stop-gap: doc glibc 2.31 RES_TRUSTAD/trust-ad

In glibc from release 2.31 onwards (change added in their commit
446997ff14) setting `dns_dnssec_ok` will not be sufficient.  glibc has
added a new `options trust-ad` toggle for `/etc/resolv.conf` and a C
macro `RES_TRUSTAD`.

This will break existing deployments and binaries.

Our current mechanism for enabling DNSSEC is with an option named to
closely match the DNS feature required, so it is probably inappropriate
to tinker with a second option there.  Instead we probably need a new
meta-option for the concept of DNSSEC, add the second new flag there,
and move `dns_dnssec_ok` to a legacy deprecated option.

That will only work if the machine Exim is built on has the new C macro,
but will need to be conditional upon that macro being defined, so
binaries built aren't going to be forward-compatible to other systems
with newer glibc.  There is no good solution to solve this.

In the meantime, document the issue and point administrators at how to
work around the issue with a setting in `/etc/resolv.conf`

Thanks to Viktor Dukhovni for highlighting the existence of this
problem.

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 0f46896e97172469ba7415552a04bab5dcc02cec..c54437181b2bc1bfd83c2109fd4bf07f54b88550 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -8120,7 +8120,7 @@ daemon as in the other SQL databases.
 
 .new
 .oindex &%sqlite_dbfile%&
-The preferred way of specifying the file is by using the 
+The preferred way of specifying the file is by using the
 &%sqlite_dbfile%& option, set to
 an absolute path.
 .wen
@@ -13290,6 +13290,18 @@ library, by setting:
 dns_dnssec_ok = 1
 .endd
 
+.new
+In addition, on Linux with glibc 2.31 or newer the resolver library will
+default to stripping out a successful validation status.
+This will break a previously working Exim installation.
+Provided that you do trust the resolver (ie, is on localhost) you can tell
+glibc to pass through any successful validation with a new option in
+&_/etc/resolv.conf_&:
+.code
+options trust-ad
+.endd
+.wen
+
 Exim does not perform DNSSEC validation itself, instead leaving that to a
 validating resolver (e.g. unbound, or bind with suitable configuration).
 
@@ -15418,6 +15430,18 @@ default. A value of 0 coerces DNSSEC off, a value of 1 coerces DNSSEC on.
 
 If the resolver library does not support DNSSEC then this option has no effect.
 
+.new
+On Linux with glibc 2.31 or newer this is insufficient, the resolver library
+will default to stripping out a successful validation status.
+This will break a previously working Exim installation.
+Provided that you do trust the resolver (ie, is on localhost) you can tell
+glibc to pass through any successful validation with a new option in
+&_/etc/resolv.conf_&:
+.code
+options trust-ad
+.endd
+.wen
+
 
 .option dns_ipv4_lookup main "domain list&!!" unset
 .cindex "IPv6" "DNS lookup for AAAA records"

diff --git a/src/src/dns.c b/src/src/dns.c
index d036162cb873b44a10f7a9927174411fd90a88da..e13aa74e24b17d880885bfd5e5cdad393df628d4 100644 (file)
--- a/src/src/dns.c
+++ b/src/src/dns.c
@@ -501,6 +501,8 @@ const uschar * auth_name;
 const uschar * trusted;
 
 if (dnsa->answerlen < 0) return FALSE;
+/* Beware that newer versions of glibc on Linux will filter out the ad bit
+unless their shiny new RES_TRUSTAD bit is set for the resolver.  */
 if (h->ad) return TRUE;
 
 /* If the resolver we ask is authoritative for the domain in question, it may