Looks like a copy/paste :)

Denis

Index: sys-bsd.c
===================================================================
RCS file: /cvs/src/usr.sbin/pppd/sys-bsd.c,v
retrieving revision 1.32
diff -u -p -r1.32 sys-bsd.c
--- sys-bsd.c	24 Oct 2021 21:24:19 -0000	1.32
+++ sys-bsd.c	18 Feb 2024 11:53:15 -0000
@@ -1123,7 +1123,7 @@ dodefaultroute(g, cmd)
     rtmsg.gway.sin_len = sizeof(rtmsg.gway);
     rtmsg.gway.sin_family = AF_INET;
     rtmsg.gway.sin_addr.s_addr = g;
-    rtmsg.mask.sin_len = sizeof(rtmsg.dst);
+    rtmsg.mask.sin_len = sizeof(rtmsg.mask);
     rtmsg.mask.sin_family = AF_INET;
 
     rtmsg.hdr.rtm_msglen = sizeof(rtmsg);