What causes a TCP/IP reset (RST) flag to be sent?

Question

I'm trying to figure out why my app's TCP/IP connection keeps hiccuping every 10 minutes (exactly, within 1-2 seconds). I ran Wireshark and discovered that after 10 minutes of inactivity the other end is sending a packet with the reset (RST) flag set. A google search tells me "the RESET flag signifies that the receiver has become confused and so wants to abort the connection" but that is a little short of the detail I need. What could be causing this? And is it possible that some router along the way is responsible for it or would this always come from the other endpoint?

Edit: There is a router (specifically a Linksys WRT-54G) sitting between my computer and the other endpoint -- is there anything I should look for in the router settings?

Answer 1

RST is sent by the side doing the active close because it is the side which sends the last ACK. So if it receives FIN from the side doing the passive close in a wrong state, it sends a RST packet which indicates other side that an error has occured.

Answer 2

If there is a router doing NAT, especially a low end router with few resources, it will age the oldest TCP sessions first. To do this it sets the RST flag in the packet that effectively tells the receiving station to (very ungracefully) close the connection. this is done to save resources.

Answer 3

I've just spent quite some time troubleshooting this very problem. None of the proposed solutions worked. Turned out that our sysadmin by mistake assigned the same static IP to two unrelated servers belonging to different groups, but sitting on the same network. The end results were intermittently dropped vnc connections, browser that had to be refreshed several times to fetch the web page, and other strange things.

Answer 4

A 'router' could be doing anything - particularly NAT, which might involve any amount of bug-ridden messing with traffic...

One reason a device will send a RST is in response to receiving a packet for a closed socket.

It's hard to give a firm but general answer, because every possible perversion has been visited on TCP since its inception, and all sorts of people might be inserting RSTs in an attempt to block traffic. (Some 'national firewalls' work like this, for example.)

Answer 5

Run a packet sniffer (e.g., Wireshark) also on the peer to see whether it's the peer who's sending the RST or someone in the middle.

Answer 6

Some firewalls do that if a connection is idle for x number of minutes. Some ISPs set their routers to do that for various reasons as well.

In this day and age, you'll need to gracefully handle (re-establish as needed) that condition.