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feat!: DKIM verifier (#35)

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* feat!: DKIM verifier

This implements a DKIM verification as well as a strict
DKIM signature alignment check with domain in `From`
header address.

Caches the retrieved RDATA using in-memory LRU.

BREAKING CHANGE: incoming messages now require DKIM signatures
  aligned to domain of the `From` header address.

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

* chore(license): License binaries under GPLv3

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

---------

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>
This commit is contained in:
Jagoda Estera Ślązak
2026-02-16 09:50:55 +01:00
committed by GitHub
parent 3bf8395ef6
commit a096d0550f
12 changed files with 2072 additions and 40 deletions
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filtermail/Cargo.lock
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filtermail/Cargo.toml
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@@ -19,6 +19,10 @@ mailparse = "0.16.1"
log = "0.4.29"
env_logger = "0.11.9"
governor = "0.10.4"
viadkim = { version = "0.2.0" }
hickory-resolver = "0.25.2"
lru = "0.16.3"
parking_lot = "0.12.5"
[dev-dependencies]
rstest = "0.26.1"
filtermail/LICENSE-GPL
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@@ -0,0 +1,674 @@
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(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:
<program> Copyright (C) <year> <name of author>
This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate
parts of the General Public License. Of course, your program's commands
might be different; for a GUI interface, you would use an "about box".
You should also get your employer (if you work as a programmer) or school,
if any, to sign a "copyright disclaimer" for the program, if necessary.
For more information on this, and how to apply and follow the GNU GPL, see
<https://www.gnu.org/licenses/>.
The GNU General Public License does not permit incorporating your program
into proprietary programs. If your program is a subroutine library, you
may consider it more useful to permit linking proprietary applications with
the library. If this is what you want to do, use the GNU Lesser General
Public License instead of this License. But first, please read
<https://www.gnu.org/licenses/why-not-lgpl.html>.
filtermail/README.md
+6
View File
@@ -7,3 +7,9 @@ Rust drop-in reimplementation of chatmaild's filtermail.
```plain
filtermail <config> (incoming|outgoing)
```
## License
Code licensed under [MIT](LICENSE).
Binaries distributed under [GPL-3.0-or-later](LICENSE).
filtermail/src/config.rs
+1 -1
View File
@@ -25,7 +25,7 @@ pub struct Config {
pub passthrough_senders: Vec<String>,
#[serde(default, deserialize_with = "deserialize_sequence")]
pub passthrough_recipients: Vec<String>,
mail_domain: String,
pub mail_domain: String,
mailboxes_dir: Option<PathBuf>,
}
filtermail/src/dkim_verifier.rs
+221
View File
@@ -0,0 +1,221 @@
use crate::utils::get_domain_from_address;
use hickory_resolver::name_server::TokioConnectionProvider;
use hickory_resolver::{Name, TokioResolver};
use lru::LruCache;
use std::io;
use std::num::NonZeroUsize;
use std::pin::Pin;
use std::sync::Arc;
use std::time::Duration;
use viadkim::VerificationStatus;
use viadkim::message_hash::BodyHasherStance;
use viadkim::verifier::LookupTxt;
// ~500kB when fully saturated (~420B per RDATA + selector).
// "top 1000 relays" is much more than enough, the limit is mostly to prevent DoS attacks.
const LRU_CACHE_CAPACITY: NonZeroUsize = NonZeroUsize::new(1000).expect("1000 != 0");
/// DNS resolver for DKIM TXT records, that caches RDATA in memory.
#[derive(Clone)]
struct CachedResolver {
