Revamping the README file

README.md

@@ -1,130 +1,136 @@
+iodine - <http://code.kryo.se/iodine>
+=====================================
 
-iodine - http://code.kryo.se/iodine
-
-***********************************
 
 This is a piece of software that lets you tunnel IPv4 data through a DNS
 server. This can be usable in different situations where internet access is
 firewalled, but DNS queries are allowed.
 
 
-COMPILING:
+COMPILING
+---------
 
 Iodine has no configure script. There are two optional features for Linux
 (SELinux and systemd support) that will be enabled automatically if the
-relevant header files are found in /usr/include. (See script at ./src/osflags)
+relevant header files are found in `/usr/include`.
+(See script at `./src/osflags`)
 
-Run 'make' to compile the server and client binaries.
+Run `make` to compile the server and client binaries.
-Run 'make install' to copy binaries and manpage to the destination directory.
+Run `make install` to copy binaries and manpage to the destination directory.
-Run 'make test' to compile and run the unit tests. (Requires the check library)
+Run `make test` to compile and run the unit tests. (Requires the `check` library)
 
 
-QUICKSTART:
+QUICKSTART
+----------
 
 Try it out within your own LAN! Follow these simple steps:
-- On your server, run: ./iodined -f 10.0.0.1 test.com
+- On your server, run: `./iodined -f 10.0.0.1 test.com`.  
-  (If you already use the 10.0.0.0 network, use another internal net like 
+  If you already use the `10.0.0.0` network, use another internal net like 
-  172.16.0.0)
+  `172.16.0.0`.
-- Enter a password
+- Enter a password.
-- On the client, run: ./iodine -f -r 192.168.0.1 test.com
+- On the client, run: `./iodine -f -r 192.168.0.1 test.com`.  
-  (Replace 192.168.0.1 with your server's ip address)
+  Replace `192.168.0.1` with your server's ip address.
-- Enter the same password
+- Enter the same password.
-- Now the client has the tunnel ip 10.0.0.2 and the server has 10.0.0.1
+- Now the client has the tunnel ip `10.0.0.2` and the server has `10.0.0.1`.
-- Try pinging each other through the tunnel
+- Try pinging each other through the tunnel.
 - Done! :)
+
 To actually use it through a relaying nameserver, see below.
 
 
-HOW TO USE:
+HOW TO USE
+----------
 
 Note: server and client are required to speak the exact same protocol. In most
 cases, this means running the same iodine version. Unfortunately, implementing
 backward and forward protocol compatibility is usually not feasible.
 
-Server side:
+### Server side
-To use this tunnel, you need control over a real domain (like mydomain.com),
+To use this tunnel, you need control over a real domain (like `mydomain.com`),
-and a server with a public IP address to run iodined on. If this server
+and a server with a public IP address to run `iodined` on. If this server
-already runs a DNS program, change its listening port and then use iodined's
+already runs a DNS program, change its listening port and then use `iodined`'s
--b option to let iodined forward the DNS requests. (Note that this procedure
+`-b` option to let `iodined` forward the DNS requests. (Note that this procedure
-is not advised in production environments, because iodined's DNS forwarding
+is not advised in production environments, because `iodined`'s DNS forwarding
 is not completely transparent.)
 
-Then, delegate a subdomain (say, t1.mydomain.com) to the iodined server.
+Then, delegate a subdomain (say, `t1.mydomain.com`) to the iodined server.
 If you use BIND for your domain, add two lines like these to the zone file:
 
 	t1		IN	NS	t1ns.mydomain.com.		; note the dot!
 	t1ns		IN	A	10.15.213.99
 
-The "NS" line is all that's needed to route queries for the "t1" subdomain
+The `NS` line is all that's needed to route queries for the `t1` subdomain
-to the "t1ns" server. We use a short name for the subdomain, to keep as much
+to the `t1ns` server. We use a short name for the subdomain, to keep as much
-space as possible available for the data traffic. At the end of the "NS" line
+space as possible available for the data traffic. At the end of the `NS` line
-is the name of your iodined server. This can be any name, pointing anywhere,
+is the name of your `iodined` server. This can be any name, pointing anywhere,
 but in this case it's easily kept in the same zone file. It must be a name
-(not an IP address), and that name itself must have an A record (not a CNAME).
+(not an IP address), and that name itself must have an `A` record
+(not a `CNAME`).
 
