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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.


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)

Run 'make' to compile the server and client binaries.
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)


Try it out within your own LAN! Follow these simple steps:
- On your server, run: ./iodined -f
  (If you already use the network, use another internal net like
- Enter a password
- On the client, run: ./iodine -f -r
  (Replace with your server's ip address)
- Enter the same password
- Now the client has the tunnel ip and the server has
- Try pinging each other through the tunnel
- Done! :)
To actually use it through a relaying nameserver, see below.


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:
To use this tunnel, you need control over a real domain (like,
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
-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 completely transparent.)

Then, delegate a subdomain (say, to the iodined server.
If you use BIND for your domain, add two lines like these to the zone file:

t1		IN	NS		; note the dot!
t1ns		IN	A

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
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,
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).

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:

t1		IN	NS	; note the dot!

Then reload or restart your nameserver program. Now any DNS queries for
domains ending in will be sent to your iodined server.

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, and the second argument is the assigned domain (in this
case 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
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.

Resulting commandline in this example situation:
./iodined -f -c -P secretpassword

Client side: 
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 ( 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
server's address as first argument (in the example: or 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).

The client's tunnel interface will get an IP close to the server's (in this
case 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.

Resulting commandline in this example situation:
./iodine -f -P secretpassword
(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 from the iodine client, and or .3 etc. from the iodine server.


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
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.

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
gateway with the iodined server's IP address inside the DNS tunnel, and
configure the server to do NAT.

However, note that the tunneled data traffic is not encrypted at all, and can
be read and changed by external parties relatively easily. For maximum
security, run a VPN through the DNS tunnel (=double tunneling), or use secure
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

The iodined server replies to NS requests sent for subdomains of the tunnel
domain. If your iodined subdomain is, send a NS request for to see if the delegation works. dig is a good tool
for this:
dig -t NS

Also, the iodined server will answer requests starting with 'z' for any of the
supported request types, for example:
dig -t TXT
dig -t SRV
dig -t CNAME
The reply should look like garbled text in all these cases.

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.

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
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"
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
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
bytes max) in only 16 fragments, requiring at least 75 real data bytes per

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
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
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,
CNAME and A (returning CNAME), in decreasing bandwidth order.  Normally the
"best" request type is autodetected and used. However, DNS relays may impose
limits on for example NULL and TXT, making SRV or MX actually the best choice.
This is not autodetected, but can be forced using the -T option.  It is
advisable to try various alternatives especially when the autodetected 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
lookups by "smart" caching nameservers to get an actual IP address, which may
either slow down or fail completely.

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
that hostname exceeds ca. 180 characters. In these and similar cases, use 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
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
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
interval to 1 second anyway (-I1) to reduce the number of error messages. This
may not help for very impatient DNS relays like (ultradns),
which time out in 1 second or even less. Yet data will still get trough, and
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).
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
speed this up by generating some upstream traffic (keypress, ping). If this
happens often, check your network for bottlenecks and/or run with -I1.

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
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.
Delayed duplicates that arrive at the server even later, get a reply that the
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
can also show received and sent queries.


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 
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)
for more than 60 seconds. Similarly, iodine will exit when no downstream
data has been received for 60 seconds. In case of a long network outage or
similar, just restart iodine (re-login), possibly multiple times until you get
your old IP address back. Once that's done, just wait a while, and you'll
eventually see the tunneled TCP traffic continue to flow from where it left
off before the outage.

With the introduction of the downstream packet queue in the server, its memory
usage has increased with several megabytes in the default configuration.
For use in low-memory environments (e.g. running on your DSL router), you can
decrease USERS and undefine OUTPACKETQ_LEN in user.h without any ill conse-
quence, assuming at most one client will be connected at any time. A small
DNSCACHE_LEN is still advised, preferably 2 or higher, however you can also
undefine it to save a few more kilobytes.


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
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
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
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
and mean deviation (indicating spread around the average), in milliseconds.

Situation 1:
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

iodine -> Home server :53
  -Tnull (= -Oraw)          1174    48.0    305.8   26.6    5.0   26.9    8.4

iodine -> DSL provider :53  
  -Tnull (= -Oraw)          1174    56.7    367.0   20.6    3.1   21.2    4.4
  -Ttxt -Obase32             730    56.7    174.7*
  -Ttxt -Obase64             874    56.7    174.7
  -Ttxt -Obase128           1018    56.7    174.7
  -Ttxt -Oraw               1162    56.7    358.2
  -Tsrv -Obase128            910    56.7    174.7
  -Tcname -Obase32           151    56.7     43.6
  -Tcname -Obase128          212    56.7     52.4

iodine -> DSL provider :53  
  wired (no Wifi) -Tnull    1174    74.2    585.4   20.2    5.6   19.6    3.4

 [174.7* : these all have 2frag/packet]

Situation 2:
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

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,
but it would likely cause serious overload for the intermediary DNS servers.
The current protocol scales performance with DNS responsivity, since the
DNS servers are on average handling at most one DNS request per client.


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 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


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.


- To kuxien for FreeBSD and OS X testing
- To poplix for code audit


Copyright (c) 2006-2014 Erik Ekman , 2006-2009 Bjorn
Andersson . Also major contributions by Anne Bezemer.

Permission to use, copy, modify, and distribute this software for any purpose
with or without fee is hereby granted, provided that the above copyright notice
and this permission notice appear in all copies.


MD5 implementation by L. Peter Deutsch (license and source in src/md5.[ch])
Copyright (C) 1999, 2000, 2002 Aladdin Enterprises.  All rights reserved.