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comment What is the mandatory information a HTTP Request Header must contain?
Content-Length or Transfer-Encoding are only mandatory if an entity is delivered with the request or response, and in many cases a request or response will lack an entity (like a GET request, or a 302 response).
comment Can Https work without a certificate?
Anonymous Diffie-Hellman, as noted in another answer, did permit a connection without a certificate — but modern OpenSSL versions are usually compiled without any support for ADH.
comment Can Https work without a certificate?
To test whether your OpenSSL can support certificate-less connections, run openssl ciphers and look for an ADH protocol like ADH-AES256-SHA — if such a protocol is present, then you can technically set up a connection without any certificates involved.
comment nginx terminates connection after 65k bytes
It would be helpful to see (a) how gunicorn is choosing to frame replies from 110k to 120k bytes in size, and (b) how nginx then chooses its framing for that same range of sample payload sizes between 110k and 120k bytes. The three ways that HTTP can frame data: provide content-length; do chunked encoding; or give no framing at all except to promise to close the socket when the body is complete.
comment Does the TCP source port have to be unique per host?
I think the question was not about the upper limit on the number of listening applications — which, as you point out, is limited by the number of unique port numbers — but about the number of sockets operating at a time. Web servers often have dozens of sockets all addressed to their single port 80 or 443. If many servers on a host did that, then there is no reason that the number of open sockets could not exceed 2^32.
comment Which cloud services allow layer 2 link level connections between servers?
One other question, @mfarver: does OpenVPN encapsulation tend to cause any MTU-related problems, as incoming packets have to be fragmented to fit the last hop from one LAN machine to its peers? Or are efficiency and service not harmed by the limits of that last hop?
comment Which cloud services allow layer 2 link level connections between servers?
You are right, of course — IPIP is layer 3. I only mentioned it as a possible alternative to accomplish what I was aiming for: to get Internet-bound packets to and from a “gateway” without Amazon being offended that their IP destination was not that of the gateway itself. Thanks for the OpenVPN link — I had never thought of using it for features besides the encryption! (Since I am the first one to ask this question on Server Fault, I cannot help thinking that maybe I am going at this whole problem wrong? How do other people do load balancing without losing customer IP addresses? Not Amazon?)
comment Which cloud services allow layer 2 link level connections between servers?
An interesting idea, @mfarver! And if I were not worried about my in-cloud network packets being captured by other customers, could I also construct a solution like this using Linux's native IPIP tunneling, imposing less CPU load?
comment What are the exact conditions based on which Linux swaps process(s) memory from RAM to a swap file?
If a program suddenly asked for > 205 MB of memory, Linux would not have to get a chunk of memory "out of swap". Instead, the system could simply decide that some of its disk cache pages were not as important as supplying this request for RAM, mark those pages as unused and wipe their content, then hand them to the process. After all, everything in the disk cache can just be read back in later if someone looks at the cached file again! Swap wouldn't even be involved.
comment Send email alert on log file entry?
I've run logcheck every minute ("* * * * *" in the crontab) for years, and it's been great.
comment pptp on linux - I do not want to save passwords to chaps_secrets
Is the Linux server in this question serving as a PPTP server, with machines out in the field connecting back to get inside the local network on which the Linux server lives? Or, is the Linux server wanting to connect as a client of a PPTP server somewhere else?
comment su to user that doesn't exist any longer?
Great idea! If he just throws new lines into /etc/passwd and /etc/shadow he should get the user back for a moment and be able to use them.