-If your iodined server has a dynamic IP, use a dynamic dns provider. Simply
+If your `iodined` server has a dynamic IP, use a dynamic DNS provider. Simply
-point the "NS" line to it, and leave the "A" line out:
+point the `NS` line to it, and leave the `A` line out:
 
 	t1		IN	NS	myname.mydyndnsprovider.com.	; note the dot!
 
 Then reload or restart your nameserver program. Now any DNS queries for
-domains ending in t1.mydomain.com will be sent to your iodined server.
+domains ending in `t1.mydomain.com` will be sent to your `iodined` server.
 
-Finally start iodined on your server. The first argument is the IP address
+Finally start `iodined` on your server. The first argument is the IP address
 inside the tunnel, which can be from any range that you don't use yet (for
-example 192.168.99.1), and the second argument is the assigned domain (in this
+example `192.168.99.1`), and the second argument is the assigned domain (in this
-case t1.mydomain.com). Using the -f option will keep iodined running in the
+case `t1.mydomain.com`). Using the `-f` option will keep iodined running in the
 foreground, which helps when testing. iodined will open a virtual interface
 ("tun device"), and will also start listening for DNS queries on UDP port 53.
-Either enter a password on the commandline (-P pass) or after the server has
+Either enter a password on the commandline (`-P pass`) or after the server has
 started. Now everything is ready for the client.
 
 If there is a chance you'll be using an iodine tunnel from unexpected
-environments, start iodined with a -c option.
+environments, start `iodined` with a `-c` option.
-
 Resulting commandline in this example situation:
+
 	./iodined -f -c -P secretpassword 192.168.99.1 t1.mydomain.com
 
-Client side: 
+### Client side
-All the setup is done, just start iodine. It takes one or two arguments, the
+All the setup is done, just start `iodine`. It takes one or two arguments, the
 first is the local relaying DNS server (optional) and the second is the domain
-you used (t1.mydomain.com). If you don't specify the first argument, the
+you used (`t1.mydomain.com`). If you don't specify the first argument, the
 system's current DNS setting will be consulted.
 
-If DNS queries are allowed to any computer, you can directly give the iodined
+If DNS queries are allowed to any computer, you can directly give the `iodined`
-server's address as first argument (in the example: t1ns.mydomain.com or
+server's address as first argument (in the example: `t1ns.mydomain.com` or
-10.15.213.99). In that case, it may also happen that _any_ traffic is allowed
+`10.15.213.99`). In that case, it may also happen that _any_ traffic is allowed
 to the DNS port (53 UDP) of any computer. Iodine will detect this, and switch
 to raw UDP tunneling if possible. To force DNS tunneling in any case, use the
--r option (especially useful when testing within your own network).
+`-r` option (especially useful when testing within your own network).
 
 The client's tunnel interface will get an IP close to the server's (in this
-case 192.168.99.2 or .3 etc.) and a suitable MTU. Enter the same password as
+case `192.168.99.2` or `.3` etc.) and a suitable MTU. Enter the same password as
 on the server either as commandline option or after the client has started.
-Using the -f option will keep the iodine client running in the foreground.
+Using the `-f` option will keep the iodine client running in the foreground.
+
+Resulting commandline in this example situation, adding -r forces DNS tunneling
+even if raw UDP tunneling would be possible:
 
-Resulting commandline in this example situation:
 	./iodine -f -P secretpassword t1.mydomain.com
-(add -r to force DNS tunneling even if raw UDP tunneling would be possible)
 
 From either side, you should now be able to ping the IP address on the other
-end of the tunnel. In this case, ping 192.168.99.1 from the iodine client, and
+end of the tunnel. In this case, `ping 192.168.99.1` from the iodine client, and
-192.168.99.2 or .3 etc. from the iodine server.
+`192.168.99.2` from the iodine server.
 
