To cut a long story short, that ACK was sent when the socket didn't belong to anybody. Instead of allowing packets that pertain to a socket that belongs to user x, allow packets that pertain to a connection that was initiated by a socket from user x.
The longer story.
To understand the issue, it helps to understand how wget and HTTP requests work in general.
In
wget http://cachefly.cachefly.net/10mb.test
wget establishes a TCP connection to cachefly.cachefly.net, and once established sends a request in the HTTP protocol that says: "Please send me the content of /10mb.test (GET /10mb.test HTTP/1.1) and by the way, could you please not close the connection after you're done (Connection: Keep-alive). The reason it does that is because in case the server replies with a redirection for a URL on the same IP address, it can reuse the connection.
Now the server can reply with either, "here comes the data you requested, beware it's 10MB large (Content-Length: 10485760), and yes OK, I'll leave the connection open". Or if it doesn't know the size of the data, "Here's the data, sorry I can't leave the connection open but I'll tell when you can stop downloading the data by closing my end of the connection".
In the URL above, we're in the first case.
So, as soon as wget has obtained the headers for the response, it knows its job is done once it has downloaded 10MB of data.
Basically, what wget does is read the data until 10MB have been received and exit. But at that point, there's more to be done. What about the server? It's been told to leave the connection open.
Before exiting, wget closes (close system call) the file descriptor for the socket. Upon, the close, the system finishes acknowledging the data sent by the server and sends a FIN to say: "I won't be sending any more data". At that point close returns and wget exits. There is no socket associated to the TCP connection anymore (at least not one owned by any user). However it's not finished yet. Upon receiving that FIN, the HTTP server sees end-of-file when reading the next request from the client. In HTTP, that means "no more request, I'll close my end". So it sends its FIN as well, to say, "I won't be sending anything either, that connection is going away".
Upon receiving that FIN, the client sends a "ACK". But, at that point, wget is long gone, so that ACK is not from any user. Which is why it is blocked by your firewall. Because the server doesn't receive the ACK, it's going to send the FIN over and over until it gives up and you'll see more dropped ACKs. That also means that by dropping those ACKs, you're needlessly using resources of the server (which needs to maintain a socket in the LAST-ACK state) for quite some time.
The behavior would have been different if the client had not requested "Keep-alive" or the server had not replied with "Keep-alive".
As already mentioned, if you're using the connection tracker, what you want to do is let every packet in the ESTABLISHED and RELATED states through and only worry about NEW packets.
If you allow NEW packets from user x but not packets from user y, then other packets for established connections by user x will go through, and because there can't be established connections by user y (since we're blocking the NEW packets that would establish the connection), there will not be any packet for user y connections going through.
You need to change this line
sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
To
sudo iptables -A INPUT -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
Because your rule matches only established (syn-ack) and related connections, not new ones (syn)
So complete rule should look like:
sudo iptables -A INPUT -i lo -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 22 -d 34.195.109.193 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
sudo iptables -A INPUT -j DROP
sudo iptables -A OUTPUT -p tcp -m state --state ESTABLISHED,RELATED -j ACCEPT
sudo iptables -A OUTPUT -p tcp -j DROP
The last output rule, will allow established connections to your host.
Best Answer
IPTables is working on IP and TCP level, so it doesn't actually know DNS.
When a client creates a TCP connection to a DNS name it first looks up the IP address corresponding to the DNS name and then connects to the IP, not to the DNS name.
This means IPTables can't possibly know which DNS name a client is connecting to, it only knows which IP the client connects to.
If you specify
--destination sub.domain.comas a parameter, IPTables will simply do a name look up to the IP that corresponds to this name, and then use the IP in it's rules.If your port 1000 serves HTTP it would be possible to configure an HTTP server on that port to not handle requests to domains other than sub.domain.com, but IPTables doesn't have the necessary information to this.
If you could put sub.domain.com and sub.domain2.com onto two different IPs on the same server, then IPTables would be able to deny access to one but not to the other, because it could decide based on the IP.
This illustrates nicely that the
Domain Name Systemis on top ofTransport(TCP) andInternet(IP), for more details you can read the wikipedia article about it: