Hi Alan,
Thank you so much.

when i set one client server to my radius server ,the vpn users work fine. but while there are two client servers ,the second client cant connect the radius server.
radius server : client.conf
--------------
client [vpn1-IP]{
        secret = secret1
        nastype = other
}
client  [vpn2-IP]{
        secret = secret2
        nastype = other
}
-------------
radius.log
-------------
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Connected new DB handle, #0
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Attempting to connect rlm_sql_mysql #1
Wed Sep 17 04:07:16 2014 : Info: rlm_sql_mysql: Starting connect to MySQL server for #1
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Connected new DB handle, #1
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Attempting to connect rlm_sql_mysql #2
Wed Sep 17 04:07:16 2014 : Info: rlm_sql_mysql: Starting connect to MySQL server for #2
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Connected new DB handle, #2
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Attempting to connect rlm_sql_mysql #3
Wed Sep 17 04:07:16 2014 : Info: rlm_sql_mysql: Starting connect to MySQL server for #3
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Connected new DB handle, #3
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Attempting to connect rlm_sql_mysql #4
Wed Sep 17 04:07:16 2014 : Info: rlm_sql_mysql: Starting connect to MySQL server for #4
Wed Sep 17 04:07:16 2014 : Info: rlm_sql (sql): Connected new DB handle, #4
Wed Sep 17 04:07:16 2014 : Info: Loaded virtual server <default>
Wed Sep 17 04:07:16 2014 : Info: Loaded virtual server inner-tunnel
Wed Sep 17 04:07:16 2014 : Info:  ... adding new socket proxy address * port 54648
Wed Sep 17 04:07:16 2014 : Info: Ready to process requests.
Wed Sep 17 04:18:23 2014 : Error: Ignoring request to authentication address * port 1812 from unknown client [IP address] port 47118
Wed Sep 17 04:18:33 2014 : Error: Ignoring request to authentication address * port 1812 from unknown client [IP address] port 47118
Wed Sep 17 04:18:43 2014 : Error: Ignoring request to authentication address * port 1812 from unknown client [IP address] port 47118
-------------

the second client server log
-----------
Sep 17 04:18:19 vpn2 pppd[8096]: pppd 2.4.5 started by root, uid 0
Sep 17 04:18:19 vpn2 pppd[8096]: Using interface ppp0
Sep 17 04:18:19 vpn2 pppd[8096]: Connect: ppp0 <--> /dev/pts/1
Sep 17 04:18:53 vpn2 pppd[8096]: rc_ip_hostname: couldn't look up host by addr: %lX
Sep 17 04:18:53 vpn2 pppd[8096]: rc_send_server: no reply from RADIUS server unknown:1812
Sep 17 04:18:53 vpn2 pppd[8096]: Peer vpn2test01 failed CHAP authentication
Sep 17 04:18:53 vpn2 pppd[8096]: Modem hangup
Sep 17 04:18:53 vpn2 pppd[8096]: Connection terminated.
Sep 17 04:18:53 vpn2 pppd[8096]: Exit.
------------------

If there is only vpn1 client, the client server and radius all ok ,while two , the vpn1 still wor fine, but the vpn2 is not.
Why the radius server cant make response to the vpn2 client server?

Thanks.