 
-MISC. INFO:
+### MISC. INFO
 
-IPv6:
+#### IPv6
 At the moment the iodined server only supports IPv4. The data inside the tunnel
 is IPv4 only.
 
 The client can use IPv4 or IPv6 nameservers to connect to iodined. The relay
 nameservers will translate between protocols automatically if needed. Use
-options -4 or -6 to force the client to use a specific IP version for its DNS
+options `-4` or `-6` to force the client to use a specific IP version for its DNS
 queries. The client has to force IPv4 if it has dual-stack connectivity and
-the hostname handling the tunnel domain has both A and AAAA records.
+the hostname handling the tunnel domain has both `A` and `AAAA` records.
 
-Routing:
+#### Routing
 It is possible to route all traffic through the DNS tunnel. To do this, first
 add a host route to the nameserver used by iodine over the wired/wireless
 interface with the default gateway as gateway. Then replace the default
@@ -138,70 +144,77 @@ shell (SSH) access, possibly with port forwarding. The latter can also be used
 for web browsing, when you run a web proxy (for example Privoxy) on your
 server.
 
-Testing:
+#### Testing
-The iodined server replies to NS requests sent for subdomains of the tunnel
+The `iodined` server replies to `NS` requests sent for subdomains of the tunnel
-domain. If your iodined subdomain is t1.mydomain.com, send a NS request for
+domain. If your iodined subdomain is `t1.mydomain.com`, send a `NS` request for
-foo123.t1.mydomain.com to see if the delegation works. dig is a good tool
+`foo123.t1.mydomain.com` to see if the delegation works.  
-for this:
+`dig` is a good tool for this:
-dig -t NS foo123.t1.mydomain.com
+
+	% dig -t NS foo123.t1.mydomain.com
+	ns.io.citronna.de.
 	
 Also, the iodined server will answer requests starting with 'z' for any of the
 supported request types, for example:
+
 	dig -t TXT z456.t1.mydomain.com
 	dig -t SRV z456.t1.mydomain.com
 	dig -t CNAME z456.t1.mydomain.com
+
 The reply should look like garbled text in all these cases.
 
-Operational info:
+
+Operational info
+----------------
+
 The DNS-response fragment size is normally autoprobed to get maximum bandwidth.
-To force a specific value (and speed things up), use the -m option.
+To force a specific value (and speed things up), use the `-m` option.
 
 The DNS hostnames are normally used up to their maximum length, 255 characters.
 Some DNS relays have been found that answer full-length queries rather
 unreliably, giving widely varying (and mostly very bad) results of the
-fragment size autoprobe on repeated tries. In these cases, use the -M switch
+fragment size autoprobe on repeated tries. In these cases, use the `-M` switch
-to reduce the DNS hostname length to for example 200 characters, which makes
+to reduce the DNS hostname length to, for example 200 characters, which makes
-these DNS relays much more stable. This is also useful on some "de-optimizing"
+these DNS relays much more stable. This is also useful on some “de-optimizing”
 DNS relays that stuff the response with two full copies of the query, leaving
-very little space for downstream data (also not capable of EDNS0). The -M
+very little space for downstream data (also not capable of EDNS0). The `-M`
 switch can trade some upstream bandwidth for downstream bandwidth. Note that
-the minimum -M value is about 100, since the protocol can split packets (1200
+the minimum `-M` value is about 100, since the protocol can split packets (1200
 bytes max) in only 16 fragments, requiring at least 75 real data bytes per
 fragment.
 
