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IODINE(8) System Manager's Manual IODINE(8)


iodine, iodined - tunnel IPv4 over DNS


iodine [-v]

iodine [-h]

iodine [-4] [-6] [-f] [-r] [-u user ] [-P password ] [-m fragsize ] [-t chrootdir ] [-d device ] [-R rdomain ] [-m fragsize ] [-M namelen ] [-z context ] [-F pidfile ] [-T dnstype ] [-O downenc ] [-L 0|1 ] [-I interval ] [ nameserver ] topdomain

iodined [-v]

iodined [-h]

iodined [-c] [-s] [-f] [-D] [-u user ] [-t chrootdir ] [-d device ] [-m mtu ] [-l listen_ip ] [-p port ] [-n ( auto | external_ip ) ] [-b dnsport ] [-P password ] [-z context ] [-F pidfile ] [-i max_idle_time ] tunnel_ip [ /netmask ] topdomain


iodine lets you tunnel IPv4 data through a DNS server. This can be useful in situations where Internet access is firewalled, but DNS queries are allowed. It needs a TUN/TAP device to operate. The bandwidth is asymmetrical, with a measured maximum of 680 kbit/s upstream and 2.3 Mbit/s downstream in a wired LAN test network. Realistic sustained throughput on a Wifi network using a carrier-grade DNS cache has been measured at some 50 kbit/s upstream and over 200 kbit/s downstream. iodine is the client application, iodined is the server.

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.


Common Options:

Print version info and exit.
Print usage info and exit.
Keep running in foreground.
Drop privileges and run as user 'user' after setting up tunnel.
Chroot to 'chrootdir' after setting up tunnel.
Use the TUN device 'device' instead of the normal one, which is dnsX on Linux and otherwise tunX.
Use 'password' to authenticate. If not used, stdin will be used as input. Only the first 32 characters will be used.
Apply SELinux 'context' after initialization.
Create 'pidfile' and write process id in it.

Client Options:

Force IPv4 DNS queries
Force IPv6 DNS queries
Skip raw UDP mode. If not used, iodine will try getting the public IP address of the iodined host and test if it is reachable directly. If it is, traffic will be sent to the server instead of the DNS relay.
Use OpenBSD routing domain 'rdomain' for the DNS connection.
Force maximum downstream fragment size. Not setting this will cause the client to automatically probe the maximum accepted downstream fragment size.
Maximum length of upstream hostnames, default 255. Usable range ca. 100 to 255. Use this option to scale back upstream bandwidth in favor of downstream bandwidth. Also useful for DNS servers that perform unreliably when using full-length hostnames, noticeable when fragment size autoprobe returns very different results each time.
DNS request type override. By default, autodetection will probe for working DNS request types, and will select the request type that is expected to provide the most bandwidth. However, it may turn out that a DNS relay imposes limits that skew the picture, which may lead to an "unexpected" DNS request type providing more bandwidth. In that case, use this option to override the autodetection. In (expected) decreasing bandwidth order, the supported DNS request types are: NULL, PRIVATE, TXT, SRV, MX, CNAME and A (returning CNAME). Note that SRV, MX and A may/will cause additional lookups by "smart" caching nameservers to get an actual IP address, which may either slow down or fail completely. The PRIVATE type uses value 65399 (in the 'private use' range) and requires servers implementing RFC 3597.
Force downstream encoding type for all query type responses except NULL. Default is autodetected, but may not spot all problems for the more advanced codecs. Use this option to override the autodetection. Base32 is the lowest-grade codec and should always work; this is used when autodetection fails. Base64 provides more bandwidth, but may not work on all nameservers. Base64u is equal to Base64 except in using underscore ('_') instead of plus sign ('+'), possibly working where Base64 does not. Base128 uses high byte values (mostly accented letters in iso8859-1), which might work with some nameservers. For TXT queries, Raw will provide maximum performance, but this will only work if the nameserver path is fully 8-bit-clean for responses that are assumed to be "legible text".
Lazy-mode switch. -L1 (default): Use lazy mode for improved performance and decreased latency. A very small minority of DNS relays appears to be unable to handle the lazy mode traffic pattern, resulting in no or very little data coming through. The iodine client will detect this and try to switch back to legacy mode, but this may not always work. In these situations use -L0 to force running in legacy mode (implies -I1).
Maximum interval between requests (pings) so that intermediate DNS servers will not time out. Default is 4 in lazy mode, which will work fine in most cases. When too many SERVFAIL errors occur, iodine will automatically reduce this to 1. To get absolute minimum DNS traffic, increase well above 4, but not so high that SERVFAIL errors start to occur. There are some DNS relays with very small timeouts, notably (ultradns), that will give SERVFAIL errors even with -I1; data will still get trough, and these errors can be ignored. Maximum useful value is 59, since iodined will close a client's connection after 60 seconds of inactivity.

