%Error reading tftp://mytftpserver/anyconnect-win-3.1.14018-k9.pkg (Undefined error code)
On the tftp server in /var/log/messages, I saw:
Mar 6 14:49:19 mytftpserver atftpd: Serving anyconnect-win-3.1.14018-k9.pkg to 192.168.47.5:32947
Mar 6 14:49:19 mytftpserver atftpd: Requested file to big, increase BLKSIZE
I had installed tftp a while ago... specifically atftp-server. After looking at the man page for in.tftpd, I concluded there was no good way to set BLKSIZE on this package. The fix was:
sudo yum erase atftp-server
sudo yum install tftp-server
I then edited the xinet.d/tftp file to enable it and change the target directory and restarted xinetd. After this, I was able to copy the pkg file over successfully.