TXT record signals the presence of a MTA-STS policy, and an effectively random policy
# ID is used to signal that a new policy may (or may not) be deployed any time the DNS is
# updated.
# A "_mta-sts" TXT record signals the presence of a MTA-STS policy. The id field helps clients
# cache the policy. It should be stable so we don't update DNS unnecessarily but change when
# the policy changes. It must be at most 32 letters and numbers, so we compute a hash of the
# policy file.
#
# The policy itself is served at the "mta-sts" (no underscore) subdomain over HTTPS. Therefore
# the TLS certificate used by Postfix for STARTTLS must be a valid certificate for the MX
# domain name (PRIMARY_HOSTNAME) *and* the TLS certificate used by nginx for HTTPS on the mta-sts
# subdomain must be valid certificate for that domain. Do not set an MTA-STS policy if either
# certificate in use is not valid (e.g. because it is self-signed and a valid certificate has not
# yet been provisioned).
# yet been provisioned). Since we cannot provision a certificate without A/AAAA records, we
# always set them --- only the TXT records depend on there being valid certificates.
mta_sts_enabled = False
mta_sts_records = [
("mta-sts", "A", env["PUBLIC_IP"], "Optional. MTA-STS Policy Host serving /.well-known/mta-sts.txt."),
("mta-sts", "AAAA", env.get('PUBLIC_IPV6'), "Optional. MTA-STS Policy Host serving /.well-known/mta-sts.txt."),
]
if domain in get_mail_domains(env):
# Check that PRIMARY_HOSTNAME and the mta_sts domain both have valid certificates.
for d in (env['PRIMARY_HOSTNAME'], "mta-sts." + domain):
@ -332,11 +338,15 @@ def build_zone(domain, all_domains, additional_records, www_redirect_domains, en
# 'break' was not encountered above, so both domains are good
mta_sts_enabled = True
if mta_sts_enabled:
mta_sts_records = [
("mta-sts", "A", env["PUBLIC_IP"], "Optional. MTA-STS Policy Host serving /.well-known/mta-sts.txt."),
("mta-sts", "AAAA", env.get('PUBLIC_IPV6'), "Optional. MTA-STS Policy Host serving /.well-known/mta-sts.txt."),
("_mta-sts", "TXT", "v=STSv1; id=%sZ" % datetime.datetime.now().strftime("%Y%m%d%H%M%S"), "Optional. Part of the MTA-STS policy for incoming mail. If set, a MTA-STS policy must also be published.")
]
# Compute a up-to-32-character hash of the policy file. We'll take a SHA-1 hash of the policy
# file (20 bytes) and encode it as base-64 (60 bytes) but then just take its first 20 bytes
# which should be sufficient to change whenever the policy file changes.
with open("/var/lib/mailinabox/mta-sts.txt", "rb") as f:
mta_sts_policy_id = base64.b64encode(hashlib.sha1(f.read()).digest()).decode("ascii")[0:20]
mta_sts_records.extend([
("_mta-sts", "TXT", "v=STSv1; id=" + mta_sts_policy_id, "Optional. Part of the MTA-STS policy for incoming mail. If set, a MTA-STS policy must also be published.")
])
# Rules can be custom configured accoring to https://tools.ietf.org/html/rfc8460.
# Skip if the rules below if the user has set a custom _smtp._tls record.
if not has_rec("_smtp._tls", "TXT", prefix="v=TLSRPTv1;"):