dns_resolver: TokioResolver,
// Note: Arc is required despite we are holding the whole handler in an Arc,
// because viadkim will internally clone the resolver (LookupTxt + Clone + 'static)
// to parallelize lookups in case of multiple signatures...
cache: Arc<parking_lot::Mutex<LruCache<Name, Vec<u8>>>>,
}
impl CachedResolver {
/// Creates a new [`CachedResolver`].
pub fn new() -> Result<Self, crate::error::Error> {
// Use resolv.conf
let dns_resolver = TokioResolver::builder(TokioConnectionProvider::default())?.build();
let cache = Arc::new(parking_lot::Mutex::new(LruCache::new(LRU_CACHE_CAPACITY)));
Ok(Self {
dns_resolver,
cache,
})
}
/// Normalizes a TXT record RDATA by removing whitespace characters.
///
/// Some DKIM key records use e.g. LF + WSP line breaks.
/// This is technically not correct, and `viadkim` fails to parse such records,
/// but in practice this is accepted by many implementations.
fn normalize_rdata(txt_data: &str) -> String {
txt_data.replace([' ', '\t', '\n', '\r'], "")
}
/// Invalidates the cached RDATA for a given selector and domain.
///
/// Fails silently.
fn invalidate_cache(&self, selector: &str, domain: &str) {
let selector_domain_str = format!("{}._domainkey.{}.", selector, domain);
if let Ok(selector_domain) = Name::from_ascii(&selector_domain_str) {
let mut cache = self.cache.lock();
cache.pop(&selector_domain);
log::debug!("Cache invalidated for {}", selector_domain_str);
} else {
log::warn!(
"Failed to parse selector domain for cache invalidation: {}",
selector_domain_str
);
}
}
}
impl LookupTxt for CachedResolver {
type Answer = Box<dyn Iterator<Item = io::Result<Vec<u8>>>>;
type Query<'a> = Pin<Box<dyn Future<Output = io::Result<Self::Answer>> + Send + 'a>>;
fn lookup_txt(&self, domain: &str) -> Self::Query<'_> {
let name = Name::from_ascii(domain);
Box::pin(async move {
let name = name.map_err(|_| io::ErrorKind::InvalidInput)?;
{
let mut cache = self.cache.lock();
if let Some(rdata) = cache.get(&name) {
let txts: Self::Answer = Box::new(std::iter::once(Ok(rdata.clone())));
log::debug!("Using cached RDATA for {}", name);
return Ok(txts);
}
}
log::debug!("Trying to resolve TXT for {}", name);
let txt = {
let lookup = self.dns_resolver.txt_lookup(name.clone()).await?;
// viadkim would filter out non-DKIM TXT records,
// but we filter it here anyway so that we know which one should be cached.
lookup
.into_iter()
.find_map(|txt| {
let rdata_raw = txt.txt_data().concat();
let rdata = String::from_utf8_lossy(&rdata_raw);
// naive check, but this can't be an attack vector,
// and selector domain should only have a single TXT record anyway.
if rdata.contains("DKIM") {
Some(Self::normalize_rdata(&rdata).into_bytes())
} else {
None
}
})
.ok_or(io::ErrorKind::NotFound)?
};
{
let mut cache = self.cache.lock();
cache.put(name, txt.clone());
}
let txts: Self::Answer = Box::new(std::iter::once(Ok(txt)));
Ok(txts)
})
}
}
/// DKIM verifier using a pre-configured [`viadkim`] verifier, a [`CachedResolver`] for DNS lookups,
/// and strict domain name alignment check.
pub struct DkimVerifier {
resolver: CachedResolver,
config: viadkim::Config,
}
impl DkimVerifier {
/// Creates a new [`DkimVerifier`] with the provided resolver.
pub fn new() -> Result<Self, crate::error::Error> {
let resolver = CachedResolver::new()?;
let config = viadkim::Config {
lookup_timeout: Duration::from_secs(60),
..Default::default()
};
Ok(Self { resolver, config })
}
/// Verifies the DKIM signature of a raw email message and alignment with the domain of
/// provided `From` address.
pub async fn verify(&self, raw_mail: &[u8], from_address: &str) -> Result<(), String> {
let (headers, body_start) = {
use viadkim::{FieldBody, FieldName, HeaderField};
let Ok((headers, body_start)) = mailparse::parse_headers(raw_mail) else {
