[ntp:questions] sendto( Invalid argument

Edrusb edrusba at free.fr
Fri Aug 12 20:09:51 UTC 2005

Dear all,

The problem I met concerns ntpd when a changes in the network interface
set occurs.

My Linux system (on which runs ntpd-2.4.0) has two links to Internet:
one is permanent (except upon hardware failure), the second is a backup
modem line.

Actually the "permanent" link is broken so the dialup modem line is in
action, and a ppp0 interface goes up and down upon request. If I launch
ntpd when the link is down, ntpd will never synchronize with peers even 
when the link comes back short after: it has a wildcard UDP socket 
opened on port 123 and for each other interfaces that existed when ntpd 
was launched, it has an UDP interface specific socket on port 123 too 
(information given by the netstat command). The problem is that when the 
link goes up then ntpd does not seems using the wildcard socket to try 
to reach peers and does not create a new UDP interface specific socket 
for this new interface (ppp0) so peers stay unreachable as reported by 
ntpdc, even long after the link came back (tried more than 2 hours).

In the other way, if I launch ntpd when the link is up, it properly
exchange time information with its peers, but when the link goes down,
it keeps its UDP interface specific socket opened on ppp0 interface
while this ppp0 interface is now down. At that time I start having

	"ntpd[11911]: sendto(x.x.x.x): Invalid argument"

messages as reported by syslog (with x.x.x.x rotating with all the IP
address of the configured peers). The worse thing is that when the link
comes back, ntpd still has his old socket opened, but still are reported
a lot of "Invalid Socket" messages and of course, ntpd cannot anymore
synchronize with ntp peers.

- What is the use of this wildcard UDP socket (it does not seems used)?
- Why does ntpd uses theses UDP specific interfaces sockets, while the
configuration file does not specify any interface to listen on or to not
listen on?
- How to have ntpd be able to work with a dialup link?

Any help is (very) welcome. :-)


edrusba at free.fr
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