Question

I have an application which talks to server over HTTP. I have written a code to control connect timeout (amount of time it will wait before server replies) . But I am finding it hard to generate a test case to test my connect timeout code. Could you please help me.

Basically, TCp handshake will contain
Host A sends a TCP SYNchronize packet to Host B

Host B receives A’s SYN

Host B sends a SYNchronize-ACKnowledgement

Host A receives B’s SYN-ACK

Host A sends ACKnowledge

Host B receives ACK.
TCP socket connection is ESTABLISHED.

In my application, if server does not complete TCP handshek in x seconds, applications moves to next server. But to test this code, I need a server stub which will probably accept SYN packet from client but will not set SYN+ACK packet to client. Thus making client wait for server’s reply.

Could you please help me to create small server stub which will listen to particular port but will not complete handshake.

Answer 1

Given you mentioned RHEL I think you’re best off using iptables to help test this. For example you could call:

iptables -I INPUT -s hostb -d hosta -p tcp --dport $port --tcp-flags SYN,ACK SYN,ACK -j DROP

calling that before running the test (or even during it perhaps?) and an equivalent matched -X to delete it seems to be by far the simplest way of breaking a handshake halfway through.

Drop all SYN+ACK (warning, WILL break new SSH connections):

iptables -I INPUT -p tcp --tcp-flags SYN,ACK SYN,ACK -j DROP

Drop all from or to 10.10.22.34:

iptables -I INPUT -s 10.10.22.34 -j DROP
iptables -I OUTPUT -d 10.10.22.34 -j DROP

Personally I would use the most specific match you can possibly write to avoid accidentally breaking remote access or anything else at the same time.

You could get fancier and use the -m owner match to only apply this rule for packets to/from the UID you run this test as.