return Err("500 Failed to parse message headers".to_string());
};
let mut viadkim_headers: Vec<HeaderField> = Vec::new();
for header in headers {
match (
FieldName::new(header.get_key()),
FieldBody::new(header.get_value_raw()),
) {
(Ok(name), Ok(body)) => viadkim_headers.push((name, body)),
(Err(e), _) | (_, Err(e)) => {
log::debug!("Failed to parse header {header:?}, skipping: {e}");
}
}
}
let viadkim_headers = viadkim::HeaderFields::new(viadkim_headers).map_err(|e| {
log::error!("Failed to parse headers for DKIM verification: {e}");
"500 Failed to parse message headers".to_string()
})?;
(viadkim_headers, body_start)
};
let Some(from_domain) = get_domain_from_address(from_address) else {
return Err("501 Invalid From address".to_string());
};
log::debug!("`From` header domain: {from_domain}");
let Some(mut verifier) =
viadkim::Verifier::verify_header(&self.resolver, &headers, &self.config).await
else {
return Err("554 5.7.1 No DKIM signature found".to_string());
};
'hasher: for chunk in raw_mail.get(body_start..).unwrap_or_default().chunks(8192) {
if verifier.process_body_chunk(chunk) == BodyHasherStance::Done {
break 'hasher;
}
}
for res in verifier.finish() {
log::debug!("Signature {}: {:?}", res.index, res.status);
let Some(signature) = &res.signature else {
log::debug!("Signature {}: No signature found, skipping", res.index);
continue;
};
if matches!(res.status, VerificationStatus::Failure(_)) {
log::debug!("Signature {}: Verification failed, skipping", res.index);
// We only invalidate cache on actual validation error, and not alignment error.
// TODO: ideally we should retry without cache and swap cached value only on success.
self.resolver
.invalidate_cache(signature.selector.as_ref(), signature.domain.as_ref());
continue;
}
if !signature
.domain
.to_string()
.eq_ignore_ascii_case(&from_domain)
{
log::debug!(
"Signature {}: Domain different than in From header, skipping",
res.index
);
continue;
}
return Ok(());
}
Err("554 5.7.1 No valid DKIM signature found".to_string())
}
}
filtermail/src/error.rs
+2
View File
@@ -8,6 +8,8 @@ pub enum Error {
Config(#[from] serini::Error),
#[error(transparent)]
Io(#[from] std::io::Error),
#[error(transparent)]
Resolve(#[from] hickory_resolver::ResolveError),
#[error("OpenPGP packet header is truncated - can't validate!")]
TruncatedHeader,
#[error("Unable to send email, Error during {context}, server said: {raw_smtp_answer}")]
filtermail/src/inbound.rs
+28 -19
View File
@@ -2,22 +2,26 @@
use crate::ENCRYPTION_NEEDED_523;
use crate::config::Config;
use crate::dkim_verifier::DkimVerifier;
use crate::message::{check_encrypted, is_securejoin};
pub use crate::smtp_server::Envelope;
use crate::smtp_server::SmtpHandler;
use crate::utils::extract_address;
use async_trait::async_trait;
use mailparse::{MailHeaderMap, parse_mail};
pub use crate::smtp_server::Envelope;
use crate::utils::extract_address;
/// Handler for incoming SMTP messages.
pub struct IncomingBeforeQueueHandler {
config: Config,
dkim_verifier: DkimVerifier,
}
impl IncomingBeforeQueueHandler {
pub fn new(config: Config) -> Self {
Self { config }
pub fn new(config: Config) -> Result<Self, crate::error::Error> {
Ok(Self {
config,
dkim_verifier: DkimVerifier::new()?,
})
}
}
@@ -27,7 +31,7 @@ impl SmtpHandler for IncomingBeforeQueueHandler {
Ok(())
}
fn check_data(&self, envelope: &Envelope) -> Result<(), String> {
async fn check_data(&self, envelope: &Envelope) -> Result<(), String> {
log::debug!("Processing DATA message from {}", envelope.mail_from);
let message = match parse_mail(&envelope.data) {
@@ -35,6 +39,21 @@ impl SmtpHandler for IncomingBeforeQueueHandler {
Err(e) => return Err(format!("500 Failed to parse message: {}", e)),
};
let from_header = message
.headers
.get_first_value("From")
