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.
The issue with doing a DNAT to 127.0.0.1:5000 is that when the remote side responds, these packets return into the routing engine as if they were locally originated (from 127.0.0.1) but they have an outside destination address. SNAT/MASQUERADE matching the outside interface would have caught them and rewritten them, but the routing decisions that have to be made for the packets to arrive at that interface come first, and they disallow these packets which are bogus by default. The routing engine can't guarantee you'll remember to do that rewrite later.
The thing that you should be able to do instead is reject any outside connections to 192.168.1.1:5000 at iptables INPUT other than those coming from 192.168.1.10 using the ! argument before the -s source address specification. If you use TCP reset as the rejection mechanism (-j REJECT --reject-with tcp-reset, instead of the default ICMP destination unreachable), it will be largely identical to the situation where nothing was even listening on that address:port combination as far as the outside world is concerned.
Best Answer
The Debian way of setting up iptables on boot is by using the iptables-persistent package.
Simply install the
iptables-persistentpackage, set up the iptables rules like you want them, and then runnetfilter-persistent save. (Note that the command starts withnetfilterand notiptables.)See the man page for netfilter-persistent for more details.
The method for saving the tables has changes since Debian 7 (Wheezy). In Wheezy one would do something like:
invoke-rc.d iptables-persistent save.