 The upstream data is sent gzipped encoded with Base32; or Base64 if the relay
-server supports mixed case and '+' in domain names; or Base64u if '_' is
+server supports mixed case and `+` in domain names; or Base64u if `_` is
 supported instead; or Base128 if high-byte-value characters are supported.
 This upstream encoding is autodetected. The DNS protocol allows one query per
 packet, and one query can be max 256 chars. Each domain name part can be max
 63 chars. So your domain name and subdomain should be as short as possible to
 allow maximum upstream throughput.
 
-Several DNS request types are supported, with the NULL and PRIVATE types
+Several DNS request types are supported, with the `NULL` and `PRIVATE` types
-expected to provide the largest downstream bandwidth. The PRIVATE type uses
+expected to provide the largest downstream bandwidth. The `PRIVATE` type uses
-value 65399 in the private-use range. Other available types are TXT, SRV, MX,
+value 65399 in the private-use range. Other available types are `TXT`, `SRV`,
-CNAME and A (returning CNAME), in decreasing bandwidth order.  Normally the
+`MX`, `CNAME` and `A` (returning `CNAME`), in decreasing bandwidth order.
-"best" request type is autodetected and used. However, DNS relays may impose
+Normally the “best” request type is autodetected and used. However, DNS relays
-limits on for example NULL and TXT, making SRV or MX actually the best choice.
+may impose limits on for example NULL and TXT, making SRV or MX actually the best
-This is not autodetected, but can be forced using the -T option.  It is
+choice. This is not autodetected, but can be forced using the `-T` option.
-advisable to try various alternatives especially when the autodetected request
+It is advisable to try various alternatives especially when the autodetected
-type provides a downstream fragment size of less than 200 bytes.
+request type provides a downstream fragment size of less than 200 bytes.
 
-Note that SRV, MX and A (returning CNAME) queries may/will cause additional
+Note that `SRV`, `MX` and `A` (returning `CNAME`) queries may/will cause
-lookups by "smart" caching nameservers to get an actual IP address, which may
+additional lookups by "smart" caching nameservers to get an actual IP address,
-either slow down or fail completely.
+which may either slow down or fail completely.
 
-DNS responses for non-NULL/PRIVATE queries can be encoded with the same set of
+DNS responses for non-`NULL/PRIVATE` queries can be encoded with the same set of
 codecs as upstream data. This is normally also autodetected, but no fully
 exhaustive tests are done, so some problems may not be noticed when selecting
 more advanced codecs. In that case, you'll see failures/corruption in the
 fragment size autoprobe. In particular, several DNS relays have been found that
-change replies returning hostnames (SRV, MX, CNAME, A) to lowercase only when
+change replies returning hostnames (`SRV`, `MX`, `CNAME`, `A`) to lowercase only
-that hostname exceeds ca. 180 characters. In these and similar cases, use the
+when that hostname exceeds ca. 180 characters. In these and similar cases, use
--O option to try other downstream codecs; Base32 should always work.
+the `-O` option to try other downstream codecs; Base32 should always work.
 
 Normal operation now is for the server to _not_ answer a DNS request until
-the next DNS request has come in, a.k.a. being "lazy". This way, the server
+the next DNS request has come in, a.k.a. being “lazy”. This way, the server
 will always have a DNS request handy when new downstream data has to be sent.
 This greatly improves (interactive) performance and latency, and allows to
 slow down the quiescent ping requests to 4 second intervals by default, and
@@ -209,24 +222,24 @@ possibly much slower. In fact, the main purpose of the pings now is to force
 a reply to the previous ping, and prevent DNS server timeouts (usually at
 least 5-10 seconds per RFC1035). Some DNS servers are more impatient and will
 give SERVFAIL errors (timeouts) in periods without tunneled data traffic. All
-data should still get through in these cases, but iodine will reduce the ping
+data should still get through in these cases, but `iodine` will reduce the ping
 interval to 1 second anyway (-I1) to reduce the number of error messages. This
-may not help for very impatient DNS relays like dnsadvantage.com (ultradns),
+may not help for very impatient DNS relays like `dnsadvantage.com` (ultradns),
 which time out in 1 second or even less. Yet data will still get trough, and
-you can ignore the SERVFAIL errors.
+you can ignore the `SERVFAIL` errors.
 
