Our client to HAProxy establishes single TCP connection and continues to send messages. We would like to know, is there are way to load balance those messages across the services sitting behind HAProxy. Please advise.
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I am trying to setup a loadbalancing lab for HAproxy in single-arm mode (when actual frontend IP and backend servers reside in same subnet, while actual clients are always remote). Another request is to make client source IPs visible to backend nodes. As we load-balance custom tcp-based app, it seems that option 'source 0.0.0.0 usesrc clientip' is a right choice here. Also, I have configured backends to have default-gateways pointing to HAproxy's IP address.
Although strange things happen once I enable this backend option: I see connection to frontend VIP was properly done and 3-way handshake formed. But when HAproxy server is trying to build a 2nd session to reach out to backend servers with spoofed IP of a client, I see exactly this happening:
Proxy is sending SYN with spoofed Client's IP address to one of the backends;
Backend is normally repsonds with SYN-ACK packet;
Proxy is NOT sending last ACK, just blindly sends SYN packets after timeout with same outcome;
On a proxy I see this connection is marked as SYN_SENT in netstat output, so it looks like proxy server doesn't accept actualy SYN-ACK packet for some reason.
Any comment would be appreciated.
The source option makes HAProxy bind to a specific IP address before it relays the request to the server. If you just need to load balance servers over TCP/IP (not HTTP), then you do not need this.
Set mode tcp in your frontend and backend, which enables load balancing of TCP-enabled applications.
To forward the client's IP address to the server, can you modify your custom application to support the Proxy Protocol? https://www.haproxy.com/blog/using-haproxy-with-the-proxy-protocol-to-better-secure-your-database/
I am trying to wrap my head around the ssl Tunneling process which is performed by an http proxy after receiving the CONNECT method from a client.
Stuff I can't seem to find or understand in docs, blogs, rfcs:
1) when setting up the tunnel, are the two connections from client-proxy and proxy-destination two separate connections or just one and the same? E.g. is there an tcp handshake between client-proxy and another between proxy-destination?
2) when starting the ssl handshake what node is targeted (ip address/hostname) by the client? The proxy or the destination host? Since ssl requires a point-to-point connection to make the authentication work my feeling tells me it should be the destination host. But then again that wouldn't make sense since the destination host isn't (directly) accessible from the clients perspective (hence the proxy).
when setting up the tunnel, are the two connections from client-proxy and proxy-destination two separate connections or just one and the same? E.g. is there an tcp handshake between client-proxy and another between proxy-destination?
Since the client makes the TCP connection to the proxy there is no other way than that the proxy is making another TCP connection to the server. There is no way to change an existing TCP connection to be connected to a different IP:port.
when starting the ssl handshake what node is targeted (ip address/hostname) by the client? The proxy or the destination host?
The SSL handshake is done with the destination host, not the proxy.
Since ssl requires a point-to-point connection to make the authentication
It doesn't need a point-to-point connection. It just needs that all data gets exchanged unmodified between client and server which is the case when the proxy simply forwards the data.
I have around 20 clients communicating together with a central server in the same LAN. The clients can make transaction simultaneously with the server. The server forward each transaction to external appliance in the network. Sometimes it works, sometimes my application shows a "time out" message in a client screen (randomly)
I mirrored all traffic and found TCP Retransmission after TCP Reset packets for the first TCP Sequence. I immediately thought about packet loss but all my cables/NIC are fine, and I do not see DUP ACK in the capture.
It seems that RST packets may have different significations.
What causes those TCP Reset?
Where should I focus my investigation: network or application design ?
I would appreciate any help. Thanks in advance.
Judging by the capture, I assume your central server is 137.56.64.31. What's happening is the clients are initiating a connection to the server with a SYN packet and the server responds with a RST. This is typical if the server has no application listening on that particular port e.g. the webserver application isn't running and a client tries to connect to port 80.
The clients are all connecting to different ports on the server, which is unusual for an central server, but not unheard of. The destination ports the clients are connecting to on the server are: 11007, 11012, 11014, 11108, and 11115. Is that normal for the application? If not, the clients should be connecting to whatever port the application server is listening on.
