diff --git a/artiq/firmware/Cargo.lock b/artiq/firmware/Cargo.lock
index 6ef0db1fb..c9cb970ef 100644
--- a/artiq/firmware/Cargo.lock
+++ b/artiq/firmware/Cargo.lock
@@ -157,7 +157,7 @@ dependencies = [
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "logger_artiq 0.0.0",
  "proto 0.0.0",
- "smoltcp 0.4.0-pre (git+https://github.com/m-labs/smoltcp?rev=bc2a894)",
+ "smoltcp 0.4.0-pre (git+https://github.com/m-labs/smoltcp?rev=5cb2dcd)",
  "std_artiq 0.0.0",
 ]
 
@@ -183,7 +183,7 @@ dependencies = [
 [[package]]
 name = "smoltcp"
 version = "0.4.0-pre"
-source = "git+https://github.com/m-labs/smoltcp?rev=bc2a894#bc2a894c00c9e0cbc51d67c01f456e45b45abcc2"
+source = "git+https://github.com/m-labs/smoltcp?rev=5cb2dcd#5cb2dcd1aad1fabc6b4ee3959873b98e78b0cc7e"
 dependencies = [
  "byteorder 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -221,7 +221,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum log_buffer 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ec57723b84bbe7bdf76aa93169c9b59e67473317c6de3a83cb2a0f8ccb2aa493"
 "checksum managed 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "61eb783b4fa77e8fa4d27ec400f97ed9168546b8b30341a120b7ba9cc6571aaf"
 "checksum rustc-cfg 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "56a596b5718bf5e059d59a30af12f7f462a152de147aa462b70892849ee18704"
-"checksum smoltcp 0.4.0-pre (git+https://github.com/m-labs/smoltcp?rev=bc2a894)" = "<none>"
+"checksum smoltcp 0.4.0-pre (git+https://github.com/m-labs/smoltcp?rev=5cb2dcd)" = "<none>"
 "checksum walkdir 1.0.3 (registry+https://github.com/rust-lang/crates.io-index)" = "dd7c16466ecc507c7cb5988db03e6eab4aaeab89a5c37a29251fcfd3ac9b7afe"
 "checksum winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
 "checksum winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"
diff --git a/artiq/firmware/runtime/Cargo.toml b/artiq/firmware/runtime/Cargo.toml
index 069a8da1f..f0879db36 100644
--- a/artiq/firmware/runtime/Cargo.toml
+++ b/artiq/firmware/runtime/Cargo.toml
@@ -32,6 +32,6 @@ features = ["mem"]
 
 [dependencies.smoltcp]
 git = "https://github.com/m-labs/smoltcp"
-rev = "bc2a894"
+rev = "5cb2dcd"
 default-features = false
 features = ["alloc", "collections", "log"]#, "verbose"]