Output Address Selection
Select the output address based on cmdline.
oas is a library wrapper around the connect() syscall and can be used to influence the IPv6 source address for outgoing TCP connections.
In IPv6, nodes might be using multiple IP addresses simultaneously. In fact, this is the normal case in many networks. Different IP networks might be configured with different privileges, making it important to use the correct source IPv6 address for outgoing connections.
While many programs allow selecting IPv6 source addresses, others don't. Also, the source address mechanisms provided by the operating system (see RFC6724, ip addrlabel on Linux) have some known shortcomings. For example, Linux takes the routing decision for locally originated datagrams before doing IPv6 source address selection, which might cause the system to send the outgoing datagram to the wrong default gateway where it might be dropped.
oas influences the IPv6 source address of an outgoing connection early enough for the routing decision to be taken correctly.
Any nonhistorical Linux System with /proc and glibc should do. If it doesn't work on Debian Stable, it's a bug.
TCP/IPv6 outgoing Connections. Future versions may support UDP. There are no plans to implement IPv4 support.
Basically it is a wrapper around connect(2). Before calling the real connect(), oas tries to bind an address to the socket, according to the configuration. If no available address matches the configuration or the bind(2) fails for any reason, your program should work like it does without oas.
oas is not needed on a router. When a datagram comes in from another system, its source address is already set and thus the in-kernel mechanisms can work as designed.
If all your software allows to set the IPv6 source address, you probably don't nee oas as well. It might be, however, easier to have source address selection configured in a central place.
- Build oas.so with ./compile
- Place it at your convenience
- Let the environment variable LD_PRELOAD point to it
- Create a configuration file
- Let the environment variable OAS_CONF_F point to it
- Adjust the environment variable OAS_LOGLEVEL to your needs
- Start your programm
Lines starting with # or containing nothing but whitespaces are ignored. A line starting with cmdlines: starts a list of Posix Extended Regular expressions, each on a line of its own. If your cmdline contains one of them the corresponding address pattern list will be used and all further cmdline-patterns will be ignored.
A line starting with addresses: starts a list of address Pattern, again one per line. If your system has a configured address matching one of these patterns, it is chosen for the output address.
An address pattern specifies the value of an address to match and a mask for relevant bits. The mask may be given as prefix length. Address and mask are separated by "/".
You may have a look at config_example.
There are currently no sanity checks for the configuration.
Logging goes to syslog. You can set the level with the environment variable OAS_LOGLEVEL. Default is NOTICE. Available are EMERG,ALERT,CRIT,ERR,WARN,NOTICE,INFO and DEBUG.