 If you are running on a local network without any DNS server in-between, try
--I 50 (iodine and iodined close the connection after 60 seconds of silence).
+`-I 50` (iodine and iodined close the connection after 60 seconds of silence).
 The only time you'll notice a slowdown, is when DNS reply packets go missing;
-the iodined server then has to wait for a new ping to re-send the data. You can
+the `iodined` server then has to wait for a new ping to re-send the data. You can
 speed this up by generating some upstream traffic (keypress, ping). If this
-happens often, check your network for bottlenecks and/or run with -I1.
+happens often, check your network for bottlenecks and/or run with `-I1`.
 
-The delayed answering in lazy mode will cause some "carrier grade" commercial
+The delayed answering in lazy mode will cause some “carrier grade” commercial
 DNS relays to repeatedly re-send the same DNS query to the iodined server.
 If the DNS relay is actually implemented as a pool of parallel servers,
 duplicate requests may even arrive from multiple sources. This effect will
-only be visible in the network traffic at the iodined server, and will not
+only be visible in the network traffic at the `iodined` server, and will not
 affect the client's connection. Iodined will notice these duplicates, and send
 the same answer (when its time has come) to both the original query and the
 latest duplicate. After that, the full answer is cached for a short while.
@@ -236,16 +249,19 @@ iodine client will ignore (if it ever arrives there).
 If you have problems, try inspecting the traffic with network monitoring tools
 like tcpdump or ethereal/wireshark, and make sure that the relaying DNS server
 has not cached the response. A cached error message could mean that you
-started the client before the server. The -D (and -DD) option on the server
+started the client before the server. The `-D` (and `-DD`) option on the server
 can also show received and sent queries.
 
 
-TIPS & TRICKS:
+TIPS & TRICKS
+-------------
 
 If your port 53 is taken on a specific interface by an application that does 
-not use it, use -p on iodined to specify an alternate port (like -p 5353) and 
+not use it, use `-p` on iodined to specify an alternate port (like `-p 5353`)
-use for instance iptables (on Linux) to forward the traffic:
+and use for instance iptables (on Linux) to forward the traffic:
+
 	iptables -t nat -A PREROUTING -i eth0 -p udp --dport 53 -j DNAT --to :5353
+
 (Sent in by Tom Schouten)
 
 Iodined will reject data from clients that have not been active (data/pings)
@@ -265,32 +281,33 @@ DNSCACHE_LEN is still advised, preferably 2 or higher, however you can also
 undefine it to save a few more kilobytes.
 
 
-PERFORMANCE:
+PERFORMANCE
+-----------
 
 This section tabulates some performance measurements. To view properly, use
 a fixed-width font like Courier.
 
 Measurements were done in protocol 00000502 in lazy mode; upstream encoding
-always Base128; iodine -M255; iodined -m1130. Network conditions were not
+always Base128; `iodine -M255`; `iodined -m1130`. Network conditions were not
 extremely favorable; results are not benchmarks but a realistic indication of
 real-world performance that can be expected in similar situations.
 
-Upstream/downstream throughput was measured by scp'ing a file previously
+Upstream/downstream throughput was measured by `scp`'ing a file previously
-read from /dev/urandom (i.e. incompressible), and measuring size with
+read from `/dev/urandom` (i.e. incompressible), and measuring size with
-"ls -l ; sleep 30 ; ls -l" on a separate non-tunneled connection. Given the
+`ls -l ; sleep 30 ; ls -l` on a separate non-tunneled connection. Given the
-large scp block size of 16 kB, this gives a resolution of 4.3 kbit/s, which
+large `scp` block size of 16 kB, this gives a resolution of 4.3 kbit/s, which
 explains why some values are exactly equal.
-Ping round-trip times measured with "ping -c100", presented are average rtt
+Ping round-trip times measured with `ping -c100`, presented are average rtt
 and mean deviation (indicating spread around the average), in milliseconds.
 
