In the kernel log there are hundreds of these lines. The source is the external IP of the VPS (not localhost nor 127.0.0.1), other times an IP of the network of the VPS provider. The source port and dest port is always 17500. I searched a bit but doesn't seem to be a known port. It's not used by SSH, FTP, mailserver or others on my part.
EDIT: Dropbox is not installed of the server (that runs Ubuntu Server 12.04)
Jan 2 01:17:17 kernel: [8861587.504866] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:17:40 kernel: [8861610.825311] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:17:47 kernel: [8861617.544797] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:18:10 kernel: [8861640.864049] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:18:17 kernel: [8861647.584077] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:18:40 kernel: [8861670.903856] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:18:47 kernel: [8861677.623413] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:19:10 kernel: [8861700.944182] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:19:17 kernel: [8861707.662837] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:19:40 kernel: [8861730.984200] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:19:47 kernel: [8861737.702796] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:20:10 kernel: [8861761.023621] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:20:17 kernel: [8861767.742645] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:20:40 kernel: [8861791.064367] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:20:47 kernel: [8861797.782511] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:21:10 kernel: [8861821.103867] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:21:17 kernel: [8861827.822161] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:21:40 kernel: [8861851.144209] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:21:47 kernel: [8861857.862165] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:22:10 kernel: [8861881.181915] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:22:17 kernel: [8861887.901566] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:22:41 kernel: [8861911.215488] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:22:47 kernel: [8861917.941271] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:23:11 kernel: [8861941.252756] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:23:17 kernel: [8861947.981005] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:23:41 kernel: [8861971.292991] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Jan 2 01:23:47 kernel: [8861978.021033] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:25:09:e7:9b:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=210 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=190
Jan 2 01:24:11 kernel: [8862001.333676] iptables denied: IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:10:00:05:90:ad:c2:08:00 SRC=xx.xx.xx.xx DST=255.255.255.255 LEN=151 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=17500 DPT=17500 LEN=131
Here's my iptables:
################## FILTER ##################
*filter
# Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT -i lo -d 127.0.0.0/8 -j REJECT
# Accepts all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# Allows all outbound traffic
# You can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT
# Allows HTTP and HTTPS connections from anywhere (the normal ports for websites)
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
# Allows SSH and Webmin connections
-A INPUT -p tcp -m state --state NEW --dport 50000 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 1212 -j ACCEPT
# Allows FTP access
-A OUTPUT -p tcp -m tcp --sport 20 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 21 -j ACCEPT
# Disallow ping
-A INPUT -p icmp -m icmp --icmp-type 8 -j DROP
# log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7
# Reject all other inbound - default deny unless explicitly allowed policy
-A INPUT -j REJECT
-A FORWARD -j REJECT
COMMIT
################## NAT ##################
*nat
:PREROUTING ACCEPT [59412:4936393]
:INPUT ACCEPT [41513:2484958]
:OUTPUT ACCEPT [16417:1072327]
:POSTROUTING ACCEPT [16417:1072327]
COMMIT
################## MANGLE ##################
*mangle
:PREROUTING ACCEPT [1574957:131349929]
:INPUT ACCEPT [1572501:131156748]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1669706:6747756890]
:POSTROUTING ACCEPT [1669706:6747756890]
COMMIT