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diff --git a/sbin/dhclient/tests/Makefile b/sbin/dhclient/tests/Makefile
--- a/sbin/dhclient/tests/Makefile
+++ b/sbin/dhclient/tests/Makefile
@@ -2,6 +2,8 @@
ATF_TESTS_SH= pcp
+ATF_TESTS_PYTEST+= functional.py
+
# Tests assign a common IP address.
TEST_METADATA.pcp+= is_exclusive=true
diff --git a/sbin/dhclient/tests/functional.py b/sbin/dhclient/tests/functional.py
new file mode 100644
--- /dev/null
+++ b/sbin/dhclient/tests/functional.py
@@ -0,0 +1,243 @@
+#
+# Copyright (c) 2026 The FreeBSD Foundation
+#
+# This software was developed by Mark Johnston under sponsorship from
+# the FreeBSD Foundation.
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+
+import subprocess
+
+import pytest
+import scapy.all as sp
+from atf_python.sys.net.tools import ToolsHelper
+from atf_python.sys.net.vnet import VnetTestTemplate
+
+
+def mac_to_chaddr(mac_str):
+ """Convert a MAC address string to a 16-byte BOOTP chaddr."""
+ mac_bytes = bytes.fromhex(mac_str.replace(":", ""))
+ return mac_bytes + b"\x00" * (16 - len(mac_bytes))
+
+
+def get_dhcp_type(pkt):
+ """Extract the DHCP message type from a packet."""
+ if not pkt.haslayer(sp.DHCP):
+ return None
+ for opt in pkt[sp.DHCP].options:
+ if isinstance(opt, tuple) and opt[0] == "message-type":
+ return sp.DHCPTypes[opt[1]]
+ return None
+
+
+def build_reply(iface, server_ip, client_ip, client_mac, xid, msg_type,
+ extra_opts=None, sname=b"", file=b""):
+ """Build a DHCP OFFER or ACK packet."""
+ options = [
+ ("message-type", msg_type),
+ ("server_id", server_ip),
+ ("lease_time", 3600),
+ ("subnet_mask", "255.255.255.0"),
+ ("router", server_ip),
+ ]
+ if extra_opts:
+ options.extend(extra_opts)
+ options.append("end")
+
+ return (
+ sp.Ether(dst=client_mac, src=sp.get_if_hwaddr(iface))
+ / sp.IP(src=server_ip, dst="255.255.255.255")
+ / sp.UDP(sport=67, dport=68)
+ / sp.BOOTP(op=2,
+ xid=xid,
+ yiaddr=client_ip,
+ siaddr=server_ip,
+ chaddr=mac_to_chaddr(client_mac),
+ sname=sname,
+ file=file,
+ )
+ / sp.DHCP(options=options)
+ )
+
+
+class TestDhclient(VnetTestTemplate):
+ _SERVER_IP = "192.0.2.1"
+ _CLIENT_IP = "192.0.2.10"
+
+ TOPOLOGY = {
+ "vnet1": {"ifaces": ["if1"]},
+ "vnet2": {"ifaces": ["if1"]},
+ "if1": {},
+ }
+
+ def vnet1_handler(self, vnet):
+ """
+ Run dhclient in vnet1 (the client side of the epair). Send
+ back the output of "ifconfig <iface>" once dhclient has finished
+ its exchange.
+ """
+ ifname = vnet.iface_alias_map["if1"].name
+
+ # Wait for the test body to be ready before starting dhclient.
+ vnet.pipe.recv()
+
+ try:
+ result = subprocess.run(
+ ["/sbin/dhclient", "-c", "/dev/null",
+ "-l", "dhclient.lease", ifname],
+ capture_output=True, text=True, timeout=30,
+ )
+ stderr = result.stderr
+ except subprocess.TimeoutExpired:
+ stderr = "dhclient timed out"
+ out = ToolsHelper.get_output("/sbin/ifconfig {}".format(ifname))
+ lease = ""
+ try:
+ with open("dhclient.lease", "r") as f:
+ lease = f.read()
+ except FileNotFoundError:
+ pass
+ vnet.pipe.send({"ifconfig": out, "stderr": stderr, "lease": lease})
+
+ def _setup_server(self):
+ """Configure the server side of the epair and return its name."""
+ ifname = self.vnet.iface_alias_map["if1"].name
+ ToolsHelper.print_output(
+ "/sbin/ifconfig {} inet {}/24 up".format(ifname, self._SERVER_IP)
+ )
+ return ifname
+
+ def _do_dora(self, ifname, first_extra_opts=None, extra_opts=None,
+ sname=b"", file=b""):
+ """
+ Run a discover-offer-request-acknowledge exchange and
+ return (discover_count, result).
