diff --git a/man7/unix.7 b/man7/unix.7
index 05681f031..3f41ad4cc 100644
--- a/man7/unix.7
+++ b/man7/unix.7
@@ -534,6 +534,33 @@ versus
 .TP
 .B ESOCKTNOSUPPORT
 Unknown socket type.
+.TP
+.B ETOOMANYREFS
+This error can occur for
+.BR sendmsg (2)
+when sending a file descriptor as ancilary data over
+a UNIX domain socket (see the description of
+.BR SCM_RIGHTS ,
+above).
+It occurs if the number of "in-flight" file descriptors exceeds the
+.B RLIMIT_NOFILE
+resource limit and the caller does not have the
+.BR CAP_SYS_RESOURCE
+capability.
+An in-flight file descriptor is one that has been sent using
+.BR sendmsg (2)
+but has not yet been accepted in the recipient process using
+.BR recvmsg (2).
+
+This error is diagnosed since Linux 4.5.
+.\" commit 712f4aad406bb1ed67f3f98d04c044191f0ff593
+In earlier kernel versions,
+it was possible to place an unlimited number of file descriptors in flight,
+by sending each file descriptor with
+.BR sendmsg (2)
+and then closing the file descriptor so that it was not accounted against the
+.B RLIMIT_NOFILE
+resource limit.
 .PP
 Other errors can be generated by the generic socket layer or
 by the filesystem while generating a filesystem socket object.