[PATCH] Only set PAM_RHOST if the remote host is not "UNKNOWN"

Daan De Meyer daan.j.demeyer at gmail.com
Wed Apr 3 00:31:49 AEDT 2024

When using sshd's -i option with stdio that is not a AF_INET/AF_INET6
socket, auth_get_canonical_hostname() returns "UNKNOWN" which is then
set as the value of PAM_RHOST, causing pam to try to do a reverse DNS
query of "UNKNOWN", which times out multiple times, causing a
substantial slowdown when logging in.

To fix this, let's only set PAM_RHOST if the hostname is not "UNKNOWN".
 auth-pam.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/auth-pam.c b/auth-pam.c
index b49d415e7..706b02420 100644
--- a/auth-pam.c
+++ b/auth-pam.c
@@ -735,7 +735,7 @@ sshpam_init(struct ssh *ssh, Authctxt *authctxt)
                sshpam_laddr = get_local_ipaddr(
-       if (sshpam_rhost != NULL) {
+       if (sshpam_rhost != NULL && strcmp(sshpam_rhost, "UNKNOWN") != 0) {
                debug("PAM: setting PAM_RHOST to \"%s\"", sshpam_rhost);
                sshpam_err = pam_set_item(sshpam_handle, PAM_RHOST,

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