 
-Situation 1:
+### Situation 1: `Laptop  ->   Wifi AP   ->  Home server  ->  DSL provider  ->  Datacenter`
-Laptop  ->   Wifi AP   ->  Home server  ->  DSL provider  ->  Datacenter
+
 	 iodine    DNS "relay"        bind9           DNS cache        iodined
 	
 	                        downstr.  upstream downstr.  ping-up       ping-down
 	                        fragsize   kbit/s   kbit/s  avg +/-mdev   avg +/-mdev
-------------------------------------------------------------------------------
+	-----------------------------------------------------------------------------
 	
 	iodine -> Wifi AP :53
 	  -Tnull (= -Oraw)           982    43.6    131.0   28.0    4.6   26.8    3.4
@@ -314,19 +331,21 @@ iodine -> DSL provider :53
 	 [174.7* : these all have 2frag/packet]
 
 
-Situation 2:
+### Situation 2: `Laptop  ->  Wifi+vpn / wired  ->  Home server`
-Laptop  ->  Wifi+vpn / wired  ->  Home server
+
 	 iodine                            iodined
 	
 	                        downstr.  upstream downstr.  ping-up       ping-down
 	                        fragsize   kbit/s   kbit/s  avg +/-mdev   avg +/-mdev
-------------------------------------------------------------------------------
+	-----------------------------------------------------------------------------
 	
 	wifi + openvpn  -Tnull      1186   166.0   1022.3    6.3    1.3    6.6    1.6
 	
 	wired  -Tnull               1186   677.2   2464.1    1.3    0.2    1.3    0.1
 
 
+### Notes
+
 Performance is strongly coupled to low ping times, as iodine requires
 confirmation for every data fragment before moving on to the next. Allowing
 multiple fragments in-flight like TCP could possibly increase performance,
@@ -335,29 +354,33 @@ The current protocol scales performance with DNS responsivity, since the
 DNS servers are on average handling at most one DNS request per client.
 
 
-PORTABILITY:
+PORTABILITY
+-----------
 
 iodine has been tested on Linux (arm, ia64, x86, AMD64 and SPARC64), FreeBSD
 (ia64, x86), OpenBSD (x86), NetBSD (x86), MacOS X (ppc and x86, with
-http://tuntaposx.sourceforge.net/). and Windows (with OpenVPN TAP32 driver, see
+<http://tuntaposx.sourceforge.net/>). and Windows (with OpenVPN TAP32 driver, see
 win32 readme file).  It should be easy to port to other unix-like systems that
-has TUN/TAP tunneling support. Let us know if you get it to run on other
+have TUN/TAP tunneling support. Let us know if you get it to run on other
 platforms.
 
 
-THE NAME:
+THE NAME
+--------
 
 The name iodine was chosen since it starts with IOD (IP Over DNS) and since
 iodine has atomic number 53, which happens to be the DNS port number.
 
 
-THANKS:
+THANKS
+------
 
 - To kuxien for FreeBSD and OS X testing
 - To poplix for code audit
 
 
-AUTHORS & LICENSE:
+AUTHORS & LICENSE
+-----------------
 
 Copyright (c) 2006-2014 Erik Ekman <yarrick@kryo.se>, 2006-2009 Bjorn
 Andersson <flex@kryo.se>. Also major contributions by Anne Bezemer.
@@ -375,5 +398,5 @@ OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 PERFORMANCE OF THIS SOFTWARE.
 
 
-MD5 implementation by L. Peter Deutsch (license and source in src/md5.[ch])
+MD5 implementation by L. Peter Deutsch (license and source in `src/md5.[ch]`)
 Copyright (C) 1999, 2000, 2002 Aladdin Enterprises.  All rights reserved.