Please update c-ares to a newer version in order to fix the following bug. The fix is present in 1.20.0 however 1.20.1 was released shortly afterwards fixing a use-after-free issue so that version would be better. 1.21.0 is the latest version with no user-visible changes, only code cleanups and the replacement of hand written DNS parsing code with a new memory-safe parser.

The changes are all detailed in the release notes at <https://c-ares.org/changelog.html>.

The bug is: when there is no search line in /etc/resolv.conf but the systems' hostname contains a domain (e.g., the kernel hostname is literally mymachine.example.com), glibc behaves as if search example.com was written in /etc/resolv.conf.

In order to disable this behaviour, systemd-resolved writes search . in /etc/resolv.conf.

This causes name resolution with c-ares to fail with ARES_EBADNAME.

Among other impacts, this breaks sssd's ability to perform dynamic DNS updates: before it does an update it needs to resolve the system hostname in order to find out if an update is necessary; the c-ares bug prevents that from succeeding.

Reproducible: Always

Steps to Reproduce:
1. Configure a network connection without a DNS search domain
2. Confirm "search ." is present in /etc/resolv.conf
3. Compile the test program from https://github.com/SSSD/sssd/issues/6322#issuecomment-1697226764
4. Run ./ares-test anything
Actual Results:  
The following is logged:
ares_search -> Misformatted domain name (8)

Expected Results:  
./ares-test mymachine
dynamic DNS update should succeed shortly after sssd is started

To observe sssd's dynamic dns updates failing...

1. Join a FreeIPA (or possible Active Directory, untested) domain
2. Configure dyndns_update=True and dyndns_refresh_interval=30 in /etc/sssd.conf
3. Watch /var/log/sssd/sssd_<domain>.log and see dynamic dns updates fail; look for the following message:

[nsupdate_get_addrs_done] (0x0040): Could not resolve address for this machine, error [5]: Input/output error, resolver returned: [8]: Misformatted domain name

More info at https://github.com/SSSD/sssd/issues/6322