Server Options:

Disable checking the client IP address on all incoming requests. By default, requests originating from non-matching IP addresses will be rejected, however this will cause problems when requests are routed via a cluster of DNS servers.
Don't try to configure IP address or MTU. This should only be used if you have already configured the device that will be used.
Increase debug level. Level 1 prints info about each RX/TX packet. Implies the -f option. On level 2 (-DD) or higher, DNS queries will be printed literally. When using Base128 upstream encoding, this is best viewed as ISO Latin-1 text instead of (illegal) UTF-8. This is easily done with : "LC_ALL=C luit iodined -DD ..." (see luit(1)).
Set 'mtu' as mtu size for the tun device. This will be sent to the client on login, and the client will use the same mtu for its tun device. Default 1130. Note that the DNS traffic will be automatically fragmented when needed.
Make the server listen only on 'listen_ip' for incoming requests. By default, incoming requests are accepted from all interfaces.
Make the server listen on 'port' instead of 53 for traffic. If 'listen_ip' does not include localhost, this 'port' can be the same as 'dnsport'. Note: You must make sure the dns requests are forwarded to this port yourself.
The IP address to return in NS responses. Default is to return the address used as destination in the query. If external_ip is 'auto', iodined will use web service to retrieve the external IP of the host and use that for NS responses.
If this port is specified, all incoming requests not inside the tunnel domain will be forwarded to this port on localhost, to be handled by a real dns. If 'listen_ip' does not include localhost, this 'dnsport' can be the same as 'port'. Note: The forwarding is not fully transparent, and not advised for use in production environments.
Make the server stop itself after max_idle_time seconds if no traffic have been received. This should be combined with systemd or upstart on demand activation for being effective.

Client Arguments:

The nameserver to use to relay the dns traffic. This can be any relaying nameserver or the server running iodined if reachable. This field can be given as an IPv4/IPv6 address or as a hostname. This argument is optional, and if not specified a nameserver will be read from the /etc/resolv.conf file.
The dns traffic will be sent as queries for subdomains under ´topdomain'. This is normally a subdomain to a domain you own. Use a short domain name to get better throughput. If nameserver is the iodined server, then the topdomain can be chosen freely. This argument must be the same on both the client and the server.

Server Arguments:

This is the server's ip address on the tun interface. The client will be given the next ip number in the range. It is recommended to use the or ranges. The default netmask is /27, can be overridden by specifying it here. Using a smaller network will limit the number of concurrent users.
The dns traffic is expected to arrive as queries for subdomains under 'topdomain'. This is normally a subdomain to a domain you own. Use a short domain name to get better throughput. This argument must be the same on both the client and the server. Queries for domains other than 'topdomain' will be forwarded when the -b option is given, otherwise they will be dropped.


See the README file for both a quick test scenario, and a detailed description of real-world deployment.


Login is a relatively secure challenge-response MD5 hash, with the password never passing the wire. However, all other data is NOT encrypted in any way. The DNS traffic is also vulnerable to replay, injection and man-in-the-middle attacks, especially when iodined is used with the -c option. Use of ssh or vpn tunneling is strongly recommended. On both server and client, use iptables, pf or other firewalls to block all traffic coming in from the tun interfaces, except to the used ssh or vpn ports.



If the environment variable IODINE_PASS is set, iodine will use the value it is set to as password instead of asking for one. The -P option still has precedence.


If the environment variable IODINED_PASS is set, iodined will use the value it is set to as password instead of asking for one. The -P option still has precedence.


The README file in the source distribution contains some more elaborate information.


File bugs at


Erik Ekman <> and Bjorn Andersson <>. Major contributions by Anne Bezemer.

JUN 2014 User Manuals