#irchelp

Split Mode

Split mode is a feature originally added in ircd-hybrid, and also present in many forks including ircd-ratbox and charybdis.

It is similar in purpose to IRCNet’s Channel Delay, but not in the details of its operation.

Split mode was introduced originally to deal with a loophole in the TS3 protocol which was then in use on EFNet and many other networks. Opless channels were considered a special case under TS3 - to prevent a situation where a channel could be indefinitely opless, the rules of the TS protocol were reversed when the side with the older TS was opless - meaning that the side with newer TS kept its modes, and TS was fast-forwarded to the newer TS, allowing a user to ride into an opless channel with ops.

While this was originally seen as a necessary feature, it created a situation where splits were actually desirable - and by the late 90s, botnets had created the potential to make splits happen via debilitating Distributed Denial of Service (DDoS) attacks against servers.

Split mode was effectively a bandage over the problem, rather than a permanent solution. By preventing users from creating channels during a netsplit, users could could no longer exploit netsplits to retake channels, and by preventing them from joining channels at all during a split, they could no longer use a split to bypass restrictive modes such as bans, limits, key, or invite only. This effectively made splits worthless to users.

Users prohibited from joining a channel because of split-mode receive the 437 numeric (ERR_UNAVAILRESOURCE), usually with a message of Nick/channel is temporarily unavailable. This is the same error as produced by Channel Delay.

The TS5 revision of the protocol closed the opless channel loophole, however the use of a split to join a channel against its restrictive modes remains possible, so split mode is still used by some networks and servers.

Split mode is a compile-time option, the parameters of which can be tuned by the server configuration or during operation by sufficiently privileged local IRC Operators.

An enabled server can choose one of two variation:

Split mode is controlled by two “split thresholds”, one for the number of users on the network, and one for the number of servers on the network. Default values of these parameters are set in the ircd.conf, and they may be adjusted via settable parameters with the SET command by an IRC operator.

Generally, split thresholds are set slightly below the normal operating condition of the network, to avoid false positives, and so that split mode isn’t inadvertently left active for an extended period of time because of one or two missing servers - if the network normally has 40 servers and doesn’t drop below 20000 users, a might be set to 35 servers and 18000 users as its thresholds.

If the server sees less than the split threshold of linked servers or global users, it enters split mode. Split mode can also be forced on or off by an IRC Operator in the case of malfunction.

IRC Operators were ordinarily exempted from these protections, allowing them to exploit network splits to recover opless channels on behalf of users, or to gather intel on locked down channels.

Split mode is considered complementary to TS protections, rather than an alternative to them as IRCNet’s Nick and Channel Delay

While TS5 removed the opless channel loophole, a side benefit of split mode remains: users can’t take advantage of a split to join a channel from which the modes of the channel would otherwise prohibit them from joining.