iodine - https://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

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)

QUICKSTART

Try it out within your own LAN! Follow these simple steps:

• 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 172.16.0.0.
• Enter a password.
• On the client, run: ./iodine -f -r 192.168.0.1 test.com. Replace 192.168.0.1 with your server's ip address.
• Enter the same password.
• 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.
• Done! :)

To actually use it through a relaying nameserver, see below.

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

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 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, for example zone transfers will not work.) Alternatively you can forward the subdomain from your DNS server to iodined which must then run on a different port (-p).

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 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	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.

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 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 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 192.168.99.1 t1.mydomain.com

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 (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 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 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 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.

Resulting commandline in this example situation, adding -r forces DNS tunneling even if raw UDP tunneling would be possible:

./iodine -f -P secretpassword t1.mydomain.com

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 192.168.99.2 from the iodine server.

MISC. INFO

IPv6

The data inside the tunnel is IPv4 only.

The server listens to both IPv4 and IPv6 for incoming requests by default. Use options -4 or -6 to only listen on one protocol. Raw mode will be attempted on the same protocol as used for the login.

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.

If your server is listening on IPv6 and is reachable, add an AAAA record for it to your DNS setup. Extending the example above would look like this:

t1		IN	NS	t1ns.mydomain.com.		; note the dot!
t1ns		IN	A	10.15.213.99
t1ns		IN	AAAA	2001:db8::1001:99

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 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 server.

Testing

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 foo123.t1.mydomain.com to see if the delegation works. dig is a good tool for this:

% 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.

Mac OS X

On Mac OS X 10.6 and later, iodine supports the native utun devices built into the OS - use -d utunX.

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 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 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) queri