+
+ If first_extra_opts is set, it is used in the OFFER for the first
+ DHCPDISCOVER only; subsequent DISCOVERs use extra_opts.
+ """
+ discover_count = 0
+ done = False
+
+ def handle_dhcp(pkt):
+ nonlocal discover_count, done
+
+ msg_type = get_dhcp_type(pkt)
+ if msg_type is None:
+ return
+
+ client_mac = pkt[sp.Ether].src
+ xid = pkt[sp.BOOTP].xid
+
+ if msg_type == "discover":
+ discover_count += 1
+ opts = extra_opts
+ if discover_count == 1 and first_extra_opts is not None:
+ opts = first_extra_opts
+ reply = build_reply(
+ ifname, self._SERVER_IP, self._CLIENT_IP, client_mac,
+ xid, "offer", extra_opts=opts, sname=sname, file=file,
+ )
+ sp.sendp(reply, iface=ifname, verbose=0)
+
+ elif msg_type == "request":
+ reply = build_reply(
+ ifname, self._SERVER_IP, self._CLIENT_IP, client_mac,
+ xid, "ack", sname=sname, file=file,
+ )
+ sp.sendp(reply, iface=ifname, verbose=0)
+ done = True
+
+ sniffer = sp.AsyncSniffer(
+ iface=ifname,
+ filter="udp and dst port 67",
+ prn=handle_dhcp,
+ stop_filter=lambda _: done,
+ timeout=30,
+ started_callback=lambda: self.send_object(
+ self.vnet_map["vnet1"].pipe, "go",
+ ),
+ )
+ sniffer.start()
+ sniffer.join()
+
+ assert done, "DHCP exchange did not complete"
+ result = self.wait_object(self.vnet_map["vnet1"].pipe, timeout=15)
+ return discover_count, result
+
+ @pytest.mark.require_user("root")
+ def test_classless_routes_truncated(self):
+ """
+ Verify that dhclient rejects a DHCPOFFER containing a truncated
+ classless static routes option.
+
+ A malformed option 121 payload is constructed with a valid first
+ entry (default route, width=0) followed by a second entry with
+ width=24 that has only 1 byte of subnet data instead of the
+ required 3 bytes.
+
+ The test sends the bad offer first, then a valid offer, and
+ verifies that dhclient rejects the bad offer and successfully
+ acquires a lease from the good one.
+ """
+ ifname = self._setup_server()
+ bad_routes = (
+ b"\x00" # width=0 (default route)
+ b"\xc0\xa8\x01\x01" # gateway: 192.168.1.1
+ b"\x18" # width=24 (/24 subnet)
+ b"\x0a" # only 1 of 3 required subnet octets
+ )
+ discover_count, result = self._do_dora(
+ ifname, first_extra_opts=[(121, bad_routes)],
+ )
+ assert discover_count >= 2, \
+ "Expected dhclient to reject the first (bad) offer and retry"
+ assert "inet {}".format(self._CLIENT_IP) in result["ifconfig"]
+
+ @pytest.mark.require_user("root")
+ def test_filename_backslash(self):
+ """
+ Verify that backslashes in the BOOTP filename don't cause the
+ lease to be rejected. These can appear in Windows boot paths.
+ """
+ ifname = self._setup_server()
+ discover_count, result = self._do_dora(
+ ifname, file=b"\\windows\\boot\\pxeboot.com",
+ )
+ assert discover_count == 1, \
+ "Lease with backslash in filename should be accepted"
+ assert "inet {}".format(self._CLIENT_IP) in result["ifconfig"]
+
+ @pytest.mark.require_user("root")
+ def test_sname_valid(self):
+ """
+ Verify that a valid server name is preserved in the lease file.
+ """
+ ifname = self._setup_server()
+ discover_count, result = self._do_dora(
+ ifname, sname=b"dhcp.example.com",
+ )
+ assert discover_count == 1
+ assert "inet {}".format(self._CLIENT_IP) in result["ifconfig"]
+ assert "dhcp.example.com" in result["lease"]
+
+ @pytest.mark.require_user("root")
+ def test_sname_quote(self):
+ """
+ Verify that a double quote in the BOOTP server name causes the
+ server name to be discarded but does not cause the lease to be
+ rejected. Values from the lease file may be handled by
+ dhclient-script and thus are susceptible to shell injection.
+ """
+ ifname = self._setup_server()
+ discover_count, result = self._do_dora(
+ ifname, sname=b'evil"server',
+ )
+ assert discover_count == 1, \
+ "Lease with quote in sname should be accepted"
+ assert "inet {}".format(self._CLIENT_IP) in result["ifconfig"]
+ assert 'evil"server' not in result["lease"], \
+ "Server name with quote should not appear in the lease file"
+ assert "server-name" not in result["lease"], \
+ "Unsafe server name should be discarded entirely"

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