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Disallow leading lines before the ident for server (#102) Per RFC4253 4.2 clients must be able to process other lines of data before the version string, server behavior is not defined neither with MUST/SHOULD nor with MAY. If server process up to 50 lines too - it may cause too long hanging session with invalid/evil client that consume host resources and potentially may lead to DDoS on poor embedded boxes. Let's require first line from client to be version string and fail early if it's not - matches both RFC and real OpenSSH behavior.
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 15 Jun 2020 18:22:18 +0500
parents b9d3f725e00b
children 13cb8cc1b0e4
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 Tips for a small system:
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3 If you only want server functionality (for example), compile with
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4 make PROGRAMS=dropbear
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5 rather than just
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6 make dropbear
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7 so that client functionality in shared portions of Dropbear won't be included.
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8 The same applies if you are compiling just a client.
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10 ---
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11
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12 The following are set in options.h:
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14 - You can safely disable blowfish and twofish ciphers, and MD5 hmac, without
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15 affecting interoperability
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16
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17 - If you're compiling statically, you can turn off host lookups
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19 - You can disable either password or public-key authentication, though note
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20 that the IETF draft states that pubkey authentication is required.
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22 - Similarly with DSS and RSA, you can disable one of these if you know that
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23 all clients will be able to support a particular one. The IETF draft
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24 states that DSS is required, however you may prefer to use RSA.
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25 DON'T disable either of these on systems where you aren't 100% sure about
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26 who will be connecting and what clients they will be using.
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28 - Disabling the MOTD code and SFTP-SERVER may save a small amount of codesize
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30 - You can disable x11, tcp and agent forwarding as desired. None of these are
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31 essential, although agent-forwarding is often useful even on firewall boxes.
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33 ---
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35 If you are compiling statically, you may want to disable zlib, as it will use
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36 a few tens of kB of binary-size (./configure --disable-zlib).
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38 You can create a combined binary, see the file MULTI, which will put all
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39 the functions into one binary, avoiding repeated code.
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41 If you're compiling with gcc, you might want to look at gcc's options for
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42 stripping unused code. The relevant vars to set before configure are:
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44 LDFLAGS=-Wl,--gc-sections
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45 CFLAGS="-ffunction-sections -fdata-sections"
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47 You can also experiment with optimisation flags such as -Os, note that in some
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48 cases these flags actually seem to increase size, so experiment before
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49 deciding.
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51 Of course using small C libraries such as uClibc and dietlibc can also help.
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53 If you have any queries, mail me and I'll see if I can help.