diff --git a/src/tunnel.zig b/src/tunnel.zig
index b7389f0..d5423be 100644
--- a/src/tunnel.zig
+++ b/src/tunnel.zig
@@ -1,3 +1,4 @@
+const builtin = @import("builtin");
 const std = @import("std");
 const os = std.os;
 const mem = std.mem;
@@ -6,18 +7,25 @@ const log = std.log.scoped(.nyotun);
 
 const IFF_TUN = 0x0001; // <linux/if_tun.h>
 const IFF_NO_PI = 0x1000; // <linux/if_tun.h>
-const TUNSETIFF = 0x400454ca; // write a C program, run and see
+
+// git -C ~/code/go grep TUNSETIFF
+const TUNSETIFF = switch (builtin.target.cpu.arch) {
+    .x86, .x86_64, .arm, .aarch64 => 0x400454ca,
+    else => @compileError("unsupported architecture"),
+};
 
 dev: [posix.IFNAMESIZE:0]u8,
-tunFile: posix.fd_t,
+tun_file: posix.fd_t,
 
 pub fn init(devname: ?[]const u8) !Tunnel {
-    const tunFile = try posix.open(
+    const tun_file = try posix.open(
         "/dev/net/tun",
+        // NB: vtun just uses RDWR, tailscale-go adds O_NONBLOCK.
+        // Not sure why I would add O_NONBLOCK, so just not doing that.
         posix.O{ .ACCMODE = .RDWR, .CLOEXEC = true },
         0,
     );
-    errdefer posix.close(tunFile);
+    errdefer posix.close(tun_file);
 
     var ifr = mem.zeroInit(
         posix.ifreq,
@@ -29,20 +37,23 @@ pub fn init(devname: ?[]const u8) !Tunnel {
         @memcpy(ifr.ifrn.name[0..name.len], name);
     }
 
-    const err = posix.system.ioctl(tunFile, TUNSETIFF, @intFromPtr(&ifr));
+    const err = posix.system.ioctl(tun_file, TUNSETIFF, @intFromPtr(&ifr));
     if (posix.errno(err) != .SUCCESS) {
         log.err("unable to set TUNSETIFF: {s}", .{@tagName(posix.errno(err))});
         return error.OpenDevTun;
     }
 
-    var tunnel = Tunnel{ .dev = .{0} ** posix.IFNAMESIZE, .tunFile = tunFile };
+    var tunnel = Tunnel{
+        .dev = .{0} ** posix.IFNAMESIZE,
+        .tun_file = tun_file,
+    };
     const ifname = mem.sliceTo(&ifr.ifrn.name, 0);
     @memcpy(tunnel.dev[0..ifname.len], ifname);
     return tunnel;
 }
 
 pub fn close(self: *Tunnel) void {
-    posix.close(self.tunFile);
+    posix.close(self.tun_file);
 }
 
 const Tunnel = @This();