2.4 Sockets and Descriptors

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2.4.1 The native socket #

The native door is a SOCK_STREAM Unix socket at /run/resolvd/resolv.sock (PSPU §6.4), inside resolvd's runtime directory rather than directly in /run, which resolvd's account cannot write.

At startup resolvd:

1. creates /run/resolvd if peinit has not, and sets its mode to 0755;
2. replaces the directory's DACL (below);
3. removes any existing /run/resolvd/resolv.sock, logging removed a stale /run/resolvd/resolv.sock at warn level when there was one, and a removal that fails is fatal (§2.2);
4. binds the socket, sets its mode to 0666, and replaces its DACL in the same way;
5. makes the listener nonblocking.

2.4.1.1 A socket left by a previous run #

resolvd leaves its socket behind when it ends (§2.2), so a start that follows an earlier run finds one unless something else has removed it. The previous run gave its own account full access to the directory and the socket (below), so the new process removes the file in step 3, logs the removed a stale … line, and binds a fresh socket. This holds for every restart of the service: svctl restart, a stop and a start, and a restart by the policy after resolvd exits or is killed.

2.4.1.2 The DACL on the directory and socket #

The DACL written to both the directory and the socket is:

resolvd's own entry is what lets the next run start. Everyone's read, write and execute include neither DELETE nor deleting a child, so without it the next start could not remove the stale socket: it would exit with native socket: Permission denied (os error 13), and the restart policy would loop on that until the file was removed by hand. If resolvd cannot read its own identity it logs at warn level

could not read resolvd's own identity (<error>); a restart may not be able to replace <path>

and writes the DACL without that entry.

2.4.1.3 The owner of the directory and socket #

Only the DACL is written, so each object keeps the owner it was created with. peinit creates /run/resolvd before resolvd starts (§2.1), so the directory is owned by SYSTEM; resolvd creates the socket, which is owned by resolvd's account. Reaching the socket grants nothing: every request is checked against the control object (§5.2).

2.4.1.4 A descriptor that cannot be written #

If the descriptor cannot be written, resolvd logs at error level

could not set a descriptor on <path> (<error>); programs other than SYSTEM and administrators will not be able to reach the socket

and carries on. The socket then keeps whatever descriptor it was created with, and for ordinary processes the native door — and with it the NSS shim — is unreachable while the stub listener still works (§9.3).

2.4.2 The stub listener #

• Two sockets, a UDP socket and a TCP listener, are both bound to 127.0.0.53 port 53 and both nonblocking (PSPU §6.8).
• Nothing is bound on ::1 or on any other address.

Binding port 53 depends on the port reservation (§2.1).

2.4.3 Other descriptors #

2.4.3.1 Connections whose request has been read #

A connection whose request has been read keeps occupying a slot in its door's 256-connection allowance until resolution completes or a definite hangup/error abandons it. The engine's attempts and timeouts (§4.6) govern this phase; the request-delivery deadline no longer applies. A completed answer that cannot be written immediately moves to the separate bounded reply-writer queue (§2.5).

UDP questions share the one listener socket, but retain at most 256 waiting origins in total and 16 per source address and port (§6.1).

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