The reason for the retransmits is that instead of giving up on the connection upon receiving a RST from the server, the client tries to initiate the connection again so Wireshark considers it a retransmission.
I have client and server component. Server may be installed behind the firewall or load balancer. Many sites/forums suggested to use TCP keep-alive feature to avoid connection termination due to inactivity.
The question is whether the keep-alive message from client will actually reach to server?
I tried to simulate the deployment using tcptrace utility and found that the keep-alive messages does not reach to server still the client was getting ACK for keep alive message.
I am not sure whether LB/FW work in same manner.
Is the keep-alive good option to avoid connection termination due to inactivity over socket in case of firewall and load balancer?
The answer is, of course: "it depends".
Many firewalls and load balancers maintain separate frontend and backend TCP connections, e.g.:
client <-- TCP --> firewall/balancer <-- TCP --> server
For situations like this, using TCP keepalive will not work as you'd expect. Why not? The TCP keepalive works for that TCP session only, and the keepalive probe packets are more like "administrative overhead" packets that data-bearing packets. This means that a) using TCP keepalive on the client end only means keeping the TCP connection to the firewall/balancer alive, and b) the firewall/balancer does not "forward" those keepalive probe packets across to the backend connection.
So is using TCP keepalive useful? Yes. There are other types of proxies which work at lower layers in the OSI stack, and which do forward those packets; using TCP keepalive is good for keeping your idle connection alive through those types of network intermediaries.
If your client/server application uses a long-lived, possibly idle TCP connection through firewalls/balancers, the best way to ensure that that connection is not torn down (sometimes politely, e.g. with a RST packet sent by the firewall/balancer, sometimes silently) is to use a "ping" or "heartbeat" message at the application layer. (Think of this as an "application keepalive".) This is just some kind of message that is sent e.g. from the client to the server. A simple and effective technique is to have the client periodically send some bytes to the server, which the server echoes back to the client. The client knows which bytes it sent, and when it receives those same bytes back from the server, it knows that everything in the network path is still working as expected.
Hope this helps!
From what I understand, each HTTP request uses its own TCP connection (please correct me if i'm wrong). So, let's say that there are two current connections to the same server. For example, client side javascript code triggering a couple of AJAX POST requests using the XMLHttpRequest object, one right after the other, before getting the response to the first one. So we're talking about two connections to the same server, each waiting for a response in order to route it to each separate callback function.
Now here's the thing that I don't understand: The TCP packet includes source and destination ip and port, but won't both of these connections have the same src and dest ip addresses, and port 80? How can the packets be differentiated and routed to appropriately? Does it have anything to do with the packet sequence number which is different for each connection?
When your browser creates a new connection to the HTTP server, it uses a different source port.
For example, say your browser creates two connections to a server and that your IP address is 60.12.34.56. The first connection might originate from source port 60123 and the second from 60127. This is embedded in the TCP header of each packet sent to the server. When the server replies to each connection, it uses the appropriate port (e.g. 60123 or 60127) so that the packet makes it back to the right spot.
One of the best ways to learn about this is to download Wireshark and just observe traffic on your own network. It will show you this and much more.
Additionally, this gives insight into how Network Address Translation (NAT) works on a router. You can have many computers share the same IP address and the router will rewrite the request to use a different port so that two computers can simultaneously connect to places like AOL Instant Messenger.
They're differentiated by the source port.
The main reason for each HTTP request to not generate a separate TCP connection is called keepalives, incidentally.
A socket, in packet network communications, is considered to be the combination of 4 elements: server IP, server port, client IP, client port. The second one is usually fixed in a protocol, e.g. http usually listen in port 80, but the client port is a random number usually in the range 1024-65535. This is because the operating system could use those ports for known server protocols (e.g. 21 for FTP, 22 for SSH, etc.). The same network device can not use the same client port to open two different connections even to different servers and if two different clients use the same port, the server can tell them apart by their IP addresses. If a port is being used in a system either to listen for connection or to establish a connection, it can not be used for anything else. That's how the operating system can dispatch packets to the correct process once received by the network card.