.unwrap_or_default()
.trim()
.to_string();
let Some(from_addr) = extract_address(&from_header) else {
return Err(format!("500 Invalid FROM header: {from_header}"));
};
self.dkim_verifier
.verify(&envelope.data, &from_addr)
.await?;
let mail_encrypted = check_encrypted(&message, false);
log::debug!("mail_encrypted: {mail_encrypted}");
log::debug!("is_securejoin: {}", is_securejoin(&message));
@@ -50,20 +69,10 @@ impl SmtpHandler for IncomingBeforeQueueHandler {
// Allow cleartext mailer-daemon messages
if let Some(auto_submitted) = message.headers.get_first_value("Auto-Submitted")
&& !auto_submitted.is_empty()
&& from_addr.to_lowercase().starts_with("mailer-daemon@")
&& message.ctype.mimetype == "multipart/report"
{
let from_header = message
.headers
.get_first_value("From")
.unwrap_or_default()
.trim()
.to_string();
if let Some(from_addr) = extract_address(&from_header)
&& from_addr.to_lowercase().starts_with("mailer-daemon@")
&& message.ctype.mimetype == "multipart/report"
{
return Ok(());
}
return Ok(());
}
for recipient in &envelope.rcpt_to {
filtermail/src/main.rs
+5 -1
View File
@@ -25,6 +25,7 @@
clippy::bool_to_int_with_if
)]
mod config;
mod dkim_verifier;
pub(crate) mod error;
pub(crate) mod inbound;
pub(crate) mod message;
@@ -95,7 +96,10 @@ async fn main() {
process::exit(1);
}
} else {
let handler = Arc::new(IncomingBeforeQueueHandler::new(config.clone()));
let handler = Arc::new(
// We want to panic here if the handler cannot be created.
IncomingBeforeQueueHandler::new(config.clone()).unwrap(),
);
let addr = format!("127.0.0.1:{}", config.filtermail_smtp_port_incoming);
let max_size = config.max_message_size;
log::debug!("Incoming SMTP server listening on {addr}");
filtermail/src/outbound.rs
+1 -1
View File
@@ -55,7 +55,7 @@ impl SmtpHandler for OutgoingBeforeQueueHandler {
Ok(())
}
fn check_data(&self, envelope: &Envelope) -> Result<(), String> {
async fn check_data(&self, envelope: &Envelope) -> Result<(), String> {
log::debug!("Processing DATA message from {}", envelope.mail_from);
let message = match parse_mail(&envelope.data) {
filtermail/src/smtp_server.rs
+2 -2
View File
@@ -21,7 +21,7 @@ pub trait SmtpHandler: Send + Sync {
fn handle_mail(&self, address: &str) -> Result<(), String>;
/// Checks the DATA command before reinjection.
fn check_data(&self, envelope: &Envelope) -> Result<(), String>;
async fn check_data(&self, envelope: &Envelope) -> Result<(), String>;
/// Reinjects the mail back to postfix.
async fn reinject_mail(&self, envelope: &Envelope) -> Result<(), String>;
@@ -29,7 +29,7 @@ pub trait SmtpHandler: Send + Sync {
/// Handles the DATA command.
async fn handle_data(&self, envelope: &Envelope) -> Result<String, String> {
log::debug!("handle_DATA before-queue");
self.check_data(envelope)?;
self.check_data(envelope).await?;
self.reinject_mail(envelope).await.map_err(|e| {
log::warn!("Failed to reinject mail: {e}");
e
filtermail/src/utils.rs
+21
View File
@@ -26,6 +26,17 @@ pub fn extract_address(input: &str) -> Option<String> {
})
}
pub fn get_domain_from_address(address: &str) -> Option<String> {
let parts: Vec<&str> = address.split('@').collect();
if parts.len() == 2
&& let Some(domain) = parts.get(1)
{
Some(domain.to_string())
} else {
None
}
}
#[cfg(test)]
mod tests {
use super::*;
@@ -44,4 +55,14 @@ mod tests {
let result = extract_address(input);
assert_eq!(result, expected)
}
#[rstest]
#[case("t1@example.org", Some("example.org".to_string()))]
#[case("SRS1=HHH=example.com==HHH=TT=example.org=alice@example.net", Some("example.net".to_string()))]
#[case("invalid", None)]
#[case("invalid@address@com", None)]
fn test_get_domain_from_address(#[case] input: &str, #[case] expected: Option<String>) {
let result = get_domain_from_address(input);
assert_eq!(result, expected);
}
}